Model Boat Mayhem
The Shipyard ( Dry Dock ): Builds & Questions => Navy - Military - Battleships: => Topic started by: El Neave on February 03, 2011, 06:03:16 pm
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Apologies for any howlers- This is my first posting and my first howl. Advice from "those who know better" would be welcome indeed.
I have been building a 1/96 Ticonderoga for the past 20 months. I have reached the point where without the knowledge from others, ambition
risks being thwarted! Amazingly so. Question; how much lead or lead shot would one have to put along the centre line to prevent
the wretched vessel capsizing due to its inordinate top hamper? If anyone has any idea, or has built this ship, say how many pounds
or better still kilogrammes since I live in France, should I align on the centre line to avoid ignominy and the scoffing of the locals,
in the event of trying to sail it in say a 10 knot wind?
Any advice welcomed with a gratitude born of desperation!
El Neave.
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The only real way to do this is to put the completed boat in water along with installed batteries and then ballast it down to the waterline.
The ballast should be kept low and if possible fixed to the hull sides rather than keel. This will help to prevent roll. Weight in the middle of the hull acts like a yacht keel and your boat will rock and roll.
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El Neave ...please re-read the experienced words provided by DickyD :-)).......the second sentence is of particular importance O0 ....
So simple.......but if you have any question....just ask.....Derek
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Much tugging of forelock for the advice and much note taken of your second sentence, DickyD. Since Rocking and Rolling is highly unfashionable in these latter days, I will do my best to avoid it. When
you suggest aligning the ballast along the sides of the hull, I take you to mean aligned on the inner side of the hull. Such an asinine remark is a further revelation of bestial ignorance, to be sure. But in these matters it is always better to admit such virtues if only to avoid the consequences if one misinterprets excellent advice.
Your empirical suggestion, to "get thee to a pond" is more than sound. I dread it however, since the nearest pond is in front of a very well known Château which brings with it the not inconsiderable disadvantage of the event being attended by a cast of thousands of onlookers from 5 to 555 years old. Thus, the risk of parts suddenly disappearing or - worse still - being trodden under the foot of Man, Beast, Child or insect is so great that no insurance company would even give you quotes!
One other question: which, in your opinion, is better - shot in specially built silos as low as possible along the inner hull or lead plates?
El Neave, a little wiser but still a long way to go!
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Much tugging of forelock for the advice and much note taken of your second sentence, DickyD. Since Rocking and Rolling is highly unfashionable in these latter days, I will do my best to avoid it. When
you suggest aligning the ballast along the sides of the hull, I take you to mean aligned on the inner side of the hull. Such an asinine remark is a further revelation of bestial ignorance, to be sure. But in these matters it is always better to admit such virtues if only to avoid the consequences if one misinterprets excellent advice.
Your empirical suggestion, to "get thee to a pond" is more than sound. I dread it however, since the nearest pond is in front of a very well known Château which brings with it the not inconsiderable disadvantage of the event being attended by a cast of thousands of onlookers from 5 to 555 years old. Thus, the risk of parts suddenly disappearing or - worse still - being trodden under the foot of Man, Beast, Child or insect is so great that no insurance company would even give you quotes!
One other question: which, in your opinion, is better - shot in specially built silos as low as possible along the inner hull or lead plates?
El Neave, a little wiser but still a long way to go!
Ballast inside hull is best idea. {-)
I use lead plates stuck in with silicone.
For testing tank, if boat to big for bath I use white melamine screwed together to form box with joints sealed with silicone. Empty after use and keep dry for next time. :-))
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El Neave ...surely in modern France someone must have a bathtub long enough to trail ballast your 1/96 Ticonderoga {-)
An alternate method of trial and error is to take a set of bathroom scales....lay flat on your vessel :kiss: & add downwards force until the correct waterline is achieved..... :-))
:o ...at that point memorise the mass/weight displayed....this then provides you with the known mass of ballast required :police: :police:
Lead shot is OK.....could have advantages in spreading the ballast evenly........then add some epoxy resin to set the ballast........Derek
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Why is there a differeance between laying lead weight along the inner sides and laying in flat on the bottom of the hull? I would have thought it would make no differance.
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Why is there a differeance between laying lead weight along the inner sides and laying in flat on the bottom of the hull? I would have thought it would make no differance.
One for the yachties I feel :-)) Centre of mass??
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Why is there a difference between laying lead weight along the inner sides and laying in flat on the bottom of the hull? I would have thought it would make no difference.
It can affect the righting moment as the hull heels. If the weight is at the sides then it takes more effort to move the side of the hull out of the water than if it was pivoting around the weight at the centre of the hull. The same thing applies in a fore and aft direction. The turning characteristics will be different if you have the weight at the ends of the hull rather than concentrating it in the middle. In the former case the model will be more directionally stable but harder to turn, in the latter it will be more agile abut also more sensitive to the rudder and may need constant correcting to steer in a straight line.
It all comes down to the basic laws of physics with particular reference to leverage.
Colin
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But doesn't the centre of Gravity always act through 1 point of the hull? If so then placing the lead on the sides or in on the bottom wont affect the centre of gravity - it should act through the same point.
I can understand a yatch with weight in the keel to lower the centre of gravity and I can understand the centre of gravity moving fore and aft affting how a boat steers.
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Try thinking counter balances.
_V___________________________V_
I
Does that help ?
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It's to do with the pendulum effect. Imagine you have a see saw with equal weights on each end. It will go up and down relatively slowly as there is a mass at the end of each arm. If you put both weights over the pivot you can flick it up and down with hardly any effort at all because you have more leverage. So in a model it will roll more slowly if the weights are placed at the sides.
The analogy of a yacht is not quite right as in effect you have a vertical see saw here with the weight of the keel being balanced by the weight of the mast, rigging, sails and wind strength and pivoting around a point somewhere in the middle of the hull. It's also complicated by the fact that as the yacht moves forward the foil cross section of the keel also exerts a force towards keeping the vessel upright.
The forthcoming Model Boats Construction Special, due for publication at the beginning of May, will feature a comprehensive article on model boat stability along with other items on the questions which commonly crop up on the model boating forums.
Colin
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Another free ad Colin. ok2
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It's to do with the pendulum effect. Imagine you have a see saw with equal weights on each end. It will go up and down relatively slowly as there is a mass at the end of each arm. If you put both weights over the pivot you can flick it up and down with hardly any effort at all because you have more leverage. So in a model it will roll more slowly if the weights are placed at the sides.
Thanks. But if there was a see saw with the same weight distributed evenly across its entire lenght - wouldn't that be just as easy to to flick up and down and move relatively easily? An empty see saw still has the weight of the plank and that moves easily with a flick, no harder than if you place equal weights at either end.
If a boat is more stable with weight on its sides rather than laid out evenly on its bottom, there must be another explantion beccause the math only require centre of gravity, centre of buoyancy and the differance in the vertical height between them to determine the stability of of a static body in water.
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In the book "Warships and Warship Modelling by Dave Wooley there is a section on ballasting model hulls - particularly the narrower warship hull
in which it describes where to put the weight
M - metacentre
G - centre of gravity
B - centre of buoyancy
K - keel
states the weight is best placed below the centre of gravity but above the centre of buoyancy
and goes on to say this is best placed on the side of the hull.
it also states if you place it in the centre the ship will be very stiff and will look unnatural when righting from a heel
This is just another way of saying what colin and Dickie have already said
Geoff
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I'm interested in this discussion.
First of all, are we talking about this type of ship?
http://en.wikipedia.org/wiki/File:Spruance_and_Ticonderoga_lead_ships_in_class.jpg
If so then...
I agree that weight either side will dampen roll and improve initial stability.
But should we not be concerned with keeping the centre of gravity central and low given the upper weight.
Just thoughts at the moment.
Regards,
Kim
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Trying to explain in a term I understand ( call it idiotspeak :-))) imagine a destroyer model with the weight placed in the bow and stern, down to its waterline, in a chop the model would be prone to pitch into waves rather than ride easily over them if it had all its weight concentrated midships. Now turn the concept 90 degrees, so that the effect is acting across the vessel rather than fore and aft, I think that makes sense....
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In the book "Warships and Warship Modelling by Dave Wooley there is a section on ballasting model hulls - particularly the narrower warship hull
in which it describes where to put the weight
M - metacentre
G - centre of gravity
B - centre of buoyancy
K - keel
states the weight is best placed below the centre of gravity but above the centre of buoyancy
and goes on to say this is best placed on the side of the hull.
it also states if you place it in the centre the ship will be very stiff and will look unnatural when righting from a heel
This is just another way of saying what colin and Dickie have already said
Geoff
M, G, B. I can understand centre of gravity (you want this to be as low as possible to increase stability - so add weight to the lowest point in the hull), I can understand centre of Buoyancy (you cant change this as it is a function of the hull design of a ship, a sub is another matter entirely) and the you want the metacentric height to be as great as possible to increase stability.
What I don't understand is why weight on the sides of the hull will make any difference when it does not alter the centre of Gravity.
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Trying to explain in a term I understand ( call it idiotspeak :-))) imagine a destroyer model with the weight placed in the bow and stern, down to its waterline, in a chop the model would be prone to pitch into waves rather than ride easily over them if it had all its weight concentrated midships. Now turn the concept 90 degrees, so that the effect is acting across the vessel rather than fore and aft, I think that makes sense....
No it wont, if the centre of gravity of the destroyer the with weight either end is the same as the destroyer with the weight dead in the centre then both destroyers should behave in the same way. And both destroyers will ballance about their mid point. (same as the see saw analogy - doest matter if you have 2 equal weights either end or spread evenly across the the see saw, if the centre of gravity passes through the pivot of the see saw the see saw will balance)
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Are we not mixing up different things here? Where you place the weights on the see saw will affect the inertia entailed in moving the arms. The centre of gravity will remain exactly where it is. The overall stability will also remain the same but the way in which it manifests itself, i.e. quick or slow roll will change.
Colin
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Are we not mixing up different things here? Where you place the weights on the see saw will affect the inertia entailed in moving the arms. The centre of gravity will remain exactly where it is. The overall stability will also remain the same but the way in which it manifests itself, i.e. quick or slow roll will change.
Colin
Ahh... ok we migt be tying it down. So how you do you work out the initeria roll of a ship?
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Hi all,
I have seen 1:1, 1:72 and 1:96 scale Ticoderoga class cruisers and they all ROLL about and I mean ROLL
El Neave, what my friends who have built models of them have done is put the model in the water and place in your batterries
then some extra weight to finish the ballasting as far down in side the hull as possible ballast the model so the top of the waterline is at the water level.
then you should be ok to sail, watch out for strong side wind though :-))
also what did you make your mast out of ? please say plastic tubing and not brass :embarrassed:
here is a video clip of a 1:72 scale Tico I took http://www.youtube.com/user/gingyer#p/u/6/U1Etw17HGX4 (http://www.youtube.com/user/gingyer#p/u/6/U1Etw17HGX4)
I hope it helps see what I was saying about the ballasting to the waterline
Colin
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the amount of ballast depends on how you have built your upper works and what materials you have used and what type of electrics and what type of batterers you are using.
All i can say is ballast your boat to the water line and see what happens ,if that doesn't work then maybe you will have to hang some weight from a small Kiel about 2in long witch will lower the center of gravity and act like yacht to keep it up right but make it removable for showing the model at shows.
bat44
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Are we not mixing up different things here? Where you place the weights on the see saw will affect the inertia entailed in moving the arms. The centre of gravity will remain exactly where it is. The overall stability will also remain the same but the way in which it manifests itself, i.e. quick or slow roll will change.
Colin
I been doing a bit of reading and everything I've seen so far suggest the roll inertial of a ship is related to the Metacentric height and the mass of the ship, and NOT related to if weight is placed on the sides of the ship or laid flat on the bottom of the hulll. This matches my experience balasting and trimming countless submabarines that love roll by virtue of their shape, and unlike a ship, varing metacentric height as the sub progresses from surfaced to submerged trim.
This Wiki article simplipies it http://en.wikipedia.org/wiki/Metacentric_height
May be a rewrite of that forthcomming article is in order???
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Well, the guy who wrote it is chief engineer on a very large ship and a qualified marine engineer so I am prepared to defer to his expertise...
Colin
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Well, the guy who wrote it is chief engineer on a very large ship and a qualified marine engineer so I am prepared to defer to his expertise...
Colin
Maybe that very large ship had fuel tanks with waterplane inertial moments, something models dont have to contend with.
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We may be at cross purposes here. The original argument posited the placing of the ballast along the ship's centreline at the bottom of the hull as opposed to placing it AT the sides of the bottom, not ON the sides of the ship which would be higher up and thus reduce stability. In which case the see saw effect previously described would surely come into effect.
Given the cross section of a submarine hull it would be difficult to replicate this as the widest beam is around half way up the height of the hull but not so on a full bodied surface ship. However, as far as models go I don't suppose there would be very much practiical difference if you were to simply lay a lead sheet uniformly across the bottom of the hull. In reality the weight of batteries is probably spread across 75% or so of the bottom of the hull anyway.
Colin
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Yep that right with submarines. Still with flat bottomed ships, all the math and see saw analogies point to their being no differance regardless of if the weight is place on the sides of the bottom or in the centreline on the bottom. If the centre of gravity is the same both ships will act in the same way.
Water acts on the ship as a whole. It does not see the ship as made of different parts with different densities. It sees and treats is as a sold piece of metal of fixed mass fixed centre of gravity & centre of buoyancy (assuming not movable internal mass like fuel). It matters not which part of the ship is heavier than than another; it does not matter if all the weight is on the sides or centre line - mass and centre of gravity remain constant.
For a full sized ship I can well understand the need to keep fuel low down in the hull and to the sides of the hull so them can be pumped to either side to help with trim and cargo imbalance.
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Ok, I can understand the reference to the see saw on the horizontal plane. A see saw is fixed to the ground and the centre stays equal distance from one end to the other. So same downward force applied at each end ... sounds ok so far?
So sticking with the see saw idea...
Problem with a boat is the centre of the see saw effect moves as the boat heels right?
Also lets add another see saw this time in the vertical plane same idea right ? the more the boat heels the greater the force at the 'sky' end of the see saw. ... still with me?
Now this part is a guess ... she is probably a fairly shallow draft? or low draft to beam ratio
So looking at this photo of one in a high speed turn and if pushed to give advice I'd be suggesting as much ballast on the centre line and as low as possible. I suspect more than just dampening the roll will be needed even if the Superstructure is light ..... Obviously i haven't even thought about windage ...
http://www.naval-technology.com/projects/ticonderoga/ticonderoga6.html
Perhaps even better than my ramblings is Dicky's suggestion of a test tank before doing anything permanent.
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Another free ad Colin. ok2
Yep but where are the free books O0 O0 for supportive Mayhemers <:( <:( <:(
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Stability may be defined as the ability of a floating object to return to its original position when a force displacing it is removed.
Whether you put the weight at the sides of the bottom of the hull or in the centre will make no difference to the centre of gravity and thence stability but may affect the way it returns to the stable position.
Imagine you are on a rocking horse. You are sitting on the saddle over the centre of gravity and can whip too and fro to your heart's content until you lose your breakfast. If you stop rocking you will finish in an upright position. Now imagine a couple of long poles sticking out of the ends of the horse (poor creature) with equal heavy weights on them. You can still rock and you will still return to the upright position when you stop, but the motion will be slower and more ponderous. In a model hull this will tend to give a more realistic rolling effect.
Colin
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Not qiute, if you add two poles and stick weights on them you are lowering the centre of gravity even further and increasing mass, and that why you gain roll stability stablity. The same thinkg would happen if you added those two weight to the bottom of the rocking horse. This is explained mathematically in the Wiki Article I linked to. All this is because the Centre of Gravity and the centre of Boyancy all act through a single point.
Quote from the Wiki article I linked to before
StabilityGM and rolling period
GM has a direct relationship with a ship's rolling period. A ship with a small GM will be "tender" - have a long roll period - an excessively low or negative GM increases the risk of a ship capsizing in rough weather (see HMS Captain or the Vasa). It also puts the vessel at risk of potential for large angles of heel if the cargo or ballast shifts (see Cougar Ace). A ship with low GM is less safe if damaged and partially flooded because the lower metacentric height leaves less safety margin. For this reason, maritime regulatory agencies such as the IMO specify minimum safety margins for sea-going vessels. A larger metacentric height, on the other hand can cause a vessel to be too "stiff"; excessive stability is uncomfortable for passengers and crew. This is because the stiff vessel quickly responds to the sea as it attempts to assume the slope of the wave. An overly stiff vessel rolls with a short period and high amplitude which results in high angular acceleration. This increases the risk of damage to the ship as well as the risk cargo may break loose or shift. In contrast a "tender" ship lags behind the motion of the waves and tends to roll at lesser amplitudes. A passenger ship will typically have a long rolling period for comfort, perhaps 12 seconds while a tanker or freighter might have a rolling period of 6 to 8 seconds.
The period of roll can be estimated from the following equation[2] - 9see the article for the equation)
Where g is the gravitational constant, k is the radius of gyration about the longitudinal axis through the center of gravity and is the stability index.
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Thank 'ee, shipmates, one and all!
One thing of which I am certain. It is a Ticonderoga class ship, I'm building and, moreover, its masts are plastic not brass! That at least is something I appear to have got right! Finding a French
bathtub of more than 1 metre 70 is likely to be "challenging" because 1) neighbours are VERY territorial and bathrooms reveal far more than the next door neighour ought in decency to know! 2) because of the trend towards incredibly small showers. Best solution, make my own test tank.
An excellent tutorial. But like most tutorials, one has to go away and THINK, mutter and ponder over the pearls of wisdom offered. Since my math and physics are medieval, this too is going to be another and yet more 'weighty' "Challenge" Viewed the you tube videos of a Tico AND a type 23. Inspiring and encouraging. Noted the water line on the former. Noted the swan as well. The type 23 seemed bolder by far in skirting our feathered friend. I shall use lead shot, siliconed together and, after having performed the various operations suggested by DickyD and Colin, with hopefully just the right amount distributed down each side - inside! And may The Lord have mercy on the ship and all who sail her!
Once again, many thanks for such a barrage of practice and theory. Expected no less from Mayhemers and it's a real pleasure to have one's expectations so amply born out.
El Neave.
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Glad the video helped the type23 was mine and I was aiming at the swan but it moved >>:-( {-) {-)
If you put the lead shot into small bags and move them around the hull until you fond the best position before you silicone them in
I found my video I had of a 1:96th scale Tico and as you can see there is a bit of a cross wind and shows how it rolls about
http://www.youtube.com/user/gingyer#p/u/4/x2HWTp83J4k (http://www.youtube.com/user/gingyer#p/u/4/x2HWTp83J4k)
any more help please ask :-))
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An excellent tutorial. But like most tutorials, one has to go away and THINK, mutter and ponder over the pearls of wisdom offered. Since my math and physics are medieval, this too is going to be another and yet more 'weighty' "Challenge" Viewed the you tube videos of a Tico AND a type 23. Inspiring and encouraging. Noted the water line on the former. Noted the swan as well. The type 23 seemed bolder by far in skirting our feathered friend. I shall use lead shot, siliconed together and, after having performed the various operations suggested by DickyD and Colin, with hopefully just the right amount distributed down each side - inside! And may The Lord have mercy on the ship and all who sail her!
Getting back to the the original question :D You want to have all your weight as low down as possible and keep the super stucture as light as possible (may be some holees can be drilled in redundent material with the superstructure to lighten it. Go for low profile batteries like NMHydride, rather than top heavy gel cells. Can you motors be fitted lower in the hull (may be with belt drive couple rather than universals if the angles to the prop shaft are extreme). Does the Tico have hull stablizers - is so get them working (with a sub pitch controler or just hook them up to the rudder channel). And weigh the boat down toits waterline. I matters not where you place the weigth in the hull, as long as its as low as possible.
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the Ticonderogas don't have stabilizers
which is a shame as they could do with them %)
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My second model was a Billing Mary Ann trawler which I built in 1982, I fitted shingle ballast (!) into one of the front compartments, second one back I believe and the same towards the stern to balance the model to its waterline, then fitted the deck over the compartment, permanently sealing in the ballast. The model spent most of its time as a very wet boat, ploughing through the waves rather than riding over them, havn't done this since, havn't had the problem since. Theory sucks, welcome to practical applications :-)).
Actually theory has its place, but theory is usually based on practical experimentation, which i seem to have spent the last .... years doing at the local lake, with the latest insane projects :}
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This argument is rocking back and fore (pun intended) and not being seen. Unless you get the rocking horse or see-saw and do it, postulating will not get to the answer.
However, here is an example we have all seen, and I hope can make the evidence clearer.
Ice skater spinning on his/her axis. Pulls arms in and goes faster, stretches arms out and goes slower again. Mass/weight is the same. C of G is the same, otherwise they would spin out or fall over. Only the rate of change of direction/rotation has changed.
I could postulate why this is, but we have all seen it.
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I could postulate why this is, but we have all seen it.
Angular momentum.... (Doesn't she appear in 'Holiday on Ice'?) as in conservation of momentum.
Colin
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Angular momentum.... (Doesn't she appear in 'Holiday on Ice'?) as in conservation of momentum.
Colin
Uno momentum, while I look that up %)
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Angular momentum.... (Doesn't she appear in 'Holiday on Ice'?) as in conservation of momentum.
Colin
Big girl!
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all this talk about center of this and that and the of laws of this and that, how about ballast the model to the water line hope you have built big enough comings to keep out the water and have fun sailing your model, because don't forget you are sailing a scale model on not so scale water
bat44
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all this talk about center of this and that and the of laws of this and that, how about ballast the model to the water line hope you have built big enough comings to keep out the water and have fun sailing your model, because don't forget you are sailing a scale model on not so scale water
bat44
I prefer to sail on alcohol, more scale density and you have the bonus of throwing yourself in it afterwards :-))
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As one famous character is reputed to have said, "Sometimes, I sits and sometimes I sits and thinks!" After that splendid barrage of information, I shall go deep and if lucky, do the latter. The throw away
remark about combings, now that is worrying. Shall go down, carefully examine all parts of the decks and superstructures and if necessary take remedial action. Better late than never. Suspect it will indeed be necessary. Huge thanks to one and all.
And, now following equally weighty advice, back to the gin bottle. Or rather, in these parts, Calvados - a most flavoursome kind of apple brandy and much recommended for the generation of thought, though somewhat deleterious in its effects on action!
God knows what its specific gravity is, but in a misspent youth I once came across a newly distilled batch that made surgical alcohol seem like Coca Cola.
One gets one's inspiration where one can. Thanks for yours,
As ever,
El Neave.
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As one famous character is reputed to have said, "Sometimes, I sits and sometimes I sits and thinks!" After that splendid barrage of information, I shall go deep and if lucky, do the latter. The throw away
remark about combings, now that is worrying. Shall go down, carefully examine all parts of the decks and superstructures and if necessary take remedial action. Better late than never. Suspect it will indeed be necessary. Huge thanks to one and all.
And, now following equally weighty advice, back to the gin bottle. Or rather, in these parts, Calvados - a most flavoursome kind of apple brandy and much recommended for the generation of thought, though somewhat deleterious in its effects on action!
God knows what its specific gravity is, but in a misspent youth I once came across a newly distilled batch that made surgical alcohol seem like Coca Cola.
One gets one's inspiration where one can. Thanks for yours,
As ever,
El Neave.
I like this man's sense of humour............ %)
Martin doon under
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Lifeboats do self-right by pendulum effect in simple terms. Spreading the additional mass using lead sheeting helps to distribute mass across the available width (beam) to help resist rolling induced by turning or windage/wave action. In these conditions the hull enjoys greater roll resistance and the roll rate is reduced. Riders of motorcycle combinations understand this effect and quickly recognise the advantage of 'ballasting' their sidecar when riding solo.
The see-saw analogy may be misleading since there is a plank but no firm surface or pivot as such, however, concentrating weight within a hull along the centreline axis does unfortunately introduce a pivot effect and lateral roll resistance is diminished. It's true of course, that the water is not 'scaled', but this may be to our advantage since it's greater relative density probably gives greater support to our model hulls. This is my 'take' on the topics, for what it's worth. :-)
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Lifeboats do self-right by pendulum effect in simple terms. Spreading the additional mass using lead sheeting helps to distribute mass across the available width (beam) to help resist rolling induced by turning or windage/wave action. In these conditions the hull enjoys greater roll resistance and the roll rate is reduced.
Life boats self right because they have large metacentric height and bouyancy cambers higher up that are not subject to waterplane effect when upturned. Reduced roll is a consequence of the large metacentric height and the mass of the life boat. Placement of any mass across across the beam is purely for practical purposes. A ping pong ball with a piece of lead epoxied to one end will always 'self right', no need to glue a line of smaller lead bits to achieve the same effect.
Riders of motorcycle combinations understand this effect and quickly recognise the advantage of 'ballasting' their sidecar when riding solo.
In a side car, typically weight is greater through the front two wheels due the location of the centre of gravity (motorcycle and rider being heavier than the side car). Placing of weight in the side car is an attempt to shift the centre of gravity so weight is better distributed between the 3 wheels. A boat ofcouse is supported by water equally along its length as buoyancy is related to the displacement (volume) of the hull in water, not the mass of any particular part of the hull which is in the water - displacement of a hull cannot be altered by moving lead from the centreline towards the sides.
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Enough of the theorising! This morning I put the partly completed hull of my Revell QM2 (which is about three feet long and 3.75" in the beam) in the bath and ballasted her down with four identical fishing weights. First of all I laid the weights along the centreline then pushed down on one side and let go to study the roll characteristics. Then I rearranged the weights along each side of the bottom of the hull and did the same. In the first test the hull whipped upright immediately, in the second the roll period was much longer - not just a little bit but a lot. So in both instances the centre of gravity, centre of buoyancy and metacentric height were exactly the same - only the physical arrangement of the ballast in the same horizontal plane had changed and it did make a lot of difference.
Obviously roll periods are influenced by the position of the centre of gravity too. This is why damaged battleships tend to capsize as they are built to be 'tender' to give a stable gun platform and it doesn't take very much to destabilise them.
Perhaps someone else would like to have a go?
Colin
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Hi Colin I'm happy to agree with your findings -
Its bit like a rowing boat ...
put two weights either side and the boat remains stable and the boat has a gentle recovery rate - put the same weights to the centre line and the boat remains stable but the recovery rate is quicker and more 'violent'
So the theory is proved in (as i pointed towards in an earlier posting) initial or static stability -
Now, does the same theory hold in Dynamic stability and ballastring a model?
Going back to the rowing boat idea this time using two equally built and weighted people ... If conditions started to get a bit 'lively' on the water .... would the people not naturally move to the centre line?
There you go that should keep the discussion going a bit longer :)
Regards,
Kim
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would the people not naturally move to the centre line?
You can see that happening when people who have hired rowing boats bring them into the bank to get off. They half stand up in the middle causing the boat to oscillate rapidly and they then make matters worse by trying to keep their balance instead of sitting down again. Unfortunately the instinctive reaction is not the right one, sometimes with unwanted consequences....
Colin
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You can see that happening when people who have hired rowing boats bring them into the bank to get off. They half stand up in the middle causing the boat to oscillate rapidly and they then make matters worse by trying to keep their balance instead of sitting down again. Unfortunately the instinctive reaction is not the right one, sometimes with unwanted consequences....
Colin
OK, Take that idea further...
If we stick a superstructure on our rowing boat does that not have a similar effect to the standing up scenario?
Does the superstructure on our boats not contribute to roll and the need to compensate for the roll ?
Also if we put the weight out to the sides do we have a quicker onset of vanishing stability as the immersed hull shape changes pattern, centre of buoyancy moves etc...?
Rgds
Kim
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should I align on the centre line to avoid ignominy and the scoffing of the locals,
in the event of trying to sail it in say a 10 knot wind?
Any advice welcomed with a gratitude born of desperation!
El Neave.
Going back to the original question -
The advise sought was for ballasting to allow sailing in a 10knt wind (the upper end of F3 , the point when some sailors might start to think about reefing?)
Does that change the advise given ?
I'd really like to know the correct answer but with so many variables i suspect this isn't the last time that Ballasting and stability are discussed and i look forward to seeing the article - Hopefully that will answer some of my questions questions.
Rgds,
Kim
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There still seems to be confusion here. If you stand up in a rowing boat or add superstructure then you will raise the centre of gravity and reduce stability accordingly. My experiment has nothing to do with that at all. It just shows that if you move the ballast out to the sides of the hull the boat will roll more slowly and thus more realistically. The basic stability will not be affected. I suppose it is possible that if the boat rolls more slowly it may be more susceptible to being clobbered by a wave on the beam which might tend to push it over a bit further before it can recover but that's something else again.
As long as the boat has a good margin of basic stability stays fully watertight and the ballast etc. is firmly fixed down then it shouldn't matter how rough the conditions are within reasonable limits. But extreme conditions could overcome that. Just imagine your model broadside on in moderate surf at the beach. It would literally be picked up and hurled over bodily and not many models are strong enough to cope with something like that. In practice the danger often comes from short sharp waves at one end of a pond which have been kicked up by the wind blowing down the length of the water. Under these conditions the model will tend to be swamped although it will still be OK if 100% watertight. However most models are not 100% watertight in these conditions and often the superstructure is not firmly latched down. Once the superstructure is knocked off that's it! I have seen this happen on a number of occasions - example below.
Colin
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Kim,
Going back to your original question, there is no magic formula. As others have said, if you have a relatively top heavy model then make sure she is ballasted right down to her waterline with the ballast as low as possible in the hull. As my experiment demonstrates, the rolling characteristics will to some extend depend on how you spread the ballast over the bottom of the hull. To play safe spread it evenly over the hull bottom as much as you can as this will hep keep the centre of gravity down compared with piling it up in the middle or at the sides.
If the model is unhappy with the conditions this will become evident as it will roll about all over the place with the decks regularly going under - then you need to think about bringing her in! The tug in the photos above was caught out along with several other models as general conditions on the pond were OK but the wind was causing waves to pile up at one end and these were being reflected back off the concrete surround which produced a localised but very nasty chop. The tug was flung about so much that the superstructure came off and it duly sank. Fortunately it was recovered and proved to be OK when dried out.
Where was it? At Wicksteed, where the Mayhem gatherings are held..... %)
Colin
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You know there is a quick solution to this that is to send Dicky D , Coilin and me tickets to go and help Dickand Colin can help and i will stand guard and keep all the well meaning on lookers from standing on ore nicking bits of the ship when your back is turned and i know what your answer will be {-) {-) {-) >>:-( >>:-(
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Colin, personally if I were you I wouldn't bother any more, You have explained it in several different ways, all are logical and backed up by
various modelmakers/ books on the subject, but you are still going to have doubting Thomas's.
Geoff
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Well done on carrying out an experiment! I was going to do something similar myself but couldn't find the energy. Could those lead weights at the side of the hull been a little higher due to the hull curvature? If the centre of gravity is raised a little because of this the boat will have a longer roll period as you noticed. Setting the weight down the centre line is like that is very hard to get the centre of gravity dead through the centre like of the hull as you don't have any significant moment forces to help you with fine tuning by movingthe weigths around. I was thinking about placing lead shot spread evenly along the bottom of the hull, rather than just on the sides.
Yes battle ships are relatively top heavy with deck armour and heavy turret and have longer roll periods. I see if I can hijack my sons bath toy boat which is flat bottomed and repeat the experiment.
Geoff, I am out the correct answer for myself , not to proved anyone right or wrong. Until a few days ago I had done little reading on the subject. Everthing thing I have read on the matter, all the equations given suggest that the Centre of gravity, mass, and Metacentric height are what is important in determing roll stability and period ( assuming no change in hull shape) in a static vessel. Always better to be a "Doubting Thomas" than a Lemming. I have never been one to subscribe to the notion that "this is the way to do it, because this is the way it has always been done". I have yet to see anything in print that explain why placing mass laterally will make any difference (assuming no changing the CoG, yet I have come across plenty of explanations that suggest there is no difference.
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I'm still not getting it yet ....
So can someone explain to me why Yachts have keels / weights concentrated in the centre. (i know some have bilge keels leave that bit of learning for another day )
rgds
Kim
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Mankster, you are grasping at straws! The bottom of the QM2 is dead flat and the experiment was quite conclusive. I think you are mixing up two areas of physics here, stability and conservation of angular momentum! Both can result in varying roll periods for quite different reasons and are quite separate. What do you think bilge keels are for? They substitute a resistance for weight at the side of the ship to slow down the rolling period, just two sides of the same coin. I appreciate you have read up on metacentric heights et al and it's all true but it is not the only influence upon rolling periods, check out the science of levers as well.
Kim, I have already explained about yachts. The keel is designed to counter the pressure on the sails to stop the boat blowing flat. If a yacht is dismasted it becomes the most uncomfortable thing on this planet as the weight of the keel with no counterbalance jerks it upright and scrambles the brains of the crew!
Sorry guys but this is all pretty basic stuff really.
Colin
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Ok, Colin i can understand the counter balance idea, my only concern was that a superstructure might present itself as an ''over sheeted sail'' and anything that can be done to help right the boat might be a good thing.
Sorry if this is basic to you Colin but I am still learning, should we just accept and not question? I'm just surprised that the same physics applied to real boats is equally acceptable to models given that many factors are not scaled to the same degree.
In conclusion are we saying that weight spread over the bottom of the hull is an ideal to all types of boat without sails?
Happy to be wrong if we get the right answer...
Regards,
Kim
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I remember reading somewhere that a destroyer in WW2 did avoid capsize and 'righted' in spite of a registered 35 degree roll during severe weather.
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Being thoroughly confused it is time to put in ten cents worth.
The thread became confusing with the seesaw example which is not applicable to boats, ie buoyancy.
Don't think I am a doubting "Thomas" just trying to make sense of opposing view points.
A vessel is not simply supported at one point, the pivot as per the seesaw, which is more appropriate as an example of lever and fulcrum example.
It seems to me that a hull is supported along the entire length of the plank of the seesaw (there's that seesaw again) and not at one point and can also be said is evenly supported along the plank length.
The thread asked about a specific hull, namely a cruiser, it seems to me that the placement of ballast in say the QM, would be different than in the warship, as I take it that a warship has a keel and being "pointy" requires less load to get down to water line whereas the flat bottom needs more, but we are not talking about amount of ballast but where to place it.
It is evident by the comments that ballasting is not cut and dried and compromises may have to be made such as do you want a boat that steers well or goes fast or a combination.
So examples and commentary of how the hull (in this case the warship) and also say other hull shape eg tugs, would react with ballast in alternate positions is required and the the builder can choose their own weapon of destruction.
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Here we go again,
In my basic understanding i would suspect that in most model boats that the actuall CoG is higher than than that dipicted in builders drawings.
Hence my reason for advising a more central and lower positioning of ballast to counter act the Cog.
Afterall, is our primary concern not (given the investment in time and money) to keep the boat upright?
more questions .....
Regards,
Kim
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Observations on this thread.
There are suppositions that we can make, that are inherently wrong. Even if they feel right.
This is where designers and builders can get into trouble.
There was a lot of knowledge among artisans that was based on common practice.
In answer to a question, 'Why do we do it this way?'. The answer would often be, 'We've always done it that way'.
And unless the method changed it was good enough for those who did not have the resources to experiment.
And so there are two distinct questions.
First, what is the best way to ...?
Second, why is ... the best way to ...?
The first will give us a solution to a problem.
The second may give us an answer to a question, or just more questions that we do not know how to answer.
This thread has turned into the latter.
Just my 2 cents.
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OK, i have found an article that makes sense without using complicated techy formulas etc -
You can read it here -
http://casanovasadventures.com/catalog/watersports/p40104.htm
Seems to tie in with what has been suggested so far -
So my new understanding goes something like this -
- In model boats righting arm maybe a better theory to observe to improve overall stability?
- Where we place the ballast should be predicted by righting arm and hull shape?
Am i getting somewhere?
All this new theory should help me give new life to a rather tippy boat i have gathering dust lol
Regards,
Kim
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Ah well, said he, scratching his pate, and now to the Open University for a course in hydrodynamics! Clearly, the penalties of NOT getting things right are grim, as your photos of the tugboat suggested, Colin. But supposing I do
by the sheer laws of probability, manage to get things right, I will think very seriously of issuing tickets to all and sundry since I have the idea of testing the ship out on July 14th with White Ensigns flying from fore and main; This is likely to incense the St Germanois mightily. Protection will almost certainly be needed. Support for this provocative act from Mayhemers will be greatly appreciated!
Before that, the building of a test tank is obvious. Better than a bath any day and more so if one remembers the immortal words of the late O. Nash
"I test my bath before I sit
and it always moves me to wonderment.
That which chills the finger not a bit
Is so frigid to the fundament."
Test tanks do away with this form of prior verification!
And finally just as a gesture to all who have cudgelled their wits on the matter of ballasting and its location, here is a pict of the ship. There will be cries of horror at the White Ensign flying on an American hull. But, curiously after
twenty years spent over here, one grows strangely patriotic. Accordingly I have renamed this Ship. In my flotilla it will be HMS Quiberon Bay - a strategically highly significant victory on November 20th 1759 - some say almost as
important as Trafalgar - at least for us and the Canadians, though the Québecois will not take so even handed a view. The same battle is known over here as the bataille des Cardinaux, they being the rocks off which it was
fought on the Britanny coast. Not surprisingly, few if any over here remember it!
El Neave.
(http://s4.postimage.org/vhy32o4k/Jo_Mags_001_Kids_Xmas_2010.jpg) (http://postimage.org/image/vhy32o4k/)
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Thanks..... O0 about blu##y time we were invited to see .jpgs of the vessel..... >>:-( <*< why not post more?,,,,,,,, O0 Derek
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The only real way to do this is to put the completed boat in water along with installed batteries and then ballast it down to the waterline.
Test tanks do away with this form of prior verification!
(http://s4.postimage.org/vhy32o4k/Jo_Mags_001_Kids_Xmas_2010.jpg) (http://postimage.org/image/vhy32o4k/)
As i said earlier maybe that is the best piece of advise given - maybe that should be the stock answer the next time stability is questioned.
Do as we have allways done and not question...
Bon Chance! with her launch she looks good and i hope she performs on the water as you wish.
Regards,
Kim
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And finally just as a gesture to all who have cudgelled their wits on the matter of ballasting and its location, here is a pict of the ship. There will be cries of horror at the White Ensign flying on an American hull. But, curiously after
twenty years spent over here, one grows strangely patriotic. Accordingly I have renamed this Ship. In my flotilla it will be HMS Quiberon Bay - a strategically highly significant victory on November 20th 1759 - some say almost as
important as Trafalgar - at least for us and the Canadians, though the Québecois will not take so even handed a view. The same battle is known over here as the bataille des Cardinaux, they being the rocks off which it was
fought on the Britanny coast. Not surprisingly, few if any over here remember it!
El Neave.
(http://s4.postimage.org/vhy32o4k/Jo_Mags_001_Kids_Xmas_2010.jpg) (http://postimage.org/image/vhy32o4k/)
Interestingly, the Royal Navy was offered the first four of the class, when the delays set in on the Type 45 project, so you can legitimately fly the White Ensign as a ' What If' project. Also, its good to see some practical application of the theorys being discussed here.
I had a small destroyer which had to be fitted with a ballast weight on a removable rod, about 6" below the hull ( the model rolled over and sank on its first outing, a result of all that is being discussed here!). A gentleman watching the model commented on the models tendency to heel in 'all the wrong ways' on a turn. When I pulled the model out of the water to reveal the 'yacht like' keel hanging under the model, his statement was "that would explain it..." However wrong this solution was, it did result in a 39" x 4" model that could be run in a near Force 8 gale. Great Fun :-))
We staged a Battle Of Quiberon Bay event at Westerham in Kent two years ago, on the 200th Anniversary of the battle. We were the fleet out at sea, while 300 re-enactors were shooting muskets at each other on the shore, it was like Last of the Mohicans.....
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as I take it that a warship has a keel and being "pointy" requires less load to get down to water line whereas the flat bottom needs more
Most warships have flat bottoms, they just tend to have more slender hulls than merchant vessels although there are exceptions in both cases. Certainly high superstructure will increase windage and act as a sail which will tend to push the model over when the wind blows.
But having built the boat, the options, are strictly limited. You can only add ballast sufficient to bring the model down to her scale waterline although in practice you can often get away with just a bit more as an extra half inch or so may not be noticeable except in still water. In fact you could even cheat a bit and paint the waterline a little bit further up the hull, no one is likely to notice and it may give you just that little bit extra you need to keep the model stable.
Having determined how much ballast the model will take then this needs to be placed as low as possible in the hull. Where outright stability is needed then lead sheet across the bottom of the hull is the best bet as it will keep the centre of gravity as low as possible plus you will probably get a decent roll period too as the weight isn't concentrated in the middle of the hull.
The only further measure you can take is to 'add lightness'. As has already been suggested, is there any opportunity for trimming surplus material from within the superstructure to reduce weight? Any such gains can be doubled by placing an equivalent weight to that removed in the bottom of the hull as extra ballast.
If none of this is sufficient then the only alternative may be an external keel.
Colin
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My compliments to El Neave. You are a gentleman............... and a scholar. :-)
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Most warships have flat bottoms, they just tend to have more slender hulls than merchant vessels although there are exceptions in both cases. Certainly high superstructure will increase windage and act as a sail which will tend to push the model over when the wind blows.
But having built the boat, the options, are strictly limited. You can only add ballast sufficient to bring the model down to her scale waterline although in practice you can often get away with just a bit more as an extra half inch or so may not be noticeable except in still water. In fact you could even cheat a bit and paint the waterline a little bit further up the hull, no one is likely to notice and it may give you just that little bit extra you need to keep the model stable.
Having determined how much ballast the model will take then this needs to be placed as low as possible in the hull. Where outright stability is needed then lead sheet across the bottom of the hull is the best bet as it will keep the centre of gravity as low as possible plus you will probably get a decent roll period too as the weight isn't concentrated in the middle of the hull.
The only further measure you can take is to 'add lightness'. As has already been suggested, is there any opportunity for trimming surplus material from within the superstructure to reduce weight? Any such gains can be doubled by placing an equivalent weight to that removed in the bottom of the hull as extra ballast.
If none of this is sufficient then the only alternative may be an external keel.
Colin
:-)) :-)) :-)) O0 O0 O0
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Your every command, Derekwarner, is my whim. I will have a shot sending a few more picts of the Quiberon Bay. Tried yesterday, but the limitations on Mayhem' server meant that, like Mr Jagger, "I don't get no satisfaction" and
also like Mr Jagger, "I have tried, I have tried" (latter to be read in a high stragulated falsetto!) javascript:void(0); Unbuiltnautilus, so you fired off muskets in celebration of the 250 anniversary pf Quiberon Bay. My heartiest congratulations on your surviving the event. By the way, how many models (scrub that, I mean ships, unless you were also marking some definitely pagan festival) did you sacrifice?
And now for a further display of incompetence!
Best,
El Neave;
(http://s2.postimage.org/36i75g2jo/019_ship_view.jpg) (http://postimage.org/image/36i75g2jo/)
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Sorry to keep quoting from you Colin It's not personal, just you seem the most willing to debate the matter.
If none of this is sufficient then the only alternative may be an external keel.
Colin
Could we not think about pinching a bit of technology from one of the other R/C disciplines and maybe start thinking about Active Ballasting?
Now that could help fill a page or two in your Magazine?
Regards,
Kim
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The self-righting pendulum analogy may be better understood if it is considered as double ended - the superstucture or mast/sail above and the machinery mass or keel as the bottom weight. The pivot point of this pendulum is the hull at it's waterline. Viewed in cross section the pendulum is a vertical axis perpendicular to the horizontal water line. The roll rate of the model is dependant on the relative values of these opposing weights and their distance from the pivot. :-)
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Right, Perekasaman, and very especially so given the top hamper Tico's carry. What I retain from a mind widening discussion is to put as much weight on the bottom at the point where the hull goes vertical; even, as was suggested by
Colin, to a point that waterline level is slightly above the marked waterline in an effort to reduce rolling or at the very least to ensure that the Quiberon Bay does not suddenly take on all the characteristics of a U Boat!
That the erudite are now turning to Active Ballasting, is like as not to add to the number of grey hairs. I shall follow this with attention, none closer, But whatever the outocme, it will have to be for the next build.
Gentleman and scholar, hey!!?? I shall turn the other cheek. The last time I lived in UK, only the drinkers of Carling Black Label; I think it was, could make the slightest claim to such distinction. Scholarship? I' m not sure,
though I do scribble from time to time; Let's call it, obscure meanderings.
To help these irrelevances go down the gullet easier, here)s another pic of the QB,
Best,
El Neave.
(http://s2.postimage.org/15ba4ahw/004_ship_view.jpg) (http://postimage.org/image/15ba4ahw/)
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Years ago I worked for the Shetland Islands Council and had to do some work with the Fair Isle ferry. She had been converted from a fishing boat by adding a few tons of concrete ballast (and a bit of passenger accommodation). This replaced to weight of fishing gear, which had of course been removed.
She bobbed like a cork!
So the concrete ballast was removed from the bilges and its mass replaced with a concrete bulkhead - effectively a huge flywheel.
Now she would roll but very sedately, then gently (for her) roll back to upright (sort-of). She was still a pig in the for-and-aft (pitching) axis.
Geoff
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The QB looks good there :-))
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Hi,
What you are referring to are changes in 'moments of inertia'.
The effect is the same as a seesaw. If you both sit at the ends it is hard to make it rock quickly. If you both sit together over the pivot, it is easy.
As a sailing man, we have to consider whether to put heavy things, such as the anchor and chain right at one end , close to the bows, and the crew right at the other. Same weight but high moments of inertia.This makes the boat resistent to pitching so gives a smoother ride as you cut through the waves but makes a lot of water come over the boat (uncomfortable for the crew).
Or: Place heavy items close to the centre of the boat which mekes her ride easily over the waves but makes the boat pitch badly. This is uncomfortable for the crew (but it is their career choice) but worse; shakes the wind out of the sails.
Over the waves or through them?
Hope this helps
Cheers, Andrew.
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Of course for the really academically curious with lots of time you can always conduct an inclining experiment which will enable you to establish CofG, Metacentric Height, Centre of Buoyancy and crucially the righting lever (GZ sin O/ ) (theta). :}
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An inclining experiment - that takes me back, I was on one after the special refit of HMS Ashanti in Pompey dockyard. It was to be conducted in Basin 3 on a Saturday, but the wind was too high, and as HMY Britannia was alongside in Basin 3 and held off from damaging paintwork by wires right across basin 3 they refused to drop the wires to let Ashanti into the Basin. So it was postponed to the Sunday, as an apprentice it was the only Sunday I did!
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I have one of these, and I can say she looks great. :-))
The type of weights I used for this ship are the sticky weights they use on Aluminum car rims.
Make sure that you secure your battery in the hull,as I have had the gray hairs pop out when it slid to one side.
I lost the SH 60 over board when this happened.
The wind was a little over 10 knots .
Keep her close to shore when it is that windy .
Keep up the great work.