Sail Drives Forward

KenyKirschstein · Joined: 27 Jun 2000 Posts: 3

When I sail my larger cambered sails and I get overpowered in a gust the sail tries to turn and drive into the wind. To counteract it I have to twist the sail. Push with the front hand and pull with the back hand. I dont experience it with me smaller wave sails. What causes this? Larger sails, cambered sails, sailing farther upwind with larger sails. How can I compensate for it?

KenyKirschstein · Joined: 27 Jun 2000 Posts: 3

I dont believe it is an harness line issue. Usually if your harness is not on the COE you end up pulling in with either forward or back hand. In this case its like the sail wants to slam forward into the water and it takes all of my strength to keep it from doing it.

carl · Joined: 25 Feb 1997 Posts: 2674 Location: SF bay area

What brand, model sail and board are you using when this happens? Also what is approximate wind speed? are you in the straps? It sounds like a rigging or mast foot position issue but I would like to know more details. Most likely its not enough downhaul.

spyder · Joined: 24 Sep 1996 Posts: 2790 Location: oahu

My main guess would be not enough downhaul in the sail, so you arent getting enough twist off in the gusts.

Also, check your boom height, on larger slalom sails, I like my boom higher, like shoulder height, since its raked back when planing. If its too low, then the mast will tend to hang out at more of an angle off the board, and will make the pull you describe worse.

Also just check your placement of harness lines as well, should be able to balance the boom with one fist in between the harness line straps.

hobie2772 · Joined: 10 Apr 2000 Posts: 25

Definitely sounds like not enough downhaul. From what youre describing it sounds like the COE is moving when the gust hits the sail, moving you out of balance with respect to your harness lines. More downhaul will tend to lock the draft of the sail forward & that should correct it.

prev1 · Joined: 06 Jul 2000 Posts: 66

Wow, this discussion is very interesting to me since this has happened to me many times in severe overpowering conditions. As noted, the wind slams the mast down to windward. It takes great strength to fight the dumping action.

Regarding the downhaul, I doubt that it will make enough difference in these extremely overpowered conditions. I think that the problem is more likly the position of the mast.

Im dont understand the dynamics of whats happening but clearly the sail is pushing to fall to windward mast first.

Having been a sailboat sailer and knowing that I can luff a sail in any conditions, I recently tried standing the mast up straighter both front to back and side to side in those gusts and the situation was managable since you are able to spill the wind. The problem is that you cant point up wind very well. It would be great to get a better understanding of whats actually happening to my sail during these gusts and if there is a better way to handle this (other than changing to a smaller sail).

gemoore · Joined: 14 May 2001 Posts: 494

It is NOT the position of the mast.

The phenomenon happens because the increase in wind strength increases the deflection of the sail surface, which has the effect of moving the CoE towards the clew (aka - increasing back hand pressure). You have two ways of sheeting in to fight this tendency: pull with the back hand and/or push with the front hand. Race sails resist this phenomenon better because they have more battens to provide stiffness to the sail surface.

More downhaul flattens the sail, decreasing the curvature, and reducing the Bernoulli effect and thus lift that the sail develops. It also makes the sail more twitchy, since it becomes less and less like an airfoil and more like a door. Adding downhaul also makes the leach more loose and floppy, effectively reducing the chord (width) of the sail, and making the effective sail size smaller.

If the wind has picked up and is steady, the easiest / quickest fix is more downhaul. Be aware that adding downhaul increases the need for outhaul too. Many many beginners do not use enough downhaul. But sailing is almost always better with a smaller sail, so IMHO rig down if the wind is stronger and steady. That, or learn how to hold a big sail and go to Warp speed.

If the wind hasnt really picked up, and its just a few big gusts, flattening the sail will help. The idea in such situations is to have the biggest sail you can that will allow you to survive the gusts. Experience / technique pays off big time.

GEM

carl · Joined: 25 Feb 1997 Posts: 2674 Location: SF bay area

There is a circumstance that the mast foot position would make a difference, when the COE of the sail is behind the
fin. This condition feels uncomfortable because youre compensating for the COE in an unusual spot. This is why you should move the mast foot forward with bigger sails. Increasing the downhaul on any (modern) sail will make the sail more forgiving in the gusts because the sail twists off better to expell the extra power on the top of the sail. This is especially noticeable on larger sails. More downhaul also flattens the sail wich reduces power. I have noticed small wave sails get more twitchy when over-downhauled, but larger, racier sails seem to get much more stable and forgiving when downhauled to the max.
As for sheeting in, it
is much better to commit your weight to
the harness rather than sheeting in with the front hand or back hand.
With big sails, this is much easier with a seat harness.

gemoore · Joined: 14 May 2001 Posts: 494

I didnt mean the mast foot position wasnt important, its just that I dont think mast foot position has a big impact on the mechanics of getting slammed to the deck in an overpowering gust. Sitting down on the harness to sheet in is a good idea. Whether youre way behind the rigs pivot point or close to it (i.e., the mast base), the mechanics are an aftward change in CoE relative to your connection to the rig (harness and hands).

Regarding the aft-of-fin CoE: you cant go in a straight line in if the sails CoE isnt aligned with the fins CoE, because the board will either turn or spin out.

Regarding downhaul, were all in agreement but using different syntax. When the sail twists off, the leech is not contributing to the lifting surface. As a result, the aft limit of the lifting surface moves towards the mast, decreasing the chord of the sail. Since most of this effect occurs at the peak (head) of the sail, the effect is to reduce the power in the peak - as you properly note.

Little sails indeed are often twitchy when flattened, but the better ones are a lot rangier when on a softer/shorter mast. Its easy for me to say go spend more money, but one thing to think about is to have the smallest possible mast thats compatible with your smallest sail. In so doing, youll maximize the high-wind limit of your rig and your ability to control the sail.....and therefore youll be more likely to enjoy sailing in the windiest conditions.

GEM

fgelsomini · Joined: 27 Sep 2000 Posts: 4

Interesting thread - I seem to encounter this in gusty or overpowered conditions as well when using bigger sails.

Im using an 8.0 on a softer mast than the one recommended (25 IMCS 460 vs. 29 IMCS 490). Would the softer mast Im using also contribute to the sails COE going goofy on me and preventing control?