Pre bend ??

What ever you feel cofortable with and matches boat setup and spinaker luff length..... I have ran 0 pre bend to and currently about 200 mm on a 4 meter pole (38mm OD Alloy extrusion). I increased the pre bend by pulling down the pole tip to lower our pole end. I did this because we had a new high aspect roachy kite made up.

What ever works works.

The main reason for a prebend in the spi pole is the force the buckling proces to push the pole upwards and into its reinforcement. This will happen before it can move in other directions

With a straight pole, it can move in all (unpredictable) direction. Only a small prebend is required for this. When the pole is not critical (wide diameter, alot of wallthickness or stiff carbon) than she can be perfectly straight without problems.

Wouter

I sail mine and if it get's straightened out - or heaven forbid inverted, I increase the prebend so that it never quite get's straight under load. I've got a 14' aluminum pole on my 6.0 and I run about 6-8 inches of prebend.

In my case too though, I have the bridle foil that makes the pole sit pretty low to the water. I haven't been in heavy seas with it yet but I understand it can be a problem driving the pole into waves. That being the case, minimum prebend is in my best interest (until I get longer bridle wires for the foil).

Jake,

I just got a set of the longer bridle wires for my 6.0. Out of curiousity, how big is the chute on your boat? Is your pole mounted to the beam? Or the dolphin striker?

Pole is mounted on the beam but I'm still not sure how big the chute is. The previous owner said 396 sq ft but I think it's a bit smaller than that. This picture is pretty cruddy but it gives you an idea.

My pole sounds a lot like Jakes setup. I re-mounted it to the dolphin striker to get the pole a little higher without raising the foil. I have been sailing it that way for years with little problems. I sail primarily in the ocean and the pole used to kiss the waves often until I moved it. A shorter dolphin striker or (pelican striker) on the pole will help keep things a little dryer on the tramp as well. Prebend is critical for the thin wall aluminum poles on the 6.0's. Not so critical for the carbon poles or the Hobie 20 which relies more on the lines secured to the tip of the pole to reduce the amount of inversion.

AJ
Nacra 6.0 Express 425sq.ft. spin/14.4ft thin wall pole
short alum. striker

Attached is a pic of the pole mounted to the dolphin striker. This gets the pole a little higher. The pole has to be longer to do this. About 4". I bought a 18' pole, cut it and installed all the hardware myself in about a day.

Note the short dolphin striker on the pole. Keeps things dryer on the boat when it gets rough. I had a local mfr. make the striker. The original one was Stainless (heavy) and too long. It was constantly skimming the water keeping the tramp and spinnaker soaked. No fun on a long day.

AJ
Nacra 6.0 Express

I don't use a dolphin striker on my Aluminum pole (man that could be interpreted wrongly! )

Jake's feeling saucey tonight 😛

Here's a pic of my pole..... sorry spinnaker pole.

We are lucky enough to still run a bridle on the Big T. The pole is mounted 500 mm bellow the bridle using a ridged strop and it is braced with wires running from the base of the strop to the shackle points for the bridle. Another set of wire braces run from the pole tip to the bow tips. Our jib also now comes all the way down to the pole (500 mm longer).... the jib is longer in the luff/leach and shorter at the foot to allow a self tacking track to be located on the front beam.

I think we all have pole-envy now.

I'm sorry I started that now.

So, do I have to dril into the bow or can I use the forestay tang?

It's best to get the attachment point as far forward on the bow as possible. I've seen people attach them to the bridle tangs (I assume that's what you're talking about) but it looked like things were loaded at bad (worse) angles.

I glassed in shackel points into the bow tips.... I used the 2 threaded eyes from a Ronstan Seafast Turnbuckle (part number RF217)check out the top of page 3 at the following http://www.ronstan.com.au/catalogue/M_P94-97.pdf

I put 2 nuts on each thread end and drilled a hole on the inside only of each hull / bow tip. The hole must be large enough for the nuts to enter.... I then pump in epoxy resin with miro fibres using a large syringe.

This will set rock solid and the eyes should not pull out. It allows quick easy shackling of the pole tip wires to the bow tips and has no real sharp edges for the kite to snag on. Still cover with insulation tape as with anything sharp to prevent the kite catching.

>>It's best to get the attachment point as far forward on the bow as possible. I've seen people attach them to the bridle tangs (I assume that's what you're talking about) but it looked like things were loaded at bad (worse) angles.

This is a common believe that even I fell for but it isn't true.

I don't have the time to explain now. Sorry.

Simply put:

Moving the lines from the bows to the forestay points results in :

-1- Putting the hulls in high tension axial wise (Which is rather good as the hull is very strong in that direction and it even lowers the compression stresses on the inside of the hulls.)

-2- reduced vertical and horizontal stresses caused by bending. reduction is proportional to the distance by which the wires attachment points are moved back. This is very attractive as bending is what destroys hulls and not tension.

Wouter

ahh yes - I agree. But all that reduction in load to the hulls are transfered to compression on the pole. O.K. so it's probably not a big deal but if you're running a thin AL pole it might be a problem.

That is totally correct jake, the pole will come under higher compression. However long tin tubes are far more limited in loadbaring capacity due to buckling than failing in stresses. Often fixing the middle of the poll by three dyneema lines or wires will stabilize the pole sufficiently and allow it to be carried without danger of breaking it. Also I rather break my pole than my hulls.

having said this, nearly all hulls appear to be strong enough to take the loads of the spinnaker without much problems.

Wouter

Alright, you guys have got me curious. For a N6.0 that uses a bridle foil, it sounds like it would be just fine to have the spi pole tied back to the bridle stay connection at the hull as long as the pole is also securely tied in beneath the foil. Have many tried it that way?

Kevin,

I think that is not enough angle since the foil only allows the middle of the pole about 6" above the bridle connection point. In Wouter's case, he's got bridle wires that put the middle of the pole much higher leaving more angle on the lines supporting the end of the pole (what do you call those?). In order to achieve a measley 6" of prebend those lines would be horizontal and the lines, tang, and pole would then be under exponentially huge loads to support any vertical lift from the chute. That is, unless you extend those bridle foil wires.

That's what made me curious, since I added longer bridle foil wires this spring.