How strong is fibreglass?

[GreekYiannis]

I have a Colvic 20 motorboat. The foredeck bends a little across the widest span when I walk on it. I'm a big guy. I've scraped off the paint and added three layers of fibreglass but the bend is still there when I walk on it. So I'm thinking of adding two support brackets from below - one on either side. But I don't want to drill any holes in the side of the boat to support these 'brackets'. So I'm thinking of cutting plywood to shape and sort of 'glueing' the triangular pieces in place using fibreglass. Each 'bracket' would be fully covered and glassed to the inside of the hull and the underside of the deck. What I don't know - HELP me out here guys - is whether fibreglass is strong enough to fix such 'brackets' in the way I've described. The 'span' I need is 30cm up and down (the upright part) and the same 30cm for the 'tread' part under the deck. I imagine an almost right-angled cut going up the hull to meet the underside of the deck, and a curve between the two points to keep the 'bracket' kind of out of the way of passengers' heads. Question 1. Are there any structural engineers on here? Q2. What do the rest of you guys think? Many thanks.

 

[Trundlebug]

Not a structural engineer, but glassing in plywood is an accepted, indeed commonplace practice because it's very effective.

The resin soaks into the ply very well, and the ply gives added stiffness so a good solution.

I'm not sure I follow exactly what you're trying to do here but I'd say go for it, if you're adding material you're only making it stronger. If it only flexed a bit before and didn't break, it'll only be better than it was.

Of course, the thicker the plywood the stiffer it'll be. Assuming there's room.

 

[kds]

1. Use epoxy, not fibreglass.
2. if deck flexes across the span - you need a beam across - not supports at the sides.
3. epoxy-glass the beam in.
4. look on website for "west epoxy system" or similar - or on previous postings here - for advice.
ken

 

[Trundlebug]

I'd still say glassing is a better option here, as GRP allows flexibility (and is the same as the parent structure).

Epoxy is much more brittle, although ultimately stronger. It's also more expensive and less easily obtainable.

GRP would be my preferred choice here, although happy to agree to differ

 

[jfm]

GreekY, you are mixing up strangth with stiffness. It's the latter that you are short of. Adding extra layers as you have done is the worst way to create stiffness. Either a beam glassed right across under the deck, or your brackets, will be ok. Clean the old fibreglass very well (sand paper in a machine) and stick away. Epoxy might be technically better but is not at all necessary here and I would use ordinary polyester resin. It is strong enough to do what you're trying to do. If boat is small enough turn it on its side so you are not working upside down. Good luck

 

[GreekYiannis]

Many thanks

Many thanks to you all. I didn't mention because I wanted to get to the point, but I'm working on a tiny Greek island in a boatyard with only occasional electricity and working alone. We have one hardware shop on the island so I've got very limited access to resources. Anyway - thanks for the distinction between 'strength' and stiffness. You're right. Stiffness is what I'm after. I really need to get this deck stiff on either side of my hatch, so a beam across was never an option. Hence the bracket idea. I'm gonna try this as I'm encouraged that the hardening resin will adhere well to the new plywood - which is 21mm.
Thanks again guys.

 

[SC500]

Not eally familiar with the design or the actual span you neee to achive to stabilise the surface.
Assuming the outer peripheral structure is stable, you would gain very little by adding side support brackets. You need to stabilise the surface. Adding grp layers unless 12mm thick wont do much.
You need to create depth to the surface. This can be achieved by bonding strips of balsa wood (approx 25mmx25mm) at a guess to the inner surface using an epoxy filler type material or Isopon. Matrix should be approx 250mm squares. When cured an located apply 3 layers of GRP over the balsa wood and existing GRP surface to create an internal structural layer. This will stablise the surface. Before bonding the surface should be sanded to aid adhesion.
Alternatively you could purchase GRP honeycombe structural layer of about 25mm thick and bond over the entire surface in question using epoxy resin. Either way it will resolve your problem if I have understood it correctly

 

[jfm]

Many thanks to you all. I didn't mention because I wanted to get to the point, but I'm working on a tiny Greek island in a boatyard with only occasional electricity and working alone. We have one hardware shop on the island so I've got very limited access to resources. Anyway - thanks for the distinction between 'strength' and stiffness. You're right. Stiffness is what I'm after. I really need to get this deck stiff on either side of my hatch, so a beam across was never an option. Hence the bracket idea. I'm gonna try this as I'm encouraged that the hardening resin will adhere well to the new plywood - which is 21mm.
Thanks again guys.

GRP polyester resin (the ordinary stuff) will bond incredibly well to new plywood. Make sure to round off the corners of the wood brackets, because it is hard to fold the glass around a sharp corner - you get air bubbles. The part to be careful with is bonding to the old GRP surface inside the hull. You need to prepare this by sanding it and cleaning with acetone. Then it will be ok if you are careful. Good luck!

 

[GreekYiannis]

GRP polyester resin (the ordinary stuff) will bond incredibly well to new plywood. Make sure to round off the corners of the wood brackets, because it is hard to fold the glass around a sharp corner - you get air bubbles. The part to be careful with is bonding to the old GRP surface inside the hull. You need to prepare this by sanding it and cleaning with acetone. Then it will be ok if you are careful. Good luck!

Brilliant help. Many thanks. Now if this force 8 would just subside a little and the hardware store has acetone, then maybe ....

 

[VicS]

GRP polyester resin (the ordinary stuff) will bond incredibly well to new plywood

Or does it?

I mostly seem to read that the main problems with polyester resin is that it does not adhere well to wood or to cured polyester

 

[elioti]

i would say it doesnt! Epoxy bonds better, but think is fine to use polyester. You must score the ply first and rough it up, then acetone wipe it as may have some oils /dust on. Acetone wiping old grp will help a bond too but not if grp is too old, not sure exactly how ols but a few years old anyway. I did loads of research into this before building my boat. There is alot of stuff on the web by americains who swear only by epoxy, thats mainly cause its very cheap over there, polyester is fine and should last along time if surfaces prepared properly, and no water ingress allowed. Polyester will absorb water, expoxy resists it better. If you wish to bond anything before glassing over , or just want a fillet to round off a 90 degree angle, you could try some `crestomer advantage 10 or 30`from Scott Bader. Used this on my boat , is a new product and used on new lifeboats. lloyds approved.
http://www.scottbader.com/adhesives-products/structural-adhesives/crestomers.html

Cheers

 

[jfm]

Or does it?

I mostly seem to read that the main problems with polyester resin is that it does not adhere well to wood or to cured polyester

I'm sticking to my position which is that ordinary polyester resin bonds very strongly indeed to plywood, including with no scoring of the surface. I don't now what publications you're refering to but practical experience (and 70,000 mirror dinghies!) tells you that anyone who says it's a bad bond is talking bolx.

In the current case, depending on how exactly the job is done, the polyester-wood bond may be irrelevant. If OP completely covers the bracket in enough GRP he could make the bracket out of foam or something with no strength at all

 

[jfm]

i would say it doesnt! Epoxy bonds better, but think is fine to use polyester. You must score the ply first and rough it up, then acetone wipe it as may have some oils /dust on. Acetone wiping old grp will help a bond too but not if grp is too old, not sure exactly how ols but a few years old anyway. I did loads of research into this before building my boat. There is alot of stuff on the web by americains who swear only by epoxy, thats mainly cause its very cheap over there, polyester is fine and should last along time if surfaces prepared properly, and no water ingress allowed. Polyester will absorb water, expoxy resists it better. If you wish to bond anything before glassing over , or just want a fillet to round off a 90 degree angle, you could try some `crestomer advantage 10 or 30`from Scott Bader. Used this on my boat , is a new product and used on new lifeboats. lloyds approved.
http://www.scottbader.com/adhesives-products/structural-adhesives/crestomers.html

Cheers

BTW, not relevant to this job, but beware the weight of crestomer though. A great product but also a great way to add kilos

 

[VicS]

I'm sticking to my position which is that ordinary polyester resin bonds very strongly indeed to plywood, including with no scoring of the surface. I don't now what publications you're refering to but practical experience (and 70,000 mirror dinghies!) tells you that anyone who says it's a bad bond is talking bolx.

In the current case, depending on how exactly the job is done, the polyester-wood bond may be irrelevant. If OP completely covers the bracket in enough GRP he could make the bracket out of foam or something with no strength at all

I don't believe your claim that you have made 70, 000 Mirror dinghies but however many you have really made I suggest you go examine some of them and you will discover just how easy it can be to strip the tapes off.

The fact that polyester bonds so badly to plywood is the reason more and more people are now tending to use epoxy for such work.

Mirror kits are even available at significant extra cost with epoxy rather than polyester. I dont believe they would be if epoxy was not better

I must admit though for what the Op is doing I would use polyester.

 

[rwilson]

he didnt say he had made them?

 

[LittleSister]

Are you sure you don't have a balsa or foam cored deck? I don't know the Colvic, but many boats have deck stiffness provided by having two layers of GRP with a thick layer of end-grain balsa or plastic foam between them.

This 'sandwich' is a very good way of providng stiffness, in terms of cost and weight, but can fail over time. Balsa cored decks in particular are vulnerable to poor maintenance or inappropriate fixings. Unless the top layer and through fittings are kept completely waterproof, water can get into the balsa, it then rots, and the stiffening support to the deck is drastically reduced, resulting in 'springy' decks which flex underfoot.

If you have this sort of construction, adding stiffening beams or boards under the bottom layer is not going to help (the top one will still move). To find out if you have this sort of construction check how deep your deck is, e.g. by checking the thickness aroud a hatch, or if necessary by removing a though bolt. A single layer will probably be less than 1/4 of an inch, a cored deck will be much thicker.

If you do have a cored deck, and a rotten or detached core is the problem, then it is a rather different, and bigger, task - replacing the core, which will involve cutting out the relevant section of grp -either the deck on top or the underside (cabin ceiling) beneath it - clearing out and replacing the core with new balsa, foam or plywood; then glassing the cut out part back in place. There are numerous articles in books, magazines and on the web if you need to go down this route.

 

[jfm]

I don't believe your claim that you have made 70, 000 Mirror dinghies but however many you have really made I suggest you go examine some of them and you will discover just how easy it can be to strip the tapes off.

The fact that polyester bonds so badly to plywood is the reason more and more people are now tending to use epoxy for such work.

Mirror kits are even available at significant extra cost with epoxy rather than polyester. I dont believe they would be if epoxy was not better

I must admit though for what the Op is doing I would use polyester.

Vic I've actually built only two (sorry, I can't find the bit where I said I made 70,000). And I can assure you that you cannot peel the tape off assuming the job is done with modest skill and diligence. The other 69998 have been built by others but if the tape peeled off I somehow think we'd know about it. Your "The fact that polyester bonds so badly to plywood" is a false premise. Sure, people must be able to cock it up I suppose but I've done it lots and it sticks seriously well.

Reference your last sentence, sure, no-one on this thread has argued epoxy isn't better. No argument there. I merely said "ordinary polyester resin bonds very strongly indeed to plywood".

 

[GreekYiannis]

Or does it?

I mostly seem to read that the main problems with polyester resin is that it does not adhere well to wood or to cured polyester

Well, as I just started this a few days ago I'm no expert. But I will say that the tests I did before beginning demonstrated that ordinary cured resin bonded to new plywood like a new layer of plywood itself. If not enough hardner is used, then of course there won't be a bond at all.

 

[elioti]

Well, as I just started this a few days ago I'm no expert. But I will say that the tests I did before beginning demonstrated that ordinary cured resin bonded to new plywood like a new layer of plywood itself. If not enough hardner is used, then of course there won't be a bond at all.

it will still bond if less hardener used, will just take longer to cure. Be careful not to use too much hardener, think its called an exothermic reaction, sure jfm will correct me, thinks will get hot and it wont stick! Same as laying too much glass at once, dont do it! I replaced every single bulkhead in my 34ft boat, was a major task, and glassed in new ones, including the new engine beds. Was all new to me too then. I`ve even glassed in a thrust bearing plate for my aquadrive that takes an incredible force...all with polyester. The original ply bulkheads where glassed in just 3 inches around the edges even ones holding a 750kg engine and it lasted years. I ended up replacing al of them as said and glassed them in totally. Infact after finishing i then questioned the quality of my ply....for no particular reason. But as JFM said, you could used foam as if its encapsulated, your just using it as a former. Scott bader sell sheets of 8 by 4 plastic this can be used instead of ply, more flexible, differnet thicknesses and glass sticks to it better, is designed for this. Only downside i found is it soaks up alot of resin. Cheers

 

[Elessar]

Vic I've actually built only two (sorry, I can't find the bit where I said I made 70,000). And I can assure you that you cannot peel the tape off assuming the job is done with modest skill and diligence. The other 69998 have been built by others but if the tape peeled off I somehow think we'd know about it. Your "The fact that polyester bonds so badly to plywood" is a false premise. Sure, people must be able to cock it up I suppose but I've done it lots and it sticks seriously well.

Reference your last sentence, sure, no-one on this thread has argued epoxy isn't better. No argument there. I merely said "ordinary polyester resin bonds very strongly indeed to plywood".

Paul, my GRP expert, does indeed argue that epoxy is worse. He says that whilst epoxy can have a stronger bond, there are far more ways to get it wrong. So for DIY he strongly advises polyester.