Our friends at LLoyds state that "In general, well protected hatches should be at a height of not less than 1.5 times the significant height of the waves above the still waterline relevant to the limiting sea state for which classification is sought."
Our friends at ABS recommend "In addition, the distance from the waterline to the top of the coamings around hatches that may be opened with the unit afloat is not to be less than 2.5 feet with the unit upright." So there is your 30 inches.
I'd think that the first step is defining operating requirements. ??
Carsten is able to operate his micro sub on lakes with what looks like a few inches of freeboard. (Although I think he said it's challenging.) But for many other locales or situations you might need a tall coaming or even the hatch elevated a few feet.
Paul
On 9/17/06, ShellyDalg@aol.com <ShellyDalg@aol.com> wrote:
The idea of a pop-up sail has occurred to me. Did I read somewhere that 30 inches of freeboard is required? The coast guard rules are kinda vague but generally say protection from swamping must be designed for the worst case surface conditions expected at any given dive site.
Frank D.