To get more room in a hull space people often want to substitute a hat section layup for a bulkhead. So how do they compare structurally?
First, ABS ORY tells me that I can use some of the hull skin in the laminate calculations. But only a flange width of 18 times the skin thickness, plus the base width of the internal part. The skin thickness is assumed to be 0.15″.
The basic bulkhead is assumed to be an inch wide so I’m allowed to use about 4″ of the flange width. I will assume the bulkhead is 6″ tall and has 0.1″ laminate faces.
For a hat section I assume that base is about 6″. I’ll again use a skin thickness of 0.15″, giving me about 3″ so a total flange width of 9″. I give it a 3″ wide inside flange and sides at 45 degrees. Those parts are also 0.1″ thick. I assume all parts are of equal strength.
The original bulkhead gave a section modulus SM of 2.1 inches to the third power (2.1 in^3).
The hat section only gives SM of 1.33 in^3. If I thicken all the hat parts to 0.13″ (4 times 17 oz biaxial) it still only gives a SM of 1.6 in^3. 5 layers of biaxial will give a SM of 2 in^3.
Conclusion. Like magical catamaran beams, hat sections take more material to achieve the same strength as a bulkhead.