weather helm with heel - question

Discussion in 'Boat Design' started by CrunchyFrog, Jun 5, 2014.

  1. CrunchyFrog
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    CrunchyFrog Junior Member

    We all know that increased heel leads to increased weather helm. My question is why does this happen?
     
  2. Olav
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    Olav naval architect

    I'm lazy, so I just go and give you a link to this thread where the issue has been discussed already. ;)
     
  3. daiquiri
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    daiquiri Engineering and Design

    There are several things involved in the increase of the weather helm when a boat heels, but the most prominent one is this:

    Weather helm.gif

    Hope the drawing is clear enough.

    Cheer.
     
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  4. CrunchyFrog
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    CrunchyFrog Junior Member

    Thanks, that makes sense.
     
  5. messabout
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    messabout Senior Member

    OK Crunchy, you have opened a can of worms. Daquiris example is one of the reasons for weather helm. It is about the position of the sail force and the influence of the lever arm. That is explanatory as far as it goes, but when you are boomed way out you are not necessarily heeled. You will have some weather helm from the leverage or offset thrust from the sails. That is not the primary cause of helm when the boat is leaning over to leeward as it might be on a beat to windward where the sails are sheeted close to the centerline.

    Think of the boat being cut laterally like bread slices. Now you have a lot of sections, each of whom have a center of area (technically, the centroid). If all of those centroids line up in a nice straight line then weather helm will not be much in evidence. With a typical rounded bottom on a dinghy like an O'day Day Sailor, for example, The centroids do not make a straight line when the boat is heeled. They do not line up and they represent a curve. That curvature forces the water to move in a curved pattern and that causes one end of the boat or the other, usually, but not invariably, the aft end, to want to steer the boat to windward. I did not pick the Day Sailor as a lonely offender It is a very good dinghy but it must be dealt with. The phenomena occurs with the likes of a Sunfish, Lasers, 49ers, FDs, and many others too.

    For dinghys in general, it is best to sail them flat. That's why there are hiking straps, trapezes and strained gut muscles. Sail 'em flat and the centroids line up nicely. Heel them and you have to put in some corrective rudder which slows the boat. This is not a phenomena exclusive to dinghys. It is in full force with many big boats too.
     
  6. Milehog
    Joined: Aug 2006
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    Milehog Clever Quip

    A primary way of turning kayaks is to lean them them in the opposite direction of the desired turn.
     
  7. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Could be, but only if we want it so. :)

    The question was "what is the reason for the increase in weather helm when the boat heels?" The answer can be very articulated and omni-comprehensive one, or it can get to the physical core of the problem.

    A broad answer should take into account the hull shape, the volume distribution, the position of the appendages and the shape (type, rake, twist etc.) and longitudinal position of the rig.
    A short answer should IMO bypass the secondary elements and go to the core, by addressing principal contributors to the increase in weather helm, which are: sails and keel. As shown in the drawing attached to my previous post.

    The shape of the canoe-body, especially if it is a modern round-bilge hull with a modest draft, is of secondary importance for the generation of the side force. See, for example, the classical and easily available text by Oossanen "Predicting the Speed of Sailing Yachts" published by SNAME: http://www.oossanen.nl/beheer/wp-co..._-_predicting_the_speed_of_sailing_yachts.pdf. It has a chapter dedicated to the hull-generated side force, which shows that the canoe-body contributes with no more than approx. 10% to the total hydrodynamic side force generated by the heel.

    Sure, if there were no appendages, then the contribution of the canoe body would be 100% - like in the cited case of kayaks.

    Cheers
     

  8. jehardiman
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    jehardiman Senior Member

    If we need more worms, we could also drag out the old metacentric shelf theories. ;)
     
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