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Mast de-rotating


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On the weekend in a 20 knot north easter, I sailed super sloop with no trap.

Only on the port work my mast kept de-rotating. If I let the jib of it helped. I tried no down haull and then some down haull and then lots of down haull. Nothing worked.

My rig is fairly tight but not drum tight.

I checked to see if my mast was bent but it wasn't.

Any suggestions please.

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How much traveller did you have let out?

How much mainsheet were you dumping?

The Hobie 16's had (may still do) a problem with the mast derotating when you dumped mainsheet in a gust.

Easy fix... put the trap on and then you dont have to dump mainsheet near as much. You'll be faster and happier...

[This message has been edited by TornadoSport260 (edited 08 December 2009).]

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The Great Darcy told me that happens if the mainsheet block isn't far enough back, I fixed mine this weekend and it wasn't too bad (although wasn't very windy). Needed a bit of a kick if the mast didn't rotate on a tack occasionally.

Too far back and it puts a lot of pressure on the gooseneck.

Other thing might be batten tension.

[This message has been edited by knobblyoldjimbo (edited 08 December 2009).]

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The major problem that causes this is:

As the jib releases wind off the leach it "throws" wind at the leeward side of the main towards to front 30%, this applies an opposing load to the pressure on the windward side. This opposing force tries to backwind the main (if you've ever sailed a yacht you note how the main backwinds ALOT upwind in windy conditions). As the main tries to backwind it will also try to force the mast to rotate and yes it will counter rotate occasionally.

So what stops this occuring in say 10-15knts?

The mast in its natural state will always try to face into the wind because of the elliptical profile. But forces such as stay tention, sails etc will try to fight its natural tendency to face into the wind.

The main sheet applies a load onto the boom which in essence tries to force the boom forward because of the sheeting angle. This does 1 of 2 things, it helps keep the mast rotated by applying alot of load onto the mast via the gooseneck. So the further aft the sheeting position of the top block inreference to the bottom block then the more load it applies through the boom onto the gooseneck and subsequently the mast, keeping it rotated correctly.

Noting that the mainsheet provides the major force that keeps the mast rotated, if your out in 20+knts and you have traveler down say 10-20cm to depower, you've shortened the distance aft that the top block of the main sheet is in relation to the bottom block, hence there is less forward pressure on the boom/gooseneck/mast. Add in a gust that requires you to dump mainsheet and now you've not only got less pressure forcing the mast to stay rotated (courtesy of the traveler & easing sheet) but you've also got more pressure on the leeward side of the mainsail that's applying an opposing load in that front 30% that's trying to counter rotate the mast.

As a side note, the battens will provide some resistance against the mast the tighter they are tied into the sail, because the tighter they are tied the more they try to flatten the lengthen/flatten the sail but the mainsheet tension/position is the biggest factor here.

So the fix?

Well if you sailed with trapeze, you wouldnt have to depower as early or as much, hence there would be more forward load on the mainsheet/boom/mast etc.

The other one as darcy suggested, open the slot in the head of the jib to reduce the amount of opposing force on the leeward of the mainsail.

Ie. if you have a clew board use the bottom hole remembering that in a blow you don't need power in the head of the sail to go fast. Using the bottom hole will help the head spill air and throw less at the mainsail. Remember that height comes from the bottom 1/3 of the mainsail leech, healing moment comes from the top of the sail, so if you make the sail flat up top using a lot of downhaul and open the foot a little with less outhaul. You'll get a lot of drive with much less healing moment, equaling more forward drive. (Note: in flat water use more outhaul)

Wait what am I doing? I'm helping the enemy (Maricats) sail faster... hahaha

[This message has been edited by TornadoSport260 (edited 09 December 2009).]

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Opening the 'slot' would be similar on a dinghy, considering i was taught all that off some of Australia's best dinghy coaches inc. 1 Olympic 49er coach.

You'll still get backwinding on a dinghy just because you sail closer to the wind than we do but it can be minimised.

The reason the yachties get so much of it is that their masts don't rotate, which causes alot of turbulent air around the leeward side of the mast & they prefer to sail off the heady in the windy stuff. It seems to work for them but then we're not sailing yachts and it sure as hell doesn't work on a cat...

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