Crack in aluminum mast : options?
 

Hello,

I have a 33' masthead sloop. As i was doing a routine inspection of the rig, i found two cracks just above the second spreaders. The cracks originate at the bottom screw holes for the spin topping lift sheave box and spread aft on either side for about 2 inches. The cause of the crack i assume is from possible corrosion coupled with the loss of structural integrity with the housing of the sheave, and the fact that the mast pumps when sailing to weather putting stress on this weak point. I have and use running backs.

The mast is a proctor 47' x 4"x6"

I have been advised to drill stopper holes at the ends of the cracks, then grind the cracks and have a pro TIG weld them. Next add a plate ( cut from the a forespar section of a broken mast) to be welded over the housing box to stiffen the mast. The plate is an elongated diamond with a rectangular midsection and rounded corners about 15 " top to bottom. The mid section spreads aft to cover the ends of the cracks. The plate is a 1/16" thicker than the mast walls.

Any feedback would be greatly appreciated!

I can only chime in here from an engineering standpoint as I am not a rigger. I am sure Brion will have a better reply. The holes you drill are standard practice to stop a crack from spreading. They spread the stress out over a larger area so it no longer has enough force to crack the aluminum. The diamond piece you are welding on has a similar function. You don't want to make one part of the mast too strong compared to the section right next to it.
That would put too much stress right at that junction. The diamond will make your weak point very strong and then gradually reduce the strength as you go up and down the diamond. The radius does the same thing although personally I would not see a problem with a sharp point going up or down as it would not be a stress concentrator but a stress reliever. I like the radius on the sides of the diamonds. But in general, no sharp corners, no abrupt transitions.

I personally have had good luck with TIG wilding of some aluminum although they were not boat related. I had a very good welder and he did an excellent job. There must be a lot of variation in welders as I had people tell me both jobs were impossible.

Allen

Ouch
 

Hello,
A tricky one. First, if corrosion is a contributing factor, something else is really, really wrong, like stray current or weld sickness, in which case these cracks might be indicative of problems elsewhere.
Next, this sounds like a classic stress riser failure, at least as far as the crack origin goes, but there are lots of topping lift mortises with corners, and they don't all crack like this, so these cracks, once again, might be indicative of problems elsewhere.
Next, drill stops can only do so much. I have a picture, courtesy of ace surveyor John Koon, of a similar crack that had been drill-stopped three times; the crack just kept bypassing the stop. The length of the cracks, in this case, do not speak well for the mast's remaining integrity.
Next, why does the mast pump when going to weather? This is not a good thing. It is possible that the tune is off, and/or that the mast scantlings are off. Tune you can fix, but scantlings, not so much. So if you weld that patch on, you might just be welding in a big stress riser, the strengthening effects of which are overridden by the break-here effects. Have you run the numbers on this stick?
Finally, it is very, very difficult to make a non-damaging weld on an aluminum mast, especially at a location, like this one, that is in a bend-intensive section of the column.
I'm inclined to say replace that mast. But if the numbers come out okay, and if a sleeved section won't compromise efficient bending too much, then a complete sleeve would be a much better fix.
Fair leads,
Brion Toss