Altering trim in light airs?

Discussion in 'Sailboats' started by dinghydan, Mar 24, 2014.

  1. dinghydan
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    dinghydan New Member

    Hi guys, I have been struggling to understand something for a long time. Anyone know why in light winds it pays to slide body weight forward in the boat to sink the bow and lift the stern? Having learnt about the various drag components I understand that at low speed (light winds) the main component of resistance is skin friction not wave making resistance or pressure drag, so surely by sinking the bow (narrowest part of boat) skin friction is increased??? I have been thinking about this for so long! Any ideas? :D
     
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Hi Dinghydan, Welcome to the forum. Actually, sinking the bow does indeed reduce wetted surface and thereby reduce drag so that speed is inhibited the least. This is because generally there is more wetted area at the back of the boat than at the front of the boat when the boat is in level trim. And as boats move forward, they naturally want to trim down by the stern, thereby increasing wetted area. So by moving the crew forward, you are actually lifting some of the stern out of the water, thereby reducing wetted area, reducing drag, maintaining a higher speed.

    Eric
     
  3. Doug Lord
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    Doug Lord Flight Ready

    When I was racing we would move forward in light air AND heel the boat substantially for the reasons Eric says.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Welcome to the forum Dan.

    Hone your sailing skills in light and very light air, it will reward you with repeatable wins on the dinghy courses. Anyone can sail pretty good in 5 to 15 knot wind strengths, but you really have to work to gain advantages in light winds, which tends to separate the men from the boys around the buoys.
     
  5. tom28571
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    tom28571 Senior Member

    Ditto the thoughts already expressed. Another important factor is the force of the center of effort of the sails, which is always well above the boat hull. This is obvious in high wind where the crew may be forced to move their weight aft to prevent sail force from driving the bow down and becoming a submarine. Drag of the hull and foils creates an opposite aft force down low and adds to the bow down force.

    These are less apparent in light wind but are still there and the conditions are reversed with centered (normal) crew position too far aft which may cause dragging of the stern. The point being that you have to be aware of the conditions and adjust accordingly. This includes sail adjustments for different conditions also.

    Heeling the boat can also reduce wetted surface and drag but its a search for the optimum. Heeling also exposes less sail to the wind and lowers the sail driving force but makes it easier to get a good sail shape due to gravity on the sail. Its a constant search for little improvements that dictate performance in light air.
     
  6. gggGuest
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    gggGuest ...

    Bethwaite volume 1 is very good on all this stuff, but basically the nearest you can get the immersed part of the boat to half a sphere then the lower the wetted surface will be. You're right that the bows are deeper and narrower than ideal, but the aft sections are a *lot* shallower and flatter - in both directions - than the ideal, so weight forward is generally good. How far depends on the shape of each particular design. But really it isn't so much sinking the bow as lifting up part of the wide flat stern.
    Julian Bethwaite gives a tip that's often forgotten - to use your ears. By listening to how much noise is coming from the wash in front and behind you it will give you clues as to where energy is being lost. It seems to me that if one is much noisier than the other then possibly you need to move a bit in the other direction.
     
  7. SukiSolo
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    SukiSolo Senior Member

    One other benefit is by pushing the bow down a bit the mast becomes more vertical in the londitudinal plane. This is faster in light air too, but don't forget to allow twist too and careful control of kicker tension and sheet is required when the wind is strengthening and dying away. Most people leave too much tension on when the wind dies.

    Another good 'test' is to sail through some small waves. It shows up the extra 'pushing' around of the hull by flat and maybe fat stern sections. Noticeably less if trimmed a bit bow down. You can overdo it, though in really light stuff say 1-2 knots wind speed it is worth lifting the transom out slightly. Boat dependent, you need to find an optimum as others have said. As PAR says a bit of breeze is easy, the trick is partly to keep concentration and smooth movement. A lot of people point too high before accelerating and also don't forget to alter sheet/boom angle if sailing on a decent swell where 15 deg of angle of attack variation from the wind is common on one wave.
     

  8. rwatson
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    rwatson Senior Member

    I didn't see mentioned the number one reason for heeling the boat in light airs - to steer the boat with minimal use of the rudder.

    The objective is to attempt to keep the rudder totally straight ahead, thus avoiding drag created when the rudder is turned.

    You can adjust the amount of heel on a small boat by shifting your weight to the sides, and this can be used to steer the boat without using the rudder at all.

    I have never heard of a boat sailing faster because the mast is not as vertical. edit - just for steering and sail shape. The amount of heel should be as small as possible in light airs apart from these two considerations.

    The other light air trick is to tension (usually loosen) the outhaul on the mainsail boom, to ensure that the telltales on the outer, curved face of the sail are flowing horizontally. This is one of the major performance improvers in light airs.
     
    Last edited: Mar 29, 2014
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