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[Popeye]

 Is there a 'rule of thumb' method of determining how much ballast is likely to be required for a model other than calculating the estimated underwater volume and converting this into weight. eg., cubic inches to pounds
  :-\

[Bryan Young]

There is...but why do it? The easiest way is to put the model into water and play around with it. Using "proper" formulae does not work with models. (Previous posts apply).
1. Get the weight down to the bottom.
2. Shift the weights around until the model sits right.
3. Is this another of those questions that could have been answered by the questioner?

[Bunkerbarge]

Hi Popeye and welcome to the forum.  I have never been a great believer in trying to calculate the ballast required for a model as it is all a bit hit and miss and estimation.

I am a great believer in doing the experiment in the bath, which gives you a perfect answer to work with every time.

Simply place your bare hull in the bath and either fill with something like sand or just random weights until the hull sits perfectly as you want it in the water.  Remove the hull and weigh it on an accurate set of scales.  Take the weights out and weigh the bare hull so that all you have to do is subtract the hull weight from the ballasted weight to give you the actual ballast you require.

All you then need to do is estimate the weights of all the items that you are planning on fitting such as motors, electronics, battery, accessories, construction materials including fittings, servos etc...etc...  The total weight required tom get it to it's marks minus the fittings you are going to add is the amount of additional ballast you need to fit.

I would go for about 75% of this weight to be placed as low down into the hull as possible as lead shot or sheet before completing the model.  You should then require a small amount after the model is finished to trim it to exactly where you want it.  This allows for the odd thing you forgot in your weight estimation plus any inaccuracies.

[Colin Bishop]

The amount of ballast will depend upon the other weights in the model. If it is true scale then you should be able to scale the full size displacement down to get the overall model weight. But it is the distribution of this weight that matters in a model. As Bryan says, you need to get as much as possible as low as possible which means batteries on the bottom of the hull. The superstructure needs to be built as light as possible consistent with strength. You need to weigh all the working bits, servos, motors, speed controllers etc. and place these as low as possible in the hull. If you have some spare "weight" left over then put it in the bottom of the hull in the form of lead sheet to make the model more stable. in a model you can usually get away with a degree of over ballasting to improve stability as the top layers of the water will usually be transparent and if the model is sitting a little deeper than it ought to then this will not be very noticeable.

[Ghost in the shell]

another factor is the hull shape, a rounded hull that is 30" long and 5" wide will require a different amount that one that is of identical size but having a flat bottom

[Ghost in the shell]

also, do you sail in salt water or Fresh water, taking my Graupner neptun as an example, the difference in density between salt and fresh water equates to 1/10th inch difference in draft, with the model pulling 1.9" in salt, 2" in fresh

[Bryan Young]

another factor is the hull shape, a rounded hull that is 30" long and 5" wide will require a different amount that one that is of identical size but having a flat bottom

Rounded hulls (as in "Galleons") have an entirely different stability equation. I don't think you were talking about the old sailing ships though..or were you? The last "rounded" hulls I ever saw were some RN Destroyers/Frigates...and these were really just exagerrated bilge curves and a steep "rise of floor". To all intents and purposes modern ships have a flat bottom. (Not true, but near enough). However, I have wondered if the hull shape of (for instance) HMS "Victory" was designed with any knowledge of stability. With the rounded shape of the hull I think that the "righting lever" would change drastically when the ship rolled beyond the "rounded" part of the hull.

[Ghost in the shell]

rounded hulls in general bryan, because the guy hasnt stated what he is building. a round hull can also refer to a corvette or a conveted surface running revell u-boat, or a large north atlantic trawler similar to the lindberg boat, and yes gallions too

[Colin Bishop]

The pre dreadnought French and French inspired Russian pre dreadnoughts were designed with tumblehome in their hulls (wider at the waterline than at upper deck level). This gave them good initial stability but, as was found in the Russo Japanese War, once the hull became partially flooded and the ship listed, the degree of reserve stability diminished very quickly and the ship was prone to capsize.

British pre dreadnoughts were also susceptible to capsizing but this was because they were built with a longitudinal bulkhead down the middle of the ship dividing the engine and boiler rooms. If they hit a mine or were torpedoed one side of the ship flooded and over she went.

In the case of Nelson's ships the same principle applies except that battle damage did not usually lead to flooding and if the ship was pushed over by gale force winds the masts, sails and rigging would usually go by the board before the hull could be capsized. Once the topweight had gone the hull would be very stiff indeed with all the ballast in the bottom no longer balanced by the sailing rig.

[Popeye]

Many thanks to those kind folk who have responded to date.

I wasn't thinking properly when I framed the question as a result of which I omitted to mention several other critical points which would prevent me from adopting the tried and trusted post-construction test tank - alias,Bath - method:-

1) I'm researching an 18th Century navy brig which I plan to build and need to have a rough idea of it's likely all up weight before I start work coz I don't want to end up with something too heavy to  handle without the aid of a mobile crane or Shiplift - or two accommodating club members.
2) ballast would be removable - either bolt on fin keel and/or full length keel weight  (dependent on required ballast weight).
3) my calculations, using fresh water conversion factors (cubic inches to pounds), indicate that approx 80lbs, inclusive of model weight, would be needed to bring the hull's displacement  down to its design Water Line. This is a tad too much hernia inducing weight  for my liking.
 
So, is there any other method, other than (3) above, I can use to achieve my objective prior to committing myself to the workbench? Reducing the scale from 1:24 to 1:32 would of course be an alternative way of reducing all up weight to manageable - for me -proportions.

The Jury's still out.
 :-\

[Bunkerbarge]

Fit a ballast tank with an open valve outside the hull.

Place the model unballasted in the water with the valve open.  The internal tank will flood to the required amount at which point you close the valve.  After sailing either lift the boat out and open the valve allowing it to drain or have an electric pump which will pump the tank out and allo you to remove the model easilty.

The beauty of this is that you can ballast the ship to exactly the level you require but the downside is that you will have to ensure that the tank is segregated to prevent "free surface" effects, which cause instability.  The most effective method is to have the tank split into sections with drain holes between them.  The water will only move from one compartment to another slowly and the tank can be arranged to be in the bottom of the model and a flat shape such as a normal ships double bottom tank.

Alternatively if you fill the tank completely it will not be an issue.

[Bryan Young]

Fit a ballast tank with an open valve outside the hull.

Place the model unballasted in the water with the valve open.  The internal tank will flood to the required amount at which point you close the valve.  After sailing either lift the boat out and open the valve allowing it to drain or have an electric pump which will pump the tank out and allo you to remove the model easilty.

The beauty of this is that you can ballast the ship to exactly the level you require but the downside is that you will have to ensure that the tank is segregated to prevent "free surface" effects, which cause instability.  The most effective method is to have the tank split into sections with drain holes between them.  The water will only move from one compartment to another slowly and the tank can be arranged to be in the bottom of the model and a flat shape such as a normal ships double bottom tank.

Alternatively if you fill the tank completely it will not be an issue.

I seem to have "hit a nail on its head" here. From the tone of the question I wondered if it was a round-hulled sailing ship. I do not think that your problem is to do with stability. I believe it will be more due to "windage". Is this model going to have sails? If it is then I guess that the only way to keep the model anywhere near upright is to fit a removable deep keel with a hefty weight on the bottom. Talk to the yachting guys. But looking back I would suggest that the real ships were so bottom-heavy that the wind pressure on the sails was a minor consideration. And they could only go along at a snails pace (OK, some were built for speed...but the big buggers were slow). As I have said before, you cannot scale wind and water so you have to learn to live with it...and adjust.

[Ghost in the shell]

you cannot scale wind and water so you have to learn to live with it...and adjust.

true, and if the density of scale we sail in were to be applied to somehting like the QE2, it'll be sailing in somethjing with the consistanmcy of treacle

[Bryan Young]

A slight diversion. RFAs "Regent" and "Resource" used to use many tons of WW2 500lb. bombs as ballast. (Neat thinking, what), but much of this lot were used up trying to destroy the "Torrey Canyon". The 2 "Rs" from then on were pretty sensitive when rolling a lot. But then, the RFA has a history of making their ships uncomfortable in bad weather! Cheers. BY.