Squirrelly feeling == lee helm?

Discussion in 'Sailboats' started by Skippy, Feb 20, 2005.

  1. Skippy
    Joined: Nov 2004
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    Skippy Senior Member

    Some time ago, I jimmied up a really clunky storm rig for my Laser, about the size of a radial rig. I took an old worn-out full-size sail about halfway apart, and put it back together shorter and flatter. Dropped a 3-4 ft piece of lumber down the lower mast section, dropped in the upper section upside down, and widened the upper end of the sleeve to recieve the collar now near the top of the mast. Maybe the biggest problem is that it makes the mast almost perfectly straight. That shouldn't help the plan shape any, and should move the CE forward. I remember when sailing it, it felt "squirrelly", or unstable. Any time I allowed the boat turn a little in either direction, it would tend to veer off in that direction even more. Maybe more so on a run, but that's basically what I remember. Is that just a simple lee helm, or does it sound like there's something else going on? Thanks much for replies.
     
  2. gonzo
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    gonzo Senior Member

    A little weather helm has two advantages: firstly it uses the rudder to produce lift, and secondly, because the tiller is pulling, gives you better feel and control. Neutral helms oversteer.
     
  3. Skippy
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    Skippy Senior Member

    Thanks gonzo. The regular sail is too big for heavy wind, so I decided to make a smaller sail out of the old one. It's just a cheap fix & experiment. I've never sailed in a boat that had the helm misadjusted, so I don't know if that squirrelly behavior is due to the helm or something else in the sail shape. I guess "oversteering" might be an accurate way to describe it.
     
  4. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    Bullish for lee helm?

    Hi Skippy

    My guess is that you are dealing with lee helm, but you haven't given me much to go on. Why do you think the CE has moved forward? Did you shorten the boom? Did you shorten the foot of the sail? None of this seems at all clear in your post. I suppose if I were more familliar with Lasers I would know.

    The reason I suspect lee helm is that with a weather helm the amount of course variation is relatively small except when sailing off the wind. What happens is that the boat rounds up and the sail flutters. Once it flutters, it loses all or most of its drive. And since it the drive thats causing the weather helm in the first place, the weather helm diminishes quickly once the boat rounds up.

    If the weather helm is severe enough, the boat will be just as useless as if it had severe lee helm. Such a boat (I once had one) will whip right into the wind as soon as the sail produces any drive at all. And this can happen long before there is enough forward movement for the rudder to be effective. The more wind there is, the worse the situation becomes. If the weather helm is small to moderate this problem becomes at worse mannageable and at best desireable.

    With little to moderate lee helm, the situation is much different. When the sail fills, the bow falls off. When that happens, some rudder is used to correct it. If it is enough, the boat sails straight. If it is too much, the boat starts rounding up. When that happens, the sail starts losing some of its drive. Now a little too much rudder becomes way too much rudder and the boat rounds up even further.

    If, on the other hand, not enough rudder is used, the boat falls off even further. This puts more drive into the sail which causes the boat to fall off even more. As you can see, this sets up a wide feedback loop that covers everything between too much rudder to too little rudder, which, of course, leads to too much excitment. At least on windy days. Most design texts I have read warn against allowing even any lee helm at all. And probably for this reason.

    I would imagine the precceeding scenario would effect your boat from any were from tack to a broad reach. Sailing any further down wind than that, The boat should become more stable as the increasing drive of the sail, as it rounds up, tends to point the boat back down wind. Boats that were on long blue water passages used to be sailed primarily down wind and were diliberately, though temporarily, set up with extreme lee helm (only head sails set). But, it is interesting to note, that they were usually set up this way with two head sails of equal or near equal size with one on the port and one on the starboard side. Maybe this was to insure adequate sail area, but I suspect other reasons.

    I know from both theory and practical experience tha a typical fore and aft rig tends to have a strong rounding up moment when saild down wind. That is probably because the the CE of the sail is to the lee and the CLA is to windward and once the boat rolls or changes its heal angle, there is either too much or too little correcting rudder. This, especially with an inexperienced helmsman, can start a vicious feed back loop where the boat starts swaying and rolling. And the more it sways, the more it rolls. And the more it rolls, the more it sways. Throw in some waves and you can add pitching to the equation too to the extent that the boats motion resembles that of a mechanical bull.

    I presume that you are an experienced helmsman, at least for your boat. What I think could be happening is that the behavior further up wind with your boat is now so changed that it is throwing your down wind judgement off.

    At least thats my guess (which is all this whole post really is).

    Best of luck

    Bob
     
  5. Skippy
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    Skippy Senior Member

    sharpii2: Why do you think the CE has moved forward?

    Normally the Laser mast is very bendy, so the luff & the leech both angle up & in toward the center. To shorten the mast, I overlapped the two mast pieces 5-6 feet rather than the usual 1 foot, making it much too strong to bend. That pulls the upper area of the sail forward. I tried to compensate somewhat by putting in plenty of roach.

    sharpii2: With little to moderate lee helm ... this sets up a wide feedback loop that ... leads to too much excitment.

    Lee helm can also make the boat's steering “twitchy” and unpredictable.

    Boy, there sure are a lot of things that go on with sailboats. The sail and the rudder tend to turn the boat in opposite directions around the keel/hull CLR, which acts as a fulcrum. The force on the sail depends on the heading, but with the boat's momentum, the force on the rudder doesn't. So if you want the helm to be stable, then when the boat rounds up, you need a net leeward restoring moment to make it fall off. This requires that either (a) the force turning the boat off the wind increases, or (b) the force heading it up decreases. Since the sail's force decreases and the rudder's force is constant, we're stuck with (b): The sail must be the element producing the windward moment. So the sail's CE must be leeward of the CLR, and the rudder must be resisting the windward moment, which is what you have with a weather helm. Ya learn somethin' new every day. :)
     
  6. Doug Lord

    Doug Lord Guest

    veering off either way

    Since you have a Laser this is probably irrelevant but here goes anyway: if you had a rudder that had the center of pivot AFT of the quarter chord point the symptoms you describe could happen regardless of weather or lee helm. When you turned such a boat it would probably feel like it wanted to keep on going which ever way you turned..
     
  7. CT 249
    Joined: Dec 2004
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    CT 249 Senior Member

    When Ian Bruce and Hans Fogh designed the Radial,they specifically introduced a flexier bottom section that bends easily, moving the C of E aft to keep the boat balanced. It also allows the Radial to have decent gust response; the original small Laser sail, the M rig, used a different top section and had the stiff lower section of the big rig.

    Ian says that “The minute we started taking roach out of the (M) sail (to reduce the area) we had to make sure the sail had a closed leach, to get the balance right- but in a breeze the closed leach made it actually harder to sail upwind than the full rig." Initial trials in light winds were so successful that the rig was launched without extensive testing.

    With the Radial, Ian says, "Hans and I were talking one day and I was looking at my old Finn spar one day….right about the gooseneck we planed them in, to get them to hinge back. That’s when I started talking to Hans, I said what we really need to do is to use a bendy section and peel off the back end of the sail."

    "I went back to the original section on the original weekender which sailed in the original Teacup regatta, which happened to be the original section of 4 mm, 2 and 3/8" outside diameter irrigation tube. We kept the normal top section, the reason being the longer the top section the easier it is to bend the bottom section."

    So the Radial is designed to bend aft to keep a bit of weather helm (it feels like a lot of weather helm because the rudder is raked, but it's not too much when you're sailing well) and to de-power. The 4.7 has a permanently-bent lower section to achieve the same effect. Unfortunately your stiff two-in-one section has the opposite effect.

    I wonder how much a bit of un-anodised water pipe costs and if it's strong enough?

    PS - many of the Laser's problems can be cured by using more vang, and keeping the boat flatter.
     

  8. Skippy
    Joined: Nov 2004
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    Skippy Senior Member

    That sure makes it sound desirable to be able to reposition the mast. The AR on the 4.7 must be very low.
     
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