Hi guys I just got a second mast and was just wondering on my 430 it's IMCS is 21 but on the 460 it's IMCS is 25
what dose this mean? and what effect (if any ) will this have on a
weekend clux like me...both are normal dia.

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Originally posted by basher

International Mast Check System.

This relates mast stiffness to mast length and has resulted in standardisation of mast stiffnesses over mast sizes to allow cross-compatibility between sail and mast brands.

A 460 length mast usually has a stiffness or IMCS 25 whereas a shorter 430 mast has a typical IMCS of 21.

21 is less stiff than 25. The idea being that the smaller size sails benefit from setting on bendier masts.

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Have a look at the foot of your sail. It will detail the desired mast characteristics to match.

The old system in the late 80's was MCS (mast check system or something). It was useless as it relied on the following analogy: think about a broomstick 1m long and another one of same diameter but 6m long.
Obviously the 6m one is "floppier" ..... but under the MCS system it is assigned the same stiffness number as the same length section of broomstick (mast) is the same stiffness.
That is a bit silly so they changed to IMCS

IMCS is the stiffness relative to length. Thus they measure the amount of deflection under load (weight hung in the middle) ... and it is relative to how long the mast is.
A 400/17 has the same deflection (bend in the middle) as a 430/21 and same deflection as a 460/25.

That may not make much sense but in the old days: a sail line may have said you need a stiffness 21 mast. But in the smaller sizes in the same sail range a 21 didn't bend enough, and in larger sizes a 21 bends too much. People wondered why their sails didn't work well.
Now you see a sail range may specify for 3.5-4.5 use a 400/19. From 5.0 - 6.0 use a 430/21 and from 6.5 - 7.5 use a 460/25.

Therefore: if your masts are the same brand, one is a 430/21 and the other a 460/25....... if the 430/21 works on the smaller sizes in a particular model of sail, you are totally guaranteed that the 460/25 will work in the larger sails in that same range.

Clear as mud?

quote:

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Originally posted by wayneg

You may also want to look at www.fibersparsports.com/msguide/imcs.html

Remember when doing the calc's that length on this site is listed in cm, whereas deflection is listed in mm (on their example).

Don't know why someone would use cm?? Do yourself a favour and use all mm or all m. Next they'll use decimeters More likely inches!

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As an addition to this.. The fiberspar site doesn't tell you how to load the mast to work out the IMCS calculation yourself. (just in case the label has come off, or, heaven forbid, you think that the manufacturers may have got it wrong)

I emailed fiberspar a while back - the bend should be induced by supporting the mast horizontally on two 'points' 50mm from either end. The edge of two tables, or saw-horses will work. Measure to the middle of the mast lengthwise, and then hang 30kgs from that mid point. Then run a string line from the top edge of each end of the mast. The distance from the string line to the top of the mast where the weight is attached will be the 'midpoint deflection' that they are refering to.

Thanks guys that covers my limit and at my level too
it,s just that I was told that a 21 felt a bit stiff for a 5.8
but the sail and mast are from the same manufacture
and the sail foot calls for a 21 no biggie just got me thinking
thats all..

The info and instructions came from one of the fiberspar pro sailors (can't remember his name at the moment though!)

If the string line is pinned to the supports, do you measure to the top of the mast tube, or the bottom? I would have thought the top would again cause a problem in that the deflection would be decreased by one mast diameter?

The more modern masts I have measured using the method outlined came out extremely close to their rated IMCS.