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Old 29-09-2009, 15:36   #1
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Rebedding Chainplates

Currently 6 of my chainplates are each bolted to a hanging knee (tab) which is glassed to the hull, and the chainplates are through bolted to it. The chianplates are then heavily glassed over which makes it impossible to inspect.

When I rebed the chainplates I would rather have them exposed so they are more easily inspected. Is it best to inspect the hanging knee and then re-glass the knee to the hull, and then install the chainplates over top of the glass?

It seems that bolting over top of the glass would make it stronger, as it is then anchored to the glass. When the glass is over top of the plate does it give it the same strength? Doesn't glass over the chainplates allow the SS to erode easier? Although the top and the bottom aren't glassed, which may allow water to drain out??

This job should be fun.

Any good books on this? I have a Don Casey book, but it doesn't really talk about this in it.
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Old 29-09-2009, 16:46   #2
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Free rider,

I can't follow you with these hanging knees and chain plates. Are your chain plates on the inside or the outside of the hull? You write that they are glassed over so I guess they are on the inside? What are the knees and where are they exactly?

In general, I don't like chain plates glassed into the hull. I like to see one side and the edges, and a neoprene gasket on the other side between the plate and the hull.

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Old 29-09-2009, 17:04   #3
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I would first of all remove the glass over the chainplates and fair the surface. One will have to assume that the knees were cut from good solid cored plywood, and that there are no voids. Check them for rot, and the joint between them and the hull and deck for cracking or any sign of problems. Perhaps add another layer of glas over the face of the knee and around the joint onto the hull for a few inches if you want to beef that up. Then take the chainplate and make a backing plate for it that stops just under the deck. Thru bolt the chainplate and backing plate, with sealer where needed. Seal the chainplate where it exits the deck and rebed the cover plate which usually goes round the chain plate and bolts to the deck.

My Grampian had a similar set up, and that was how I did the job.


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Old 29-09-2009, 18:05   #4
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Quote:
Originally Posted by s/v Jedi View Post
Free rider,

I can't follow you with these hanging knees and chain plates. Are your chain plates on the inside or the outside of the hull? You write that they are glassed over so I guess they are on the inside? What are the knees and where are they exactly?

In general, I don't like chain plates glassed into the hull. I like to see one side and the edges, and a neoprene gasket on the other side between the plate and the hull.

cheers,
Nick.

The chainplates are on the inside of the hull, bolted to the knees. The knees are some how attached to the hull and or deck, they meet the hull at a 90 degree angle.

The chainplates have HEAVY glass over them, should I re-apply the same amount of glass. Does it matter how much glass is in between the chainplate and the backing plate?

This boat will be going offshore and I'd like to have it as strong as needed.
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Old 29-09-2009, 18:21   #5
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This sounds like the way Tartan orginally did with the T27. It's been an unholy mess to correct. The knees were plywood with glass over and then the chain plates were also glassed in. Because of leaks, the plywood has rotted on most of them. That requires grinding the glass away from the chainplates and removing them. Then the knees had to be split open, the rotten wood dug out, and some non compressible, rot free core added or the whole smear built up with new glass laminates without any core. A real PITA but doable by any amatuer who has monkey genes, for contortion ability.

Hopefully Hughes would not have used wood or some other rot prone material for the core of the knees. The chain plates should not be glassed over. Perfect environment for crevice corrosion of the SS if they are.
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Old 29-09-2009, 23:30   #6
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Well, I would start with drilling a small 1/8" hole through the knees, catching what comes out in the process and checking it. If it's plywood and it's rotten or just wet, there's trouble and they should be replaced.

You can use plywood without getting into that trouble but it takes more effort than builders want to put in. If you ever want to replace them (or anything load bearing with plywood or plywood core), this is the way:

Cut & shape the part from top quality plywood like Bruynzeel. Wrap some 1x1 wood stock with plastic wrap and put the parts on top of that on a workbench. Adjust a heatgun temperature setting so that it doesn't discolor the plywood (using scrap left over piece) and heat the top surface and sides of the part with it. Mix epoxy & hardener and coat top surface and edges. When this cures & cools down, the epoxy is drawn in by vacuum of the contracting air inside the wood. Before cured, you can turn and repeat for the other surface. Sand it with 120 grit (dry) after that.

Now, bond the pieces in place but don't add any fiberglass yet. Fit the hardware (i.e. chain plates) and dril the holes. Remove hardware and enlarge the holes to double the size (i.e. for a 1/2" bolt you make 1" holes). Tape one side of the holes, wet the hole with epoxy and a small brush. Now make a thick peanut butter consistency mixture using high density filler (the same as you used for bonding, high density filler is the way to go for this) and fill the holes.
After that, glass the part in and redrill the holes that go through solid epoxy instead of plywood. Make gaskets for both sides (one behind the chain plate and the other behind the backing plate) out of 1/8" thick neoprene sheet with the holes for the bolts oversized (use hole punch). If the surface is very smooth, you can use 1/16" neoprene. Use nylock nuts and tighten using a torque wrench. You use the torque for a bolt of it's diameter in shear loading. You must compensate the torque value for using a nylock nut, most tables show a percentage compensation or even have a 2nd table for nylock. This torque is very important because the bolts must be tensioned to (from memory) 20% of their breaking strength for shear loading.
The neoprene gaskets will compress to almost nothing and spread the load over the complete surface without crushing fibers. Use no other bedding compound. The advantage of neoprene is that you can't squeeze it all out plus it's re-usable.

cheers,
Nick.
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Old 30-09-2009, 00:30   #7
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Nice post Nick
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Old 30-09-2009, 08:59   #8
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Thanks Nick!!!

When you say 'bond' the pieces in place, what should I bond it with? I'm not sure
how they're bonded at the moment since they are glassed it.

Is it critical how this step is done? Or does the glass over top provide all the structural strength??

Also, is West System - 404 high density filler, appropriate filler for the drilled holes?
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Old 30-09-2009, 10:21   #9
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Freerider,

You should bond it with a thick mixture of epoxy + high density filler, the same as used for filling the holes. Prepare the surfaces well, finishing with something like 60 grit sandpaper to increase surface area. Use a strut (is that the correct English word?) to push it into the corner so that the surfaces show no voids. Scrape away the excess epoxy because with this filler it gets very hard and almost impossible to sand. When it is cured, put a strong fillet around the joint. Use Colloidal silica filler for that. This greatly improves the strength of the joint and it provides a nice radius for the glass that goes over later.

The reason for using high density filler is that the joints and "filled bolt-holes" are in compression and the higher the density of the mixture, the more resistant to compression force it gets.

About the glassing: many use mat + woven roving but I wouldn't. Matt is really just a filler and doesn't provide much strength. Also, you are working on vertical and upside down surfaces which isn't the easiest thing to do. This is what I do for good results while making it as easy as possible:

Make a syrup consistency mix using both micro fibers and colloidal silica fillers in 50/50 ratio. The micro fibers filler is very good for laminating and the colloidal silica provides anti sagging properties. Coat the area to be glassed with this mixture. Next, use fiberglass woven cloth, pressing it into the wet epoxy, trimming it with scissors where needed. Use multiple pieces: one piece on each side for knee-to-hull joints, one piece on each side for knee-to-deck joint (which is the second layer on the knee already) and a 2" wide tape around the edges. Keep working wet on wet: wet out & fill the weave of the cloth with just epoxy/hardener mix with a roller. Next is woven roving and next is cloth again. Make each layer bigger so that it completely covers the previous layer. You're gonna use a lot of rollers, you can cut them into three smaller pieces.

cheers,
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Old 30-09-2009, 12:31   #10
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Thanks for the help so far, its greatly appreciated. I'm able to visualize the job now, and that makes everything much easier.

One of the issues I can see having is that because I will now be placing the chainplate over the fibreglass, that now it won't line up directly with the slot in the deck. How thick do you recommend layering the plywood with fibreglass? I want it as strong as possible, so I guess cutting a new slot in the deck is a real possibility.

Also, re-drilling the the holes through the fibreglass and finding the center of the high density filler will take some very exact measuring, but I should be able pull it off.


I will be performing this job in the winter, when the temperature will be around freezing level, is there any draw backs to this?


Thanks again everyone!!
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Old 30-09-2009, 13:57   #11
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It sounds like the system was not cheaply done to start with. ("Heavy glass over the plates") Can you inspect any part of the plates? Any sign of past wetness etc? They are most likely to have corrosion at or near the deck. If there is no corroded away metal etc from deck level down an inch or two.... a rebed may be all you need. The glass on the knees and over the plates does little. The deck essentially holds the knee from moving. The glass that holds the knee to the hull does a lot to strengthen the system. It keeps the deck from having to hold all the strain of the rig. If you do disassemble it, there is no need to glass over the chainplates. If the glass is thick and well bonded to the hull and deck, the old plywood inside may mean little really, just something to glass over when it was built and keep the bolts from squeezing the glass together........
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Old 30-09-2009, 14:54   #12
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I'm hoping a rebed is all I need, its just I'm preparing for the worst. I can see about 1-1 1/2" of the chainplates just below the deck before it enters the glass. I did notice a small amount of rust stains around 1 of the nuts at the top of the plate.

The deck is slightly raised at both aft shrouds. The port one looks to be repaired as there were several holes drilled, (for drying I assume) then re-filled. There looks to be re-enforcement below the deck at this location however; the deck is still raised. The starboard one is less raised and it doesn't look to be repaired. When I rebed this chainplate I plan on checking the core for moisture and repairing if necessary.

Is there anyway to flatten the raised deck, besides sanding it flat; I can't think of anything. Am I right to assume that drying out and re-glassing from below the deck will provide sufficient strength even though the deck its still slight raised?
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Old 30-09-2009, 15:17   #13
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Are the knees tabbed into the hull sides with fiberglass? are they "raised" 1/8" or more?
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Old 30-09-2009, 15:37   #14
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Ya they are tabbed to the sides and yes they are raised no less than 1/8". The port side as I said is raised more, but it looks to be repaired; although I plan on re-doing it.

How do you think this could have happened? They don't appear to be pulled away from the hull so I'm not sure what went on here. If the holes are elongated, shouldn't the plate just pull up through, and not bring the deck with it?
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Old 30-09-2009, 16:59   #15
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1st, I'd ditch the plywood and use some kind of plastic like the bread board material. Better yet would be to cut away one side of the knee and laminate up a solid fiberglass knee. This may ultimately be the simplest solution as you can laminate up so that the edge of the knee lines up with the puka in the deck for the chainplate. Chainplates leak, it's a decree by God. Using plywood, you risk water get into the wood and repeating the rot process you are correcting. No mater how careful you are sealing the plywood, you risk water getting in and causing rot.

The problem with working in cold weather is getting the resin to go off. Polyester resin is a bit easier to work with as you can use more hardener to get it to kick off quicker. Epoxy has to be mixed to a specific ratio. There are cool wx hardeners available for Epoxy so be sure you get the right one. In any case, you will need a heater to keep the temperature of the laminate to at least 50 degrees, warmer is better, or whatever the resin manufacturer suggests. If you don't keep the temps up, the resin will drain out of the laminate before it goes off making for a weak, resin starved layup.

Mat adds stiffness to the lay up. It also fills in the depression in the woven roving so you get a stronger laminate. I would never do a lay up without mat. Lay your pieces of roving and or cloth on a piece of plywood. On top of that, put the mat and wet out the whole smear resin. the cloth/roving holds the matt in place and makes it as easy as possible to laminate on the vertical. On an overhead, may have to thicken the resin to get it stick till it goes off.
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