While I have no experience with steel hulled boats, I do have some observations that may help. Dissimilar metals are not subject to galvanic corrosion if electrically insulated from each other so a stainless or bronze valve might work if threaded into a steel, internally threaded, thru-hull with a threaded plastic bushing between them. I have seen water boiled in a paper cup held over a flame. I know a pipe or tank is almost impossible to weld when containing a liquid inside. Perhaps a plastic thruhull or hose would be similarly protected in a fire if full of water. And why worry about the thruhull and seacock when the hose connected to it could have it's water boiled out and then melted. I must point out that stainless does not necessarily lose it's stainless properties underwater. Seawater is rich in oxygen - ask any fish. I usually haul out in a commercial yard (Rose Marine in Gloucester MA) and get to see lots of commercial fishing boats out of the water. I have seen many stainless rudders and other underwater parts in perfect condition after years of use. I have a stainless propeller shaft with bronze CP prop, bearing housing, and stuffing box made by a Danish company that has been doing so for generations. This gear has pushed my vessel 38K miles in the 30 years since launching without fault. I have many stainless parts handling seawater today, including hot seawater in my genset exhaust elbow. The original cast iron elbow failed early on but the stainless one I made years ago is still working properly. My rudder and it's two pivot points, the bottom of the rudder shaft and where the rudder shaft passes through the hull at the top of the rudder, were originally made of steel painted with epoxy. At my first haulout in 1988 (7 years after launching), the top fitting was wasted at it's lower end, and there was about a cubic inch or two of steel wasted from the lower hinge socket. (I made the mistake of relying on the epoxy paint for protection and attaching no zincs). The rudder itself also had several holes in it which I repaired by welding new steel in and then fiberglassing the entire rudder. I repaired the lower hinge wastage by stick welding stainless material to replace the missing steel material and replaced the top fitting with a stainless one. I attached zincs to both the rudder and to the upper and lower hinge points. Both stainless bits are liberally slobbered with bottom paint on each haulout. Even though I have mixed stainless and mild steel underwater on the rudder I have detected no damage to the underwater steel since then to which I credit the fiberglassed rudder and the zincs. Type 304 stainless (previously called 18-8) is 18% nickle, 8% chrome, and the remainder iron. I suspect that in a piece of stainless underwater, the nickle and chrome cause the iron near the surface to electrolyze out leaving a layer of mostly nickle and chrome on the surface protecting the deeper iron. But when stainless is attached to steel perhaps the nickle and chrome continue to attack the iron the stainless piece is attached to. On the other hand, I have seen many steel boats that have stainless bits welded on above the waterline at wear points (cleats, fairleads, toe-rails, etc), with no problems. The problems I have seen with stainless appear to be due to how it is used and the particular stainless's properties. Norm S/V Bandersnatch Lying Julington Creek FL N30 07.68 W081 38.47 > Actually, Ed, I don't feel lucky at all about this issue. (Am damned lucky to have a boat and good friends with lots of knowledge on the list.) But, I don't know the best way to solve the problem. Marlon is pretty strong for most conditions and certainly does solve the electrolysis problem, but it has this one weakness. Bronze would solve the fire danger but definitely cause electrolysis problems. Stainless would solve both problems but looses its "stainless" properties under water. Hmmm?? Having just written that last sentence makes me wonder. If stainless essentially becomes mild steel under water, so what? My whole boat is mild steel. Now it is time to tap a different deep well on knowledge possessed by the list members, metallurgy. Does stainless steel under water just act like mild steel or does it become weaker or more brittle? And, are any losses in strength or flexibility minor enough to be outweighed by the fire and electrolysis advantages? BTW do I ass! > ume correctly that mild steel seacocks are not available in the sizes that I would need? Thanks in advance for all of you willing and able to share your knowledge and experience on these questions. > > Lee Huddleston > s/v Truelove > _______________________________________________ Liveaboard mailing list [email protected] To adjust your membership settings over the web http://liveaboardonline.com/mailman/listinfo/liveaboard To subscribe send an email to [email protected] To unsubscribe send an email to [email protected] The archives are at http://www.liveaboardonline.com/pipermail/liveaboard/ To search the archives http://www.mail-archive.com/[email protected] The Mailman Users Guide can be found here http://www.gnu.org/software/mailman/mailman-member/index.html
