There is no simple explanation. Brion Toss's "Rigger's Apprentice" and Dave
Gerr's "Nature of Boats" have a fairly straight forward explanation. Both
have charts to help you estimate the RM. Larsson & Eliasson "Principles of
Yacht Design" has a much more complete explanation but requires more math.
Read all three in that order and you will have a pretty good grasp of the
subject.

Best way to get accurate numbers is to send your hull dimensions to a spar
maker and ask them to specify a mast. They all have computer programs that
can crank out a report in seconds.

--
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

"pete" wrote in message
...
Hi folks,

Can anybody explain in very simple English the moment of inertia of a
mast, and how the Ixx and Iyy numbers work - I am a little like Pooh
Bear, I have a poor brain so any help is appreciated.

A million grovelling thank you's in advance

Pete