I have managed to pick up a Gaastra mast 70% carbon content 4 metres long with an IMCS of 22.5 instead of the standard 19 for this size. Does this mean the mast is softer or harder then normal for that size ? I understand the formulae to calculate IMCS is

(MCS or deflection in mm ? at midpoint with 30 KG weight) x mast length(cm)/over 460 (cm)

This should mean that a larger deflection for a given size will give a higher IMCS and therefore for a 4 metre a 22.5 IMCS would be softer then a 19 IMCS, which would mean less power but more control in strong winds, or if I have this back to front, the rig will most probably have a more limited wind range in strong winds and be a handful ..... Can a mast guru please set me straight

No.
MCS = L/D
IMCS = MCS x L^2/460^2
ie IMCS= L^3/D 460^2

So higher number = stiffer

Sick of limping home with a broken mast, so I pick up a nice strong Fiberspar 25 IMCS 460 30% carbon for my Gaastra 6.5.
Can't go wrong, as Gaastra recommend Fiberspar masts?
Aha, it is a "Quicktip", and I have ended up after much fiddling with the sail adjustable top at maximum extension to make it rig like on older masts. Does anybody have any expert experience with these sort of masts? (Reflex 3200) ?

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Originally posted by Haggar

a 22.5 IMCS would be softer then a 19 IMCS, which would mean less power but more control in strong winds, or if I have this back to front, the rig will most probably have a more limited wind range in strong winds and be a handful ..... Can a mast guru please set me straight

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Think you have this back to front as well, as the mast flexes downwind under load, the leach tightens up and the middle of the sail fills out. Surely that means a softer mast gives more power, and a stiff mast less????

confucious say,
there is correct setting for all sail and mast combination.
you need is goody eye and much knowledge of eagles wing.
rest is skill!

Sorry guys I'm not convinced, think about the line between mast tip and foot, without mast flex this passes thru center line of boom, when mast flexes downwind the line moves closer to downwind boom. This is also the line the sail has to follow, so as the mast flexes the sail gets more shape. That's why the tip has to flex to conteract the bottom of the sail powering up.
With your sail rigged get a mate to stand on the tip of the mast and push down on the foot. This simulates mast flex, watch what happens to the sail, leech tightens sail shapes up

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Originally posted by mkseven



It does?

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Well I haven't got a sail rigged in front of me, and it's a while since I last did it, but I'm fairly sure that's what happened!

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No you cant say because the mast is moving towards the downwind side of the boom that more fullness is being put into the sail. Remember the sail twists off and all that the whole sail dosent follow that line of the mast.

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Agreed the head doesn't but the middle of the sail does, which is probably 70% - 90% of the sail depending on width of head.

Had no idea I would cause so much mast debating

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Originally posted by mkseven

OK but it goes against all convention.

Just a final question, are you doing this with a raf sail and is it possible that it is taking up the cloth and setting the sail into the correct shape?

It's been ages since i've done the deflection test and never on a non cambered sail.

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Depends on whose convention?

Yep it's a wave sail, if you hadn't asked the question I was going to mention this anyway, as it won't show up to the same extent in a cambered sail, but you should still see the leach tighten I think and I'm sure the sail will still fill out a bit, but no where near as obvious as a wave sail does.

You had me doubting myself a bit, so I got the wife to stand on the tip before I went for a sail today, and it's as I said.

Note, this doesn't mean a heavier person should use a softer mast to get more power. As there will be more power in the sail to deflect the mast with a heavier person. It's probably the opposite, heavier rider needs a stiffer mast, to get the right deflection.

The other point of confusion may be a mix up with the effect of mast bend due to downhaul, that will cause more sail twist and less power.

Thanks Mark, it's nice to have that sorted out, not that it's going to make much differnce to most people.

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Originally posted by oldie

... But surely the lack of a strap extension on the top of all new sails is removing an extra tuning tool which would allow one to change the amount of twist off, say...

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Yes, but it sells more masts