working out ICMS equivalents

Im trying to work out an ICMS equivalent for the landyacht masts Im making, but am struggling with how the number comes about .
the weight in the middle , etc is easy, but the calculation seems vague.
can anybody help with a good dummies explanation. until I have an idea of an equivalent, I can go shopping for your secondhand masts

so the 3 lengths are the mid deflection, 1/4deflection and the 3/4deflection?
thats the vague bit

Are you wanting the ICMS measurement or the Bend curve characteristic?

ICMS for a 430 mast with a mid deflection of 179mm for instance would be
430 (cubed) divided by 460 x 460 x 17.9 which equals an ICMS of 21

Here are some of my own Bend curve characteristics calculations below.
Mast bend curves Middle top bottom Bend curve characteristic
2011 Naish SDM 430 C90% 179mm 135mm (75.4%) 117mm (65.4%) 10 Constant curve
2010 Sunshine 430 C80% 191mm 144mm (75.4%) 118mm (62%) 13.6 Constant Curve Flex top
2007 Severne Redline 460 C100% 191mm 149mm (78%) 118mm (62%) 16 Flex Top
2007 Tushingham 490 C100% 206mm 163mm (79%) 128mm (62%) 17 Flex Top

Bend Curve Categories (www.peterman.dk/mast-imcs-formulas.htm)
0 -   6      Hard Top      
7 -   9      Hard Top - Constant Curve
10 - 12     Constant Curve                                                                         
13 - 15     Constant Curve - Flex Top
16 - 18     Flex Top    
19 - 21     Flex Top - Super Flex Top  
22 -         Super Flex Top

30 is REALLY stiff...

But as you recut sails anyway this probably doesn't matter, as you'd recut the sail to suit your mast.

This also goes for bend curve... remember you need to measure the deflection at the 1/4 and 3/4 point to figure out whether it's hard top constant curve, you can't figure it out by looking at it ("Constant curve" actually means it bends more at the top than the bottom)

And just to make life interesting I'd only measure the part of your mast that has a sail wrapped around it, rather than the whole thing (remember there's a good 50cm or so of supported length at the bottom of the mast).

Arrr now I understand...

A stock 490 mast will have an IMCS of around 29. By chopping off the top you can make it behave as if it were a smaller, stiffer mast. By chopping off the bottom you can make it smaller (but I think it keeps its imcs -- not sure about this).

If you get yourself a 490 carbon windsurfing mast, and cut 30 cm off the top, then you might be pretty close to your 460 in terms of stiffness.

You may need to get a 520 (pretty rare) to match your 460 aluminium mast. Not a very common size unfortunately.

I would see if I could find a 490 mast, and measure it from the bottom as if you'd chopped the top 30cm off, and figure out the stiffness from that. Then measure it again as if you'd chopped off 60 cm.