Are self-tapping screws good enough to hold windows in?

[MaxCG]

After much deliberation, I'm resealing my frameless acrylic windows this weekend with Scapa 3507 tape. The windows are screwed through the hull into plywood backing pads with self-tappers. My question is - Are self-tapping screws enough to securely fasten the windows in given that I plan to take the boat offshore? (Perhaps 40 screws per window)

On another note - I cannot easily replace the screws with bolts as the screw holes are so close to the window aperture that widening them isn't possible. I've considered drilling some extra holes for bolts further out but am not sure if this is really necessary.

 

[Boater Sam]

Stainless screws? If it was made like that why not continue?

 

[MaxCG]

Stainless screws? If it was made like that why not continue?

Yes stainless, and I figure if it has lasted 40 years it should be fine. I'm really just after confirmation that there isn't some rule that you need fully bolted windows to handle weather offshore.

 

[RunAgroundHard]

Yes, assuming there is a flange on the outside to transfer force into the coachroof coaming.

 

[RunAgroundHard]

Reading your OP, I would have expected the self tappers to cut a thread in the GRP, not a ply backing pad. How thick is the ply backing pad?

 

[MaxCG]

Reading your OP, I would have expected the self tappers to cut a thread in the GRP, not a ply backing pad. How thick is the ply backing pad?

12mm marine ply stuck on with CT1

 

[BobnLesley]

We sailed halfway around the world in a boat whose windows were overlapped acrylic tap screwed into the grp. What may be relevant to your current task: When we replaced four of eight in the USA, I changed all the screw locations by a couple of cm. That way I was fixing into fresh grp rather than the original holes, I filled those with grp repair paste and bedded the new acrylic on butyl tape

 

[RunAgroundHard]

12mm marine ply stuck on with CT1

I don’t know what the maximum penetration is required for full strength. Worth checking.
Gudgeon Brothers recommend filling wood holes with Epoxy, allowed to soak, then fitting self tapper. As epoxy sets, turn screw to prevent bonding. They did tests which demonstrated that hole strength in ply, some wood types was stronger than self tapper tensile strength.

Data in The Gougeon Brothers on Boat Construction: Wood and WEST System Materials.

 

[Aja]

I would consider interscrews.

 

[MisterBaxter]

I would second the idea of getting some epoxy into the screw holes, to prevent water ingress into the ply as well as for extra strength.

 

[[193211]]

Another vote for interscrews. I used them on my Hunter when I replaced the windows. They were built with self tappers screwed directly into the coachroof. They were prone to leaks due to the differing expansion rates of the acrylic, grp and screws.

A 6 or an 8 self tapper has a diameter of 3.5ish or 4.2ish mm. An M5 machine screw would be more than adequate. Maybe a max of 1.5mm of lost material closest to the edge? I’m no engineer, but I suppose that it’s the clamp force of the fixings that secures the assembly to the coachroof.

Some good info here:

http://www.hadlowmarine.com/Pages/Downloads.html

 

[Stemar]

Interscrews need a larger hole than a. say, 3mm bolt. On Jissel, I used 3mm machine screws with domed nuts on the inside. Jissel (Snapdragon 24) wouldn't have been my first choice for off-shore cruising, but the windows wouldn't have been a concern.

With a 3mm ply trim, I don't reckon they looked too bad

 

[MaxCG]

I don’t know what the maximum penetration is required for full strength. Worth checking.
Gudgeon Brothers recommend filling wood holes with Epoxy, allowed to soak, then fitting self tapper. As epoxy sets, turn screw to prevent bonding. They did tests which demonstrated that hole strength in ply, some wood types was stronger than self tapper tensile strength.

Data in The Gougeon Brothers on Boat Construction: Wood and WEST System Materials.

Downloaded the book last night. Makes good reading. I'm very keen on that idea for the extra strength and as MisterBaxter suggests it would ease some anxiety over water ingress through the screw holes leading to rot.

My only concern is if it would make it difficult to adjust the screws to tighten the seal or remove them for resealing.

 

[MaxCG]

Interscrews need a larger hole than a. say, 3mm bolt. On Jissel, I used 3mm machine screws with domed nuts on the inside. Jissel (Snapdragon 24) wouldn't have been my first choice for off-shore cruising, but the windows wouldn't have been a concern.

With a 3mm ply trim, I don't reckon they looked too bad

View attachment 171465

Very tidy! I think it might be very tight for 3mm bolts but I'll remeasure on the weekend just to make sure.

 

[RunAgroundHard]

Downloaded the book last night. Makes good reading. I'm very keen on that idea for the extra strength and as MisterBaxter suggests it would ease some anxiety over water ingress through the screw holes leading to rot.

My only concern is if it would make it difficult to adjust the screws to tighten the seal or remove them for resealing.

Once the epoxy starts setting you can back out the screw and reinsert. I think the screws can also be coated to stop epoxy adhering. The screws do not have to be glued in for the benefits.

Good luck with the project.

 

[Poignard]

Once the epoxy starts setting you can back out the screw and reinsert. I think the screws can also be coated to stop epoxy adhering. The screws do not have to be glued in for the benefits.

Good luck with the project.

Frylight olive oil spray is what I used.

£1.50 in Tesco.

 

[MaxCG]

Frylight olive oil spray is what I used.

£1.50 in Tesco.

Haha brilliant! Shall have to pick some up.

 

[thinwater]

Sounds like a trick question.

Properly bonded, no fasteners are required, or even advised. My last two boats had bonded frameless windows. I replace the later at ~ 20 years, and went back with bonded windows.

 

[William_H]

OP is concerned for the windows in open ocean sailing. Worse case scenario boat laid down with great pressure on the window or wave hitting it.Will it hold? This would much depend on the unsupported size of the window largest dimension, thickness of acrylic and attachment. As said sealer/ glue should be very strong given a decent over lap seal area. Just what is adequate for open ocean is a variable question. I would suggest if area is large unsupported and you are concerned that you consider support from in side, to the middle of the acrylic. ol'will

 

[thinwater]

OP is concerned for the windows in open ocean sailing. Worse case scenario boat laid down with great pressure on the window or wave hitting it.Will it hold? This would much depend on the unsupported size of the window largest dimension, thickness of acrylic and attachment. As said sealer/ glue should be very strong given a decent over lap seal area. Just what is adequate for open ocean is a variable question. I would suggest if area is large unsupported and you are concerned that you consider support from in side, to the middle of the acrylic. ol'will

Removing the old windows tore out some gelcoat. One had to be broken out. I've never heard of this problem. Also, certainly stronger than the hinged hatches on the boat (weak locks and the hatch glazing is only bonded--no screws), so kind of irrelevant.

That said, nothing wrong with some screws or bolts, if just for the nerves.