imcs rating question

[huefner]

When the mast is longer than the sail luff length, does that increase or decrease the effective imcs rating of the mast being used?

For example, I am using a Neil Pryde Wavepro mast, 430cm length, with a imcs rating of 21. If I use that mast on my 4.2 North Zeta wave sail its about 10 longer than the luff length, and hence I have to use the vario top, and extend the luff length out the top about 10.

So, would that change the effective imcs up to say 22 or 23, or down to 20 or 19?

[johnmartin]

IMCS is independent of the sail being used - it is only useful for comparing the relative stiffness of masts of different lengths - with a 460 being the normal sized mast. As length increases above 460 the IMCS goes down relative to the MCS and vice versa. Windwing has a good page that describes MCS and the conversion to IMCS numbers (windwing.com). A 430 for a 4.2 should be fine unless it has an unusually high or low IMCS rating.