Chainplate design/redesign

[alanro]

My new to me Windward 850 is a 30 year old 28' sloop.

I love just about everything about her except the back stay, or more accurately, the "chainplate" for the stay. Rather than a conventional chainplate, a U bolt is used, through the rear deck.

I think the boat was designed with a split stay in mind, but some previous owner had gone with the U bolt.

What they didn't do was add a backing plate, just some big washers. Worse, they didn't attach it to the hull in any way.

The end result is that the desk has deformed, with a graceful "bulge" under the U bolt, so graceful it almost looks normal.

However, the deck has also lifted away from the hull in the area of the U bolt by about 3/4" of an inch.

This clearly has to be fixed.

Sadly, engineering texts on the subject seem to be sorely lacking, so I thought I would just take a tour of the marina and see what other designers of similar sized boats had done.

And what I found left me even more confused. Looking at external chainplates only, at one extreme I found one that used a 1" x 1/8" x 6" strap held on with a couple of 1/4" bolts.

At the other extreme, I saw a 2" x 3.8" x 12" strap held by six 1/2" bolts.


My tentative plan is somewhere in the middle.


A piece of 1/2" x 1" x 10" will be bolted (on the narrow edge) to a 6" high x 8" wide x 1/4" plate. It will also be welded ('cuz I'm chicken).

A similar plate will be used as a backing plate.

Four 3/8" bolts, holes drilled in a "V" pattern, will be used to fasten the two plates to the transom.

After that, it's all conventional; 1/2" hole, clevis holding a toggle, etc.

Am I off base here? Overkill? Brave?

Any thoughts or opinions on this plan gratefully accepted.

And pointers to anything technical also much appreciated.


Thanks in advance.


Alan

[philSweet]

The necessary cross section area of the chain plate can be determined from the size of the stay wire. Its attachment to the transom is based on the shear strength of the lamina and its service or damage tolerance required. Any rigging shop should be able to hand you what you need across the counter, and the specs for most boats rigging can be found on the web from owners associations websites or the factory. There is usually structural reinforcing, not just a backing plate in most of the boats this size I have looked at. If this boat was designed for a split backstay, thats kinda important to know.

[masalai]

Is the hull material wood, Glass over wood CSM or what?

A grp type hull could be given a grp type chainplate by taping a SS cylinder to the hull and grinding out a neat pair of slits to fit the turn-buckle assembly and fabricate a SS pin to hold in place... Looks quite elegant...

The first 2 are for the midships stays and the latter images are of one of the fore-stays... Each is capable of holding more than 4600kg...

[alanro]

Originally Posted by philSweet

The necessary cross section area of the chain plate can be determined from the size of the stay wire. Its attachment to the transom is based on the shear strength of the lamina and its service or damage tolerance required. Any rigging shop should be able to hand you what you need across the counter, and the specs for most boats rigging can be found on the web from owners associations websites or the factory. There is usually structural reinforcing, not just a backing plate in most of the boats this size I have looked at. If this boat was designed for a split backstay, thats kinda important to know.

Thanks for the reply!

Sadly, the company that built my boat is long gone and there's just NO information to be found online. (Believe me, I've looked).

What I was able to find are some pics of another Windward for sale and this one had a split backstay.

And looking at my Windward, it looks like it _could_ have had a split stay at some point. I say "could" because when staring at the inside of the transom, there are some little "worms" of sealant visible (as if plugging some holes from the outside), right in the area of where a split stay chainplate would be.

But there's no sign on the outside whatsoever that there was ever anything there.

That leads me to believe that this particular boat has had a single stay for a long time.

But your suggestion to visit a rigger is well taken. I've been hesitant to do so because my single experience with a "professional" rigger was .. disappointing.


Alan

[philSweet]

found this. Make a call and see if you can get photos/ specs of rigging.
http://www.thunderbirdmarine.com/boat-info.php?ID=3325
or contact the designer, John Simpson.
http://www.simpsonmarinedesign.com/sail.htm

[masalai]

Your Windward 850 looks quite a sweet boat... May you enjoy lots of good sailing...

[Stumble]

I would go ask at http://www.briontoss.com/spartalk/fo...sc&daysprune=7

It is Brion Toss' website who is one of the few truly professional riggers. He normally responds in a couple of days at the longest, and likely has seen your boat before.