Stripped Threads on Mast

[estarzinger]

^^ I have been puzzled by this. The galvanics charts are one fact that certainly make monel look worse, but it is also a fact that many of the best riggers use monel rivets in preference to both stainless and aluminum (I have seen that first hand in the best riggers in NZ and Holland - and just as an aside I was on the management team that for the parent of Southern Spars and involved in their technical process and know in detail what they do) . And it's a third (less significant) fact that the monel rivets on Hawk have caused no corrosion problems, while the stainless ones have (both bedded in 4200) - they are in different applications and installed by different people so may not be apples to apples. So, I can't figure out how to square all that.

But again, not to dance to finely, I would suggest that both stainless and monel are reasonable for 'lightly loaded' parts, and aluminum for 'very lightly loaded' parts. And the OP has not even asked about rivets at all.

Originally Posted by transmitterdan

Copper is closer (less corrosive) to aluminum than either stainless or monel.

This may be true on the galvanic charts but is definitely 100% NOT the experience of aluminum boat builders. Regarding thru hulls for instance, copper is absolutely forbidden while stainless is the preferred material if engineered plastic is not desired/allowed.

And that's not the opinion of just a few hack boat builders. I project managed the refit of a 112' Royal Huisman (perhaps the best aluminum builder in the world) and one of the things we did was replace plastic bits with stainless (because the boat was going to be commercially classed and for fire reasons the original engineered plastic was not allowed). Stainless was the only option approved, copper alloys/bronze was not allowed - all agreed on that: RINA, MCA, Royal Huisman, and the designer.

[transmitterdan]

It really is good to hear first hand experience. Jedi is right that there are variables in the galvanic series. The biggest variable is the electrolyte solution. Most charts are based on sea water electrolyte and then it is a matter of oxygen content. Less oxygen usually being worse. But a mast is not immersed in low oxygen content seawater. It's possible that the environment of masts makes a difference. If someone can find research on the typical electrolyte solution found in mast locations it would be nice. Probably no one has done the research.

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.

[dannobee]

Boy, talk about getting into minutiae.

WRT fine thread vs coarse thread, the fine thread will strip out at a lower torque, but that should be weighed against the fact that fine thread bolts have 1.4 times the clamping force than coarse thread bolts. So, torque less but have more overall clamping pressure when using fine threaded bolts.

[Celestialsailor]

Originally Posted by s/v Jedi

It'll be in plenty engineering handbooks as all tapped holes in aluminum are course thread (like engine blocks that get high load). Into soft materials like aluminum, fine thread strips out easier.

Let me just point out that in most Engineering handbooks it does say that about fine threads stripping easier. However...they are generalizing. The opposite is true for thin metals, such as a mast where there is a larger percentage of fine threads than coarse. On a mast thickness of .150", a 3/8"-16TPI bolt would have 2.4 threads purchase in the aluminum. A 3/8"-24TPI would have 3.6 thread purchase, giving it greater gripping strength. Now...having said that...in general if the aluminum thickness is that of the minor diameter of the bolt then it changes back to what you are pointing out. So the minor of a 3/8"-16 is 5/16".

Originally Posted by estarzinger

This is common knowledge among almost anyone who has mechanical/machining knowledge. . . . But here is a link to one technical paper/site that agrees with Jedi's comment http://www.eisc.com/support/coarsefine.htm. There are dozens of others on the web.

...and here is a link for you... 5/16-18 Coarse Thread Tap and Letter F Drill Bit
I am a Machinist of 32 years and a Mechanical and Field Engineer of 10+ years.

[estarzinger]

Originally Posted by Celestialsailor

...and here is a link for you... 5/16-18 Coarse Thread Tap and Letter F Drill Bit

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 and equipment. 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 and leaks 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.

[s/v Jedi]

Originally Posted by estarzinger

I don't see your link mention aluminum or 'soft metals' - would you please point out what I am missing.

It's just Jedi bashing, they love it so why not let them have their fun?


Nick.

[Celestialsailor]

We can beat this topic until the cows come home.
Realistically, I rarely see any failures on a mast. My boat is almost 40 years old without an issue. You want monel...use monel. Inserts...use inserts. There are many different ways of doing it. All strong...all successful. I even have monel rivets on a lot of my stuff on the mast.
To think that someone is bashing you because they might think their way is better is over stated.