I’m building a Claudio ETNZ and having some fun doing so. (I’m a complete newbie, don’t even know how to sail). In reading all over the net about different techniques I decided to try pouring a bulb into High Temperature Gasket Maker, as some have asked about, but not posted results. Well, here are my results.
I has a friend with a lathe turn me an bulb, slightly oversized as I was paraniod about coming in under weight. I coated the bulb with some paint and some wax and cast it in a small casting box, in plaster. This gave me the half profile sticking above the plaster as per Claudio's description in his AC33 build. I covered the bulb and some of the plaster with a thin layer of petrolium jelly (vasoline) and had another box ready to put on top of the already cast one. I picked up one tube of Permatex Red High Temperature Gasket maker. I layed down a thin bead of the rtv, patting it down with a wet finger to try to eliminate air bubbles, all over the bulb. I tried to keep the layer relatively thin, because this one part RTV cures using humidity in the air. I let this layer set for a day. I then added more layers in the same manner, until the tube was empty. This did not fillup my casting box, so I poured plaster and fibreglass scrap cuttings in as a mother mold, to fill the rest of the upside down casting box (I never did put a bottom on it). Once set, I carefully separated the two casting boxes and had my mold. The internal surface fo the RTV mold was not perfectly smooth, however, as the caulking type RTV does not pour, but it was perfectly acceptable as far as I was concerned. About 2 weeks passed before I was ready to pour the lead. These two weeks were more of a timing thing for me, and the weather outside. I do not know how soon after puring the plaser I could have started. The RTV will shield the plaster somewhat from the heat. I melted my Dive weights and did the pour. First pour was poor, as I am new and did not have the understanding of how the lead sets up. I had one "Pop" where a small piece of lead was spit up (I expected worse and was well protected). As I was melting my second batch of lead, I slid the first pour out of the mould (about 15 minuites I think) and it came out easily. The inside of the mould looked bubbly! I suspect trapped air in the RTV caused bubbles which inturn imparted surface defects on the cast. The cast was pleanty usable, but it had several small craters. My second pour went really well, as I started to understand the dynamics of solidification, and miniscus forming. I remelted the first pour, and did a third (which I again was not happy with) then remelted that ind did a fourth, which was pretty good. Over all my 2 halves of the bulb came in at 3.3 kg with a target of 2.850, so the poor surface condition has already been filed down to a bulb I am happy with. Currently sitting at 3.13 KG awaiting fin weight and slot cutting before final weight adjustment.
High temperature Gasket Maker can be used at $10 a tube.
Thin layers must be put down an allowed to cure. Thick air cure RTV may never cure to the centre.
The plaster backing did not cause eruptions, but did steam a little.
The RTV did cause one pop, so not perfectly safe. Trapped water or air in the RTV (wet fingers remember?)
The RTV held up well during the pour, but produced a poor, cratered surface on the cast.
1 week later the RTV feels gummy and may be degrading already.
Not recomended over 2 part RTV, and I have no experience pouring into plaster, so I cannot comment on that.