Mast head rig/mast bend

[cameronke]

I have a mast head rig and noticed that I have a fair degree of mast bend even with no back stay tension. The bend appears to be due to the sweep back/tension of the spreaders/cap shrouds. Could the spreaders possibly have to much aft sweep?

All comments greatly appreciated

Regards
Cameron

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[Aja]

Cameron

Pre-bend is not neccessarily a bad thing. On a mast-head rig you should induce pre-bend by using the cap shrouds. This will stop the mast 'inverting' or pumping.

How much pre-bend is the question? Well On my 10 meter mast I have about 2" (nice mixture of metric/imperial there..) which is not excessive. I can also wind a bit more on my adjustable baby stay. This is before backstay tension.

One problem when inducing pre-bend is that your forestay becomes slacker. You are not bending the mast back rather you are compressing it in column.Thats when backstay tension comes into effect.

The spreaders need to be swept back slighty to get pre-bend. Mine are not adjustable fore and aft.

Regards

Donald

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[boatless]

As Donald says, helps to stop 'inversion' (mast getting negative bend). An absolute maximum is 3 diameters of bend, but one diameter is normal.

(A diameter meaning the fore and aft length of the mast - and a line stretched between the aft end of the foot to the aft end of the head would be one diameter away from the aft face of the mast at mid-height) Hope that's clear, makes sense to me?

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[William_H]

I can't resist commenting although I am a fractional owner and so not necessarily in the know. There must be a lot of spreader sweep back to induce mast bend with backstay loose. Most mast head rigs that I have seen have very little sweep back as they rely on the inner forestay and intermediate shrouds to control fore and aft position and movement. The simple answer is more tension on the intermediate shrouds assuming the chain plates are reasonably aft of abeam the mast. This should pull the bend out and leave the middle of the mast very well stiffened. (no pumping)
If the spreaders do swing a long way aft you may find that the inner forestay can ber disconnected for light weather making tacking of the jib much easier as in a fractional rig however be very carefull here as mistakes could endanger the mast. Watch out for the pumping or fore and aft movement of the middle of the mast. I would sufggest you always use the inner forestay when any real backstay tension ios applied assuming thisd is adjustable. regards will
PS with aft swept spreaders watch out for chafe on the mainsail when running put patches obn the sail and don,t let the main right out.

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