: I am buiding a Pygmy and am making hatches and bulkheads on my own without
: the kit they sell. I have 3mm mahogony ply for the bulkheads and 4mm for
: the hatch lips. Do these parts need to be glassed on each side or just
: sealed with epoxy?

It depends on how strong you need them/want them. At a minimum you'll encapsulate the wood parts in two coats of epoxy resin. That doesn't add any strength to speak of, but it keeps the plywood from rotting. If you want any additional strength you add glass cloth. At a minimum, seal the wood with a scant quantity of resin, apply a layer of glass and squeegee on one layer of resin. You don't need to fill the weave unless you are doing this on the side of the hatch lip that makes contact with the rubber seal. Then you want to avoid the waffle pattern of the weave.

: Do the bulkheads need to be glassed in place or will a fillet of thickened
: epoxy in the corners be OK? The boat is intended for expedition use.

You are using 3mm plywood -- thinnest and weakest stuff around -- for your bulkheads. WHY?

I'd use the 4mm for the bulkheads, too, if I had enough. Or, if stuck with 3mm, I'd cover both sides with at least one layer of glass cloth and resin. No need to fill the weave. For economy, and a slight weight saving (fraction of an ounce) you can cut the cloth so it comes to within an inch of the edge of the bulkhead. Then, when you install the bulkhead, with a fillet and at least 2 inch wide bias-cut cloth strips or glass tape, covered by a second layer of wider tape ( 3 or 4 inches wide) the overlapping glass will provide additional strength.

There is a school of thought that preaches against tightly fitted bulkheads, saying they might promote stiff areas of the hull, where stresses can concentrate (these areas they call "stress risers") and that damage to the hull would occur at these points. If you follow that line of reasoning, then forget the fillets and attach the bulkhead with just the glass cloth or tape and resin, allowing the bulkhead to be at least 1/8th inch away from the sides ( up to maybe 3/8ths) of the boat on all sides. This way the bulkhead's joint with the hull can flex, wliminating the stress risers. Along the same line, your 3mm ply is going to flex plenty, and a single layer of glass will just stiffen it a little bit, while adding some strength.

The bulkheads take stress and abuse from all directions: impacts to the hull, weight on the deck, expasion of air in the sealed areas, resistance to water pressure from shipped water or totally flooded cockpits, and gear sliding back and forth as the boat rocks. (I dearly love cooking in my cast iron frying pan, and despite the weight, I take it camping -- but it is an effective sldege hammer when it rolls around.) Anyhow, you want strong bulkheads. If you put 2, 3 or 4 layers of glass on both sides of your bulkheads no one would blame you for your extravagance.

Hope this helps.

PGJ