Is this what They Mean when They Say to Taper the Hole ?

[off-the-grid]

Doing my very first below the waterline patch.

Of course I'm going to sand down to bar glass before filling, but how am I looking so far?

[David M]

That looks pretty tapered to me. There is no way the pressure from a boat stand is going to push that patch through.

[SabreKai]

Ok, so far you have the right idea. However, in order to do a decent patch you need to feather that out much more. I believe the general rule of thumb is 10:1 minimum.

So if your hull is 1/4 inch thick you will need to grind out a dish at least 2.5 inches all around the perimeter, feathering to zilch at the hole. This means you need to use a backing plate when starting the layup. Once the first couple of layers are in and hard, you can remove the backing plate.

Alternatively you can grind a dish in from one side, half way thru the skin and fill from the outside, Then go back inside and do the same. Fill from inside. But remember, that 10:1 is pretty important.

Sabre

[DeepFrz]

There is some good information about it on the West Site

WEST SYSTEM | Projects | Fiberglass Boat Repair and Restoration - Custom fiberglass boat repair

Actually that was the wrong one. This is the article I was thinking about.
http://www.westsystem.com/ss/assets/...1practical.pdf

[off-the-grid]

Wow! Good thing I asked. Never realized it had to be feathered that much.
So what you're saying is I could do 5:1 overall thickness with both the inside and the outside tapered?

I was planning on grinding and building up just from the outside and then a few layers built up on the inside completely overlapping the hole.

[SabreKai]

Yeh, 5:1 on the inside, then 5:1 on the outside. This way you get a nice wide patch the will not be over the thickness of the original skin. Use alternating layers of chopped strand mat and woven roving. Cut the first circle just big enough to cover the hole with a bit of overlap, then the next one a half inch larger, and so on til you are done. Try and figure out how many layers you will be doing so the outer one is woven roving. When done, sand it all down and paint with epoxy and sand again. Then paint on gelcoat and and sand again. rinse n repeat until you have it as smooth as you like.


Sabre

[James S]

Its all about surface area for a good bond....the more the merrier.

[GordMay]

The WEST Systems tutorial, previously linked, specifies a 12:1 bevel, on BOTH sides, if the backside is accessible.

[forsailbyowner]

I'd always gone by 12:1. I'm a believer in doing both sides. My last boat I did a bunch of mods to cockpit while it was removed and upside down. After flipping it over I faired the joints. Turns out a few years later the fairing had a crack at the joints and required "tapering" and laying up on the outside also. It held and looked good for 10 yrs after that till I sold the boat.

[Christian Van H]

Originally Posted by James S

Its all about surface area for a good bond....the more the merrier.

James is right. Also, IMHO, You should use the largest piece of glass first...followed by progressively small patches 'til its all filled up. Reason is, that large piece will have the greatest surface area for bonding to the hull. Using the smallest first means you are only bonding the edge...

[S/V_Surya]

Originally Posted by grunzster

Wow! Good thing I asked. Never realized it had to be feathered that much.
So what you're saying is I could do 5:1 overall thickness with both the inside and the outside tapered?

I was planning on grinding and building up just from the outside and then a few layers built up on the inside completely overlapping the hole.

I have pictures in my album of my hull repairs fixing holes and and cracks. Fiber glassing from inside is good if you can get to it. Taper edges as much as you can, the more taper the stonger the bond even 20:1. I used 16 oz cloth sanded edges then filled with colloidal silica.

[osirissail]

I use the opposite technique of laying up layers of mat and roving putting in the smaller circles first and then laying up increasing larger circle of material until you are up to the original suface level. The theory being that it is the loose hairs at the edges of the circles of cloth that are attaching to the existing hull.
- - Most important though is to use liquid styrene painted on the old fiberglass a few minutes prior to starting your layup layers. Styrene is the "vehicle/solvent" that softens the old resin and allows the new resin to chemically "grab" better. Be sure to try to determine exactly what resin was used in the original hull and then use the same for your patching. Different resin types have different shrinkage rates on curing and you want to try to match the old stuff. If you don't know and the boat is quite old then they probably used orthophthalic resin which was commonly available. Isophthalic is newer and much stronger but has a different shrinkage rate.