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Old 28-10-2012, 15:39   #1
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Paradoxical Lee Helm Puzzle

Here's a mindbending brainteaser: (at least, it bent my mind, the one time it happened to me)


You're close reaching in strong conditions, in waters sheltered from the open sea. It's a reasonably tender sailboat with a tall rig, so you're under triple-reefed mainsail only.
There's a bit of a tendency to round up as you'd expect but it's not unmanageable if you feather the sail, in a measured way, in the puffs. (Which is a big advantage of any sail which sets on a boom: in contrast with a jib, which gets more camber as it's eased)

Then conditions deteriorate further. The wind picks up.
A killer gust comes through. The wind strength is enough that you're no longer fighting to stop the boat rounding up, in fact (still on the same heading relative to the wind) it's all you can do to stop it bearing away.

! ? ! ? ! ? ! ?

There is no problem with the rudder or the steering mechanism, or any failure of the sails or rigging. If anything the seas are flatter, as the crests are now being bulldozed off into the troughs.
What's happening, and what can you do about it?

- - - -

(By way of back-story: I woke early this morning, my mind travelling pleasantly over sailing lessons which nature had dished out to me at various times. This one took place so long ago that, lying there enjoying the dawn chorus of native birdlife, I could no longer remember exactly what happened, or (more worringly) what it had taught me.
As wakefulness gradually seeped back, I mercifully gained the upper hand on the nocturnal ringwraiths, and patched together a memory which seems to ring true, even though the situation seems so counter-intuitive.
But I'd had a sneak preview, in my groggy state, of a time when some of those lessons learned in magical moments would be largely lost.

So ... I thought I'd put it down here, and if it helps one other sailor, so much the better. I've always felt that counter-intuitive outcomes are the ones really worth analysing and trying to understand, given that informed intuition is a reliable guide to the rest.
And if it doesn't persuade anyone else to consider a new angle, at least I have the consolation that when (not if) I can no longer summon it up, I might still be able to use Google-assisted recollection!
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Old 28-10-2012, 16:37   #2
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Re: Paradoxical Lee Helm Puzzle

[QUOTE=Andrew Troup;1070538]
<snip>
What's happening, and what can you do about it?

- - - -

The way I see it: Ultimately in strong enough winds 2 things come into play: 1)Your center of pressure moves forward as you feather the sail and 2) the mast itself becomes a source of significant drag. Both of these will be ahead of your center of lateral resistance (keel/center board) which will push you off the wind. With only a small main up you will only get so far off the wind until the sail regains its significance then the center of pressure again moves back and you will once again tend to round up. (That is if the main hasn't shred in the process.)

If the wind is strong enough it will overpower the helm. Trying to steer becomes useless. If these conditions were expected to persist then this might be a good time to consider deploying a drag device or para-anchor.

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Old 28-10-2012, 17:05   #3
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Re: Paradoxical Lee Helm Puzzle

It would help to know the details of the boat - Length, rig height, keel type, keel depth, keel chord, keel location. Jib setup as well (masthead, 7/8 whatever)

There are a lot of variables here. Here in my mind is the main variable.

The rig is balanced with pressure on the full main and jib. You remove the jib and you remove pressure forward of the center of rotation which is somewhere on the keel.

Now you have a full main with a center of pressure aft. When you reef that main the center of pressure moves towards the tack, forward and down.

At some point depending on design the center of pressure and rotation are pretty darn close to each other. Then add in whatever is happening to the relationship between the keel, mast, center of pressure, direction of sail forces & rudder and I can see how a closely balanced boat can gain lee helm when reefed with no jib.

I am not a boat designer but imagine you want a fast boat. You'd like that boat to be balanced with a full rig, at full power and have little "extra" weather helm.

Start moving the center of pressure forward and you can have predictable but not necessarily desirable results.

Aerobatic airplanes design a lot of stability out to improve turning rates around all axes. Maybe something similar with that boat.
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Old 30-10-2012, 18:35   #4
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Re: Paradoxical Lee Helm Puzzle

Anyone else care to come up with an alternative explanation, or to toss around ideas on the ones already suggested?
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Old 30-10-2012, 18:59   #5
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Re: Paradoxical Lee Helm Puzzle

It appears you were heading towards a Chinese Gybe "perhaps". Was the boat healing to windward as she lost control? My guess is that rolling motion allowed your mast to lean out to windward, moving your COE outside of the hull itself.

Notice that on the image below, the mast has moved to windward of the hull.
If you were to draw a vertical line down to the centre of the boat, you will see that there is more sail to windward. (on the starboard side in this case). The COE will be momentarily well outboard of the hull and lee helm would be increased

http://surfabike.files.wordpress.com...t-20-59-06.png
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Old 30-10-2012, 19:18   #6
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Re: Paradoxical Lee Helm Puzzle

Ummm... Starboard side is leward in this case.

In fact this is a crazy photo - Why haven't they gybed? This could go south in a hurry... If that boom gybes the chick on the coach roof and perhaps the bowman are gonners.
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Old 30-10-2012, 19:46   #7
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Re: Paradoxical Lee Helm Puzzle

Well, it seems to me all the ingredients are now in the pot for the solution I came up with. Anyone see a way of combining them which is plausible? I'm not saying mine's conclusive: if you can come up with a better one, you'll have done us all a considerable favour.

To address a few specific questions or possible misunderstandings:

1) Dan: If I'm right, the causes are more general than your quest for detail would be needed to support.
Another way of saying this: I think this is potentially observable across a wide range of the variables you mention....
although I hope never to experience this in a boat larger than say 40', because by then I'm guessing it would be blowing "harder than any sail can withstand". Unless the variables I did mention were at an extreme end of the spectrum...

2) Ozskipper: We were close reaching. No heeling to windward, and I can say that with some emphasis ;-)

3) "Ex Calif: Ummm... Starboard side is leward in this case."

Are you inferring that from their 'heel'?

Or are you looking at a mirror image of the photo I'm seeing? I'm seeing their Mainboom to port, spinnaker pole to starboard....

I'm interpreting this as a boat, broad reaching or much more likely running on starboard gybe, snapped at the maximum windward extent of one of a series of what theorists often refer to as "self-excited downwind rolling" and sailors call "Death Rolls"
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Old 30-10-2012, 20:00   #8
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Re: Paradoxical Lee Helm Puzzle

I find myself agreeing with Ex-Calif's first post. Sail presure very far forward combined with wind presure on the forward portion of the hull causing the lee helm.
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Old 30-10-2012, 20:01   #9
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Re: Paradoxical Lee Helm Puzzle

Andrew,

What you are describing is exacally why boats headed into really bad weather should always have a storm trisail (if you are not familure with them, do a quick google search). Because of the trisail's design it brings the COE back to roughly the same as a normal main, while reducing the sail area significantly, and lowering the COE. Obviously they are pretty inefficient sails, but in 40kn of breeze who cares.
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Old 30-10-2012, 20:03   #10
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Re: Paradoxical Lee Helm Puzzle

Well I see a boat on Starboard tack (boom to port) sailing by the lee (spinnaker to starboard).

The deduction of windward vs. leward is the position of the spinnaker which will always fly downwind. i.e. the wind is coming from the port quarter.

Port is windward and starboard is leward - Unless I am totally screwed up...

The skipper may be doing this on purpose, I have no idea. But my vote is to gybe the pole and boom if skipper plans to maintain this heading.

Barring that if I am on the bow or coach roof I am clipping on.
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Old 30-10-2012, 20:07   #11
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Re: Paradoxical Lee Helm Puzzle

A simpler explanation perhaps - We can all agree basic spinnaker handling is pole square to the wind and boom square to the wind.

Looking at the angle between spinnaker tack and clew the pole needs to come way back and the boom needs to go way forward - i.e. impossible.

The spin sheet is in front of the forestay...

The mainsail is inverted.

If the boom was gybed and the pole swapped the guy would be hauling the spinnaker back in front of the boat. I am guessing they are sailing 160-170 and the gear needs to be rigged for port tack.
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Old 30-10-2012, 20:20   #12
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Re: Paradoxical Lee Helm Puzzle

Quote:
Originally Posted by Ex-Calif View Post
Well I see a boat on Starboard tack (boom to starboard) sailing by the lee (spinnaker to starboard).

The deduction of windward vs. leward is the position of the spinnaker which will always fly downwind. i.e. the wind is coming from the port quarter.

Port is windward and starboard is leward - Unless I am totally screwed up...

The skipper may be doing this on purpose, I have no idea. But my vote is to gybe the pole and boom if skipper plans to maintain this heading.

Barring that if I am on the bow or coach roof I am clipping on.
Main boom to starboard is Port tack by right of way. So this is a port tack boat sailing by the lee.

From online Colregs
(b) For the purposes of this Rule the windward side shall be deemed to be the side opposite that on which the mainsail is carried or, in the case of a square-rigged vessel, the side opposite to that on which the largest fore-and-aft sail is carried.

I see a big wave that probably affected steering, combine that with the spin too squared aft for the conditions, so too far to starboard side of the boat causing it to round down.
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Old 30-10-2012, 20:25   #13
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Re: Paradoxical Lee Helm Puzzle

CalJohn - I corrected the boom position in my post (turned myself around for a minute).

However this isn't a colregs exercise.

This boat is on starboard tack and the wind is coming from the port quarter.

Skipper needs to steer to starboard - #1 rule, stay under the spinnaker - if anything he is rounding up - down in this case is towards the spinnaker.
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Old 30-10-2012, 20:26   #14
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Re: Paradoxical Lee Helm Puzzle

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Originally Posted by Ex-Calif View Post
A simpler explanation perhaps - We can all agree basic spinnaker handling is pole square to the wind and boom square to the wind.

Yes except when there is too much wind, then pole forward enough to keep from rounding down.

Looking at the angle between spinnaker tack and clew the pole needs to come way back and the boom needs to go way forward - i.e. impossible.

The spin sheet is in front of the forestay...

The mainsail is inverted.
I don't think it's inverted, I see it wrapped around shrouds

If the boom was gybed and the pole swapped the guy would be hauling the spinnaker back in front of the boat. I am guessing they are sailing 160-170 and the gear needs to be rigged for port tack.
I don't think that they're trying to sail it that way, its an out of control moment. Either the rudder will bite and he'll come back up, or he'll chinese jibe.
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Old 30-10-2012, 20:27   #15
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Re: Paradoxical Lee Helm Puzzle

The clew is in the first paragraph of the OP's post. The intentional misspelling of clue in my 1st sentence is a 2nd clue.
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