How much bal­last will I need?

Canal Boat - - Back Cabin -

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.

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