Filleting technique question

[Dale R. Hamilton]

I'm planning on S&G joint between 1/4" plywood bottom and sides- actually two layers. I will use a 2"wide fillet over the joint. Question is, do I really need to lay a 2" wide strip of fiberglass cloth into the fillet- ON THE INSIDE? I know FG cloth is used for abrasion protection- so how much abrasion would such a inside joint encounter. What do you guys do and wht? Many thanks in advance

[NormMessinger]

With knowing the angle your joint forms a 2" fillet seems a bit wide. However, my preconcieved misconceptions about glassing the joint is that the glass should bond to the wood, thus I wouldn't think a 4" tape over a 2" fillet would satisfy me. A 1/2" radius fillet and a 2" tape should suffice, given the above caveat. You need a fillet that will allow the glass to conform to the joint.

[DavesFlatsBoat]

Yes - two inch tape on the inside - because then you have created a composite joint. The putty alone may crack as the hull is stressed - the addition of the cloth adds strength and creates a matrix over its 2-in surface as compared to half that surface for a putty joint alone. Better distribution of the stress.

However, I am not an engineer, just a amateur designer.

[Bruce Hooke]

I agree with what Norm said (but I think he meant to say "withOUT knowing..."

As I see it, the point of the glass on the inside is to add tensile strength in that area.

One way to determine whether you have a suitable design for your fillet joint is to do a test where you make the joint between two samples of the plywood you plan to use. Make the joint as planned, let the epoxy cure, and then try to break the joint. You want the wood to break, not the epoxy. What I like to do is make up, say, a 12" long sample of the joint and then cut it into 3" chunks so that I can break the joint in various ways (e.g., folding inward and outward). A refinement is to cut off 1" sample and try to break it; if it breaks in the joint then add some more reinforcing to the right part of the rest of the sample, let it cure and then cut off another sample and test it. For the final test I like to have a longer sample, say, at least 3 or 4" to make sure the test is not being distorted by some small imperfection or variation. Make sure the pieces of plywood you use to make the test joint are wide enough so that you have some leverage when you try to break the joint.

[plyboat]

The fillet size seems way big to me as well. Whatever size fillet you use the glass should cover it and go on to the plywood a couple of inches. This glass isn't about abrasion, it is a structural joint. When glassing a joint the fillet only needs to be big enough to allow the glass to make the curve.
---Joel---

[JimConlin]

Think of the fillet joint as a sandwich, with strong glass skins and a light weak core. The fillet radius doesn't need to be large. 2x or 3x the plywood thickness is OK. The glass skins carry the load of the joint and should extend far enough beyond the fillet to pass their load to the plywood.
It saves lots of work to lay up the whole joint at one time, from wetting the plywood to the glass and peelply.

[Dale R. Hamilton]

Ok guys- convinced me. I'll use FG tape.

[Dale R. Hamilton]

Ah sorry- many thanks for the comments- I can always count on that.

[paul oman]

Many folks here will probably disagree with me but...

on a small boat using 1/4 ply your foot will go thru the wood before any sort of epoxy seam lets go. So, in my point of view, just taping the inside of the hull is more than enough strength, although I have used a 'fillet' over the tape to hide my messy newby tape job.

paul oman
progressive epoxy polymers

[NormMessinger]

Fillet OVER! the tape? That's a new one on me. Sorta seems like putting the weak side outside.

[George Roberts]

The width of the fillet is related to the angle of the parts.

If you have a 90 degree angle between panels, a 2" fillet is a lot of filling material. On the other hand a 150 degree angle between the panels would require very little material.

Put down some plain epoxy to wet the surface, put down some thick epoxy and shape it with spoon shaped tool (lots of different radii to chose from). Lay the glass on top.