Even with all the righting moment calculations spar sections for a given
mast height tend to fall in a fairly narrow range. A competent designer can
calculate the requirements very closely but the spar makers tend to put in
more safety margin and you are practically limited to the available
sections. Here is a "rule of thumb" for aluminum masts from Dave Gerr's
book you might use as a double check.:

For a single spreader rig the transverse mast section should be about 1/90
of the length from cabin top to masthead. The fore and aft section should
be about 1.4 times the transverse section and the thickness of the aluminum
should be about 1/35 of the transverse section.

Double spreaders reduce the required transverse section by 10 to 15% but not
the fore and aft or thickness. Heavy displacement and blue water boats
might go up 5 to 10% in all dimensions.

The only way to drastically reduce the weight of a spar is to go carbon but
brace yourself. After I got a price for composite spars at the Annapolis
show I went directly to Pussers for a #4 painkiller. :-)

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com



Thanks, I don't really want to calculate it myself, I'm trying to
understand the correlation between figures given to me by the
architecht, and the tree trunk mast sections that the mast suppliers
want to give to me.