Correcting permanent distortion of GRP

[penfold]

A question for any GRP experts; is it possible to correct permanent distortion in GRP, and how would you do it? There's a dent under the waterline, about 12" in diameter and perhaps 20-25mm in depth. I suspect it was caused by poor shoring at some point in the past, as there is no gross crazing on the outside(caveat; the AF hasn't been removed to get a look at the gelcoat yet) and no gross cracking in the flowcoat or tabbing to adjacent bulkheads on the inside(again close inspection hasn't been done yet as there's a layer of what looks like danboline slathered over everything). I suppose it's possible it was a manufacturing flaw, but as the boat is a 1975 contessa 26 I'm discounting this. The absence of any visible damage other than the dent itself leads me to think that it could safely be ignored, but I'm curious what corrective action may be available.

 

[William_H]

AFAIK the only possible way to unbend the polyester resin is to heat it to the temp needed to make the resin soft. This would be very iffy and not worth considering.
Either ignore the dent or fill it with glass layers and polyester or epoxy until dent is flush.
However it is hard to imagine such a dent could be made without damage to the GRP or internal structure. If GRP is cracked and crushed then best cut it out and replace with a patch or perhaps just patch over the top. good luck olewill

 

[Graham_Wright]

A question for any GRP experts; is it possible to correct permanent distortion in GRP, and how would you do it? There's a dent under the waterline, about 12" in diameter and perhaps 20-25mm in depth. I suspect it was caused by poor shoring at some point in the past, as there is no gross crazing on the outside(caveat; the AF hasn't been removed to get a look at the gelcoat yet) and no gross cracking in the flowcoat or tabbing to adjacent bulkheads on the inside(again close inspection hasn't been done yet as there's a layer of what looks like danboline slathered over everything). I suppose it's possible it was a manufacturing flaw, but as the boat is a 1975 contessa 26 I'm discounting this. The absence of any visible damage other than the dent itself leads me to think that it could safely be ignored, but I'm curious what corrective action may be available.

I had a rudder post bottom bearing that was glassed in out of line with the one at the top (by me).

Warming with a hot air gun allowed it to be pulled over into alignment. Whether it was good practice or not, time will tell.

 

[vyv_cox]

The theoretical answer is that cured polyester is a thermoset resin, not a thermoplastic, so heating will not allow it to be repositioned. So I would not expect it to be possible, without denying that those who have done it are correct.

 

[macd]

12 inches by 1 inch deep sounds like quite a deformity, and you'll obviously wish to check fully for any crazing or cracking.

I have seen a similar-sized deformation (actually, bigger) in a GRP hull, when a boat toppled onto a Bene 345 in our local yard. The resulting ding was akin to a diaphragm spring and popped out quite easily with a suitably and gently applied bottle-jack (a process OK'd by the insurance surveyor). For peace of mind the whole area was then reinforced with epoxy cloth. There was no sign of any crazing or cracking.

I'd hesitate to suggest this as a remedy in your case without much clearer information about the nature of the ding, but it does illustrate what might be possible.

 

[bikerbill]

Without seeing the dent, I can only suggest that if water is not getting in, I would probably leave it. However, as suggested before, you could fill the dent with fibreglass / gellcoat to make flush again. I would probably chip off the gellcoat to let me see if there was some kind of damage, then repair by applying patches of fibreglass cut to shape and finish with gellcoat then apply AF.

 

[JumbleDuck]

I believe there is a Mr Rogers in Lymington who knows rather a lot about Contessas. Might it be worth giving him a call?

 

[bernie_ni]

What ever you do, dont attempt to fix it by heating. With polyester, which the Contessa must be as its that age, any heating will make the resin very brittle and weaken the entire structure. It will de-laminate and the resin will go all crystalized and crack.
As suggested, use a bottle jack and brace it against something else on the inside and apply gentle, increasing pressure - even leaving it for a few hours. If that doesnt work, then your own title describes it - 'permanent distortion ', and just leave it

 

[nimbusgb]

Buy a big tin of filler.

Fill

Gelcoat

Sand back and paint.

 

[penfold]

What ever you do, dont attempt to fix it by heating. With polyester, which the Contessa must be as its that age, any heating will make the resin very brittle and weaken the entire structure. It will de-laminate and the resin will go all crystalized and crack.
As suggested, use a bottle jack and brace it against something else on the inside and apply gentle, increasing pressure - even leaving it for a few hours. If that doesnt work, then your own title describes it - 'permanent distortion ', and just leave it

I'm curious about this; given that many anti-osmosis treatments include the use of either infrared lamps or heating pads of some kind, is there a temperature range beyond which damage can occur?

 

[William_H]

A safe rule of thumb with polyester resin is if you can't hold your hand on it cos it is too hot then it is too hot. The best anti osmosis treatment to get moistuur out is probably some warth along with vacuum over the moist area. But also needs lots of flushing with fresh water. olewill

 

[Avocet]

I wouldn't be too worried about heating it, but am not sure I will achieve the desired effect and might have some other undesirable effects. I used to work with fibreglass sports cars and it was pretty common to have a bonnet that was too hot to put your hand one. Certainly, a black car parked in the Florida sun could get hot enough to fry an egg on! HOWEVER, although the fibreglass itself (just ordinary polyester resin) wasn't damaged by the heat, it could distort permanently. They'd look great when they came out of the mould and after a couple of years somewhere hot and sunny would look all ripply and the fibreglass carried on curing and changed shape slightly. With something as thick as a boat, I'd be worried about changing the temperature quickly, and not having the hull warmed completely through. I'm also not sure it would go back to the right shape - probably just a "different" wrong shape!

Is it really 1" in 12"? It might be worth holding something flexible (like an old saw blade) across the "dent", taking care to curve it to match the (correct) profile of the line the hull SHOULD have made, and then measure down to the bottom of the dent. Also, it it were my boat, I think I'd want to scrape away all the internal paint until I got down to laminate (or flowcoat) and do something similar on the outside to be sure that there wasn't any damage to the laminate. Assuming there wasn't, I'd be more inclined to fill (with an epoxy-based fairing compound) rather than further disturb the laminate.

 

[basil421]

However, as suggested before, you could fill the dent with fibreglass / gellcoat to make flush again. I would probably chip off the gellcoat to let me see if there was some kind of damage, then repair by applying patches of fibreglass cut to shape and finish with gellcoat then apply AF.

I agree with Bill.
Assuming the distortion has been present for years not weeks I would perform a glass mat epoxy repair from the outside. Sand off the existing gelcoat and build up ever increasing discs of glass mat and epoxy. If there was any structural damage within the grp this will reinforce it where it is required. Fair the surface with Flowcoat or Gelcoat with an addition of wax solution (1%).
I would not apply heat.
Inspect again at next lift out.

 

[West Coast]

I'm curious about this; given that many anti-osmosis treatments include the use of either infrared lamps or heating pads of some kind, is there a temperature range beyond which damage can occur?

The heating route is taking you into the realms of Glass Transition Temp for resin materials - from memory this is around 60 deg C for polyesters, but this varies widely from resin to resin, even for the same family of resins. Orthopthalic polyester will be different to Isopthalic, and so on. I would not recommend heating to Tg temp etc as you may permanently affect the laminate performance.

I have seen FRP change shape, but this has been structural FRP beams under significant continual stress, so creep is more the issue here. From what you describe, this is perhaps the possible cause - if there was a continual point load applied over a long period of time where the poor shoring was?

So - as others have suggested, check for damage to the structural laminate in behind - if all OK, then focus on fairing the indentation.

 

[JumbleDuck]

Assuming the distortion has been present for years not weeks I would perform a glass mat epoxy repair from the outside. Sand off the existing gelcoat and build up ever increasing discs of glass mat and epoxy.

I would be a little apprehensive about stiffening a disk of the hull, the boundary of which is the circle which has probably suffered most internal damage. I'd use something rather less stiff on the outside (i.e filler) and built up the grp on the inside too.

 

[Keith 66]

The dent problem described was common in Contessa's particulary the bigger 32's, you will sometimes see one described as having a faired hull.
It would seem that they were selling so well that they were ripped outof the mould while still green & put in cradles that didnt fit, whence the dents. My father in law owned one & she had four dents, one on each quarter. The worst dent on the starboard bow was nearly 3ft long & 2ft wide & at its deepest point was 1 1/2" deep, the frd bulkhead had been scribed round it & glassed in. It took several buckets of low density epoxy filler to fill them, glassed over with cloth & epoxy.
It amazes me that owners would accept this in a new boat!

 

[penfold]

On closer examination the dent is more like 12mm deep, although the exact depth is hard to measure without having something to use as a spline tool and some extra hands to hold it on while I wave a ruler around. paint removal and inspection of the gelcoat inside and out is the next step, although not urgently. I'm sanguine about the integrity of the laminate as knocking on the hull in and around the dent reveals no difference in tone to knocking elsewhere. The dent is on the starboard quarter, pretty much where a prop might be on a cradle like Kieth66 describes.