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Wednesday, April 22, 2009

More Metal

Yes, the blog has been a bit quiet. Not much physical work on the boat has been going on, save for resetting some t-track backing plates. Focus has been on the next round of metalwork like the chainplates. This means more face time with CAD programs. As dictated by the updated rig design, not all chainplates will be as was original from the factory. Specifically there will be some "double" chainplates to accommodate the swept mizzen spreaders and the continuous running cap shrouds on the main mast. In addition, we have decided to place the staysail deck attachment as forward as possible atop the caprail to help keep the foredeck clear (the factory originally had these on the foredeck, a few feet back from the very front, creating a toe stub hazard). Also, though this will be a ketch, we will have the backstay chainplate used for the sloop. Figure this will provide some utilitarian use and perhaps help with managing the mizzen boom. We will be sending drawings out this week for estimates. Including jet cutting. Edit: updated to take flat head socket cap screws instead of carriage bolts.
From Building A Westsail42:Drawings


Colin&Lizzy said...

I have always understood that square holes tend to fail ( cracks form in the corners.)
Why not round ,drill ,ream and countersink?

rj said...

That is a good point. I'll look into it.
Thanks for that!

Ron Sheridan said...

I've been trying to follow as much info as possible for 20 years on stainless steel chain plate failures. I believe there are 3 main reasons.
1. Inferior metals. Use 316 or 316L(if welding).
2. Welds.
3. Inadequate material mass. Skinny straps fail much more often. For instance, if you take a 2" piece of metal and drill a 1/2" hole in the middle of it for the bolt, you now have (2) 3/4" pieces holding your rig. When masts 'pump', and rigs/boats flex, there is a cycling loading on each 3/4" portion. This work hardens the metal eventually and will add to crevice corrosion opportunities.

It costs so little extra, to make the strap maybe 2 1/2" or 3" wide, instead of 2" and then you have overkill, rather than accepting the risk of failure.

I strongly suggest wide tangs on any chain plate, or extra width in the entire strap. The 3rd plate from right in your drawings might be suspect, in my opinion. Hope you don't mind my thoughts. ron

rj said...

Thanks for that Ron. Comments appreciated. Yes, the material will be 316 L.

On the qty of material, according to our designer we should be ok. He says the critical part for mass is the top where the rigging attaches. Apparently we could go thinner in the "strap" and double layer the tang at the rig point and that would be just as good. But we wont do that.

We are at least as thick on the material (more in some cases) over the factory and using cap head screws over carriage bolts eliminates crevace corrosion risks at the square corners.