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Old 23-04-2013, 00:32   #16
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Re: Excessive weather helm even on reach

Well, it really sounds like a classic case of blown out sails. Draft gradually increasing in depth and creeping aft, you not paying attention until one day it blows a bit harder than you expect and wham... too much heel, not enough drive and a big weather helm developing..."all of a sudden".

The cure is obvious, but expensive! In fact, we have just gone through this with a mainsail that looked sorta ok but had 42,000 miles of use. The new one is sure a lot flatter!

Cheers,

Jim
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Old 23-04-2013, 00:53   #17
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Re: Excessive weather helm even on reach

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Originally Posted by s/v Jedi View Post
No, the obvious solution is to reduce heel.
And how to you intend for him to do that when he is reaching so deep his main is pressed against the shrouds?
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Old 23-04-2013, 00:54   #18
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Re: Excessive weather helm even on reach

I agree that heel is the single most important cause of weather helm, but it's not just about hull form.

But before I address that: I think its misleading to look at a side view of a yacht rig, sitting bolt upright, which is what generates the widespread contention that correcting helm imbalance is done solely by moving the centre of effort fore and aft.
In other words, by changing relative sail areas, where the draft is, and which sail is sheeted harder.

As proof that this does not get you far: You can get weather helm with NO mainsail up, on a normally balanced boat, if you put up a genoa which is too big for the windstrength. The diagrams of centre of effort cannot explain this, unless they resort to 3D and 'get with the program' in respect of heel.

The weather helm resulting from heel is not just because of the asymmetrical underwater shape caused by heel, which generates a hydrodynamic turning force.

Despite this hydrodynamic force dropping away when the yacht hits a bad sea and slows, the weather helm will continue to build if there is a simultaneous gust.

The reason is that the driving force is a vector acting at the center of each sail.
Focussing for simplicity on the mainsail only, this force can be thought of as a rope, essentially perpendicular to the sailcloth at that location, being pulled, as if by an obliging towboat, and trying to pull the boat with it. Because it's a rope, not a rigid pole, it can only pull in the direction it's pointing.
The keel resists the sideways component, so that the boat moves forward.

Despite being perpendicular to the sail at the point where it attaches, the 'rope' is slanted slightly forwards even when closehauled, because the centre of effort is ahead of the deepest point of the draft, so the sailcloth at that point is curving inboard towards the mast.
If the notional rope were not slanted forwards, (say because we've inhauled the boom past amidships) the boat would make no forwards progress.

But here's the thing: because of the heel, the rig is not standing up, as it is in the simplified diagram in the book on boat design; it's out beside the hull.

So the force in that 'rope' acts on the hull through the rig, as if it were tied part way up a spanner, with the hull as the nut.

Essentially the rig is trying to slew the boat on the surface of the ocean, just as a towboat with a rope tied to the sail's centre would do.

ON EDIT (having now read posts which appeared since I started composing this)

If the cause of the heel is the genoa, one way to alleviate it and flatten the genoa (provided it has a high clew) is to pole it out to leeward on a reach.

(As Dockhead says, but is unable to explain, a genoa which is too full can cause weather helm, purely because it induces too much heel in relation to a given amount of forwards drive)

Failing that, if the problem IS heel induced (you don't give enough info to gauge that) and yet the main is appropriately sized, you need to reef the genoa or change to a smaller headsail to stand the boat up.
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Old 23-04-2013, 01:15   #19
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Re: Excessive weather helm even on reach

That stopped all farting in church!
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Old 23-04-2013, 02:14   #20
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Re: Excessive weather helm even on reach

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Originally Posted by Jim Cate View Post
Well, it really sounds like a classic case of blown out sails. Draft gradually increasing in depth and creeping aft, you not paying attention until one day it blows a bit harder than you expect and wham... too much heel, not enough drive and a big weather helm developing..."all of a sudden".

The cure is obvious, but expensive! In fact, we have just gone through this with a mainsail that looked sorta ok but had 42,000 miles of use. The new one is sure a lot flatter!

Cheers,

Jim

If the sail is otherwise in good shape, maybe it could just be re-cut ...
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Old 23-04-2013, 02:53   #21
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Re: Excessive weather helm even on reach

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Originally Posted by Andrew Troup View Post
(As Dockhead says, but is unable to explain, a genoa which is too full can cause weather helm, purely because it induces too much heel in relation to a given amount of forwards drive)
Thanks for the explanation, which sounds logical, but I have noticed this strange (to me) phenomenon even with a moderate heel angle.

As you said, you can have weather helm even with the main furled altogether, which refutes the idea that the location of the center of effort is the only factor involved in generating weather helm. In a situation similar to Steve's, where I had a lot of weather helm on a beam reach, even taking away the main altogether (as an experiment) did not solve it. What helped in my case was trimming the yankee well by getting the sheet lead right and closing up the leech. After fixing the yankee, I was able to pull the main out again, reefed well down, and with very hard vang, very hard outhaul, and very hard halyard, and fairly loose sheet, traveller let all the way down, sailed quite well. Maybe something like this would help in Steve's case.

There was little heel when I was messing around on yankee alone, and I had the impression that the yankee was generating not too much drag, but too much lift -- i.e., too powerful shape --which was pulling the boat around, but this may have been a hallucination on my part.

My experience may not be that useful to anyone -- my boat has an extremely powerful rudder, taller than I am and semi-balanced, which is capable of making up for a multitude of sail trimming sins, so that I often don't feel my crap sail trim through the wheel until the wind gets well up. I have been working on it by watching the rudder indicator on the autopilot head. There are still many mysteries.

In my case, all these problems occur in a good blow -- 25 - 30 knots and more. In a nice F4 - F5, I have never had any difficulty in trimming my sails for a reach.
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Old 23-04-2013, 03:27   #22
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Re: Excessive weather helm even on reach

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Originally Posted by Rakuflames View Post
If the sail is otherwise in good shape, maybe it could just be re-cut ...
Yes, RAku, sometimes that can help. However, if the material is stretched enough to cause the problem, it is likely to continue to stretch after recutting and soon one is back to where you started.

Might be interesting to get Island Planet's take on this if he is around... his advice is usually quite sound IMO.

Cheers,

Jim
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Old 23-04-2013, 03:39   #23
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Re: Excessive weather helm even on reach

Regardless of how much the headsail has blown and the draft has shifted back, the vector of the headsail will still be in front of the CE for the sail-plan.

With a reefed main and full genoa your CE should be forward of optimal causing LEE helm - likely even if the main was over-trimmed.

However you mention weather helm. The only variable left would be the heel angle as others have mentioned.

DO you normally leave a full genny with reefed main? Some boats can carry a full genny with the first reef, some cant.

I would do the following to reduce heel:
1. Ensure traveller is all the way out
2. If you have an adjustable backstay bang it on hard
3. Try either of the extremes on your vang tension - either on hard or really loose to open the leech
4. Most important - if you are unable to reduce heel and correct weather helm using sail controls then you need to reduce sail

edit: I am assuming that halyard tension on your headsail will have little affect on draft position since you describe what you think is blown sail, but increased halyard tension on both main and headsail can help shift CE forward as well.
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Old 23-04-2013, 04:37   #24
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Re: Excessive weather helm even on reach

too much head sail, roll it it up some or put a smaller one on
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Old 23-04-2013, 04:55   #25
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In the typical case you would not reef the main until the genoa is struck. When the main is reefed the headsail is typically down to some sort of non-overlapping jib.

Most cruisers have long forgotten what a flat sail is. Blown out baggy is the rule. Baggy sails are the leading cause of trim problems. As Andrew pointed out all that drag is way off to the lee side pulling the bow to weather. If the sail cannot be trimmed flat with only a few percent belly (5% ?) it is too old.
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Old 23-04-2013, 05:21   #26
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Re: Excessive weather helm even on reach

I will try a few of the suggestions flattening the main more, yes we have been fairly heeled over recently more than i am used to for reaching ie apparent wind 90 degree plus or minus 15-20, just the last few hundred miles we have done in these conditions and its been baffling me, wind has been maybe average 12-16kn with gusts in the low 20s, but im struggling to get the rudder sensor to show less than 4-6 degrees , wehn normally i can get it to zero fairly easily on a reach an maybe 2-4 degrees close to the wind.

We recently crossed the atlantic and we did vristually from gibralatar to cananries to cape verdes to antigua under poled out genoa, so i am wondering it it has bagged slightly depowering the genoa and too much in the main, but i still think surely a bagged out genoa should be less of an issue with the wind on the side, than on the nose?
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Old 23-04-2013, 05:41   #27
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Re: Excessive weather helm even on reach

I wouldnt regard 4-6 degrees as "excessive" weather helm in many boats, 10 degress yes. Id regards 4-ish degrees as about right. You want some weather helm for safety

Dave
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Old 23-04-2013, 05:49   #28
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Re: Excessive weather helm even on reach

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Originally Posted by goboatingnow View Post
I wouldnt regard 4-6 degrees as "excessive" weather helm in many boats, 10 degress yes. Id regards 4-ish degrees as about right. You want some weather helm for safety

Dave


when reaching, anything less than 5 degrees whether helm on a cruising boat is acceptable. You can find that when going to weather you will actually have less helm than when reaching.

Still - you need to address your heel, even if your helm is not bad your leeward drift would be with excessive heel and your VMG to your course will be dramatically reduced with alot of heel.
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Old 23-04-2013, 05:59   #29
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Racing, we trim for about 5 degrees of rudder on a weather leg. Powered up reaching might have a bit more just because of the huge forces at play.

On a performance hull there won't be any annoying helm forces though. The rudder may just point up on its own. On some boats the down wash from the keel is about 5 degrees. Or, with a performance rudder one puts some angle on it simply to add some lift to weather - it's down there creating drag it might as well work a little.

While racing a good main trimmer may be trimming to the tiller or wheel position - one eye on that.
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Old 23-04-2013, 06:07   #30
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And how to you intend for him to do that when he is reaching so deep his main is pressed against the shrouds?
Reduce drag. Flatten sails. These are the very basic sail trim skills. But like said before, when the sails are past their useful life, they need to be recut or replaced to attain correct trim again. When they are otherwise in good condition, an improved construction might be usefull, like tri-radial cut, hydranet material, tapedrive etc. so that they keep their shape longer, extending their useful life.
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