Pre bend on IOM mast

What would be the process for pre bending a IOM mast?

I sent you a PM.

thanks don


I was under the impression that this site was a FORUM, not just a minor message page. Could you please pass on your ‘pearls of wisdom’ to the general worldwide readership as well as to the initial requester. We might then develop this into an educational discussion.


A.B. Just be a bit careful with the prebend.
Not like me this week as I built a new A rig mast for my IOM, using the 10 point whatever alloy groovy mast section.

I bent mine gently across my chest, trouble is im either too muscular or too stupid, but whichever, I put in far too much bend and when I tried to set the mainsail on the mast yesterday it looked dreadful. I had to put in huge amounts of backstay and ended up with an S bend in the mast. The boat was not quick and so I had my arse kicked by all and sundry even after getting some cracker starts. ( If you cant sail faster then you have to sail smarter )
Today`s job is to remove some of the bend, without ending up with permanent S curves in the unmounted mast.
I have some other challenges with my clever gooseneck fitting to deal with as well. (Pictures at eleven )
Remember with brebend, "A little goes a long way "

How much is too much… see photo, that`s too much.

The gooseneck fitting is experimental, made from a knitting needle tapped out to 3mm and fitted to the mast on an angle so that as the sail goes out the leech is made tighter keeping the sail drawing downwind.:lol:

I have now corrected the excessive pre-bend.:cool:

I used to straighten trombone slide in a previous life so have some idea of what I am doing. :scared:

Lester Gilbert has a piece in his huge website with reference to mast bending.

I have bent a few using a ‘Three Roller Tool’ which a friend in the tool making trade lent me. It consisted of a length of alloy channel with two rollers, suitably shaped to nest the mast tube, spaced about 8" apart, and a third roller built into a yoke between the two fixed rollers, which can be screwed down to grip the tube. With the tool held securely in a bench vice, and by drawing the tube back and forth through the tool, a permanent bend can be introduced into the tube as the centre roller is gradually screwed down.
This method gave very good control of the radius required. Bending the tube over a knee or round the chest is very hit or miss.
To find the centre line on the forward part of the mast so that the forestay hole can be drilled,(do this AFTER bending) lay the bent mast flat on the bench, ‘black’, with soft pencil lead, the blade of an engineers square ,or something similar that is at right angles and with the body of the square on the bench and the blade vertical, offer it up to the mast in the region where you want to drill the hole and rub it against the mast. This gives you a very accurate centre line. Do the same for positioning the gooseneck
I only bend mine over the top half of the mast and rely on the mast ram to control the lower part. It looks like the bend in Ians mast is only near the middle, whereas the centre of the arc of the bend should be up near the forestay attachment. and the bend should wash out to straight at arounds the spreaders.



Sorry Ralph
AB and I both belong to a small Yahoo Group that is password protected. One of the members had posted a mast bending procedure there. In my PM to AB I was pointing it out to him. I didn’t have permission from the writer to post it here. I guess I shouldn’t have publically announced that I was sending him a PM but I know that I have had PMs waiting for days before I notice the indicater. I will try and get the authors permission to post it here.

He said go ahead

“NOTE, this is for 1/2” tubes.
OK here’s what I do. Make a plywood jig about 1 m long with a curve on one side that has a radius of 1250 mm. place the jig in your vice with one end down below the top surface of your bench by about 30 mm. place a screw in the front of the bench just beyond the end ofyour jig and 1/2" above the extension of the curve of the jig. Now hook the top of the mast under the screw and bend the mast over the jig in one smooth steady motion. Go all the way to the end of the rig for the A rig and stop short of the end for the B and C rig. When you let the mast up again at first you will think that nothing happened and that the mast just sprung back to straight, but when you sight down the mast you will see that there is a lovely even bend in it, it is enough to make you weep with joy. However there is something to be very
very careful about. Make sure the bend is EXACTLY fore and aft, not slightly off to one side or you will spend many frustrating hours trying to get your mainsail to look the same on each tack later on (experience talking here). I end up bending the mast just past straight with the backstay so that I sail with about 2 or 3 mm of mast bend which suits the way I cut my sails but isn’t enough to cause the other problems I mentioned earlier. Without prebend, you will end up with about 8 mm of mast bend to get sufficient fore stay tension, and most sails can’t tolerate this much mast bend.
Cheers Graham"


Thanks Don,

Like the old saying KISS, ‘keep it simple, stupid.’

There is no problem keeping the bend fore and aft if the first operation is to bend the mast and THEN to drill and attach the fittings to suit. BUT, remember which is the forard side of the mast before drilling the forestay hole. I once aquired a mast which had already been bent by ‘the man himself’, G. Bantock, only to have to rebend it back in the opposite direction, AFTER I had attached the gooseneck, forestay AND the shrouds (flat wire). The mast is still in use three years on with the spreader arms streamlined in the wrong direction!.

Any idea what the jig radius would be for 11.1mm (7/16) masts.

Ohh! The confessional!! Let everyone learn by others mistakes.