Thin is good, it saves weight & epoxy (money). Ideally, the fillets should be just wide enough to keep the glass from pulling off when applied. Any more than that is waste. The strength comes from the glass & epoxy, not the fillet material.

If your glass is on with no bubbles, you're done. Move on and prosper.

Laszlo

Laszlo:

Thank you for your answer and help.  I am sending much good karma your way  :-)   I will move on and glass the underside of the deck.  I am hoping to marry the two pieces by the end of this long weekend!

The real purpose of the fillets are (besides 'welding' the joints) is to round out the seams to provied a rounded surface for the glass tape. Fiberglass is strong but not at 90 degree angles.

I have asked this question before - where lies the greater part of the strength of a well-made conventional fillet plus glass/ epoxy overlay joint?   Does the fillet contribute most strength, or the tape/ epoxy?   Does the answer vary according to the cross-section of the joined panels at each given point - a very flat angle as at the bottom of the hull in the cockpit area: 90 degrees or so as at bulkhead-to-hull joints: very sharp as at the ends of the craft?   I fully understand the 'smoothing' effect of the fillet under the epoxy/ tape layup, but does it (the fillet) contribute much in terms of overall strength?

Thanks for the anticipated explanations!

Wordsmith  

Warning: long digression from anything remotely practical follows.

"Which determines ultimate strength: the fillet, or the glass tape + epoxy?"  In other words, if the joint is stressed to failure, which one fails first?

I'd bet a cup of coffee on this answer: 

Neither.  Well-made filleted-taped joints never fail at the fillet, and never fail at the glass tape.  It's always the plywood adjacent to the joint that fails.  I think this observation has been posted several times on this forum, with Lee G. being the first I recall to actually test it.

Once the boat is shattered, who cares what happens next?  But I suppose you could ask the academic question: if there were some way to continue the experiment, by putting the fillet/tape combo with its shards of broken plywood dangling from it into a new fixture and bend it to failure, which would fail first?

I was just an EE in school,and they didn't ever trust us electrical types to go into the ME lab, but I suppose that when you bend anything, a surface fails first.  A fillet removed from the wood would have a taped side and two untaped sides.  I suspect that Glass-epoxy is way stiffer than cured putty even with a little fused silica mixed in, so I reckon that most of the stress would be concentrated in the tape, and it would fail first.

Here's another way of analyzing it:

What holds the pieces of wood together is the epoxy that's in contact with the wood. So what determines the strength of the joint is the total surface area that's glued together.

Click on the picture to enlarge it and you'll see a taped seam. The fillet is thin enough that it fits between the stitch holes. The tape has way more surface area than the fillet, so it's doing most of the work.

Making the fillet wide enough to even begin to challenge the tape just adds weight and consumes epoxy. You get more glued surface area per unit weight by letting the tape do the work.

The fillet is more brittle than the tape, has less tensile strength and weighs more, but is superb at bridging gaps and smoothing transitions. If there's a gap, it also distributes the panel contact forces more evenly than if the panels touched each other.

The tape has more tensile strength, is more ductile and lighter, but peels away if the bending radius is too small.

The filleted seam partners these two such that each does what each does best, cancelling the weaknesses and reinforcing the strengths. It's classic composite structure.

Laszlo

Yes - I get all that.   I guess it's another example of the whole being greater than the sum of its parts.   Intuitively, one can see that a relatively wide tape+epoxy overlay would be stronger than a simple stand-alone fillet of appropriate width.

To answer J-L's query more specifically, I now read in a specialist publication that "joints (fillets) will consistently outlast the plywood if the fillets are made with a radius:thickness (of ply) ratio of 2.5 or more".

If I understand correctly, this implies that with 4mm ply a fillet radius of >10mm (4mm x 2.5, or almost half-an-inch) is OK for use under a tape+epoxy overlay.   Again, 'looks' and sounds about right.

Thank you.

Twenty-five years ago, when S&G construction (then called "tack and tape,") was first beginning to catch on, Maine boatbuilder Harold "Dynamite" Payson filleted and taped two scraps of plywood together at an angle, then ran over the piece with his pickup truck, destroying the plywood, but not the joint.

I rest my case.