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Old 03-04-2010, 14:37   #1
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Electrical-Grade Fiberglass Backing Plates

I'm contemplating using GP03 electrical grade fiberglass for making backing plates for seacocks (McMaster-Carr #8549K59). Is there any downside to this material for this application?
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Old 03-04-2010, 15:46   #2
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You don't say what thickness you would use. The 6 inch wide pieces may be too narrow. It's also not going to be easy to cut a round hole in it since it requires carbide tools to work it. At 1/2 inch thick it's also a bit expensive. At 3/4 inch it's wow! I'm not seeing reasons that jump out and say it's a good choice.
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Old 03-04-2010, 16:07   #3
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I was going to use 3/4" thickness and cut them out from a 12" X 24" sheet.
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Old 03-04-2010, 16:20   #4
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I wouldn't say it is a poor choice other than price and ease of working. Structurally it would be good against compression and rot. The hull would need to be totally flat as this stuff won't be bending.
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Old 03-04-2010, 16:52   #5
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It's not cheap, but neither is marine plywood. I should be able to get 6 to 8 backing plates from a 12"X24" piece (less than $10 per seacock). Epoxy putty should compensate for a slight hull curvature and provide a good bond.
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Old 03-04-2010, 17:10   #6
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I see it's indoor rating only so that puts it off for intended use.

Like Paul wrote, the 2nd problem is that you need a perfect flat piece of hull which is also exactly parallel to the outside surface. I wouldn't use it.

I have used King Starboard or mostly just epoxy with filler to prepare seacock installations. The epoxy method is the best imo:

Clean and sand the inside area; wrap thru-hull fitting with household plastic wrap; stick it through hull and prop it up with a batten under tension so that it's flange sits flat on the hull; mix epoxy with hardener and add colloidal silica filler until it's peanut butter consistency; scoop it around the fitting; take another piece of plastic wrap and punch a hole in the middle and force it over the fitting (enlarging the hole in the process) so that it drops over the epoxy; screw the seacock onto the fitting until it touches the epoxy all around, squeezing excess out where needed; screw the seacock back up until it doesn't touch the epoxy/wrap anymore and let cure; cleanup and mount hardware with sealant and bolts/screw whatever you prefer.

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Old 03-04-2010, 17:54   #7
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Nick,
I think your method would work well to provide a flat mounting surface on a curved hull, but would not necessarily provide the desired degree of stiffening and reinforcement, especially if the hull is nearly flat at the seacock location.

I also noticed the "indoor use" qualification on the GP-03 tech data. I'm wondering if this is because isophthalic polyester resin is more prone to absorb moisture thus degrading its effectiveness as an electrical insulator. If so, I would think a generous coating of epoxy should provide an adequate moisture barrier.

An alternative would be the "Structural Fiberglass" (Structural Fiberglass ) which is rated "indoor/outdoor". But apparently the greatest available thickness is 1/2". Do you think that would provide adequate strength and rigidity?

King Starboard does not have very good structural properties and it is difficult to bond to anything so I eliminated it from consideration.
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Old 03-04-2010, 18:26   #8
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Yeaaaah, No. Go with proven...Jedi has the answer, but you can use west system's new Six10 in the mixing tube instead of having to mix your own and it works with a standard sealant gun. Works great, easy cleanup (almost none) and comes pre-thickened.
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Old 03-04-2010, 18:37   #9
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i would just make them out of regular wood and glass them in, i should get some pictures but here is how i did the last few.

cut the circle out of 3/4'' ply. hit the perimeter with a router w/roundover bit. drill a generously oversized hole for the thru-hull and bolts for the seacock.

grind up the hull on the inside where it will be glued, mix some epoxy with colidal slica thicken to peanut butter consistency. glue it down the epoxy and fill the oversize holes you drilled with the epoxy also so they are flush with the top of the wood. put a layer or two of glass over the whole thing and redrill the holes the proper size through the epoxy filler...

sounds easy enough, right?

i though long and hard about the solid fiberglass backing plates (a la MaineSail) but this way just seems considerably easier and im pretty sure theyll never rot out...
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Old 03-04-2010, 22:18   #10
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Quote:
Originally Posted by Ziggy View Post
I think your method would work well to provide a flat mounting surface on a curved hull, but would not necessarily provide the desired degree of stiffening and reinforcement, especially if the hull is nearly flat at the seacock location.
Why not?

Quote:
An alternative would be the "Structural Fiberglass" (Structural Fiberglass ) which is rated "indoor/outdoor". But apparently the greatest available thickness is 1/2". Do you think that would provide adequate strength and rigidity?
I do not understand what you are trying to do? Is you hull weakened in these areas? 1/2" fiberglass is more than most hulls are so why would you want to add that to the existing hull thickness? For thru-hull fittings of 2" or smaller diameter, after you mount the fitting in the hole, the hull will probably be stronger instead of weaker.

Quote:
King Starboard does not have very good structural properties and it is difficult to bond to anything so I eliminated it from consideration.
But you can shape it. Also, bonding isn't needed because you use mechanical fasteners in the form of the fitting+seacock plus bolts/screws through the seacock flange, backing plate and into or through the hull. As long as the hull is in good condition, even starboard should be fine and it's better than plywood which I find in use so often.

If you want it tougher and harder, you can use high density filler with the epoxy but I think it's overkill and it's not as easy to work with (but you can mix it with coloidal silica).

I would propose you cast some epoxy with coloidal silica, let it cure for a week and then try to destroy it. They use this between keel and hull and the forces there are way higher than the clamping force of the fitting + seacock.

If you want it to look better, I started out with a technique that is good for that: I used a metal ring like for cutting cookies from dough to contain the epoxy. But after a couple of fittings I can get so close without it that I don't bother anymore.

ciao!
Nick.
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Old 03-04-2010, 22:21   #11
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grind up the hull on the inside where it will be glued, mix some epoxy with colidal slica thicken to peanut butter consistency. glue it down the epoxy and fill the oversize holes you drilled with the epoxy also so they are flush with the top of the wood. put a layer or two of glass over the whole thing and redrill the holes the proper size through the epoxy filler...
So how do you ensure that the backing plate is exactly parallel with the outer hull at the point of the hole in the hull? Because getting that right is the primary function of it.

cheers,
Nick.
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Old 04-04-2010, 06:04   #12
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I do not understand what you are trying to do? Is you hull weakened in these areas? 1/2" fiberglass is more than most hulls are so why would you want to add that to the existing hull thickness? For thru-hull fittings of 2" or smaller diameter, after you mount the fitting in the hole, the hull will probably be stronger instead of weaker.
There are two reasons for putting backing plates under seacocks. One is to provide a flat mounting surface. Second is to distribute the load as required by ABYC H-27 ("A seacock shall be securely mounted so that the system will withstand a 500-pound static force applied for 30 seconds to the inboard end of its connecting fitting.") Most fiberglass hulls are not designed to withstand this kind of load, especially where the radius of curvature is large, precisely where seacocks are likely to be installed. Very large boats or those built in the 1960's when it was common to lay up a 1" thick hull may be OK without backing plates.

When you spread thickened epoxy around the thruhull and use the flange of the seacock to compress it, you will ensure a flat interface, but you will likely squeeze out much of the epoxy and do little to distribute the load.
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Old 04-04-2010, 07:25   #13
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Hulls flex, some just barely, others quite easily. Using a more rigid backing plate that doesn't fit the curvature creates stress risers at the edges of the backing plate. Bad.

If there is any expansion of the material from water absorbtion, loads on the seacock will increase, possibly breaking the the fixture or cracking the compression nut. Bad.

Stick with the suggestions above, to avoid both issues. The carbide cutter alone will set you back much worse that making a couple training sessions to get comfortable with the process.
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Old 04-04-2010, 09:48   #14
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Second is to distribute the load as required by ABYC H-27 ("A seacock shall be securely mounted so that the system will withstand a 500-pound static force applied for 30 seconds to the inboard end of its connecting fitting.") Most fiberglass hulls are not designed to withstand this kind of load, especially where the radius of curvature is large, precisely where seacocks are likely to be installed. Very large boats or those built in the 1960's when it was common to lay up a 1" thick hull may be OK without backing plates.

When you spread thickened epoxy around the thruhull and use the flange of the seacock to compress it, you will ensure a flat interface, but you will likely squeeze out much of the epoxy and do little to distribute the load.
What the specs tries to prevent is the fitting breaking when one steps on it by accident. It is the fitting that is the worry much more so than the hull.

When you look at the construction with a fitted thru-hull and seacock, the most important thing is sufficient clamping force between the two bronze parts and the flange that is bolted or screwed onto/into the hull. This keeps the force away from the thin wall of the thru-hull fitting.
If the hull is cored, the core must be removed and replaced with solid laminate or epoxy. Good builders have a small section of solid already; for an extra fitting or a bad builder, you have to do that yourself. It must be done because a cored hull will crush or at least deflect under the clamping force.
Next is the backing plate which works like an extension of the flange to distribute the forces over a larger area. Like Sandy writes, hard spots must be avoided and the best way to do that is by casting.

You write that much of the epoxy will be squeezed out, but that isn't the case so you must have misread the method I described. You decide yourself how much thickened epoxy to put in and there is no squeezing at all. You fix the thru-hull fitting in place from the outside so that it's flange is flat on the hull and that it can't move. The part of screwing the seacock on is just for transferring the shape onto the epoxy. When the epoxy is distributed wrong, or it is not enough, you take the seacock off and redistribute or add epoxy again. Just screw it on until the flange touches the epoxy all around plus one extra turn to compress (squeeze) it a bit and then back it off so that it is clear again. You can control how much epoxy is there. I tend to end up with 3/4" thickness which is plenty strong and think 1" would become silly.

About the filler: I would not use a ready made thickened epoxy product because I want to control the properties. The colloidal silica prevents sagging much better than anything else so that you can make it "thinner" without it starting to run. This helps avoid cavities. If the hull isn't particularly strong or flexes a lot, you can add microfibers to the mix, using the colloidal silica to prevent running and the microfibers for a tougher end result. Both fillers will cope with the compression forces easily so a high density filler isn't needed.

You underestimate the strength of epoxy used in this manner. Think about cast bases for winches: they cope with much much more than 500 pounds of force in the worst direction thinkable (at about 90 degrees). It is common to cast epoxy for that although I (West System really because they wrote the book I read ;-) recommend the high density filler in that case (you use a mold so running is no issue).

MaineSail uses a self made layup of fiberglass with epoxy for backing plates. I love the looks of the end result but the only big advantage I see in it is that he taps the fiberglass for machine bolts. When you use a plate like that, you only use the casting epoxy to provide a flat and parallel surface. If you want that you should just do that layup yourself because it is quick and easy... you need no previous experience because it really is that easy. I think he has the instructions for it on his site. I do not know if he clamps it immediately after layup to improve the glass to resin ratio but that is how I do it. Just lay it up, cover with plastic wrap and put plywood with weights on top (poor man's vacuum in reverse system).
But, in the end, I think he is just a show off ;-) I would do it that way when I want to show it to people but I don't think the extra strength he builds in makes any difference because strong enough is strong enough. I can also use taps and machine screws (I do). Just drill and tap the epoxy. If you see voids or other imperfections in the drilled hole, drill it oversize and use a syringe to fill with epoxy thickened with high density filler syrup consistency. Drill & tap after cure. This method is used commonly, even for attaching genoa tracks and other deck hardware.

cheers,
Nick.
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Old 04-04-2010, 18:32   #15
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So how do you ensure that the backing plate is exactly parallel with the outer hull at the point of the hole in the hull? Because getting that right is the primary function of it.

cheers,
Nick.
pretty much eyeball it, have not had any problems, "close enough" engineering
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