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[NoNuFink]

Given a hull that's a bit broad in the beam but a bit top heavy and therefore a bit tender in the turns, obviously ballast needs to be as low as possible in the hull. 

If the hull is flat bottomed does it make any difference whether the ballast is central or split in two and placed as far out as possible?  Would one or the other make for a less tender setup?

If the hull was a shallow 'V' would that in itself make the hull less tender and would ballast placement make any difference? (assuming that the chine level wasn't too much higher than the keel level).

All comments much appreciated

NNF

[Martin (Admin)]

This should licit some interesting answers!!   

[derekwarner]

NNF....

If the ballast is placed at extreme sides [lowest port & stdb], the effect is the same as a pendulum but in the horizontal plane, so if a wind hit the stdb side, the vessel will list to the port side and stay there until the wind abates

If the ballast is placed on axis lowest mid ships, there is no real horizontal pendulum effect, and so if the same wind hit the stdb side, the vessel will list to port side and stay there until the wind abates, however the real difference is the speed of recovery

A vessel with the ballast on axis and high up within the hull will bob around like a cork
A vessel with the ballast at either side and low down will display slower and more lifelike rolling/rocking movement from not only windage, but also from turning moments.........

Google metacentric point of balance......you will find a few hours good reading .....Derek

https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CB0QFjAAahUKEwjplcf1ifXIAhVEHaYKHbG-DhE&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FMetacentric_height&usg=AFQjCNEERBlehAbYLv6sxrRqVlJky2slNQ

[NoNuFink]

Thanks for that Derek. - A lot of reading to be done there.  Am I right in thinking that a shallow 'V' hull would be more stable than I similar flat bottom hull?

NNF

[mrlownotes]

Metacentric point of balance......that was not a good idea to read that at 01:00hrs.

I have a very large flat-bottomed hull about to be ballast tested. I have assumed, thus far, that keeping the ballast as low as possible and on the keel line is the best way to set it in.

I'll read that again in the morning daylight with fresh eyes and refreshed grey matter.

[derekwarner]

NNF.....that's a very interesting question about differing stability between V profile or flat bottom hulls  ...however there a number of answers so let’s consider a few points

a.   A hull of V profile having a 10kg displacement and being subjected to a horizontal wind force X as shown
b.   A hull of flat bottom profile having a displacement of 10kg and being subjected to the same horizontal wind force X as shown
c.   The draft of the flat bottomed hull is some 1/5 the depth of the V profile hull, due to the surface area presented to the water

So what happens [with all things being equal?]

•   The wind force against the V shaped hull will rotate the hull around it’s MPoB counter clockwise until the wind abates
•   The wind force against the flat shaped hull will also tend to rotate the hull around it’s MPoB counter clockwise however to a far lesser degree & with the principal action being the displacement of  the hull to the left
•   This is simply due the larger surface area of the flat shaped hull under the water, hence the force X takes the least line of effort in the movement to the hull to the left over rotation of the hull

An example of this is say a 30,000 ton general cargo vessel ….load it with granulated iron dross which will be placed very low in the holds and have a very low MPoB ….the vessel when underway at sea will have a near continual port to stdb pendulum swing
Alternatively, load the same vessel with limestone dust with  the same total gross mass, however the limestone having a far lower density, stacked to the near top of the holds and hence a far higher MPoB
The result here is the vessel with the limestone cargo will understandably have a smoother or lower roll in the seaway

So whilst we read in so many build recommendations to ‘keep the superstructure light & the batteries as low as  possible’, such comments do need to be considered however relative to the type of vessel and hull design

 I hope this make sense………..Derek

[Tug Fanatic]

.................An example of this is say a 30,000 ton general cargo vessel ….load it with granulated iron dross which will be placed very low in the holds and have a very low MPoB ….the vessel when underway at sea will have a near continual port to stdb pendulum swing
Alternatively, load the same vessel with limestone dust with  the same total gross mass, however the limestone having a far lower density, stacked to the near top of the holds and hence a far higher MPoB
The result here is the vessel with the limestone cargo will understandably have a smoother or lower roll in the seaway................

0630 is a bit early to be thinking about this but what you conclude about the limestone verses granulated iron is exactly the opposite of what I would have expected. I would have thought that keeping the weight low made for a much lower roll. When I wake up there is definitely some reading to be done.

The other question is about the difference between whether the ballast is at the bow/ stern or in the lengthwise middle of the hull. I have tended to try & put ballast at the ends - especially the bow - so that it minimised "bobble". I am probably wrong there as well although experience suggests that I am not totally wrong!

[RAAArtyGunner]

 I also will need to read and digest the link (article).

 However one question comes to mind, the possible loss of the model if the freeboard is insufficient and as it heels as shown in the examples it may take on water and sink.?????

[Jerry C]

Derek, as part of my Masters ticket I studied stability up to a level sufficient to operate a ship safely. I have never heard of "Metacentric Centre of Balance" so I googled it. The only mention of it was a link to your previous post which was top of the list. Looks like a googlewack!
Jerry.

[derekwarner]

OK Jerry .....there are many analogy's of which branch or service onboard a ship and the relative importance of the respective goup

One does not need a Deck Officer qualification to understand basic engineering studies by those down stairs to understand Metacentric_height....as you well know !........... Derek
_________________________________________________________

https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CB0QFjAAahUKEwjl-_DBtfbIAhWCMqYKHS4kA4M&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FMetacentric_height&usg=AFQjCNEERBlehAbYLv6sxrRqVlJky2slNQ

[Colin Bishop]

I don't think one should get too hung up on the theory here as there are practical considerations which govern the performance of scale models.

Firstly, the effects of wind and water on our models in normal operating conditions are out of scale with their full size counterparts. A moderate breeze and a slight chop on the pond probably equates to a force 8 gale at full size.

Secondly, the superstructures of almost all model boats are proportionately very heavy compared to their full size counterparts. But to build them to scale lightness would mean paper thin sides which would be extremely fragile. This is why it is so difficult to construct a working scale model of a cruise ship for example. The result is that model boats inherently tend to instability and many are found to be downright unstable. So they will roll about horribly in the waves and if they have a high superstucture it provides lots of windage for even a slight breeze to push them over. You can see this time and time again on the pond. To achieve realism we should be showing our scale crew members lining the rail and giving up their breakfasts.

So the crucial rule in most models is to keep the ballast as low as possible thus ensuring a low centre of gravity as our models have to regularly cope with conditions which would be considered extreme at full size - a gust of wind for example could equate to hurricane force which might capsize even full size ships.

As far as the distribution of ballast is concerned you can indeed affect the sailing qualities of the model as mentioned in an earlier post. Putting it in the bilges rather than along the line of the keel will indeed damp down the rolling and make the model look more realistic.

In a long thin hull in particular, you will also notice a difference in steering characteristics between concentrating the ballast at the ends of the model as opposed to the middle. My preference is to try and distribute the internal weights as evenly as possible along the length of the hull to encourage neutral handling characteristics although the actual shape of the hull will exert a greater influence.

Sometimes the layout of the model gives you only limited choice as to where to put the weights but you really MUST keep them as low as possible to help offset the unscale weight of decks and superstructure. Many people do not appreciate that plasticard when used in structural thicknesses is actually quite heavy compared with equal strength and much thinner plywood!

My thoughts anyway!

Colin

[Jerry C]

I apologise to Derek, my post not cleverly put. No offence intended. Colin's post much better.
Jerry.

[derekwarner]

It's good evening from me Jerry......hope you & yours are all well

Your PM idea of a thread on stability would be great, why not consider it?...even though you say you are not a teacher, I suggest you are one of our formally qualified members who could conduct this subject

I was simply trying to offer a few 'out of the square' thoughts on the basics from the question this morning

Typical complications are those offered by Colin, which when considered from an overall view appear acceptable
However Colin uses the same rhetoric that we cannot scale elements so makes it all too hard etc

I am not suggesting that all model design should be based on full sized theory, however having an understanding of the same allows us to take elements out of the shoe box and apply them to our models

Having said this, I am also loathe to offer any text beyond a simple Wikipedia type extract
If the member reads the one offered, I hope he would at least read something that is comprehensible [& to a lay person such as myself]

Derek

https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CB0QFjAAahUKEwjl-_DBtfbIAhWCMqYKHS4kA4M&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FMetacentric_height&usg=AFQjCNEERBlehAbYLv6sxrRqVlJky2slNQ

[Colin Bishop]

'However Colin uses the same rhetoric that we cannot scale elements so makes it all too hard etc'

Derek, that is not what I said at all! You can certainly work it all out in theory, no problem at all. We did run a long article about all this in one of the recent Model Boats special issues.

You can do all the calculations you like but you still have to be pragmatic. One of the most common problems with scale model boats is lack of stability and this is because people do not fully understand it and build their models top heavy without realising it. In that sense an understanding of the basic theory is a good thing but just as important is to know what practical steps to take to avoid problems and two of the most important are to keep the top hamper as light as possible and the ballast and internal weights as low as possible. That will give you some working room if you want to experiment further. There isn't a great deal you can do about windage as this is a fixed area which you cannot change but you can mitigate the effects by keeping the centre of gravity as low as possible. Basically for the models that we normally operate that is all you need to know, much as people love to delve into all the theoretical nitty gritty which can often confuse people further.

At full size the subject can certainly become very interesting. For example the old R class battleships carried their deck armour a deck higher than the preceding Queen Elizabeth class, the idea being to give them a slower roll and thus make them more steady gun platforms. The downside was that it compromised their margin of stability in event of damage and also the ability to accept further topweight (inc. deck armour against air attack) which is one of the reasons that the QEs were rebuilt and the Rs were not as they could not accommodate the necessary changes.

Colin

[Jerry C]

Hi NNF, sorry if it seems you have opened a can of worms with your original question. We all want to help you but as you may have noticed every answer leads to more questions. To simplify ballasting, you need to add weight to a model to bring it down to its designed waterline. It's best to use lead in some form or other because it has the most effect for its volume. Put strips of lead progressively into the hull at the bottom and roughly at mid length until the waterline on the outside of the hull corresponds with the surface of the water. Next, move the whole weight athwartships until the model floats upright. Now move the whole weight forward or aft until the waterline is parallel to the surface of the water. You now have the exact amount of ballast needed. It may be that there is an obstruction already fixed in the boat that stops you from doing this so you need to split the ballast and spread it evenly round the boat. Winging out the weight to the sides increases the moment of inertia of the ballast and effects the amount of roll. Keeping the ballast low shortens the period of the roll, stiff. Raising it lengthens the period of the roll, tender. Raise it too high and the boat will become unstable. Splitting the weight fore and aft will increase its moment of inertia and will effect the pitching and steering. Steering will start slowly and be hard to stop the swing. With the exception of the total weight and the centre of gravity with regard to fore and aft and low none of the rest is worth worrying about. Boat upright, at even keel or a fraction down by the stern and stable is all that really matters. Its most important to secure all ballast so it can't move.
On the subject of me doing a thread on idiots guide to stability I'll give it some thought. Someone needs to teach me how to present diagrams in a post first. The only way I know at present is to draw one and photograph it before posting. Let me know who wants it.
Jerry.

[Colin Bishop]

I will have a look at adding Richard Simpson's original article on stability as a feature on the Model Boats website. As chief engineer of a large cruise ship as well as being a keen modeller he knows the subject inside out.

Colin

[jarvo]

HI Guys


Just to add another problem, my tug Amsterdam has a round hull not flat or V, my batteries are low in the hull but laid down. (gel cells are heavier at the top of the cells) my question is the moment of roll??? she heals into a turn (both ways) and returns upright quickly, is this right???? i have heard that the full size Amsterdam would roll on a mirror sea. Is this due to hull shape????


Mark

[Mad Scientist]

Mark,

Yes, a round hull will roll more than a flat-bottom or v-shaped hull - all things being equal.  Of course, all things are never completely equal.

Perhaps I should say that a round-bottom Amsterdam will (probably) roll more than a flat-bottom Amsterdam. Once the hull shape is changed, then you get into all sorts of possibilities concerning location of ballast and batteries.

I hope this helps - at least a little bit!

Tom

[jarvo]

Hi Tom


Thanks for the reply, would it help to add, bilge keels to the hull, dont know how this would make a difference


Mark

[Netleyned]

Gel Cells are Lighter at the Top not Heavier
If laid flat in line with the hull no problem.
if laid athwartships either side of the keel
best laid with tops facing outboard.


Ned

[malcolmfrary]

Hi Tom
Thanks for the reply, would it help to add, bilge keels to the hull, dont know how this would make a difference
 Mark

Bilge keels act as roll dampers rather than roll preventers.  To figure out how this works, put your flat hand sideways on into water.  It goes in easily.  Now move your hand sideways. it takes more effort.  Essentially, that is how a bilge keel works.

[jarvo]

Sorry Ned


You are right, i wrote it wrong, sadly i cant fit the batteries length ways in the hull, 1 not wide enough, 2 not long enough end to end, also there is a ballast problem with the batteries in the widest part of the hull it is also leaving the stern slightly higher, so i get some cavitation when going astern


Mark

[NoNuFink]

It will come as no surprise that I'm more confused now that when I started this thread.  Just to clarify for me .....

For a flat bottomed boat.
Jerry

It may be that there is an obstruction already fixed in the boat that stops you from doing this so you need to split the ballast and spread it evenly round the boat. 

Are you saying that central is the best choice if obtainable?

Winging out the weight to the sides increases the moment of inertia of the ballast and effects the amount of roll. Keeping the ballast low shortens the period of the roll, stiff. Raising it lengthens the period of the roll, tender. Raise it too high and the boat will become unstable.

This is the confusing bit. The first sentence tells me that moving the ballast horizontally away from the keel affects the amount of roll but not how. The second sentence tells me the effect of raising the ballast which, while of interest, is irrelevant to a flat bottomed boat where the ballast is very likely to be on the floor.


Help I think it's me that's becoming unstable - Oh no I'm sinking!!!!!

NNF

[Jerry C]

NNF, a practical example of how increasing the moment of inertia of the ballast. Take a broom stick, find its balance point, clamp a G cramp either side of the balance point such that you can hold the stick between the cramps. Hold your arm out straight in front of you with stick horizontal. Now twist the stick backwards and forwards and note how it feels. Shift the cramps to the ends of the stick and repeat the experiment. It feels different. Much harder to twist.
A ship rolls when an external force (a wave) acts on it. If the period of the waves is different to the period of roll of the ship then the ship will roll to varying amounts also in cycles but manageable. If the period of roll and the period of the waves are equal then the you have a resonant frequency situation. The angle of roll will keep increasing and the ship will capsize. This is theory only and hardly ever occurs. 
"Raising the ballast is irrelevant because the ballast will be on the floor ". The ballast will be wherever you decide to put it. As previously said by others, a ship loaded with the c of g of the cargo low down ( iron ore) has a very short period of roll and is very uncomfortable. A ship loaded with c of g cargo high (cotton bales) has a slower roll period and is much more comfortable. Passenger vessels have a roll period around 20 seconds.
Many models in a seaway look unrealistic because they pitch and roll too quickly. Indeed there is a video app available that films in slow motion to give realism to models. Raising the ballast (on a pedestal) will slow down the roll too. It's not important but it's not irrelevant.
Jerry.

[NoNuFink]

Thanks Jerry - I'm with you now 

My comment about the vertical placement being irrelevant was in the light of the point that Colin B made about superstructures on models being disproportionately heavy (Reply 10).  It would appear that the ballast would be unlikely to be anywhere other than as low as possible in a model.
 
Cheers

NNF