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Nacra Bridal Foil Warning!


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After walking around and looking at other peoples boats I have noticed an alarming number of Nacras fitted with a bridal foil being rigged and sailed with little or no foil tension. Some of these sailors are new to the class and had no idea that the foil should have a 25mm - 30mm pre bend while off the boat.(Not while the mast and sails are up).

Sailing your Nacra without pre bend in the foil could result in the foil inverting making your rig to come down and causing injury to the crew and skipper and damage to your boat.

The easiest method of measuring your foil pre bend is to use a string line and ruler. Stretch the string line across the upper side of the foil from tang to tang and measure in the centre at the compression post. If the measurement is under 25mm you need to adjust the tension on the foil by using the adjustment nuts on the compression post until you have pre bend of 25mm - 30mm than nip the lock nut.

With the correct pre bend the only way it will fail is if the foil tension wire breaks. This wire should be inspected regularly and if there is 1 strand broken replace the wire immediately. I normally replace this wire every 2 years whether it needs replacing or not. Some sailors might replace this wire yearly.

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Did you know the metal tangs at the end of the foil can break too? My brother broke a foil that way.

Upon closer inspection of the unbroken tang, the tang had streched where the holes were drilled and the the holes had become elongated. I would not have picked it to be a weak spot.

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Thanks for the tip Hood on the tangs. I thought that part of the rigging would be indestructible. To view the tangs properly you need to remove the foil wire for closer inspection. Loosen off the locking nut and back off the tension on the foil with the other nut. Remove split pins on the cleavis pins on the terminal forks and get 2 pieces of 4" x 2" and place on either end of the foil and use your foot to put weight near the centre of the foil to take the tension off the wire than remove the cleavis pin on one side. Once it is removed the other side is easy as there is no presure on the wire.

Assembly is reverse of the method described.

Here is a couple photos of measuring the pre bend of the foil.



[This message has been edited by GRACELAND1216 (edited 11 October 2007).]

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With this sort of pre-bend, does the foil become straight under full rig tension in heavy conditions?

If not, it's a waste of time putting that much tension in. All you're doing is pre-stressing the alloy in the foil itself and reducing it's cyclic life, which could lead to catastrophic compression failure of the foil itself.

From an engineering perspective, it only needs as much pre-bend as the maximum load experienced whilst sailing.

Hope this helps and also hope that i haven't stated the bleeding obvious.

Best Regards,

Leroy wink.gif

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Hey Leroy, anything but bleeding obvious!!

The way i look at it (may not be 100% right) is the foil should never be allowed to come straight. As soon as the foils straightens out it becomes the maximum length it can be. after that it will shorten and effectivley make the wire longer and allow it to bend upward further, until colapse.

While the foil is pre bent, every upward movment is making the foil longer and increasing the tension on the wire.

30mm of prebend is maybe a little much, but by the time your sailing the foil has probably moved 20mm up so there really is only 10mm margin for error. give or take.

yeah great invention the foil.... just another thing to go wrong smile.gif

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I have spoken with a couple of people in the know, apparently 25mm is the standard and yes they do straighten out. WOW!!!, some pretty heavy ("heavy mannnn") loads on that bit of gear.

What is actually happening is the wire is stretching to it's maximum in combination with the alloy in the foil compressing. They are working "with and against each other" if that makes sense.

Aluminium is an amazing metal capable of some extremely interesting feats. I have seen Structural Grade T6063 T6 ally GROW!!! Powdercoated a neutral colour (not light, not dark) on the west elevation of a building the vertical length increases by up to 19-20mm in only 3 metres!! So to compress a 2 (and a bit?) metre piece of foil 3-5mm would be entirely feasible and enough to start straightening things out!

Makes my previous post fairly redundant, but the principles are correct.... Still only needs as much pre-bend as the maximum loads likely to be had or expected.

Having sailed the 5.8 for a number of years (mid to late 80's into the early 90's, all "pre-foil" days), i think that the foil is the best invention yet.

The reduction in the loads around the bows and even as far back as the main beam (which is a torsional load/pivot point) would be huge. This reduction can only lead to boats remaining stiffer for longer and therefore competitive, which can only be a good thing?

Just my fitty sents werf!


Leroy wink.gif

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