Originally Posted by Ziggy
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