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    • #25454
      JimSumberg
      Participant

      A124 Helene’s cockpit instrumentation is pretty basic — echo sounder (absolutely essential for the Essex rivers) and compass (incorporating an inclinometer). I have been keeping an eye on the inclinometer over the last few weeks, and have come to the tentative conclusion that 10-15 degrees of heel is the sweet spot. This got me thinking: is there any way to determine this more systematically?  I assume that more or less heel will affect the sailing efficiency of the hull shape, keels and rudder, but in practical terms. how much does it really matter?

      Thanks for any insights!

    • #25455
      Mike Dixon
      Participant

      Interesting observations Jim.

      Not being a qualified naval architect, (but I know who is) I’m not sure I can comment.

      From my dinghy sailing days, I recall that the main object was to always keep the boat upright so as to maximise drive from the (upright) sails.  Maybe an old wive’s tale, but it seemed to work.

      The other drag is the rudder; there are those who advocate a tad of weather helm. I would have thought that neutral helm (ie with the rudder right onthe centere line) would be best so as to eliminate drag.

      Then the really keen racing Atalanta skippers would advocate lifting one or other of the keels.  My advanced years precludes this option.

      To answer your question though, there must be someone who can interpret the underwater shape of the 10 to 15 degree heeled hull, displacement, sail plan etc and reach a definitive answer.

       

      Mike

    • #25458
      JimSumberg
      Participant

      Mike,

      And there is more to this story. Being based in Manningtree, I can go out for a sail around HW, and if I stay away from the channel below Mistley, I will never have much more than 2m depth (and at times, and in some places, quite a lot less). My strategy is to lower the keels only partially, on the basis that some additional leeway is better than going aground. On the water this morning I got to thinking… The keels are there for both ballast and leeway control. So, in a light to medium breeze, given that there are two keels, and that the depth is adjustable independently, how much would we expect leeway to increase if one or both of the keels are raised (i.e. by a third or by two thirds)? Would there be some way to test this using a chart plotter?

      Jim

    • #25461
      AOA
      Keymaster

      ‘Le Bateau Ivre’ David – it would be very interesting to your views on this as a practising Naval Architect.

      My own views are not based very much on Naval Architectural theory but rather the conclusions drawn (rightly or wrongly) from my sailing over the years.

      On the matter of angle of heel. As Mike says keeping the boat level is the ideal in dinghies where it is possible to move the centre of gravity sideways by moving the crew. For keelboats this is more difficult because the relative weight of movable ballast (crew) is a much smaller proportion of boat weight, even lined up on the windward rail wellies waving. Also as the boat heels the centre of buoyancy of the underwater volume moves to leeward creating a ‘righting moment’ when coupled with the weight of the boat acting at the centre of gravity nearer the centreline. There is a simple explanation of ‘righting moment’ at https://www.sailboat-cruising.com/righting-moment.html

      So, that righting moment works against the wind pressure to ‘stabilise’ the yacht in equilibrium at an angle of heel determined by the relative forces. In our cruising yachts I believe your empirical 10-15 degrees is a sensible sweet-spot, although it might vary slightly in other designs. Any more than this and the boat presents a less favourable shape to forward motion, bigger bow/stern waves, and more drag. Also helm angle tends to increase – more drag.
      I have often found when sailing against similar boats in a bit of a blow that although it is exhilarating to sail ‘gunwale under’ a reef to return to that sweet-spot makes you go faster.

      As to leeway increasing with raised keels. As you suggest you could measure leeway with different amounts of keel by noting the angle between a constant course steered and the ‘course made good’ on a plotter or GPS (corrected for Variation and Deviation of course). Ideally in identical conditions – waves, point of sail, wind strength, helm.
      I would expect other factors to be more impactful in light winds but as the wind increases progress will definitely be negatively impacted with less keel.
      Unless of course the extra keel is buried in the mud!

      Nick

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