Replacing chainplate structure - keelboat

[RickEdmonds]

The boat is an IOR 3/4 tonner, 34', about 12000lb - 5500kg. masthead rig. The existing chainplates are 1/2" s/s plates (one each for cap shroud and lower shroud - spaced apart by about 7" fore and aft) that pass through the deck and bolt to a fabricated double hanging knee arrangement. I just discovered there is a rust issue with the port hanging knee, so strength is suspect (stbd appears OK but will get the treatment just in case).

My plan is to have a plate made that bolts between the cap and lower straps and fit a strainer down to the hull. The strainer will have to be wire as I can't get rod in there without destroying all the existing furniture. Rigging is 1/4", rigging screws are 1/2" thread and 1/2" pin - about 12000lb breaking point - which agrees with my understanding that each chainplate should be capable of individually lifting the boat.

The original knee will be retained 'cos it's too hard to remove it, but it will be redundant (or that's the idea). However, the whole lot fits hard under the deck, so the deck obviously contributes to the strength - it would have to tear a big hole for the structure to pull out, and the new plate will be bolted to the old structure anyway.

So, it seems the wire strainer (one strainer each side, carrying both cap and lower loads) would need to be 5/16", using 5/8" pins to be comensurate with the strength of the actual rigging - does that sound right?

I'll have a S/S "ladder" made up of 3/4" s/s rods, with a plate strongback joining them and glass the rods to the hull at the bottom end of the strainer, but does anyone know what area of bonding would be necessary (original hull is polyester, but I'll use epoxy for this job), and how thick would the bonding glass have to be?
The bonding area is the main concern - not much point having the S/s all at 18000lb strength if the ladder rips off the hull at 10000lb! I think I can achieve roughly 2' x 2' - would that be sufficient? One rod can "tuck" below a stringer which should help anchor everything in place, and the bonding will also wrap over the existing bottom end of the knees, increasing surface area and avoiding a straight, flat glass pad.

What thickness plate would be necessary to "hang" the strainer off at either end? I'm thinking of using a rigging screw to tension the wire strainer - 5/8" thread and 5/8" pin again.

Any tips will be much appreciated.

Cheers

Rick Edmonds

[SailDesign]

Rick,
I can't figure out your system. What is a "strainer"?
If you can post a sketch, that would be great, or a digipic with squiggles on it would do fine as well.

Steve

[PAR]

'Sail, I think he's looking to put a wire tie rod in there. It would work in tension only, but may make him feel a little better when the wind is blowing good.

I'm not real sure how he's intending to bond this arrangement to the hull, but it better be straight and in alignment with the shrouds or something is going to tear loose when he doesn't want it to.

RickEdmonds, I'd strongly rethink removal of the hanging knee, plus reinforcement if you desire. Rust means fastener sickness, this can only get worse. It also means water is getting in and this ain't no good either. Address the problem(s) then re-engineer some stuff if you like.

Transferring the rig loads to the hull is difficult to back engineer after construction. I'm not sure how old this boat is (or what it is) but a backing plate on the knee to increase the load base to the hull, or beefed up knee and it's attachment seems a better way to go then drilling holes in the furniture to run flexible tie rods.

You'd likely do less damage to the cabin (liner?) grinding out the bonding to the knee and fitting a backing plate or two (one on the deck side, one on the hull) and reinstalling the modified knee, then tabbing in a cable (with perfect shroud alignment) to a bilge stringer and surrounding hull area.

[RickEdmonds]

PAR - thanks. You are right that the "strainer" is a wire tie rod between the through-deck fittings and a structure glassed to the hull - as you put it, to a bilge stringer and surrounding hull area. The wire will be tensioned by a rigging screw, so load take-up is at all times. The hanging knees run down the hull for about 4', so removal is a big job. The chainplates are set well inboard of the gunwhale, thus the need for the knee or a tie rod. I've removed the problem rust (and am in the process of rebuilding the deck after the excavation!). I think the problem started before I bought the boat 6 years ago, and just got worse until it was visually obvious that something was going on - the deck was bulging! However, I'm not worried about further water ingress, that's easy to prevent and the whole deck at this area will be rebuilt, so no damp cores etc. The tie rod is to emulate the other way in which these boats were built (instead of the knees). I was concerned about how large the glassing area needs to be to anchor the tie rod against the bilge to prevent it pulling away from the hull.

However, I've just been in contact with another owner who has re-engineered his structure (originally a tie rod, where it did separate from the hull) and I've picked up some good ideas from his new arrangement (partially picking up your idea of "beefing up" the knee - I'll convert the bottom end of the knee into quarter bulkheads, through which I can bolt "tabs" welded to the glassed in rods - it'll be bullet proof!

Cheers

Rick