interesting failure- dyneema rig terminal

[markxengineerin]

when people were critical of your ideas and methods you listened to them, admitted your mistakes, and responded with respect

To that I credit the very humbling experience of messing up a perfectly good mast ; )

To slow down corrosion, I am planning to use lots of TefGel. There is not much (maybe no) room for tape due to tight clearance. The cavity is also completely sealed and there will be no relative motion between any of the parts- the advantage of high pre-load on the bolts.

Stainless bolts probably would have worked, but without knowing the coefficient of friction between the mast and the fitting, it's impossible to know how much pre-load is needed to prevent slip for a given load. We also don't know the actual load on the shroud, so, I feel safer making the design as strong as reasonably possible, which I have done.

I have seen the effects of steel corroding aluminum on cars, and stainless corroding aluminum on boats. I am curious if anyone knows if one or the other is worse, specifically for the aluminum.

[wyb2]

According to this galvanic potential chart “low alloy steel” is closer to aluminum than 316 stainless, so if that’s right, it should be better, not worse.

https://www.ssina.com/wp-content/upl...n__figure2.gif

I think the problem with alloy steel is just how quickly the rust becomes a cosmetic issue. The part may last 20 years, but it will be leaving rust stains after 3 or 4. Stainless hardware can fail unexpectedly from crevice corrosion, but most often it stays serviceable and looking pretty good for multiple decades.

[markxengineerin]

That's a good point about the rust stains- hopefully they are as easily removed as the ones stainless can cause. A little Ospho sprayed on 1x a month seems to do wonders.

Do rust stains happen even when steel is sealed in epoxy and paint? I am guessing a tiny amount of water does get through, but no experience with how long this takes to become ugly.

[wyb2]

Someone who has owned a steel boat will know way better than me. Whatever coatings they use seem to do a great job for the most part, but even well maintained ones always seem to have a few trouble spots with visible rust staining.

[markxengineerin]

Mostly done- all cosmetic stuff from here
Half the lower assembly ready to go in

And all done

Upper done


The hardest part was how messy it was, between the epoxy, 4200 on the back of the backing plates to seal water out, and Tefgel on the fasteners. My tools are a little worse for it but I think it came out well.

[Pauls]

Regarding your question about the corrosion issues between aluminum and steel or stainless, by far the best combination on an aluminum mast for fittings, tangs, etc is a moderately high strength steel which is hot dip galvanized, then painted. Galvanizing is coating the steel with a layer of zinc. Zinc and aluminum are immediately adjacent on the galvanic scale, they are a very close match and the combination is very good for protecting the mast from galvanic corrosion. In addition, this approach allows the use of steels which are much higher in strength than 316 stainless. 316 has a max tensile strength of 60-70K PSI. Steels compatible with hot dip galvanizing can go up to 125K PSI, basically double the strength. With these fittings as high above the water as they are, with galvanizing and paint you will not have corrosion problems.

[markxengineerin]

The large bolts that are gold looking are zinc yellow chromate, 150ksi tensile
The small black ones are black phosphate, 150ksi tensile- chose them because they are flanged, to avoid needing to deal with 100 washers in addition to 100 bolts.

Galvanized sounds good, but would have been tougher to find the sizes. McMaster has nothing under 1/4" for galvanized, and limited to coarse thread. Good to know this for future reference though, thanks.

[Sailer_Med]

Originally Posted by markxengineerin

The large bolts that are gold looking are zinc yellow chromate, 150ksi tensile
The small black ones are black phosphate, 150ksi tensile- chose them because they are flanged, to avoid needing to deal with 100 washers in addition to 100 bolts.

Galvanized sounds good, but would have been tougher to find the sizes. McMaster has nothing under 1/4" for galvanized, and limited to coarse thread. Good to know this for future reference though, thanks.

Good choice of fasteners!!

[wyb2]

Originally Posted by markxengineerin

The large bolts that are gold looking are zinc yellow chromate, 150ksi tensile
The small black ones are black phosphate, 150ksi tensile- chose them because they are flanged, to avoid needing to deal with 100 washers in addition to 100 bolts.

Galvanized sounds good, but would have been tougher to find the sizes. McMaster has nothing under 1/4" for galvanized, and limited to coarse thread. Good to know this for future reference though, thanks.

I’d get those black oxide fasteners coated with something asap. In my experience black oxide is to prevent rust from forming while it’s sitting in the warehouse, and sometimes it’s not even good enough to do that. Yellow chromate is better.

I think if you do a good job coating it, and the joint doesn’t move enough to break the coating where different parts meet, then you should be good for a long time. Thinking about my earlier comment about steel boats, the trouble spots seem to be around joints or point of wear (like anchor rollers).

[markxengineerin]

The repair seems good, it survived rounding Cape Hatteras in 40 knots from the North, so probably won't get a whole lot worse than that.



I ended up using 4200 over the big fasteners for schedule/convenience reasons, ugly but seemingly effective.