: I'm planning a rear bulkhead/hatch, ala Betsie Bay, from 1/4" solid-core
: ply. Does that change anyone's opinion regarding the need for glass?

: Thanks.

A sandwich of 1/8th inch plywood resting between layers of glass cloth reinforces the thin plywood so that it can substitute for 1/4 inch plywood. Adding more glass, or using thicker wood increases the strength and the weight.

You probably don't need to increase either.

Consider your reasons for installing a bulkhead and plan your engineering around those reasons.

If you want to create watertight compartments either to create some areas for flotation, or to reduce the volume of the cockpit should it take on water, then the thickness of the bulkhead's wood is irrelevant. What is most important would be the integrity of how it was attached to the inside of the hull.

A diaphragm- or membrane-like bulkhead made from a single layer of cloth (canvas, dacron, polyester, or fiberglass) installed as a bulkhead, would be strong enough to keep in air, or keep out water if it was sealed properly. These fabrics do this job when used as the skins on skin-on-frame kayaks, and they are certainly up to the task here.

If the hull needs to be braced to prevent crushing from water pressure then a solid bulkhead is called for. In this case a thin (1/8th inch) bulkhead might seem fine for most loads, but for exceptionally heavy loads in a larger kayak you would want to upsize the bracing. Even so, you wouldn't need anything exceptionally large.

Consider the simple bracing in a canoe. A thwart or two -- or on very large canoes you might have three thwarts. When these boats are heavily laden they sink deeper in the water, and water pressure pushes in on the sides of the boat. Without the thwarts, the boat would get narrower.

In kayaks the deck, or the deck beams, provide the same bracing that the thwarts provide in canoes. There is one additional stress here. As the sides of the boat would be forced in by external water pressure should the boat be very heavily loaded (and thus sink lower in the water) at the same time this squeezing would tend to make the boat taller. Any bracing you install should deal with this.

Try this: go to the local fast-food outlet and get a drink cup and a matching plastic lid. After enjoying the beverage, take the lid off of the cup and squeeze it in your hand. Notice how the cup goes from being circular to being oval as you squeeze it. Now replace the lid and see how it braces the cup so that it stays round. Take the lid off again and rip in between your thumb and middle finger, and squeeze gently. Watch it bend easily. The very thin lid provides a great deal of reinforcement to the cup when it is in place, even though by itself it is very weak. It is kinda like how the thin spokes in a bicycle wheel, can support the rim and tire -- as well as the rider and the cycle.

the nice thing about building your own kayak is that you can modify things as much as you want. You'll be using the boat for years, and you'll be in constant contact with it. You don;t have to build it like something that NASA wqould ship off to Msrs, or the moon. You'll have lots of opportunities to tinker.

Start with thin bulkheads and if you see that they are deforming because you are subjecting them to too much of a load, then put in additional reinforcement. This can be an additional layer of wood on your existing bulkhead -- maybe a row of strips laid at 90 degrees to the first layer, or maybe a piece of plywood. Or, add a strip of 1x3, or a foot rail which would serve like a thwart.

Your options are open. See what works best for you by building light and then adding on only as necessary.

Just some thoughts

PGJ