How much fillet? Experts please respond.

I have seen many self made boats using the stitch and glue method, both from kits and plans. And I have seen fillet material size ranging from small, 3/8 inch radius to very large 2 to 2 inch radius where the builder thought that if some epoxy is good, more must be better.

My experience has been that only the amount of fillet material that will allow an overlaying fiberglass reinforcement to lay smooth and remain attached to the substrate wood is required. Anything more just adds unnecessary weight.

I prefer epoxy and cotton fibers as the filler and keeping the fillet small. The cotton adds some structural strength but lays well and is color neutral.

Others prefer wood dust or mico balloons of glass or phenolic, cabosil, etc.

For the experts at CLC: How much fillet is necessary? What is the strongest fillet material? And what are the weight/strength benefits? 

8 replies:

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RE: How much fillet? Experts please respond.

Great Question! I might be building this winter and would like to save as much weight as possible and still have a tough boat. Looking forward to the replies.


RE: How much fillet? Experts please respond.

although a relative newbie to the stitch & glue building I've done lots of glass work on other big boat projects. On the 2 Kaholo paddle boards I'm building I did the epoxy/cabosil filet with a 3/8'' radius then used 2'' bi-axial tape instead of the wood flour/epoxy fillets. I think the tape will be much stronger with a neglible weight increase.


RE: How much fillet? Experts please respond.

That's most interesting Ron, I'd like to keep up with your Koholo projects as I'm going to be ordering a full kit soon, and I know I'll have questions. Did you use knitted fabric, or cut tooling cloth on the bias? I've cut many a bias tape from 7781 style cloth.

I spent years in the glider repair business working mostly on German made sailplanes made of fiberglass, carbon and kevlar.  I built a few kits too. But the project that improved my wood working skills most was a broken wooden glider made in the 60's in Finland. The craftsmanship and technology used way back then was incredible! 



RE: How much fillet? Experts please respond.

I run masking tape about a half inch down each side of the joint, dab in a wood flour mix (peanut butter consistency) and smooth it out with about a 1" radius tool. Leaves a small but strong fillet. Before it cures, but after it sets up a bit, pull the masking tape and lay on the 2" tape and wet it out. Never had a joint fail so it's strong enough, and my boats are pretty light. I have also added a bit of Cabosil to lighten the color a bit on some builds but normally don't worry about it. Adding it doesn't seem to effect strength and the few ounces of weight saved would be negligible.

George K 

RE: How much fillet? Experts please respond.

There is another consideration before you use a pencil thin line of material for your fillet to save weight and that is what are you going to do with the outside of the joint (how strong do you need the joint to be?)

Some people don’t like the “very hard chine” (almost square outside corner) look so they round that corner as much as possible, even going through almost all 3 layers of plywood. If you are going for this “max rounding” of the outside of the joint you will need bigger fillets on the inside.

If you are going to leave the “very hard chine” look, you can get away with fairly small fillets as they really aren’t supporting much.

RE: How much fillet? Experts please respond.

I have only built one boat but have learned a lot from it. What I found was the easiest and very effective was to use the stitch holes as a guide for my masking tape to make a nice clean fillet, and small size too. Then put down the 3in tape. Worked well, would do it again. The tape makes all the difference.

RE: How much fillet? Experts please respond.


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