Repair to 20mm thick GRP.

Depends on the nature of the repair.
If you need to re-create a strong sheet with properties near-identical to the original, you might be talking about a scarf angle of 15:1.
If you can overlap a thicker repair, then a much smaller join area can be strong enough.
If you are just filling a hole and the repair does not need to contribute much to the structure (e.g. filling a redundant sea-cock hole), very little grinding away is needed.
 
You don't say what the dia of the hole is??
if it on the hull below waterline or to take any load I would countersink the hole from both sides and if 20 mm thick you need to do it in stages
 
The log hole I would just chamfer a little to expose fresh GRP and cover with a 100mm disc inside, epoxied on. Then fill.

Th e 8 x 10cm hole, is it in a stressed area? Is it underwater? If the panel was strong enough with the hole there, then it just needs a watertight solution. If it's damage and proper design required the panel to be 20mm thick in the first place, then you need a strong repair, with lots of overlap.

My first thought would not be to grind away lots of GRP, making your 80mm wide hole lots bigger.
 
I tend to use a 12:1 scarf. You don’t say whether or not you need to repair from both sides. Google the West systems resources on repairs. It will tell you all you need to know.
 
TernVI said:
Depends on the nature of the repair.
If you need to re-create a strong sheet with properties near-identical to the original, you might be talking about a scarf angle of 15:1.
If you can overlap a thicker repair, then a much smaller join area can be strong enough.
If you are just filling a hole and the repair does not need to contribute much to the structure (e.g. filling a redundant sea-cock hole), very little grinding away is needed.
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Just so. 20mm thick grp at 15:1 would be a 30cm scarf all round. A total of 60cm x 46cm repair. Do you have access to all of that area? And does the internal side of the reair need to be flush?

I'm not a professional shipwright, so I'm prepared to be put right by those with more knowledge and experience than me. But my own approach would be that level of scarfing is required only if the repair has to be flush on the inside. If the inside can be built up with glass mat I would chamfer the edges from outside inwards to around 10cm around the edges of the hole. Depending on the location of the hole, I might also round out any hard corners to reduce stress points on the repair. I'd then build up the inside of the repair by five mm or more thick (5 layers of biaxial mat cos I have loads spare and I find it easy to work with ) and a 10cm overlap increasing to 20cm by the last layer. All with epoxy resin so it sticks to the original grp as best as possible.
I'd get a professional to do the gelcoating if it shows, cos matching and finishing gelcoat anywhere close to them is far beyond my rudimentary skills.
 
An 8 X10 isn't going to weaken the hull significantly unless it's somewhere really critical like just forward or aft of the keel. I'd feather it as much as I conveniently could, up to, maybe 5 or 6 to 1, just because that will make the job easier, but wouldn't break my neck over it. If you can taper from both sides, great, if not, one side will be fine

The 50mm hole, I'd taper a little on the outside, epoxy a bit of cloth onto the taper then fill from the inside with epoxy thickened with microfibres. You can then make good on the outside with a bit of gelcoat repair. If the inside needs to look good, stop the filler a bit short and treat the same as the outside.
 
many thanks for your views, unfortunately the inside is difficult to gain access and the damage is above the waterline, but i think needs strength. I'll see once initial cleaning out/in is done.
 
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