Gelcoat over aluminum over fiberglass in head - how?

[rls8r]

I am replacing the toilet in the head of my boat. The current toilet is bolted directly into a fiberglass platform/base. I'm thinking about providing a 1/4" thick aluminum base for the new toilet (in my mind the aluminum would provide some extra holding power for the machine screws holding the toilet down).

My questions are regarding the best way to attach the aluminum plate to the existing fiberglass platform and then re-doing the gelcoat on the existing fiberglass platform (to include the new aluminum plate) to restore the appearance of the existing platform and to result in a platform where the aluminum plate is not conspicuous. Should I 3M 5200 the aluminum plate to the existing fiberglass platform? Perhaps use some other adhesive? Once the aluminum is down - how best to prep it for the application of the gelcoat?

[David M]

Can the plate be thru-bolted through the fiberglass? Do you have clearance to tighten the nuts on the backside? Thru-bolting would be the most secure method. Of course you would want bedding compound around the bolts and where the plumbing for the head goes through the deck, if you have that kind of head design.

[model 10]

Do not use aluminum in a head. The tail of an airliner breaks off at the bathroom for more than one reason. The corrosion under an airliner Lav is terrible.

[Jim Cate]

I think the whole idea is poorly thought out. If the extant FRP platform has survived as is, additional strength is not really needed. If it really is too weak, adding a couple of additional layers of glass seems an obvious means of adding rigidity. If that glass work is beyond your skill set, use of either structural glass sheet or G-10 sheet rather than aluminium removes the corrosion issues mentioned upthread. Finally, if you insist on use of aluminium, gel coating is not appropriate at all. Rather painting, using appropriate undercoating and prep work would be the obvious finish.

IMO, some serious rethinking is in order.

Jim

[rls8r]

David - I don't have access to the space below the current fiberglass platform so thru-bolting is not really an option without completely tearing apart the head.

Ecos - I was hoping that encasing the aluminum in epoxy (or something) would address the corrosion issue. I could always go to stainless steel if folks think that would be better.

Jim - I'm certainly willing to re-think things. That's why I posted the questions. My concern is not really for the strength of the FRP platform - which seems to be in fairly good shape - just scratched and worn from use over the years. Rather - I'm concerned that the three #14 self-threading bolts used to install my new Raritan Elegance toilet may not be adequate to hold the toilet in place when subject to the pounding and heeling of a sailboat in a seaway. I was looking to increase the 'holding power' of the bolts by giving them something more substantial (e.g., metal) to tap into than FRP. There were four 1/2" bolts into the FRP platform holding the original W-C Skipper toilet. Is my concern not warranted? If there is a cause for concern - is there another way to increase the holding power other than the way I've been thinking about?

Thanks all for your comments and help.

[Copacabana]

Just an idea... perhaps a fiberglass or G-10 riser plate with ss nuts embedded and epoxied in place on the underside of the plate? Then you can epoxy the plate in place and paint over it. The toilet is then bolted down on the plate. This works well for through-hull backing plates.

[Bill O]

The aluminum plate will not be good w/urine, etc. nor w/SS machine screws.

If I'm reading this correctly, it seems like the base is firm/strong enough, so no real addition of structure is needed and you don't have access to get behind it to thru bolt it.
If what you want is metal to thread into, why not use a well nut (or something similar, the name is escaping me at the moment for them). These will do the job you want and then probably zero cosmetic work to deal with either.

Edit: They are called blind self tapping threaded inserts in SS
(the link above have metric internal threads but they have SAE also)


Bill O.

[JC Reefer]

G-10 glassed to the platform would work. I used to thread G-10 so that machine screws could be attached directly. All Thread makes SS options that where appropriate for the marine environment.

The holding strength will come from the layers of glass being added and not from the G-10 itself.

All that being said, what specifically is happening that you’d be considering such drastic measures?

[rls8r]

JC - Nothing is happening right now. I haven't installed the toilet yet. I am simply a bit nervous about trusting the three #14 bolts that come with the toilet to hold it in place in a listing/rocking/rolling boat. I am trying to calm those nerves by providing what additional 'holding power' I can.

The FRP platform seems to be strong enough to hold the toilet. I was worried about how well the FRP 'threads' would hold - which is why I was considering adding some metal threads. The suggestions to treat the FRP platform as a through-hull backing plate (thanks, Copa) - perhaps by epoxying threaded inserts into it (thanks, Bill) - will provide that extra holding power without the addition of the metal plate. Does that sound like a plan? Perhaps I'm worrying too much, but I'm afraid that's my nature.

This is why I like these forums - I get lots of help from experienced sailors and builders/repairers. Thanks.

[s/v Jedi]

Are you threading into old holes or do you have to tap new ones? How thick is the fiberglass?

As a rule of thumb, if the fiberglass is 1.5x as thick as the bolt, you get full strength.

Without access to the bottom, if the fiberglass is too thin (less than 1/4" would get me worried) then adding glass on top is the best way to go. I would use a couple of layers of 1708 laid down with epoxy and finished with peel-ply. After cure, use fairing compound to make it pretty, then a 2-component epoxy primer and a topcoat paint or 2. No gelcoat.

Check Totalboat products or the Jamestown Distributor website. Totalboat is cheaper than West System, Awlgrip etc.

[Breaking Waves]

Your concern is fastner holding - there are a couple options:

1. Various types and styles of threaded inserts - like https://www.ezlok.com/threaded-inserts-for-plastic - these can be press-in, tap-in and either can also be glued in.

2. Just tap the fiberglass holes in your existing base - if the fiberglass is thick enough (1.5x fastener dia is ok).

3. If the fiberglass base is not thick enough, or you just want more grunt, you can cut in thicker G10 discs/pads, which hold threads very well. You can either make these flush with the current surface (a bit trickier but nicer looking) or just glue on top (if they matched the toilet feet they would not be very noticeable.

[Breaking Waves]

Your concern is fastner holding - there are a couple options:

1. Various types and styles of threaded inserts - like https://www.ezlok.com/threaded-inserts-for-plastic - these can be press-in, tap-in and either can also be glued in.

2. Just tap the fiberglass holes in your existing base - if the fiberglass is thick enough (1.5x fastener dia is ok).

3. If the fiberglass base is not thick enough, or you just want more grunt, you can cut in thicker G10 discs/pads, which hold threads very well. You can either make these flush with the current surface (a bit trickier but nicer looking) or just glue on top (if they matched the toilet feet they would not be very noticeable.

4. Use adhesive studs - like https://www.clickbond.com/product-de...arge-base-stud - there are all sorts of different sizes/strengths of these

[rls8r]

Jedi - I have to tap new holes. I'm filling the old holes for the previous toilet since they do not line up with the fastening points for the new toilet. The FRP base is about 1/2" thick. A #14 bolt is about 1/4" in diameter, so it seems like the 1/2" FRP thickness is greater than 1.5x the diameter of the screw. Perhaps the existing base would provide sufficient holding power. Unfortunately, I'm a belt+suspenders sorta guy.

It sounds like the way to go is to epoxy threaded inserts into the existing FRP without adding the aluminum base. The metal threads give me additional confidence that I won't see the toilet flying across the boat. That also saves me a LOT of work (thanks, everyone).

I'm also glad to know about the adhesive studs. I can use them for some other work I'm doing for which I was using hanger bolts.

[s/v Jedi]

Originally Posted by rls8r

Jedi - I have to tap new holes. I'm filling the old holes for the previous toilet since they do not line up with the fastening points for the new toilet. The FRP base is about 1/2" thick. A #14 bolt is about 1/4" in diameter, so it seems like the 1/2" FRP thickness is greater than 1.5x the diameter of the screw. Perhaps the existing base would provide sufficient holding power. Unfortunately, I'm a belt+suspenders sorta guy.

It sounds like the way to go is to epoxy threaded inserts into the existing FRP without adding the aluminum base. The metal threads give me additional confidence that I won't see the toilet flying across the boat. That also saves me a LOT of work (thanks, everyone).

I'm also glad to know about the adhesive studs. I can use them for some other work I'm doing for which I was using hanger bolts.

Sorry to bust your ideas but I don't think threaded inserts are better at all. I think the very best you can do in 1/2" thick FRP is drill the holes at the small side for the #14 fastener tap (some tables show drill bit size for different materials, others show how much % of thread engagement is achieved, go for maximum) then tap straight into that FRP. The strength you get with that will not be improved by threaded inserts. 1/2" fiberglass is stronger than a 1/4" bolt so it's the bolts that are the weak point, not the 1/2" thick FRP base.

"belts and suspenders" for me means to achieve maximum strength/performance in a way as simple as possible, not more gadgets for mediocre results. There, I said it and am probably the bad guy now, but I hope you can see I say this to try and help you

[rls8r]

Thanks for your insight, Jedi. No 'bad guys' here. I welcome any comments, however expressed, as long as there's no malicious intent - and yours don't strike me as malicious - just helpful.

All the discussion here has made me feel a bit more comfortable simply tapping the FRP and using the three #14 bolts. Of course, I'll keep a set of inserts in the spare parts box just in case ...

Thanks everyone for your help and thoughtful discussion. I'll be back shortly with my next problem, no doubt.