Structural techie question

flipper

flipper

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Hello All

I asked this question on the PBO forum a while back but thought I would pitch my solution here for any input:

"I have a 2' bowsprit on my GRP 25' itchen ferry. The ss wire bob stay is attached to the stem by a 3" or so d ring. The d ring fitting is a bit loose and I would like to replace it but I cant access the back of the d ring from inside the boat because it is enclosed in the stem. The stem section is solid and must be about 6" thick"

So...

The original d ring has deformed because, I think, of the lateral load when they are designed for a vertical load. Would I be better cutting off the d ring at the two loose bolts, grinding and filling (as suggested by 30 boat in my original question) but then replacing with a SS eye bolt (say 12mm thread) right through the stem with a hefty SS backing plate. I could then get at this through the chain locker.

Needs to be hefty as otherwise the pull pit and bowsprit bolts are the only thing holding the rig up forward.

cheers, Patrick
 
I wouldn't be keen on an eye bolt. It would be subjected to a lot of lateral stress, with nothing to stop it trying to bend within the stem, or where it contacts the leading edge of the stem. You're also relying on a single bolt.

I'd cut off, grind back and fill the existing bolts, as said, but i'd want a more substantial replacement. I had a look around and found a picture of the type of thing i'd use :

bobstay-3-P1010707.jpg


I'd carefully make a flat spot on the stem so the mounting plate fitted flat and flush. I would have the mounting plate cut and have three holes drilled and threaded to take M10 studs. I would then drill the stem and temporarily fit the plate with the studs, cutting them to length. Then, weld the studs to the plate and grind/sand them flush. If you want a polished finish, now would be a good time to polish the plate. Finally, weld the lug for the bob stay cable in place. You'll just have to give the welds a polish and bolt it into place, obviously using a backing plate, about the same size as the mounting plate.
 
Hi
We have something similar to Pauls description except we have bolts going through the stem.
You can get something made up by a local fabricator. PM me for one we have used in the past.

bowspritfitting.jpg
 
flipper said:
Paul, thanks for that detailed reply - really helpful.

As you can see from the pics in a previous thread...

http://www.ybw.com/forums/showthread.php?t=248138

...she has a plumb bow; do you reckon I could get a standard, off the shelf fitting as i dont have an angle of sheer to take into consideration for getting studs welded?

cheers, Patrick
Click to expand...

Dear Patrick.

2 pointers.

Roach Sailing Association
Ken Wickham.

and 2 outcomes.

Cheap
Happy.
 
I looked at the previously posted pics, you can clearly see that the bob stay would be pulling against an eye bolt at something like a 45° angle, which i don't think would be structurally sound.

The reason i suggested studs, and welding them in place before adding the lug that the cable attaches to is fitted, is twofold. Partly so that you have a fixing directly behind the lug, as well as one at each end and partly so that they could be sanded back and polished, leaving a smooth mirror finish. I could be slightly over engineering things and be overly fussy about the finish :D I did spend a whole day polishing my new radar strut after i finished making it :)
 
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