Screwing into solid fiberglass

[finefurn]

I'm going to be mounting a external strainer over the engine RW intake thru-hull. The strainer attaches to the hull with 6 #10 screws. What's the best method to do this? SS self tapping sheet metal screw, pilot hole size ?. Or Tap for coarse thread machine screw. Through bolting is not an option here. The hull is approx. 1 " - 1 1/4" solid fiberglass. Thanks

[Richard5]

I would not. You are relying only on the poor tensile strength of the solid resin to resist the forces imposed by the weight of the strainer, the hoses, and the water in the system. Plus vibration and motion. All are imposing a force which wants to strip the screw out of the hole irrespective if it is tapped.

Assuming polyester resin, epoxy is nearly twice the tensile strength. Epoxy does bond sufficiently to 'glas resins.


There are several methods which offer stronger attachment.

One, a modification of the 'West system' is to drill out a hole several sizes larger than your screw. For a #10 screw, no smaller than 3/8th inch. Fill that hole with epoxy then tap the screw hole into the cured epoxy.

Two, use a compression ring with a threaded insert. The compression ring is bonded to the fiberglass with 'glass or epoxy resin. As the screw is tightened into the insert, the ring is expanded for a very secure fitting.

[Tillsbury]

Best method is to have a few small blocks of off cut marine ply, sealed with epoxy all around, on your boat. Stick the block/s to the hull with thickened epoxy over a sanded area, then screw whatever you’re screwing to the ply.

[Jim Cate]

Hey guys, he said "external" strainer, so the screws are from the outside of the hull, and there are no significant loads on the strainer. Likely won't be removed/reinstalled often, so I'd likely drill and tap for 10-24 screws (blind holes,of course... don't go through!).

But truth is that I'm not a fan of external strainers for cooling water... just another place to get growth fouling the flow, so OP, are you sure you want to do this? There have been some previous discussions here on CF, and a search will find a lot of chat about the use of strainers.

Jim

[Pete7]

Just to add to what Jim has said, we sometimes pick up floating mats of very fine weed. Being able to shove a long screw driver down the intake seacock after taking the top off means I can clear the blockage in seconds. Useful in narrow channel or river.

Pete

[NYSail]

Ditto the don’t use external strainers. Stuff gets inside and grows....... I have two that eventually will be gone.

Greg

[Benz]

If I HAD to have the strainer, I'd tap threads into a plate of G-10 to screw the strainer to. Then I'd glue the G-10 to the properly prepared hull surface with thickened epoxy, making a nice fillet around the plate. The machine screws would be set with blue loctite, and would be the same material as the strainer, if possible, to prevent electrolysis.

[Q Xopa]

Originally Posted by finefurn

I'm going to be mounting a external strainer over the engine RW intake thru-hull. The strainer attaches to the hull with 6 #10 screws. What's the best method to do this? SS self tapping sheet metal screw, pilot hole size ?. Or Tap for coarse thread machine screw. Through bolting is not an option here. The hull is approx. 1 " - 1 1/4" solid fiberglass. Thanks

By far IMHO the best solution is drill holes oversize (especially if hull is cored), fill with epoxy.

Probably not necessary but if you want the very best practise remove more core diameter than the skin, ie undercut. This is usually done with small allen key with a sharpenen tip, or something similar. Hopefully you can understand what I am trying to say.

When cured redrill correct size hole for a 6#10 tap. Then tap the threads.

[a64pilot]

Honestly just glue it on with 5200, use tape until the 5200 dries.
Then if for some reason your where getting into the water to clear the strainer isn’t an option, a piece of broom stick or similar can knock it off from the inside and you can go on your way.
5200 is plenty strong enough by itself, but if you want screws, just drill and use sheet metal screws, no tapping necessary, it’s how almost all are held on, no boat yard worker drills and taps I can assure you.

[thinwater]

I think many responders misunderstand the problem. He is ONLY adding the strainer, not the through hull. The hull is also solid glass, without core.


Yes, screws will work fine. I've done this twice, both times while snorkeling. Neither one ever budged. The forces are trivial and the glass is thick.


As for the benefits of external strainers, let's assume he has been living without and it has cause problems. It depends on the area and the application. I have had them both ways. Head intakes don't need them, but AC intakes often do (fish get sucked in by the continuous flow while the boat is not moving). If there is a 90 inside the through hull it can jam up. It depends.

[Q Xopa]

Originally Posted by thinwater

I think many responders misunderstand the problem. He is ONLY adding the strainer, not the through hull. The hull is also solid glass, without core.


Yes, screws will work fine. I've done this twice, both times while snorkeling. Neither one ever budged. The forces are trivial and the glass is thick.


As for the benefits of external strainers, let's assume he has been living without and it has cause problems. It depends on the area and the application. I have had them both ways. Head intakes don't need them, but AC intakes often do (fish get sucked in by the continuous flow while the boat is not moving). If there is a 90 inside the through hull it can jam up. It depends.

Yes true. Its hardly worth going to so much effort for a strainer. 5200 or self tapers are more than adequate. But the OP did ask what the 'best way', which I took as best practise, way to do it was, not the quickest or easiest.

[thinwater]

^^ He said "solid glass," so there is absolutely no purpose in removing good fiberglass. A pilot hole and self-tapper is the best practice in this case.

[Q Xopa]

Originally Posted by thinwater

^^ He said "solid glass," so there is absolutely no purpose in removing good fiberglass. A pilot hole and self-tapper is the best practice in this case.

If you want the strongest joint you will get it from more surface bond area on your joint like a wood splice/ scarfe or a 12:1 taper doing any fibre glass repair like filling in thru hull holes.

But of course feel free to not believe that and do it any version of your best practise way you wish.

[Singularity]

Originally Posted by Q Xopa

If you want the strongest joint you will get it from more surface bond area on your joint like a wood splice/ scarfe or a 12:1 taper doing any fibre glass repair like filling in thru hull holes.

But of course feel free to not believe that and do it any version of your best practise way you wish.

The joint strength in this application has nothing to do with scarf joints. It's mostly a sheer/peel strength matter based on the tooth and surface area to be glued to. As Thinwater suggested it makes no sense to damage the hull structure just to go back and repair it. It won't be as strong and/or it will end up heavier.

The method you described isn't even the strongest way to make a joint. This isn't an exercise in laminating wood.

[Q Xopa]

Originally Posted by Singularity

The joint strength in this application has nothing to do with scarf joints. It's mostly a sheer/peel strength matter based on the tooth and surface area to be glued to. As Thinwater suggested it makes no sense to damage the hull structure just to go back and repair it. It won't be as strong and/or it will end up heavier.

The method you described isn't even the strongest way to make a joint. This isn't an exercise in laminating wood.

2 posts ago I was agreeing with self tapers being fine. Since then there have been underlined replies telling me how wrong I am.
Feel free to believe you are right and I am full of it, if thats what you need. Im ok with that.