The following URL was posted on rec.boats.building.  It is the most
comprehensive calculator site I have ever seen.  I suspect many on the list
will find it useful.

http://www-sci.lib.uci.edu/HSG/RefCalculators.html

Regards  Robby [robbyr@pacbell.net]
S/V Rolling Stone (In Scotland)
http://www.selfsteer.com/monitor/covers/MonitorCOV.jpg
F/V Dolores E. (In San Francisco Bay)
http://www.wingo.com/sailing/dolores.jpg

I found a very cool reproduction of an antique deck prism at Restoration
Hardware over the weekend. The same one can be seen here:
http://www.treasurequestshoppe.com/deckprisms.html

This looks to be a good way to get light below decks, and look great at the
same time. $20 for an "objet d'art" paperweight is a bit pricey, but $20 for
a "skylight" is a great deal! This thing is solid glass, and is pretty
heavy. It doesn't do the hippy rainbow diffraction thing, much to my wife's
dismay.

The one problem that I encountered is that I don't have enough head room to
mount it in its typical pointy-end-down orientation. That apex could make
for quite a bit of mischief on the old cranium. The sides taper in slightly,
so mounting it pointy-end-up requires a custom bracket and frame to be
fabricated. If anyone is interested, I can post pictures of the mahogany and
welding rod gizmo that I built yesterday. I haven't mounted it in the deck,
so I can't give a good report as to its light-gathering efficiency.

This could be the perfect deal for lighting up that dark companionway or
dismal stateroom!

Doug Barnard
rebuilding a 20' Skipjack
"Fiesta Bimbo"
trawler-crawler wannabe

An acquaintance of mine got one of these prisms and tried what you're
doing - installing it upside down. He says it had no effect until he
turned it over with the pointy end down.
If I had access to deckprisms, I'd install them off center on the
ceiling so the pointy end and my cranium would be well separated from
each other.
However last time I cut a hole in my ceiling (for a woodburning
fireplace chimney), I had to go through several inches of empty space
between inner liner and the deck. So prisms probably wouldn't work on
my boat anyway. Just have to be contended to sit in the warm glow of
a real open wood flame - tough life, eh? <grin>

George, irascible mossback on Lake Ontario