Mast corrosion - repair options?

[santomas]

Hi all,

I discovered some pitting corrosion under a stainless steel fitting on my mast.

It looks like there is more than 80% thickness of aluminum, so I'd rather strengthen it instead of getting a new mast.

I'm considering fabricating a larger and thicker stainless fitting, drilling new holes outside of the damaged area, and fixing it with monel rivets.

What do you think about it?

Tom

[C420sailor]

What is the fitting for? Does it have to be SS? Would you be able to clean up and re-use the holes?

Remember that cracks are not the only things that can be stress risers—pits can be, as well. There may be an advantage to making this surface smooth prior to reinstalling the fitting—but that also has to be balanced with how much material you’d have to remove to accomplish that.

A coating to inhibit the galvanic corrosion is a must.

[Cheechako]

More info will help, some areas in a mast are higher stress than others.
My goodness, those holes look big...? What was there?
Where is it on the mast? At the top? Near the goose neck? etc.
You may be better fabricating an aluminum fitting... etc

[santomas]

The fitting is for the inner forestay and lower shrouds. (photo attached) The holes are 6.5 mm.

I believe it used to have spreader brackets/sockets, which were ground off, with new spreader brackets fitted right above.

What would be an advantage of an aluminum plate over stainless steel apart from less potential for corrosion?

[Cheechako]

One option is to have an Aluminum tang welded to the mast for the inner forestay. Many are built that way actually.
I assume the lower shrouds go to a tang from a large bolt through the large hole?
Many masts are built with that large hole which has an aluminum tube in it which goes through the mast.
These mods would be less corrosive if you do them.

You can fill the pitting with an epoxy mix prior to coating. If you reuse the SS fitting then fill and provide some sort of barrier coat to reduce the interaction of SS and aluminum. On the rivets or screws also.

[santomas]

Thank you for the excellent drawings

Yes, the lowers go to tangs, which are bolted together through the compression tube. (not sure if it's aluminum or ss)

I thought about welding an aluminum plate over the corroded area, but I read it can weaken the mast.

I wonder what the loads are on this point, right between the spreaders and the lower tangs, relative to the masthead, gooseneck, etc.

I can't find any images of mast stress points.

[Cheechako]

Originally Posted by santomas

Thank you for the excellent drawings

Yes, the lowers go to tangs, which are bolted together through the compression tube. (not sure if it's aluminum or ss)

I thought about welding an aluminum plate over the corroded area, but I read it can weaken the mast.

I wonder what the loads are on this point, right between the spreaders and the lower tangs, relative to the masthead, gooseneck, etc.

I can't find any images of mast stress points.

I can tell you that many masts that fail at sea are near and often below the spreaders. It's because of the "slenderness ratio" and compression loading. Columns dont often fail because of ultimate material strength, but more because of going out of straightness creating side loads. Once the mast flexes enough, it fails where the spreaders are, far away from the end points.
Extensive welding within a couple feet of the mast head I would have no problem with, it's not going to bend/fail there. Minimal welding can be done most anywhere.

You could form an aluminum "doubler" over the entire area except the aft where the track is. Maybe 1/8" to 3/16" thick plate. T6 alloy is too hard to bend, maybe T4, not sure. Ask what is more formable that is available. T3 maybe. I could see wrapping a piece around gradually by fastening one end and forcing it. Especially 1/8". Then just use the compression tube/bolt method for the shrouds as shown in my drawing.
A rigging company did a doubler for me on my 47 ft boat. A long doubler almost 2 ft long. They formed it to fit, used "structural epoxy" between the doubler and mast, and riveted the doubler with a bunch of rivets before the epoxy set up. It was excellent.. albeit expensive! Chesapeake Rigging.

[Cheechako]

Are the threads in the mast holes shot? I wonder if you could re use the fitting but put a layer of Teflon sheet or etc between...? Be nice to eliminate all those SS bolts though...

[santomas]

There are no threads in the mast holes, the ss fitting was fixed with monel rivets, which I drilled through.

Very interesting points about mast stress/failures.

I'll definitely look into using aluminum rather than stainless steel as a doubler. It seems better both structurally when epoxied rather than just riveted to the mast, and as future corrosion prevention.

Why did you need a doubler on your boat and where was it on your mast?

[Manateeman]

Chee is correct. I HOPE a large doubler would do the trick without welding. You can use several all thread rods inside aluminum compression tubes and big stainless or monel rivets. Just keep different metals isolated. Tef-Gel on stainless rivets last a long time. Just keep the water out between whatever you place on the spar. I’d bend the plate off mast till it fits pretty tight...doesn’t need to be perfect. Someone in a fabrication shop can do this perfectly but I’ve seen 1/8” and 3/16 aluminum bent with a hydraulic press. You can add extra metal by TIG welding the part OFF the spar, and then attaching it with rivets or threaded rod and aluminum tube. Internal halyards won’t get chewed up and you won’t squeeze the mast extrusion.
I don’t know how to post a drawing but I’ll try to explain.
The compression tube is the exact length of the outside width of the spar. The holes in the doubler (and washers if you need them) is smaller than the compression tube but obviously big enough for the threaded rod. Use extra heavy nuts. Double nuts, star lock washers...whatever you wish. I slather up the area between the doubler and the spar with 5200 and gently squeeze it. I have my rivet holes predrilled but I let things set up before riveting.
If you need a thick piece of aluminum placed on the doubler, you can double the doubler and TIG it all on the bench so you are not welding ON the spar.
You kinda got some pretty serious sized holes. Take a look at how masts are spliced together for ideas. I’d take some more photos and run it by the engineers on Boatdesign.net and some of the spar builders. They might say yes or no.
I can tell you how to do what amounts to an external “splice” but it’s not my boat and a lot depends on what the corrosion looks like inside the spar.
Please note that a lot of spar builders routinely fabricate spars from two sections with bent and special extruded internal sleeves but...the long extrusions are new and end cut perfectly so the forces of compression are distributed evenly. I’ve never seen it done at the spreaders. This is a case for a second opinion.
Happy trails to you.
Mark and the computer illiterate manatees

[Cheechako]

Originally Posted by santomas

There are no threads in the mast holes, the ss fitting was fixed with monel rivets, which I drilled through.

Very interesting points about mast stress/failures.

I'll definitely look into using aluminum rather than stainless steel as a doubler. It seems better both structurally when epoxied rather than just riveted to the mast, and as future corrosion prevention.

Why did you need a doubler on your boat and where was it on your mast?

I had in mast furling, the big cutout at the mast base for the rope coiler started cracking at the corners of the cutout. I was having the mast repainted and rebuilt anyway so they did the doubler. Frankly it looked great, They showed me some racing masts they had done near the spreaders. Their scheme was to double near the spreaders and taper the mast above that to the top. Saving weight further up. However, had I to do it again, I would have just had the cracks welded or maybe just doubled for an inch around the cutout! Overkill. But in my head that big cutout section of the mast worried me even though it was near deck.

[thinwater]

Determine how much corrosion using a pit gauge. There are sufficient good areas to use as reference. Handy, and you can get them on Amazon. MUCH more accurate than guessing.



As others have explained, masts fail from being out of column, at the point of maximum compression. One solution, short of sleeving the mast, is to lower the spreaders below the damaged area (and shorten the mast), since that will reduce the compression in this area significantly. We're only talking 6". If everything is moved, the sails should still fit. Play with it.



Or you can just put it back together.I don't think a plate or such will do you any good, unless it were something very large.

[santomas]

Reducing the compression by moving the spreaders got me curious.

Why do you think we'd need to shorten the mast then and which end?

Thank you, as well as Chee and Mark, for all the good tips and detailed ideas

[Stewie12]

Your idea of using monel rivits is a good one, but you must also insulate the stainless steel from the aluminium mast. So put a rubber gasket on it. Use the KIS principle.