Crack in forward wishbone casting

General
1

There is a very fine hairline crack extending from the back of the choker attachment point into the main body of the forward casting on Canatara’s wishbone (see photos). The crack as seen in the first photo is almost invisible without magnification. Canatara is co-owned by Rob Collie and myself, and we would appreciate any wise advice on the appropriate follow up.

The choker attachment point had been professionally rebuilt 3 years ago involving welding on the casting. The repair showed signs of failure after the first season and the choker has been lashed around the casting ever since. (The attachment point is no longer in use).

Just to be clear, Rob and I are both engineers though neither of us has metallurgy expertise. Having said that, I suspect that this is a stress crack caused by uneven heating of the casting during the repair. Mike Quill can get a new casting made for us for $3100.00 cdn but he doesn’t know how long it will take.

Our questions are as follows:

  1. Should the casting be replaced? If so how difficult is this. It appears to be held on with large Phillips head screws, however a brief torque test suggests these may be tightly seized. Will the screws then need to be drilled out?
  2. Can anyone tell me if my theory about the origin of the crack is correct? If it is a stress crack, am I correct in believing that the internal stress can be relieved by drilling a small hole (1/8") at the end of the crack to relieve the stress and thereby stop the cracking? Is it possible that this may be the only repair needed, or is it a potential stopgap until a new casting can be obtained?
  3. Has anyone ever heard of this casting failing? If it did fail under load, what might that look like?
  4. Any other thoughts?

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Comments would be appreciated

Thanks in advance

Bill Kroes

Sail No 24, "Canatara ", Sarnia Yacht Club, lower Lake Huron

2

Hi Bill,
I feel your pain, last season I went through the same questions about a crack in Soave’s mast collar. Link to my post.

I am not a rigger, engineer, or a metallurgist. My thoughts are based on my experience with Soave and previous boats.

  1. Seized stainless screw are common on a Nonsuch. Well maintained boats use an isolation paste to prevent this. After you’ve removed a dozen or so seized bolts or screws you may feel as I do, a tub of TEF45 or equivalent belongs onboard every Nonsuch. Buy some penetrating oil, a batch of left-handed bits and some extractors. I’m not strong enough to break off a phillips head screw, which is good… it’s much easier to drill them out if they aren’t broken. Triple the time you think it will take to remove hardware unless you’ve removed it before and used isolation paste when you reassembled. With practice it gets easier.
  2. The stress relief hole might provide some short term benefit, but the cast fitting will continue to age at an accelerated rate. I’m not a metallurgist, but my old mast collar seems different from the new one. Does cast aluminum age over 40 years in a marine environment ?
  3. I have talked to Nonsuch owners that have experienced collar failures and mast failures. I have not heard of a wishbone forward casting failure.
  4. Additional thoughts ( which weighed heavily in my decision ) in 5 years will it be harder to find a replacement ? My assessment is yes. So I replaced the damaged collar.

Sorry, I doubt anything I’ve said is new or surprising.

It’s a shared experience for Nonsuch owners.

Good luck,
Rob…

3

I’m neither an engineer nor a metalurgist, just a boatbuilder, but if it was my boat and the casting needed to be replaced I would be taliking to a good aluminium welder.
This is not a space shuttle part. Its job is to hold the ends of the wishbone together and support the choker tackle. It doesnt have to be cast, look exactly like the original, or cost $3000. I would look at fitting pipe pieces inside the wishbone ends and welding them solidly together ( likely plates top and bottom) and then work on the choker support and esthetics. I’ll bet it could look pretty nice and be easily as strong as the original part.

4

What Paul said… when I made the wishbone for my 36’ trimaran it was 2 aluminum pipes separated by some 1/4" aluminum plate. All welded together in a local shop for a couple hundred bucks. Huge loads on the tri, no issues.

5

I confess to being a metallurgist. Here are some comments

The crack is not straight and branches at the top, so it is unlikely to be a fatigue crack, that is ,a crack that grows over time. As well, the crack line is dirty, which means it is not new. I suspect it is an old original shrinkage crack that occurred when the part was cast. Usually it would be left alone,

The forces in the hole area would be mostly aft and upwards, so the crack is in a relatively low stress area of the hole.

The part will be ALMAG 35 a weldable aluminum casting alloy, which is specified on most of the Nonsuch fabrication drawings This material usually is TIG welded using 5183 or 5456 filler material.

Generally, to repair a crack, you first have to gouge out the crack to sound metal then weld up the resulting grove and finally grind the surface smooth. However, welding down into the hole would be difficult and probably would result in a bad weld.

I suggest you leave the crack as is and check it regularly. To see if it has grown, you could remove the paint from the crack area then mark the ends of the crack with a line of paint. To get a sharp line, apply a strip of masking tape across the crack at its end, then paint the line.

John Barbour

Nature 26U

Toronto

6

I have some limited experience with the casting on the aft end of the boom, and have also observed how the forward casting has been handled on two Nonsuches with wear problems at the same spot. My comments are based on that.

I would agree with John Barbour’s advice about the crack being subjected to low stresses, and with his recommendation of leaving the crack alone and monitoring it.

The lashing around the casting that you describe is a very solid and effective alternative to using that attachment point. Properly done, it will last for many years and will distribute any pressure on the casting very broadly. I wouldn’t expect it to exacerbate the cracking.

Another alternative to a lashing would be a stainless strap bent to wrap around the casting. Here’s an example:

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If I was doing this approach, though, I’d have the strap made with a usable eye at the end rather than the cheesy shackling arrangement shown here.

If you were to replace the casting, I’d suggest that Paul and Ron are spot on about how to have a cheaper alternative fabricated. Thinking about the difficulty of getting the casting on and off, I wonder if it might be easier to use caps that fit over the boom tubes rather than trying to do sleeves that fit inside them.

Either way, here’s what I can tell you regarding the boom tubes: they are (I’ve been told by Mike Quill) 3.5" outer diameter, 6061 aluminum, 1/8th inch thick (i.e., 3.25" inner diameter), that the casting fits into. I’ve found in the course of buying such tubes that there’s a bit of variation in these numbers. If you build an alternative with the approach Paul suggested, you may have to either machine them a little or experiment a bit with the exact size you need for the tubes that comprise the inner sleeves. The outer diameter of those sleeve tubes should of course match the inner diameter of the boom tubes. However, they should probably be thicker than the boom tubes, i.e., 0.25" thickness rather than the 0.125" thickness. (All this based on Mike Quill advice re the aft casting.)

If they’re the same as I found on my N26, the Phillips-head screws are 1/4th in by 20 threads per inch machine screws, likely not more than 0.5" or 0.75", tapped into the tubes and casting.

I was able to successfully remove all those screws from my aft casting. The trick was largely patience, combined with the right tools. Soak them a few times a week for a month using WD-40, Liquid Wrench, and CRC Freeze-off. Then go after them with a good quality impact driver making sure that the screwdriver bit is a good match to the heads. It may seem at first that they won’t come, but in that case, just soak them again and come back at it the next day.

Bottom line: it’s a lot less work to go with John’s suggestions, and to keep using lashings.

-- Bob