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how do I work out Ballast

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Andy Stoneman14/11/2013 00:35:25
128 forum posts
12 photos

I`m in a bit of a pickle in trying to work out how much ballast I will need for the current model I am building, I`m hoping that someone might know how I do it as its driving me (k)nuts

Andy

Paul T14/11/2013 07:14:22
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7183 forum posts
1214 photos
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Andy

There are some onerous calculations that can predict how much ballast might be required however in my experience the best method is to simply float the model in the bath and load the ballast until the correct trim is achieved.

Paul

Kimosubby Shipyards14/11/2013 09:30:15
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563 forum posts
272 photos

Andy,

as Paul says, the bath is good test tank. Depending on the size of boat you can also use a large bowl or I've used a large garden plastic flower trough.

Fill completely to top with water (best done out side on a fine day). Immerse boat in the tank and hold it down to the anticipated waterline. Remove boat, then re-fill tank to top again, measuring the amount of water required. This is the displaced volume and assuming that 1litre of water has a mass of 1Kg, you can work out the displacement weight. Now to get to the ballast to add, you need to weigh your boat as constructed at the time of testing because that will be included in the displacement weight just established.

The ballast weight will then be the Total displacement weight less the boat weight.

Then try the boat with this calculated ballast (added about the hull - not as one lump) and check whether its close to the water line required. Remember to also subtract the weight of batteries from the ballast weight if not in the boat when estimating the ballast weight.

Kim

ashley needham14/11/2013 19:44:21
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6755 forum posts
198 photos

As usual Paul cuts to the bone the vexing subject of ballast.

The bath wins in my opinion. Not only do you get to see how much actual weight you need to get the vessel to the waterline, but also WHERE to put this weight, although a wobble test is v useful for determining stability and so on for which the bath may be a bit small. I know Kimo has one of those Imperial Leather advert baths, you know, the sort you walk into and so he does not have this problem.. hang on, no, I am getting this mixed up with leaked pictures of DM`s new workshop area.

Some leeway needs to be allowed, in my experience, for trim changes when on the move. My little HMS Devastation (I wish I hade built a larger one) has significant nose up static trim, due to the upwards levering forces of the twin props DESPITE that the shafts are almost horizontal, perplexing this. This then gives a nice even keel when moving at battle speeds (or a bit more perhaps).

A recently built model had to have so much ballast added I actually ran of available space available to put the ballast (on its initial bath test) and I had to redesign the interior and ditch some equipment then purchase sheet lead as this would take up less room than my bits of squashed pipe....

Ashley

Andy Stoneman14/11/2013 20:11:40
128 forum posts
12 photos

Hi Kim &Paul

I think that I`m getting the drift of it, If I use a bath and mark the level of the water in the bath then put the model into the water and hold it down the the desired waterline. Mark the new level of the bath waterline then remove the model and then measure the ammount of water required to bring the water level up to the new mark. This should give me the uncorrected ammount of ballast I need roughtly. As my model is going to sailed on a salt water pond I will have to subtract the difference due to the salt water. But thats getting too far ahead of things, My Boiler and engine weagh 3.5 kg so will have to subtract that weight. As you mentioned Paul, after googling how to calculate the ballast there are a lot of onrous calculations.

Andy

Peter Fitness14/11/2013 21:25:41
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508 forum posts
215 photos

Like Paul, I favour the KISS method (Keep it simple....) of loading ballast in until the desired trim is achieved. Onerous calculations give me a headachesmile p

Peter.

Andy Stoneman15/11/2013 08:04:50
128 forum posts
12 photos

I`m going to try the kids plastic wading pool, easy to handle and plenty of room. Like mentioned, Simple ways often seem the best way in the end, Thanks for your the help in l answer to my question about the Ballast. I hope that I dont need too much as lead is not easy to come by where I live.Stability shouldn`t be a problem as my model is 12 inches at the beam. Its odd Ashley that your model of the Devestation alters its trim when moving, Propellors with their shafts with too much downward angle usually push the nose down and stern up. But as your shafts are nearly level its odd the trim is different.

Andy

shipwright15/11/2013 11:11:02
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250 forum posts
54 photos

If you have an empty watertight hull then float that empty hull in the bath. Now, using a measuring jug (calibrated in litres/ml) pour water into the hull making an accurate note of the volume of water that you have used to get the hull down to the desired waterline. The density of fresh water is 1 gram per millilitre (cubic centimetre) hence then convert the volume of water that you have just measured to grams. This will be the total displacement of the boat not including the weight of the hull - in other words the total weight of everything in or on the boat must not exceed the weight of the water that you have just measured. Obviously this method is only suitable if you are at the start of construction and the hull is empty. Dry out the hull when the measurement has been completed.

Andy Stoneman16/11/2013 19:07:11
128 forum posts
12 photos

Thats another good idea Chippy I didn`t particularly want to get the hull wet inside but after all its balsa wood and will dry out so why not, I`ll give it a try. . I am going to glass fibre the hull interior for added strength so afte that I will try your method to determine the correct ballast weight. Thanks for the tip.

Andy

Amy jane September18/11/2013 00:56:36
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551 forum posts
515 photos

Hey guys this is getting complicated. I'm with Paul

Mark the water line on the boat, chuck it in the bath, throw in what ever weight is to hand until she sits level on her water line. Pull her out, mark the balance point fore and aft, weigh the stuff you chucked in her. Job donewink

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