Spin pole pre bend?

I have always heard it adds stiffness to the pole. But I am not an expert, anyone who knows more want to chip in?

1) It strengthens the pole because the spinnaker is not able to pull up and bend the spin pole. If the pole is left free to be pulled up the shock load can damage the pole.

2) It allows you to set your luff tension more accuratley. If you don't prebend your pole then the pole will move higher therefore reducing the tension of your luff.

You don't have to prebend it rediculously so that it's pointed down in front, usually about a foot or two. The best way to figure it out is once the spin is up you should be able to grab the luff and turn your fist half a turn to a full turn. I think most of the absurd amounts of bend you've seen were to make the luff tighter which seems to be a trend latley. Many spins aren't built to be this tight and the pole must be moved down to do so.

When the spi is hoisted the pole itself itself is put under compression by its support wires. Any elongated element under compression while have a buckling failure load added to its normal failure loads like overstressing the material. Typically buckling failures are encountered much earlier then the other failure modes. Buckling is of course initiated by the pole bending away from being straight. The bending enhances the buckline failure meaning it further reduces the load it can carry before it fails completely. Basically buckling is a failure mode with a positive feedback. The further the bend progresses the less load is required to break it.

The pole can be protected against this buckling failure by stabilizing the point that will show the most bending, this is of course the middle of the pole. For this reason we all have baby bridles runnning from the hulls to the middle of the pole. The only disadvantage of this setup is that these lines will only prevent the pole from buckling upwards, not downwards. With a perfectly straight pole you never know in which direction it will bend initially and therefor in which direction it will buckle. This is dependent on many specific details that change with situation and time. The pole may buckle upwards today and downwards tomorrow. In order to force the pole to always choose to buckle upward (in which direction the support wires will prevent it) it is given an initial upward bend in the middle. This is the prebend of the spi pole. Even a very modest prebend is enough.

An added benefit of the prebend is that it pretensions the support wires to the spi pole tip and thus firms up the pole. Just like very slack side stays the spi pole will feel mushy when its support wires have almost no tension to it. Again even modest pretensions are enough.

I think I run about 2 to 3 inches of prebend on my poles and that is enough.

But indeed as others have said, sailmakers do cut the luffs of the spis very long these days and as such alot of crews prebend the poles alot to make the luff fit. This is however also weakening the poles.

Wouter

Thanks wildtsail & wouter. Thinking about it, (with some explanation), it makes. In essence the pole is being made shorter, and it has to pull on both sets of bridles in order to get to the

straight

position. Which it can't do because of the bridles. 2-3 inches is about all I have been putting in mine. Just enough to put a curve into it. Is more tension a requirement for higher winds? I haven't had the guts to hoist it in some of the winds I've been told you can pull the spin up on an FXone. Baby steps....

No, in this situation

one setting suits all

After the spi is hoisted the bridles are loaded up with much more tension anyway. The pretension is mostly there to make the tip of the pole firm enough so it stays where it is during hoists and drops. Not too much pretension is needed for that. Basically just grap the tip of the pole and try to move it side to side, (but not down !). If the tip feels firm then no more pretension is needed.

Wouter