External Chainplate Thru-Holes: Best Approach?

[RLaird]

On the carriage bolt idea, Pacific Seacraft uses this method. They did modify from stamped to water cut to avoid failure at the corners. Smooth exterior profile, easily installed by one person, doesn't 'wipe out' the sealant that makes it into the bore.

[stormalong]

As cf23907 said caulk under the chainplates to keep water out and prevent crevice corrosion. 5200 is great. One important thing about caulk - it needs thickness to work. Don't squeeze it all out and expect it to work. Prebend the chaiplates to match the hull, then mount the chainplates but do not torque the bolts. Leave then fairly loose. Water accelerates the cure of 5200 so after initial cleanup of the over squeeze spray down the caulk with water and wait about a week before torquing them. It doesn't hurt to spray it down multiple times.

[Slainte1]

have a look at the Gougeons book on boat construction.It may be online. Basically the bolts are embedded in oversized holes filled with thickened epoxy resin. you do not need nuts on bolts if buried in wooden backing plates. Contact the West System people, they could not be more helpful

[s/v Jedi]

Some points:

Prevent stress risers. Never use square corners like for carriage bolts, this is where the cracks start. Keep everything round(ed)

The best way to keep moisture out is a neoprene gasket. Very easy to order (Amazon) and make. I recommend 1/16” thickness. Holes in fiberglass should be almost press fit like mentioned before and holes in the gasket must be very tight as well. Use Rod Collins’ butyl tape on the outside perimeter behind the chain plate as well as behind the gasket. While tightening, the gasket should compress at least halfway. If that requires more torque than the bolts should do, buy a lighter density neoprene instead.

I can recommend this because mine never leaked since 1994 that they were put on. Mine used 3M 101 instead of butyl, so today you could use Lifecalk instead of 101 which is not made anymore, but RC’s butyl is better.

Edit: keep all the bolt heads aligned in the same position. My slotted boltheads are all aligned up-down so that it’s easy to see if something has turned.

[jimbunyard]

Lots of different opinions to confuse you, so I'll add mine, but first a couple questions.

What are the dimensions of the chainplate?

Did you remove the gelcoat under the fiberglass you added?

What is the thickness of the G10 backer?


My approach would be to start at the top, drill the first hole, using the chainplate as a guide. If you have a helper, let them hold the backing plate in place after you've drilled the first hole, if not, measure and mark the position on the G10, and drill it. Vacuum or blow the the dust away; it wouldn't hurt to wipe the hole and surrounding area with some acetone, because the last thing you want is to seize the nuts and bolts when trial fitting, which fiberglass dust will do more quickly than anything else on earth.

Tighten the first bolt enough to hold the backer in position, and drill the next hole down. Clean the hole same as the first, bolt it up tight, Repeat for the rest.

Holes should be the same size as the bolts; a light push fit is what you're looking for. The bolts should not be full thread; the thread should end in the middle of the thickness of the backing block when torqued.

5200 is fine, or butyl, or most any other quality sealant that is listed for such a use. You only need it between the outside of the hull and the chainplate, though it won't hurt under the head of the bolt or as a thread lubricant on the inside.

The friction between the mating surfaces of the hull and the chainplate is what should hold the chainplates in position; the bolts should be properly torqued to provide this friction. Thick, wide washers on the inside, against the G10, will greatly aid in this.

[Mike Banks]

Ok--I will probably get some curry for this--bit here goes.

I am a firm believer in wider chain plates with staggered holes, and I do not believe a bolt angled upwards is of any value at all, so omit that one and put an additional bolt somewhere else.

Now--having obtained your wider and thicker chain plates, you need to expose the sides of the hull and grind off any surface, to expose the huill clean of any paint because you need to strengthen it in the way of these chain plates.

This is because they are no longer in double-shear as they were when originally embedded. I would add this thickness using tape I buy by the weight unit, and it is heavy biaxial stuff. Three layers of 20 cm tape are needed as a minimum. I cut one width of tape so that one piece is wider than the other, and extends from well up to the deck shelf stringer to below the lowest bolt. The thinner piece is just larger than the new chain plate twidth. The last piece of tape goes over the other two full width, making a nice strong heavy pad of epoxy glass reinforcement that is going to add a great deal of rigidity to the bolt support in the hull..

When this is done, use a drill to pass precisely through the pre-drilled holes in your new heavy duty wider chain plates right through to the inside of your reinforced hull sides. On the outside of the hull, use a countersink to put a tiny recess in the hull fibreglass around the outside of easch hole under the chain plate. Do this to each of the holes when all are drilled.

I should have mentioned it earlier, but do the same to the new holes in the new chain plates on the side which will be facing the ocean.

Next I buy the best stainless bolts I can get, and 316 are not the best, they are the minimum. There is an alloy containing more molybdenum--I have no idea what its number is, but I replaced my rigging with it, supposedly a lifetime job and allegedly no crevice corrosion.

The next job is to take a small bottle-brush and to get some thinned epoxy resin and to apply this to each of the bolt holes you drilled in the hull. You can use medium speed cure, because thinned a little to aid penetration, it takes a little while to set.

Some people fit bolts while it is un-cured, I am not one of them. DO NOT set the bolts in epoxy resin--you will never get them out again without damage to the hull if you do. If you MUST sdet bolts in resin for any reason and you MIGHT want to get them out again at some stage, wax them first with Carnauba wax.


When the sealing epoxy has set and cured, put the same drill through again to remove any excess. If there is any potential for porosity in the resin, it should have absorbed enough resin to fix it. Thgere are two schools of thought on chain plate rubber gaskets, or to use mastic or silicon sealant. A non-acid sealant is fine, and if you have used O-rings to sea the bolts, it need only go around the edges of the chain plates but well under them. When the chain plates ate tapped into position using the rubber mallet, the excess will squish out where it can be wiped away. Do not over-tighten the nuts on the bolts. After a couple of days or so it is OK to tighten them to their final tension--whch is FIRM. Never really tight.

OK--now to the bolts. I use cup head bolts that will overlap the holes, and which have either an Allan key hole or a slot for a large screw driver.

Asembly time. Take the topmost bolt to be fitted, and the chain plate. Slide an o-ring over the shaft of the bolt (make sure it is not threaded for the first two centimetres) pass it through the topmost chain plate hole, put on another o-ring similar to the first, then slide the bolt part way into the hull. Do the same until all of the bolts are fitted and the chain plate is ready to be tightened to the hull. Using a small bottle of ladies clear nail varnish, put a circle of it under each of the cup heads of the bolts, and tap the whole chain plate against the hull using a rubber mallet, until the bolts protrude through the reinforced section of hull.

Over these I put rubber gaskets and SQUARE 2 mm or 3mm stainless steel washers with the points upwards to make them water-shed, or may use a stanless steel plate and rubber gasket to match the exrterior chain plate and bolt holes. Tighten the nuts in sequence so that they are FIRM and not too tight. I use self-locking nuts for this, because they look OK--but some purists may use Lock tite and Stainless domed nuts for better appearance.

Now go ahead and do it your way.

[Boatyarddog]

My 1979 Mariner has External Chainplates SS.
They don't leak, they've never been recaulked, nothing's moved and I'm not going to take them apart to find out if they will.
Maybe I'm crazy!
😳
SV Cloud Duster