annie:
No other divers I know have a large SMB for emergency use. They seem to think they're covered with a smaller DSMB - to be honest I wonder. I don't think they're really that easy to see in a heaving swell, especially not from any significant distance away. Thoughts?
I won't get into the pros, cons, and usage of a DSMB. I will say this: the more height the better. seven feet out of the water is good. Remember that this is a small slim object and the crew are looking in a field that is about two miles out by 360 degrees.
Okay, time for arts and crafts.
Take a big sheet of paper. At a convenient scale, draw a horizontal line representing your distance from the boat--let's say 200 feet, nice and close.
At one end of that line, draw a vertical line up to represent boat freeboard--let's say 10 feet. On top of that, add six feet for height of eye of your divemaster.
At the other end of the line, draw to scale a vertical line representing your head above the water, and another representing your smb height above water.
Now assume a nice calm sea and draw a small gentle three-foot swell right in front of you, between you and the boat.
Finally, draw a line of sight from the divemaster to you.
Bigger is definitely better.
-Bryan