: Here’s my and my brother’s latest debate: How much fiberglass does a C16 S&G
: need if door skins are used for plywood.

Point of view #1:

Probably none. A lot depends on some variables you have not mentioned, and perhaps not even thought of.

Bulkheads, for example. Where are they placed, and how thick are they?

How big are the fillets?

Think about what the boat would look like if you removed all the wood parts at this time. What you would have left are the fillets, which would create a skeleton, which would resemble the frame of a skin-on-frame kayak. What would be missing would be the ribs and bracing and of course the skin. Let's ignore that skin for a minute as it does not add a lot to the structural properties of the frame.

If your fiberglass fillets are big enough to match the strength of a wood chine on a canvas skinned s-o-f kayak, and then wood panels provide as much stiffness and bracing as the ribs on this hypothetical s-o-f boat, then (theoretically) there is no reason to bother with glass at all.

Point of view #2:

Glassing the bottom is good for scratch and ding resistance. A layer of 6 ounce, or two layers of 4 ounce cloth would be wonderful protecton against sand and gravel abrasion. If your area has mostly sandy beaches, then a single layer of 2 or 4 ounce would work. If your goal is only abrasion resistance you could save some money, though, by gluing on 1 to 2 inch wide "rub strips" along the bottom. Cut the strips from your doorskin panels.

Point of view #3:

If you want to add structural strength to the boat then your greatest improvement would be in reinforcing the interior of the hull. The way that hulls flex seems to make the most out of the strength of the glass cloth when the cloth is placed inside the boat. it has something to do with the difference between how glass cloth handles the forces of tension and compression.
with this theory in mind I'd put the 6 ounce cloth on the inside of the hull, 4 ounce on the outside, and 4 ounce on both sides of the deck. Assuming I had all those different sizes and lengths. Almost as good would be 4 ounce cloth on all surfaces, in and out.

You can also add strength to the boat by epoxying in ribs, or their functional equivalents. You can cut ribs from 3/8 inch plywood but making a piece that is shaped to fit like a bulkhead, but has the center removed, leaving behind a "donut" with 1 inch to 1.5 inch thick walls. Or, you can make 1 inch high forms from cardboard, and cover them with a couple layers of glass cloth and resin, thus molding them in place. Or, you can mix up some of the stiff glop you used for the fillets and seams, and build up rings on the interior of the boat.

Minicell foam bulkheads provide some structural strength, too.

Point of view #4:

There is a lot to be said (and it HAS been said) for using multiple layers of thinner and lighter fabric. For example, instead of using a single layer of 6 ounce cloth you would use three layers of 2 ounce cloth. One feature of this approach is that if you should ding the boat so deeply that the cloth layer gets snagged, you have a chance of snagging just the outer layer, or layers, of cloth, and leaving the inner layers completely intact.

Hope this helps you understand that there is no single "right" answer. Best thing for now is to build according to the plans, and if it breaks, fix it and make it stronger.

PGJ

: Me, I’m paranoid. I want at least 6oz on the bottom and 4oz on the top.
: Brother wants to go down to 2oz on the bottom and none on the top.
: We both thinking fiberglass tape only on the inside seams.

: Your opinions please.
: Roger Nuffer