Advice sought re GRP hull repair!

[aod]

I have a couple of seperate areas about 2sq feet each under the water line where the hull has delaminated. It's solid layup fibreglass with grp stingers and frames inside. I am thinking of grinding back and repairing with either carbon or a kevlar/carbon composite using epoxy even though the this will be laid over a polyester resin.

The choice of woven carbon or kevlar/carbon composite is because the hull is a fairly thin racing hull and I thought these materials would offer greater depression resistance than that offered by ordinary chopped strand or woven roving fibreglass.

Do any laminators out there have any comments or suggestions about my idea?

Many thanks in anticipation.

Richard

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[richardandtracy]

My opinion is that it would not be a very good idea to have a section of carbon laminate in a mostly glass hull.

Carbon composite is much much stiffer than glass composite. This will give a hard spot in the hull and tend to attract load. Even if the carbon bit you put in is strong enough, the surrounding areas that take the load to/from the new patch probably won't be, especially so if it's a thin racing hull.

I'd suggest that you replace it with what was there. That way the hull structure will have the originally designed load distribution.

If the hull is too fragile for the use you're putting it to (as it seems to be with several delaminated areas), maybe it's time to consider another hull or a less punishing style of sailing. If neither option above is acceptable, I'd recommend that you sheath the whole hull in carbon, not just small discrete areas. If the whole hull is sheathed, the load distribution will be pretty much unmodified, and every part will be strengthened.

Regards

Richard.


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[snowleopard]

agree with your caution against local use of carbon.

for puncture resistance, kevlar is the stuff but it is difficult to use because it doesn't wet out as easily as glass or carbon.

carbon has high tensile strength and low modulus of elasticity. it won't give much help with stiffness unless used in some form of sandwich construction.

a hull that is too flexy could be stiffened with internal ribs or by a layer of coremat over the inside then another layer of woven glass, thereby making a sandwich. unfortunately there would be a weight penalty.

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[aod]

Yes, Richard and Snow I think you have both considered something that I didn't.

I think I will stick with plan A and grind back and repair with a mixture of CSM and woven roving.

It is a very light hull and does flex so the addition of a couple of ribs in these high stress areas would certainly stiffen it up a bit and should all but eliminate future problems.

I would love to completely sheath the whole hull but the cost and additional weight prohibits this as an option.

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[vyv_cox]

Very many years ago I was quite active in slalom canoeing, about the time that carbon fibres were introduced to the GRP market. Many of the top paddlers bought boats with carbon stiffening at various ribs and the hull-deck joint, enabling the rest of the layup to be much lighter. This construction method proved to be useless, the carbon breaking up for the reasons you describe, in very short order.

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[oldsaltoz]

G'day aod,

Good input from all above, but consider the following:

You can stiffen the hull without adding weight by using an epoxy resin and fabric designed for use with epoxies, but not chopped strand, it has too many voids and will retain too much resin and weaken the structure.

You need to remove all the antifouling (if any) to prevent the grinder carrying contamination into the area first. Then grind out all the delaminated material and another few inches to ensure you also remove the stressed areas: taper out to a feathered edge, the larger this feathered area is the better the load will be spread.

Epoxy will provide a mechanical and chemical bond, this is not the case with poly resin. Epoxy needs a ratio of one part resin to one part cloth, poly resin needs 3 parts resin to one of cloth, so it will be heavier and weaker.

Have a talk with you local resin suppliers and get the facts; you don't want to be doing this every other year, and don't want a major failure at sea....

I hope this helps, good luck with your project.



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[snowleopard]

have to take issue on the subject of polyester:

<poly resin needs 3 parts resin to one of cloth> not in fact so. with chopped strand mat, fully rolled out, the ratio is 2.5 resin to 1 glass. with multi axial fabric the ratio is 1.1 resin to 1 glass.

when polyester is applied to clean previously cured resin the strength is 70-80% of that achieved when laying over wet resin. epoxy achieves no more than this, generally less. this is experimental data from a grp boat designer/builder of 40 years experience.

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[jbate]

I agree with the sentiment above, go with glass - try and use the same weight of plies as were in the original laminate - to keep the laminate as consistent as possible and so that the 'new' plies work together with the original laminate, rather than making them work harder (carrying the strain). Also, I don't know if this is what you had in mind, but I would certainly recommend that the new laminate is vacuum consolidated, this will significantly improve the bond of the new plies.
JB


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