Replacement of the Chainplate System

[flo617]

Hello,

I removed one upper chainplate from my 29' boat. The original setup had the chainplate embedded in fiberglass. The chainplate would make a couple of near 90 degrees turns (reinforced with a welded steel at at the bend) and exit through the deck.

My original plan was to reinforce the hull a bit, make a G10 knee and strongly attach it to the hull (heavy tabbing + bolting every 2 inches).

I did most of the prep work including filling the slot on the deck and drilling at a 90 degree angle from the original slot for the new chainplate (1/4"x1.5")

However I am now worried that the system might be undersized for the upper shroud even though the length of the new chainplate is slightly longer than in the original setup. I am concerned about the moment of force on the knee.

Anyone has an idea?

Thanks.
Florent

[transmitterdan]

Why did you want to change from what the builder had done originally?

[flo617]

Because I was not sure about the state of the original chainplates since I could not inspect them. In order to pull one out I had to cut it.

The original chainplate had a sharp "S" shape and welding to reinforce it. Making an exact copy might be a little difficult.

I have not heard of anyone replacing a glassed in fiberglass chainplate with another glassed in fiberglass chainplate and I was under the impression fiberglassing chainplate had been abandonned because of the issues of water getting trapped, corrosion, fiberglass delam etc...

By the way, the original design had a different main bulkhead position and a different set of attachement (with chainplates onto knees and the upper on the bulkhead)

[Suijin]

Can you expand a little bit on your new construction? As I understand it, you fabricated a new G10 knee and glassed and bolted it to the hull. From your diagram it looks like the new plate is oriented fore and aft, and that you plan to through bolt it, through the knee, to the hull. Is this correct?

I'm not an engineer but I'll hazard a guess that a 14" chainplate bolted every 2" is going to be more than adequate for a 30' boat, provided that the hull and knee are themselves sound/thick enough.

[mike d.]

Depending on your hull thickness at the attachment point adding glass to relieve the localized stresses makes sense! Our boat if a GulfStar 37, and that is what GulfStar did. If you have the thickness you can use carriage head bolts at the attachment points. You would drill your holes for the shank diameter of the bolt, then file in the square for the head. I just replaced ours after 35 yrs. and found only 2 out of 30 that had any corrosion under the heads. This appeared due to lacking the needed amount of polysulfide bedding compound. As to the thickness of the plate 1/4 by 1.5 is more than adequate, I would think, as our lowers are the same as what you intend to use. Also note GulfStar only used 4 bolts per plate for a 12 inch exposure under the deck. If you have ceiling going over the plates it would be advisable to put inspection plate cutouts over the plates.

[flo617]

I have not glassed any piece yet. When I said I was done with the preparation, I merely meant that I had sanded the paint off a considerable section around the area and ground the corner around the pocket where the chainplate was glassed in in order to smooth it out.

I want to do all the fiberglassing in one or two days in order to get the best bonding possible.

I did get a piece of G10 roughly cut to fit (need to get a better cutting tool) but have not ordered the chainplate yet, just made a wooden mock version.

I'm assume "fore and aft" means the chainplate is essentially parallel to the hull but actually what I was thinking of doing is to glass in the knee perpendicular to the length of the boat (like a stringer or a bulkhead). That way the position of the shroud attachement would match the original one exactly while the chainplate would remain straight.

In other word the diagram I have uploaded shows a transversal cut of the boat, with the interior on the right and the endless blue ocean on the left.

I would be happy to consider other solutions. I have thought of going external, especially since the spreader is basically an aluminum tube with a probably standard size but I would rather consider an option that keeps the sheeting angle range intact.

[mike d.]

I agree with you on the shroud, chain plate, G10 angle!. When I did mine, new chain plates, I talked to people in the know at the yards, you can bend the exposed chain plate to match the shroud angle, as it clears the deck surface. This relieves the shear forces on the pin, as the angles all line up. Full contact. Make sure you fill the cavity around the plate well with polysulfide. If you have beauty rings that go over the top of the plates, I would put some butyl tape under the covers, and let it squeeze out on top. this will stop any water intrusion of the poly.

[roverhi]

Use epoxy to laminate/bond the knee to the hull. Would make the length of the knee bonded to the hull about 1 1/2 times the length of the chain plate. Run the bonding out about a foot from the knee and taper successive layers. Probably at least 4 layers. That's the way my chain plates were done by the mfg 45 years ago. Seems to worked fine.

[Cheechako]

So you are saying the original builder bent the chainplate nearly 90 degrees and then welded it? (Probably because it started to crack at the bend...?) Good thing you are re-doing them!
I'm not sure I totally understand what you are doing. But your pic indicated to me that you are putting the new chainplate inside the hull and spacing it out with G10 between the hull and the chainplate...?
I see no problem with that. 1.5 x 1/4 chainplate is probably a bit smaller than I would go on a 30 foot boat.... but is likely big enough... I've seen bigger boats with smaller cross section. The weak point is the amount of material from the edge of the bolt hole to each side.
So if you are using 3/8 bolts; 1.5 minus .375 = 1.12 x .25 thick = .28 cubic inches cross section outside the hole. If the yield strength of the SS is say.... 35 ksi, then the chainplate is good for about 9800 lbs force with no safety factor.

[flo617]

9800 That's 1400 more than the official displacement.

Here is the chainplate, I cut it just before it angled out through the deck. There is a 45degree steel plate welded to reinforce the chainplate at the bend.

The anchors are smaller squares welded to the sides of the plate.

[Cheechako]

ahh... I think I understand... so the little squares were glassed to the hull and the gusset/90 degree turn was up under the deck...?

[flo617]

Correct

[Boatyarddog]

Beef up the attachment point before placing the chain plate, and dry fit the piece you're going to fabricate in place.


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[Boatyarddog]

Chainplates glassed to the hull aren't going to be the best in terms of holding power.
I'd do a build up of the areas of attachment, the hull.
And through bolt the Chainplates to the hull.
If the tang of the new Chainplates extends through the deck, be sure to install a crib plate around the tang.
Or it will break the seal to the deck as everything flexes with stress and temp.variations.



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[a64pilot]

Actually I believe if it's done properly glassing into the hull is the strongest possible way, of course the drawback is possibly very difficult to impossible to inspect and difficult to replace. I believe a glassed in Ti chainplate is the best possible?