How much ballast will I need?
Q I am awaiting delivery of a 57ft hull, and would like to have even a rough estimate of how much ballast she will need to sink her to around 20in ( I’ve been told that this is about deep enough) Is there some sort of calculation I can use? As the boat is to be landbuilt I want to get it as close as possible before I launch. ANNITA GODDARD
A TONY REPLIES: Most modern leisure boats have a 20in stern post. The uxter (swim) plate needs to be about 1in below the water. The skeg is likely to hang down below that, so you may be looking at closer to 24in.
But assuming the 20in figure is correct, you need to do some maths. 1 cubic metre of water weighs 1 tonne, and a boat displaces its own weight in water, so you need to calculate how many cubic metres of water you displace when the boat is drawing 20 inches, or 0.5m. (The calculations will not be exact enough to worry about more accurate conversions, or about the difference between a metric tonne and an imperial ton.)
First of all you need the weight of the hull when fully fitted. I cannot help you with this, but the hull builder should be able to give you a good estimate, or the crane driver who lifted it should also be able to tell you (but you will need to add the weight of the fit-out and the possessions you will load aboard).
Now to calculate the displacement volume, divide the hull into three: the front swim, the back swim (both of which are approximated to triangles for the calculation, see diagram), and the centre (parallel sided) ‘box’ part. To calculate the approximate volume of each section, multiply its length by the beam (both in metres) by the draught you want (0.5m) but then halve the answer for both of the swims. Add all three figures together. This will tell you roughly how many tonnes of water you need to displace. (Using the symbols in the diagram it is: bxd+½(axd)+½(cxd ), all multiplied by 0.5m for the draught).
Subtract the weight of the hull and contents from this figure, and you have the figure for how much ballast you need to add.