OK to use sidedeck U bolt for turning block to lift mast?

[Vega1447]

Hi all,
I recently described my A-frame mast lift/lowering system in
Yahoo! Groups.

In that post I said that I:

Pulled on mast halyard till bowline ran up to roots of spreaders inside
inner shrouds.
Fitted a turning block to the aft-most u-bolt on stbd sidedeck.
Ran the 20m mast halyard through this turning block back to stbd cockpit
winch.

It occurred to me that I am putting a lot of load - the weight of the
mast - on that U bolt?

Then again it takes the tension of the aft shroud which must be about
1/3 of the ballast? Say 1/3 tonne?

So the 80kg of mast shouldn't be a problem?

Any thoughts?

Thanks John V1447 Breakaway

 

[vegatern]

When I re-stepped the mast on the Tern I used the guidelines from Selden Spars which suggested tensioning the cap shrouds to 15% of the breaking strength of the wire. My shrouds are 7/36" stainless and 15% of breaking strength is about 800 lbs. The U bolt chainplates did not pull out of the deck at that, so they should not have any trouble lifting the mast. (I have thought that I might back off the rigging screws a bit, but if she still had the original 3/16" wire the pull would still be about 600lbs)
Craig Tern 1519

 

[pjacobs55]

In my opinion, I would think the only way to harm the U bolt would be to pull it in a direction that might bend it. You'd have to have a horizontal pull, toward or away from the boat sideways, to cause any damage. It doesn't sound like you did that. The turning block would be sitting at the apex of the "U", so it should be OK ... those little gems are pretty strong!

Peter
#1331 'Sin Tacha'

 

[bbach1949]

The chainplates are sized for the strength of the wire, and are good for many times the weight being discussed here. I didn't calculate the tensile strength, but just take the cross-section and multiply by the strength of stainless steel (80,000 psi?). Probably good for a couple of tons of static pull at least. A bigger concern is the amount of corrosion, if any, of the backing plate and fastener under the deck. I'd do a visual inspection on that before using the chainplate for anything - including sailing. Far more stress is put on the chainplate from normal shroud load than mast raising. An a frame is the way to go - mine is just douglas fir 2 by 4s (good straight grained clear ones!) that are light, non-marring, plenty of compressive strength. I put an inward arc to them (think Eiffel Tower) so they stay in column. I hung on the attachment point and bounced up and down pretty rigorously - I weigh more than the fully rigged mast. The a frame flexes, but that's good. Bill Bach V1071