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post #1 of 8 Old 04-29-2010 Thread Starter
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A Question for higher clew ?

Hi all
I've installed a bowsprit on my sail boat ,and I would like to have an asymmetric spinneker,just an information "what is the difference having an higher clew?"
thank you first!
Best regards !

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post #2 of 8 Old 04-30-2010
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Less sail area, more visibility.

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post #3 of 8 Old 04-30-2010
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A lot of times offshore cruising boats will design their heavy weather sails with a high clew to reduce the chance of catching a wave in the foot of the sail and damaging the sail or the boat with the impact. This is especially important on sails that you are likely to use when close reaching in big seas.

In the case of an assymetrical spinnaker, the usual reasoning does not apply. You do see high clews on some assymetrics in order to be able to get a decent sheet lead angle to the position of the sheet block.

A high clew will impact performance negatively in that it does reduce sail area and also reduces the end plate effect of the deck to near zero.

Jeff


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post #4 of 8 Old 04-30-2010
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But having a high clew on an asym often means you can see under it more readily and not have a huge blind spot.

Sailingdog

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post #5 of 8 Old 05-03-2010 Thread Starter
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thank you all guys

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post #6 of 8 Old 05-03-2010
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I don't understand why having a higher clew necessarily reduces sail area. It depends on what direction the clew moves.

If the clew moves straight up, i.e., gets closer to the luff, then yes, the sail area will be reduced. Similarly, if you take a deck-sweeping headsail and move the clew up the leach of the sail, then cut off the cloth below, area will obviously be lost.

On the other hand, if the clew is moved up and aft, parallel to the luff---in other words, the sail is skewed upwards---the sail area should remain constant.

Proof: imagine cutting the sail into strips parallel to the luff. Shift each strip up by an amount proportional to its distance from the luff. Resew where the strips meet. You have a new sail with a higher clew and the same area (pretending for a second that you haven't destroyed the camber).

It's easy to see how this applies to a flat-cut jib, but in general, skew transforms preserve area. Sail area is roughly equal to luff x LP / 2.

I don't have any idea how a high-clew headsail is geometrically different from its low-clew counterpart, though. But I also don't see any reason not to have the clew of a high-clew sail farther aft, other than interference from the mast when tacking, which is a problem with low-clewed headsails anyway.

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post #7 of 8 Old 05-03-2010
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Here is the deal on that, most boats have a fixed range of positions for thier jib lead. If the sail remains the same sail area, then as the clew rises the lead would need to move aft, and so off the tracks and eventually off the aft end of the boat.

But also as the clew moves moves aft, the sail loses efficiency, losing both in terms of loss of endplate effect, and also often becoming a lower aspect ratio sail.

While these sails are generally not employed on race boats, cruisers need sails that work across as broad a wind range as possible since they do not carry big sail inventories or the crews to make sail changes. With lower efficiency comes more heeling and leeway and so the need the reduce sail sooner.

Jeff


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post #8 of 8 Old 05-03-2010
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I see your points about jib sheet lead---especially for jibs over 150%---and loss of end plate effect, but maintaining LP while moving the clew should mean that aspect ratio remains constant.

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