I don't see your link mention aluminum or 'soft metals' - would you please point out what I am missing.
I do see it say: "Coarse or Fine threads: Coarse threads are stronger, faster to install, much more common, less likely to jam, and less sensitive to dirt or damage. Fine threads have more threads per inch, and are usually only used when tapping thin material, like sheet metal"
And I think we all have completely agreed with that - There is a trade-off: course threads are stronger especially in soft material like aluminum, while fine threads useful in 'thin' material. And I think we all agree with fine threads in thin steel
sheet. BUT The question (for the OP) is specifically for a mast (aluminum 6061 and probably about 6mm or a bit more thick) where exactly where does that trade-off fall. The test we conducted at the boat yard that built my hull suggests course threads are 15% stronger than fine in 6mm 5083 plate with 1/4" fastners. The second link that Gord posted above is a similar test in 1/2" 6061 material and their results were similar.
I am delighted to learn more about this from an experienced machinist, such as yourself . . . but so far the data I have suggests course thread for the typical mast section.
Are the aluminum inserts in fact fine or course outside thread, or do they come in both? The helicoils have different inside thread options but basically only come in one outside thread (I am sure somewhere someone sells different outside theads but the common helicoils are only one in each size and they sell you the tap that goes with it.), so this discussion is really pretty moot for them.
Originally Posted by transmitterdan
I can't visualize putting stainless through hulls in an aluminum hull without insulation
between them but I am not doubting Evans at all. I would be interested to know the exact layup
. We all agree that copper below the waterline in aluminum hulls is a really bad idea. In fact it's a bad idea in pretty much any hull material.
I completely agree with all that. I hated removing the engineered plastic and putting in stainless and told the owners they should rethink the idea of commercially classing the boat if that's what was required - but there was a several $million tax advantage to getting the commercial
class so they wanted to go ahead.
The 'layup' agreed by the engineers was threaded aluminum pipe welded to the hull with stainless ball valves screwed on with some Dutch brand of Loctite copy, then with stainless pipe to the tanks
. I again disagreed, and wanted an inside and outside threaded piece of engineered plastic (G10 in this case) between the aluminum pipe and the ball valve, but the class society nixed this because of the fire possibility melting the plastic and letting the ball valve loose. I thought this was dumb because there is specific fire resistant G10 available, but the class rules were not flexible.
My back up suggestion was all aluminum - ball values and pipe, but interestingly the class society said this was also not allowed because aluminum melted at too low a temp. I told the owner that was ridiculous in an aluminum hull, but it was again explicitly stated in the class rules (which were primarily written for steel
ships). The boat yard did not like aluminum ball valves, for a different reason, they said there were no good ones available while there were excellent stainless ones, and
that they had found more corrosion
in aluminum pipe than in stainless pipe . . . the aluminum hull will corrode rather than the stainless pipe and there is a lot more of the hull than the pipe. I did agree with both those concerns.