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Old 18-03-2016, 08:01   #16
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Re: Best screws for fiberglass

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Originally Posted by rourkeh View Post
Best screws, none.
My question to you is why would you put a screw into fiberglass?
I refit a large portion of my interior wood paneling, where I was placing 1/2 marine ply with teak laminate directly onto cored fiberglass. I did not use a single screw. I bonded all of the ply with adhesives.
Placing a screw into fiberglass is a poor way to attach a wood panel for many reasons, namely it is a poor bond over time. Fiberglass shatters when you screw into it, and creates a hole that does not seal. On the exterior you are going to get water migration, then leaks.
All of the best boat builders stopped using screws in fiberglass a long time ago, and changed to adhesives.
Unless at some later date one needs or wants to remove the panel. Sometimes people change their minds, needs change. Screws, bolts and other mechanical fasteners will remain an important method of fastening, regardless of what the 'best' boat builders do (usually in search of that nebulous goal of 'profit for shareholders'), regardless of improvements in 'bonding technology'...
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Old 18-03-2016, 08:12   #17
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Re: Best screws for fiberglass

This is a base support piece of plywood for my generator. I have to use this otherwise the mounting would be a lot of trouble. I'm mounting to a fiberglass section that is designed into the boat already for mounting a generator and am going to have a rubber mat between the plywood and the fiberglass for sound damping. It is going to see vibration. I would really prefer to use bolts/nuts but there is no way to get to the underside of the shelf. BTW there's a possibly that the fiberglass has an aluminum plate mounted into it.

I think I'm going to use lag screws or structural screws. This is what I was going to use before I talked myself into the self tappers (there goes a wasted $7 in a $7,000 project).
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Old 18-03-2016, 08:16   #18
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Re: Best screws for fiberglass

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Originally Posted by rourkeh View Post
All of the best boat builders stopped using screws in fiberglass a long time ago, and changed to adhesives.
I don't think that's really accurate. There are times when adhesives are best and times when screws (or bolts) are best.

I agree that installing interior paneling over fiberglass is a place where adhesives would be best (providing you used the correct adhesive and followed the directions, but attaching a piano hinge for a door is better done with screws if through bolting is not practical.

Your navigation lights are probably attached with screws. Compass, brackets for GPS, smart phone, etc. are probably attached with screws.
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Old 18-03-2016, 08:24   #19
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Re: Best screws for fiberglass

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Originally Posted by sailorboy1 View Post
This is a base support piece of plywood for my generator. I have to use this otherwise the mounting would be a lot of trouble. I'm mounting to a fiberglass section that is designed into the boat already for mounting a generator and am going to have a rubber mat between the plywood and the fiberglass for sound damping. It is going to see vibration. I would really prefer to use bolts/nuts but there is no way to get to the underside of the shelf. BTW there's a possibly that the fiberglass has an aluminum plate mounted into it.

I think I'm going to use lag screws or structural screws. This is what I was going to use before I talked myself into the self tappers (there goes a wasted $7 in a $7,000 project).
Now that we have the project at hand we can provide some better advice. Here's mine:

If you want to dampen the sound of the generator, the plywood will be a sounding board (like a violin body) and the screws or bolts securing the plywood to the fiberglass will transfer the sound to the fiberglass anyway.

I would be looking for a special mat designed for sound reduction and use an adhesive to bond it to both the plywood and the fiberglass. No screws or bolts. If you're not comfortable with just the adhesive, make the holes in the plywood large enough that they don't touch the screws at all and use a combination of rubber and metal washers to keep the plywood in place. Don't tighten the screws tight.

See if the generator manufacturer offers a sound and vibration isolating mounting system.
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Old 18-03-2016, 08:37   #20
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Re: Best screws for fiberglass

For what it's worth this is the way I mounted mine.
The wooden stringers I guess you would call them were tabbed into the hull like a bulkhead, then the top piece of plywood was screwed into the wooden stringers, SS Wood screws of course.
Top board and stringers are encased in glass as I know I'll spill oil of fuel or something one day, it's going to happen.
Make sure the top board is well supported and it wouldn't hurt if it were two pieces thick, you don't want to build a drum head for sound / vibration to be amplified, it needs to be just ridiculously stiff.

As a LOT of generator noise is actually vibration, I contacted my generator manufacturer and bought an extra set of engine mounts, generator came with one set of mounts of course and two aluminum runners meant to be attached to structure, I placed an additional set of isolators to these runners, making the generator double isolated.
You want to use the same isolators as the manufacturer of course as if they did their homework, they are "tuned" for that application

Which genset did you buy?

People love to denigrate them, but I see ours as a real standard of living improvement
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Old 18-03-2016, 08:42   #21
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Re: Best screws for fiberglass

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Originally Posted by rwidman View Post

See if the generator manufacturer offers a sound and vibration isolating mounting system.
The thread is really about screws, but:

There already are sound mounts on the generator that will be bolted to the plywood base.

If you feel that adhesive is good enough: would you be willing to remove your main engine mounts, glue a plywood base to the hull, reinstall the engine but bolting it to the plywood only?
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Old 18-03-2016, 08:42   #22
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Re: Best screws for fiberglass

If you still intend to use screws / bolts.
I'd find out if there is aluminum in it, there likely is if it's what I think it is, and if there is I'd at least try drilling and tapping and using machine thread bolts, there may well be enough "meat" in the plate.
Personally I dislike screws especially on something subject to vibration as they always seem to loosen.
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Old 18-03-2016, 08:45   #23
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Re: Best screws for fiberglass

If we are talking about mounting a say two by three ft piece of plywood to a smooth surface of fiberglass, I guarantee adhesive would be the strongest bond, so strong in fact the biggest issue would be I believe it would essentially never be able to be removed.
Think of the bond strength of several square ft of 5200 for example.
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Old 18-03-2016, 13:00   #24
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Re: Best screws for fiberglass

Wood screws are a no no in fiberglass. Wood screws are tapered and take advantage of the compressibility/expandability of the wood to make a strong attachment to the wood. Ideally, you'd use a tapered drill sized to the screw to drill a hole. Works great for wood but not for fiberglass. Fiberglass does not compress/expand so a tapered wood screw pilot hole needs to be nearly the size of the largest point of the taper. Not an issue you say if you use oversize tapered drills. The problem is the taper drills are only high speed steel at best and dull out right quick in fiberglass. They also seem to be much more fragile than straight bits and break very easily. Love my cobalt straight bits in fiberglass. If you use a straight bit and size to fit the largest point of taper of he screw, the hole will be too large for the threads to bite.

You need to use straight shank fasteners like sheet metal screws or self tappers and drill proper sized pilot holes. Too large and the self tapper won't hold. Too small and you risk cracking the glass but more likely twisting the head off or reaming out the phillips head puka in the screw head. Straight screws will allow more torque to be applied to the fastener but can also have slot reamed out.

If you don't want/can't drill a pilot hole, there are self drilling fasteners. They are adequate for holding light loads like wire clamps etc. Have found the drill part of the screw make too large a whole and the threads of the screw are not deep enough like self tapping fasteners so so they pull out easily. Self drilling fasteners typically have a hex head which make them way better to use without a pilot hole. The hex bit keeps the fastener aligned with the drill force.
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Old 18-03-2016, 16:02   #25
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Re: Best screws for fiberglass

To stop water penetration one should really treat it similarly to a through hull fitting - overdrill, fill (eg epoxy), pilot.
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Old 18-03-2016, 17:09   #26
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Re: Best screws for fiberglass

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Originally Posted by sailorboy1 View Post
The thread is really about screws, but:

There already are sound mounts on the generator that will be bolted to the plywood base.

If you feel that adhesive is good enough: would you be willing to remove your main engine mounts, glue a plywood base to the hull, reinstall the engine but bolting it to the plywood only?
It's pretty frustrating when a poster posts half the information, waits for a dozen or so posts and then posts the rest of the information so everyone has to post again after knowing the whole story. It's waste of everyone's time.

It's past frustrating when someone posts what they feel is a good suggestion and the OP feels he has to reject it in a sarcastic manner.

You may not know this but I actually know a bit about this and by your engine comment, the second part of my suggestion flew entirely over your head. You can secure your plywood without bolting it solidly to the fiberglass while still keeping it from flying loose in the case of heavy seas or the boat overturning.
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Old 18-03-2016, 17:16   #27
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Re: Best screws for fiberglass

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It's pretty frustrating when a poster posts half the information, waits for a dozen or so posts and then posts the rest of the information so everyone has to post again after knowing the whole story. It's waste of everyone's time.
It's mainly only frustrating when it interferes with forum "experts" who want to show their expertise beyond the question asked. No one needed to "post again", you wanted to.
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Old 19-03-2016, 01:21   #28
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Re: Best screws for fiberglass

Looks like two options to me.

1) Determine the accurate mounting position and mark the place where the first hole for the motor mount goes. Drill a 3/16" pilot hole to see if you have an aluminum backing plate of sufficient thickness to tap (I'd say at least 1/4") for bolts. Use the softest motor mounts recommended by the generator manufacturer, If you're sure you never want to remove the plywood bond it to the fiberglass with 5200; you'll never get it off (without destroying the plywood and the surface finish of the fiberglass). Glue a sheet of 1/16" lead to the surface of the plywood for sound attenuation.

2) If no aluminum backing plate, start with a piece of 3/4" plywood (exterior or marine, doesn't really matter). Cut to size, glue a piece of 1/16 sheet lead to it. Set the generator on it and mark the position of the holes for the engine mounts. Remove the generator and drill the holes. Get some appropriately size rectangles of 3/8 aluminum or 1/4" stainless and glue over the holes on the underside (the non lead side) as backing plates. Cut a second sheet of plywood the same thickness as the backing plates to fit around the backing plates and bond it to the first sheet. Bond the whole assembly to the fiberglass panel. 5200 seems appropriate for both. I like to screw a panel down when bonding to make sure there is good uniform contact between the two. If one uses stainless screws, they can stay in place. Drill and tap the plates and install the generator.

Or you can just lag screw the generator down and you'll probably be fine... barring any capsizes, pitchpoles or barrel rolls...

In my limited experience lead is much better at sound deadening than any rubber or plastic materials. 1/16" is 4 lbs per square foot.
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