Forum

Casting a lead bulb keel

Please note. The photographs are in reverse order! About three years ago I started to make and cast a bulb keel for my wooden Pilot boat, Cariad. Well, I have finally done it but not without some false starts and accidents. I don’t propose to dwell too long on them! My first task was to establish how much weight I would need. The boat is larger than the fibreglass hulls produced by Chris Brown. The hull is almost complete and painted so I ran the bath (while my wife was out!) and floated her in it. I then added weights until her waterline looked right then weighed the weight which was just over 13Kg. I decided to make the bulb keel a bit less than this to leave room for trimming the model. I how needed to make a plug of the keel. Luckily I have a lathe which simplified the process although you could use an electric drill to turn down the front and rear ends. The density of lead is 11.34g/cc or there about, so it possible to calculate the size I would need using basic maths. I did this in sections, firstly the nose and tail (made from sheets of plywood glued together then turned down on the lathe), then the body (made from a plastic rainwater pipe) and finally the attachment to the hull (from wood). From the total I got the final weight. I adjusted the weight by altering the length of the body. The mould needed to be in three parts: two sides and a section to reflect the size of the keel. It was made from Plaster of Paris. To contain the mould I made up some timber boxes and a plywood sheet to hold half the plug to start the moulding process off. This had to be greased well so as to release the plug. To hold the casting to the hull I needed a threaded rod which I originally made from 1/4” brass rod which was incorporated into the moulds prior to casting. Later I changed this rod to stainless steel. The first mould I made was a bit damp even though I had let it dry for a couple of months in my dry shed. Too much air bubbles were introduced and the size just didn’t look right. I’d also included some feeder holes to allow me to fill the lead and for air to escape. They were not successful either because the lead solidified around them. The casting had to be re-melted and the mould re-made. For the second mould I made a new plug using a narrower section of drain pipe but the mould was also a bit damp but the main problem was the filling points. Again, they were just too narrow and clogged up with the lead. I decided to do away with the filling points and left the top of the mould open to the air. I did this by cutting the Plaster of Paris with a jig saw and making up the irregularities of the cutting with more plaster. The next casting caused me no end of trouble because it leaked like a sieve despite being held together with clamps. I had to stop casing the lead. Because I had about 12 Kg of molten lead to deal with, I didn’t want to leave it as a solid lump of lead. It is better to have a number of smaller bits to add to the melt pot gradually: melt a small quantity first and gradually add other bits. I therefore cast it into small ingot moulds. What I did was cast one ingot, wait a minute for it to solidify then dunk it into a bucket of water to finally cool it which resulted in the ingot and mould separating. The mould would then be removed from the bucket and set aside to finish steaming. Another mistake! I thought it had finished steaming so went to cast another ingot when it exploded and scattered molten around, including over me. I was lucky and only ended up with a burnt left arm and needing to buy another pair of glasses. After being severely told off by my wife as well as being very nervous about trying again I again made yet another mould. This one I dried off in the oven (my wife was out!) for a couple of hours at about 140 deg C. This helped no end but I still got a bit of bubbling. However the casting looked alright despite needing a bit of cleaning up. Lessons Learnt: Use fresh Plaster of Paris Grease the plugs before casting the Plaster of Paris, including greasing the mould interface to prevent sticking. Make sure your body is well protected, including a full while casting lead. Use stainless steel rod as the fixing to the hull. Brass is too bendy/soft. Make the filling hole in the mould as large as possible. Ensure the mould is as dry as possible before casting

5 years ago by cenbeth