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Mast pre bend - how


PhilW

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question in the title really, how do you pre bend a mast?

In my particular need for an IOM A and B rig, I know the offset is arount 15-20mm for an A rig and some form of jig is required but that is all i know.

Any help appreciated :)

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This is what I made. It works great.

 

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I have seen some with the outer wheels at about 8 inch on center, but that puts a lot of pressure on the mast tube and can cause it to go oval. On mine,the wheels are 12 inches on center.

It is made from mostly scrap pieces and the stainless hardware cost about $4.00. The wheels were cut out of some leftover oak planks, and I cur them with a hole cutter. Then I chucked them in a drill press and spun them and then used a dremel sanding disk to cut the grooves. Access to a lathe would be even better, but not necessary.

John

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John Ball

IOM CAN 307 (V8)

In my private capacity

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Hi Phil,

There is no one answer to that question. It depends upon the mast tube that you are using, and on the luff curve build into the mainsail.

I use the French 11 mm Hi tensile thin wall tube, and for my A rig, I have 15mm of deflection at the tip and the bend extends down 900 mm from the tip.

John

John Ball

IOM CAN 307 (V8)

In my private capacity

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So your question was not How To but How Much and Where !!!

I understand that the objective is to transfer tension into the Headsail / forestay from the back stay and shrouds without bending the mast back thereby flattening the main sail.

So with that in mind – I guess the next question will be how much tension.

Now that I understand that is – enough tension to stop the leech of the Headsail opening in a puff before the Mainsail. That is why the boat seems to have weather helm and uncontrollable luff up every time the wind thumps the sail. --- and everybody else goes flying by you. It not weather helm it is Snap Helm and mast rake will not cure it. Essentially the sails are not working together.


The mechanism opening the leech of the Headsail is the Headsail boom lifting when the tension exerted on the leech by the wind is greater than the downward force exerted by the boom. This is also why the topping lift gets wrapped around the spreaders. - another irritating indication that the forestay is too slack

It is a simple three point seesaw comprising :- Forestayand leech / toping lift balling over the swivel point. So clearly the swivel position also influences this dramatically.

So now you know the objective:- to maintain a strait mast and to get enough tension into the forestay to stop the boom jumping up to soon.

So you know what to look for.. it is as simple as that. But go easy until you get the hang of it and sail the boat to see the effect in between each bend session until you are confident about what you are doing.

Set the rig up and balance the back stay and shrouds. Tension up the rig and see where the bend in the mast is.

Theoretically if you bend the mast exactly opposite to the shape it initially took up the mast will be straight with the same tension transfer.

So take rig down and bend the mast a 'little' in the opposite direction to what you saw. Put the rig on the boat again and try again. Repeat the process until you are happy that the mast is straight and the Forestay is tensioned up sufficiently.

Be careful adjusting the mast ram or you will end up with an S curve in the mast. There is no magic dimension.. and if there is it will vary from rig to rig. It has NOTHING to do with how the Luff of the mainsail is cut.

You do not need any fancy tools - just two clamps and a table, but I must confess that it is a nice tool John

.

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Eeekk! :)

Good information though thank you, putting it into practice should be interesting :)

I had presumed that the mast was pushed through the roller producing a curve, but from Dave's comment it's purelly pushing/deflecting the tube in a controlled manner at a fixed point.

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Push the tube through.

The French tube is a 7075 alloy and is hard to bend. It has an amazing ability to ‘spring back’. So when I use the bender, I try to creep up on the setting by running the tube, once in and out, then remove and look down it – no change, add half a turn and go again.


The way I do it is

1. sight the tube for any existing bend, and then put the prebend in the same direction. I do not cut the tube until finished.

2. I measure from the bottom of the mast to the lower band, and then from the lower band to the top of the mast. Tape the tip with masking tape. Then measure down from the tip to the length for the bend and tape again. That way the extension of the tube is at the top and you can apply the bend over the entire length (just trim off the top once finished).

3. Mark the centreline of the mast on both sides at top middle and bottom fro reference.

4.Place the tip in the bender at the middle wheel and run forward until the bend limit is at the middle wheel, and then run out – hold the mast tight on vertical as it will try to twist. Pop out of the bender and sight down the tube – adjust the screw and repeat as necessary until the bend is achieved.

John

John Ball

IOM CAN 307 (V8)

In my private capacity

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One additional thought - it is easier to add prebend to a French mast, then to remove some, if overdone. So err on too little and rig the boat and try it - you can always slip off the sail and add a bit more later.

John

John Ball

IOM CAN 307 (V8)

In my private capacity

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Yes, that was my thought - that the max bend force is applied at the centre wheel, so the top 6 inches beyond the centre wheel would not get much bend. But if you have already cut the mast to length,don't worry about it.

John

John Ball

IOM CAN 307 (V8)

In my private capacity

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  • 2 months later...

You can avoid the issue of flattening the spar by making sure the grooved in the bending rollers 1) exactly as wide as the spar you are bending and 2) >50% of the spar's diameter deep. This gets the widest part of the spar down into the groove and prevents it from spreading or flattening as you bend the spar.

The distance between the rollers shorter determines how close to the end of the spar you can achieve bending.

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