Well,

It is also known.

I(polar) = I(axis 1) + I(axis 2)

So I(polar) is nothing more than the sum of I(axis 1) + I(axis 2) where the two axis are perpendicular.

Of course with a mast the I(y-axis) is very small compared to I(x-axis) ; a ratio of 1 to 30,000. So it is very convenient approximate masts (the thing we wre initially talking about) by calculating only I-(x-axis). In short Ipolar of the mast is very closely approximated by I(x-axis) alone.

In basis you are right however. I(polar) is the thing you need to do math on rotations dealing with the whole platform. However for dive decelleration calculations you'll need I(x-axis) again and not I(polar).

Things are getting easier by the post aren't they.

As Radius of Gyration was not incorrect in the strickt sense but a it is a confusing expression to use.

may point was however than reducing mast weight by a factor of 2 reduces the polar moment of the mast by 2.0000333 when looking at rotational oscillations. For decelleration dives only one of the normal moment of enertias is involved and this is reduced by 2. This is only for the mast and the total of I(polar) is determined by many components INCLUDING for example the crew itself especially when far away from the main beam. The overall net effect of reducing the weight of the mast by a factor of 2 can have a net result of about 5-15 % overall depending on several factors

I think ths wraps it up nicely for all of us.

Wouter