Topic: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser (Read 22773 times)

Mankster · « **Reply #25 on:** February 04, 2011, 09:51:51 pm »

Quote from: Colin Bishop on February 04, 2011, 09:44:03 pm

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.

Colin Bishop · « **Reply #26 on:** February 04, 2011, 10:03:03 pm »

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

Mankster · « **Reply #27 on:** February 04, 2011, 10:13:47 pm »

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.

Talisman · « **Reply #28 on:** February 04, 2011, 10:41:29 pm »

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.

RaaArtyGunner · « **Reply #29 on:** February 04, 2011, 11:42:10 pm »

Quote from: DickyD on February 04, 2011, 02:18:59 pm

Another free ad Colin.

Yep but where are the free books

for supportive Mayhemers

Colin Bishop · « **Reply #30 on:** February 05, 2011, 10:00:18 am »

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

Mankster · « **Reply #31 on:** February 05, 2011, 10:44:24 am »

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.

El Neave · « **Reply #32 on:** February 05, 2011, 11:00:29 am »

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.

gingyer · « **Reply #33 on:** February 05, 2011, 11:29:53 am »

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

any more help please ask

Mankster · « **Reply #34 on:** February 05, 2011, 11:53:49 am »

Quote from: El Neave on February 05, 2011, 11:00:29 am

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

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.

gingyer · « **Reply #35 on:** February 05, 2011, 12:12:59 pm »

the Ticonderogas don't have stabilizers
which is a shame as they could do with them

unbuiltnautilus · « **Reply #36 on:** February 05, 2011, 01:31:58 pm »

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

tigertiger · « **Reply #37 on:** February 05, 2011, 02:00:25 pm »

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.

Colin Bishop · « **Reply #38 on:** February 05, 2011, 02:04:03 pm »

Quote

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

tigertiger · « **Reply #39 on:** February 05, 2011, 02:09:12 pm »

Quote from: Colin Bishop on February 05, 2011, 02:04:03 pm

Angular momentum.... (Doesn't she appear in 'Holiday on Ice'?) as in conservation of momentum.

Colin

Uno momentum, while I look that up

unbuiltnautilus · « **Reply #40 on:** February 05, 2011, 02:28:11 pm »

Quote from: Colin Bishop on February 05, 2011, 02:04:03 pm

Angular momentum.... (Doesn't she appear in 'Holiday on Ice'?) as in conservation of momentum.

Colin

Big girl!

bat44 · « **Reply #41 on:** February 05, 2011, 10:38:45 pm »

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

unbuiltnautilus · « **Reply #42 on:** February 06, 2011, 10:26:44 am »

Quote from: bat44 on February 05, 2011, 10:38:45 pm

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

El Neave · « **Reply #43 on:** February 06, 2011, 02:10:53 pm »

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.

Martin13 · « **Reply #44 on:** February 06, 2011, 10:37:13 pm »

Quote from: El Neave on February 06, 2011, 02:10:53 pm

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

Perkasaman2 · « **Reply #45 on:** February 07, 2011, 01:53:35 am »

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.

Mankster · « **Reply #46 on:** February 07, 2011, 07:37:23 am »

Quote from: Perkasaman on February 07, 2011, 01:53:35 am

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.

Quote from: Perkasaman on February 07, 2011, 01:53:35 am

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.

Colin Bishop · « **Reply #47 on:** February 07, 2011, 11:43:52 am »

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

Talisman · « **Reply #48 on:** February 07, 2011, 12:57:59 pm »

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

Colin Bishop · « **Reply #49 on:** February 07, 2011, 01:22:49 pm »

Quote

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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Author Topic: Weighty matters and cente of gravity Ticonderoga class Aegis cruiser (Read 22773 times)

Talisman

RaaArtyGunner

El Neave

bat44

El Neave

Martin13

Talisman