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Author Topic: Formulas on how to work things out  (Read 3030 times)

J.beazley

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Formulas on how to work things out
« on: April 17, 2008, 05:44:25 pm »

Ive searched the forum and didnt find what i needed so here goes.

Has anyone any formulas for working out how much ballast is needed on a model??
feel free to add other formulas.

Jay O0
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Colin Bishop

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Re: Formulas on how to work things out
« Reply #1 on: April 17, 2008, 06:07:44 pm »

Jay, there's no magic formula to tell you how much ballast you need as all boats are different. For example, if you put in a bigger batteriy then you will need less ballast.

You can however work out how much ballast you will need. Once the hull is complete and watertight, put it in the bath/test tank/pond and add weights until it floats at the designed waterline. Make a note of the amount of weight needed.

Now weigh all the bits that are going onto or into the boat such as superstructure (if not fitted), fittings, motor(s) battery, radio gear etc. If these come to less than the first figure then the difference will be the amount of ballast needed. If they come to more then you have a problem!

Bear in mind that weights going into the boat must be distributed in such a way that she will float level and upright. This is usually done in the final stages by adding the last of the ballast in small pieces to get the trim just right.

Needless to say, all weights going into the boat should be mounted as low as possible where you have that option, this particularly applies to batteries and general ballast. Otherwise the boat may be unstable. A further point is that, if you have quite a bit of ballast to add, it's best to distribute it evenly along the bottom of the boat rather than to concentrate it in the middle or at the ends as this can affect handling characteristics.

Colin
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Bunkerbarge

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Re: Formulas on how to work things out
« Reply #2 on: April 17, 2008, 06:55:02 pm »

I honestly don't think there are set formula's for this.  When you consider the significantly different bouyancy involved with the different types of hull shape how could you possibly have a set formula?

Just consider for a given length, breadth and draught a hull of a canal barge shape as opposed to a hull of a windermere steam yacht shape.  The bouyancy of the barge is going to be a lot more than the yacht.

I am convinced though that the experimental approach to ballasting, done before you even start to put things in the hull is the best way.  It is also very easy and, if done carefully should guarrantee a correct result.  Any formula would be an estimate at best and possibly even misleading at worst.

Just in it's simplest form follow this procedure:

First of all weigh your bare hull.  Then put your bare hull in the bath and fill it with weights, evenly along the length until the hull is sat at the level you want.  You could use water for a fibreglass hull, sand or any number of solid weights for any type.  When it is where you want it remove the hull and weigh it again.

Take the two weights away and you have the amount of ballast you need.  Then you need to gain as much of an accurate estimate of what else you are going to add to the model as possible.  This is motors, electronics, woodwork, fittings, batteries etc..etc..  Everything you are going to add comes off the total weight of ballast so in the end you should be left with a figure that you are going to have to add as ballast.  Your decision is then where to put it and stability also has to be now considered.  The more stable you want your model the lower in the hull you want the ballast weight to be.  There is also a consideration that a large model may become very heavy so you might want the ballast to be removeable and also you have to think that the more ballast you can convert to batteries the longer the duration of the model wil be.  My Envoy Tug actually has very little ballast but a very large 12v battery laid flat in the bottom of the hull.  This gives a very good duration and a very stable and well balanced model.

Looks like Colin beat me to it!!
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J.beazley

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Re: Formulas on how to work things out
« Reply #3 on: April 17, 2008, 07:05:43 pm »

Should of known there wasnt an easy solution to this  :'(

looks like im gonna have to hump the hull down the lake and fill her up, not so good when the hull itself wieghs in at just over 10kg  :o and the fact its over 8 foot long  :D ::)

Jay
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malcolmfrary

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Re: Formulas on how to work things out
« Reply #4 on: April 17, 2008, 09:31:03 pm »

If its a scale model, and you know what the original weighed, there is a formula that tells you what the model should weigh.
Convert the weight of the original into pounds (tons X 2240, call it W). 
Take the cube of the scale (if 1:100, then 100 X 100 X 100, call it S). 
Divide W by S. 
You now have the required weight of the model in pounds. 
Putting the weight in the right place is another story.
Getting the boat out again is an even better story.
Happy hernia.
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tobyker

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Re: Formulas on how to work things out
« Reply #5 on: April 17, 2008, 11:43:12 pm »

If the hull was suitable, (and strong enough) could you not support it level on a garden table or something, fill it up with water to the waterline, and then siphon out the water and measure the weight/volume? This would give you most of the displacement, except for that of the skin of the hull itself, and give you some idea of what buoyancy would be left over after the weight of batteries, motor, r/c etc.
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RickF

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Re: Formulas on how to work things out
« Reply #6 on: April 18, 2008, 12:07:40 am »

Jay,

Get yourself some scrap timber and a cheap poly "tarpaulin" and build yourself a test tank, a couple of inches bigger all round than the model. Then you can do all the testing in the back garden.

Rick
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Bunkerbarge

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Re: Formulas on how to work things out
« Reply #7 on: April 18, 2008, 12:21:36 am »

Unfortunately no matter what formula you use you are still going to have to test it with a tank.  The only real method is to get the hull in water and, honestly, the whole process is far easier than messing around with formula's anyway.

Rick's test tank is a great idea, because during the build process you will need to check out not just ballasting as it goes along but also trim, stability and whether you have any leaks.
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portside II

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Re: Formulas on how to work things out
« Reply #8 on: April 18, 2008, 07:36:37 am »

I would agre with RickF this time ,but as your hull is so big this time i think toybiker's method is the best .
Put your hull on a flat level surface fill it with water to the ballast line and then measure the water 1ltr=1kg  easy  {-),then keep the main weight low down and distribute it along the keel  O0 .
daz
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dreadnought72

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Re: Formulas on how to work things out
« Reply #9 on: April 18, 2008, 08:59:46 am »

Needless to say, all weights going into the boat should be mounted as low as possible where you have that option, this particularly applies to batteries and general ballast.

Colin, as a rule of thumb that's ok - but for some hulls all ballast at the bottom is not the best solution. At worst, it could provide too much righting moment, producing a hull which is toy-like - twitchy and bobby when on the water.

I'm making a large model of HMS Dreadnought, I looked into what metacentric height really means, broke out Flash, and came up with the following tool:

Metacentric height tool

You can drag the sliders to change the CG depth and the degrees of list. If you set the list to a small amount, and slide the CG to a height where's there's a (scale) accurate 4-feet-or-so of MH, the hull looks remarkably unsafe - the CG is well above the waterline.

But it's a big beamy hull, with bilge keels, not much superstructure/windage above the deck, and a load of inertia (at 1/72nd scale). In this state, the hull is still stable until long after the decks are awash, beyond 45 degrees of list.

The righting moment is lower than the abatb state, but in reality this'll mean the ship will roll slowly and much more realistically.

Andy

(I should point out that I'm not aiming to recreate the original's MH - my ballast will ensure that the CG is nearer the waterline. I want a safety factor! But this is still well above lining the bottom of the hull with lead sheet or similar.)
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Colin Bishop

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Re: Formulas on how to work things out
« Reply #10 on: April 18, 2008, 10:11:59 am »

Andy, yes I meant it as a rule of thumb. I'm aware of metacentric heights,CG  etc. With a model boat the weight of the superstructure and fittings tends to be much heavier proportionately than the full size prototype so models are inclined(!) to be rather more tender than anticipated, hence the need to keep ballast low. I think you see far more models flopping around than you do that bob about rigidly upright. As you say, form stability is important too. Initial hull stability is very high until it goes beyond a certain point. This is something which exercises the minds of modern yacht designers! With a full size battleship it was important to strike a balance between maximising stability in case of sustaining flooding damage and giving the ship an easy motion which made it easier to lay and fire the guns.

But, as you say, it does depend on the model. I think it's best to start with the ballast low and work up a bit rather than the other way round!

A clever little gadget you've found there!

Colin
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alan colson

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Re: Formulas on how to work things out
« Reply #11 on: April 18, 2008, 03:26:59 pm »

Built a model a few years ago now for a 24 hour charity marathon, filled it with batteries (useful ballast) until it was on the waterline. It weighed in at 114 lbs. about 50 kg. had to put the hull in the water first then put the batteries into it, it would have been to heavy to launch otherwise.
Alan
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