Partly because I have a friend who is an experienced pattern maker and partly because I wanted to get a good tight fit of the ballast leads in the keel, I opted to make a pair of aluminium moulds to cast split leads.
Because I contructed the keel from an inside blank, the inside is smooth and parallel. I gave the keel template to the pattern maker to make a pair of wooden patterns for casting into alumimium moulds. The patternmaker has to judge the contraction of the lead when it cools in the aluminium mould and the contraction of the aluminium when the mould is cast from the wooden pattern.
Two photos of the moulds are attached. The split is made diagonally through the centre of gravity and the widest part of the keel, so that both halves are each shorter and less wide than the space in the keel. This makes each half lead easy to lower in place and the two halves come snugly together only at the last moment. One photo shows how the two fit together, with the rear half inverted. The rear half must be cast upside down because of the rake angle of the keel. Each half is drilled and tapped M8 at its centre of gravity to take a stainless steel bolt. The lead does not stick to the aluminium or the stainless bolt, so casting leaves a threaded hole through the lead.
Casting the aluminium moulds cost £50 each. Each lead piece weighs 20kg and cost £1,10 per kilo to cast. Scrap lead costs £0,75 per kilo in the UK. The bottom pair of leads only needed running around the bottom edge with a 10mm radius router cutter to make them sit down on the base of the keel.