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How do boat builders know what weight of ballast to put in the bilge so that the boat floats level and the right depth. I understand they know weight of most things in engine room but their are many other facts to take into equation.

Asked by: Mr Vernon Whitehair  | 9.19am, Friday 18 October

WW says:

If you look at the gauging sheets that were used to calculate the weight of cargo being carried by boats, and hence the toll due to the canal company, it is roughly 1 ton per inch of hull side. This means that the boat will be an inch deeper in the water for every ton carried, averaged over the whole length of a 70ft boat.
Any boat obviously has a weight associated with it, and it will float if it weighs less than the water displaced. The question of how much ballast to put in to achieve a desired depth is difficult to get right first time the boat hits the water. Boat builders have a wealth of experience to fall back on, but even so can get it wrong when producing something a bit different.
Generally the ballast should be evenly distributed along the base plate, the engine and equipment at the rear will help to get the stern at the right depth for the propeller, which leads to the bow being generally higher out of the water. To get the trim of the boat even consider reducing the ballast on one side (or increasing it on the other) if you are fitting especially heavy items such as granite worktops or pianos. Changing the location of the batteries for example, can offset the weight of a calorifier or wood burner.
If you are fitting a boat out from scratch, it might be as well to incorporate panels where ballast can be altered once the fully fitted boat is in the water. Ballast can be added, removed, or changed for higher density material; such as replacing concrete paving with cast iron to alter the trim of a finished boat.

Rupert Smedley  | 10.44AM, Friday 18 October

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