Hi Ben,
Re the strength comparison of the cylinder to the dome; the hemisphere is a
stronger shape than the cylinder but its a little bit like comparing apples with pears.
A cylinder at 500mm OD, 250mm long (or high) & 20mm thick would have a crush
depth of 1500ft & have a volume of 50 liters.
A hemisphere at 500mm OD, 250 mm high & 20mm thick would have a crush depth
of 2,770ft & a volume of 34 liters.
A nominal hemisphere of a similar 50 liter volume to the cylinder could be 570mm OD.
This at 20mm thick has a crush depth of 2,100 ft.
The hemisphere is more economical with space as its more compatible with the shape
of your head.
The cylinder, in a large dimension like Brian has, can act as your conning tower with a
hatch on top of it. This puts you that much higher above the water level to stop wash
coming in, whereas a dome is mounted & hinges open at the base.
Interestingly enough I just received an email to an enquiry on the price of stock acrylic cylinders.
Their 500mm OD, 20mm thick cast acrylic, at 3 meters long was US $1,300 per piece.
You could get 12 x 250mm sections out of this at $108- each plus freight from China.
I will qualify this before I get "shot down" in that you'd need to go wider for a conning tower
& possibly thicker & check the technical specifications of the material. This was an initial quick
search, but could be a way of coming up with an inexpensive tower / viewport. You could
cut your length off their standard 3 meters & sell the rest off as a fish tank.
Alan
P.S have trouble with your emails in that I can't "reply to sender" because it comes up with a
a message " you cannot send digitally encripted messages because you do not have a digital
I.D. for this account"