I have to agree with Luca on the effect of moving the rudders in from the bows towards the center.

  The original setup had 9200 mm between rudders, with the new one having 7600 mm.  This could create two problems:


1) Reduced turning moment. 

  The new distance between rudders represents a 20% decrease in turning moment that the rudders can apply without stalling.  The stalling caveat is the important part of that sentence.  Once the rudder stalls, it loses its ability to be effective at speed.  A 20% decrease is not a huge change, but if that takes you from laminar flow over the rudders to a stalled and turbulent flow over one or both, it completely changes the boat's performance.  You can go from powerful steering to very little steering once you cross over that threshold.

  It might be possible to counteract the reduction in turning moment by lengthening the rudders 20%.  I would personally try this first because it should require the least change to the boat's structure.  That extra 20% might take you from stalling to running smoothly, and therefore make the rudders more effective at speed.


2) Change in position relative to CLR of the lee hull

  I can't tell where the CLR of the lee hull is, especially given the different hull shapes of the harry's out there now.  But if it's well forward of the mast when the rig is powered up, the forward rudder might have a tough time doing double duty as leeway prevention and steering. 

  If you take one extreme case of the CLR being even with the rudder, the rudder will have almost no steering power and will be handling most of the leeway prevention.  That will make the boat quite difficult to handle -- the rudder likely won't add much turning moment, even if turned past the point of stalling.  With the jib moving the COE forward as well, and further depressing the bow, the effect worsens.

  If you take the other extreme, with the rudders at each end, they'll share the leeway prevention function more equitably, and the forward rudder will have no problem turning the boat.

  One of my boats is a 17.5' monohull with a centerboard that is perfectly balanced at many wind strengths when the board is straight down, but becomes a lumbering beast that sometimes won't tack, even with a backwinded jib, if the board is raked back 30 degrees.  That 30 degrees probably yields a change in position of the board's CLR of 30 cm or less.  So it doesn't take much to take a nicely-balanced boat and make it hard to handle.

  This would be a tougher problem to correct while keeping the rudders on the beam; hopefully lengthening them would take care of the issue.  A symmetric daggerboard/leeboard should also do the trick.

---

  I partially agree with Robert that moving the foils to windward will add drag that will contribute to rounding up.  There's no doubt that the drag will be there. 

  However, I've sailed cats with dual daggerboards and experimented with having either one, or both, in the water, and never noticed a difference in the steering balance.  The drag of the windward hull will probably dwarf the drag created by the rudders.  And if the windward hull doesn't cause the boat to round up (as predicted by the naysayers), then moving the rudders 800 mm windward probably won't make any noticeable difference.

  Please feel free to correct me if I've missed something.

       - Mike

  

Luca Antara wrote:

 

Hi Rob and Everyone,

An explanation of my theory and a caveat that I gave to Rob: I'm a mechanical  engineer, not a sailor.

The torque generated at the CLR by a rudder may be small but I figure it is the same as the opposing CofE torque. Let go the tiller and a boat luffs up naturally, hold the tiller to maintain direction on a reach and the rudder is presumably generating the same torque as the opposing CofE torque.
If lack of torque IS the problem, then consider the equation Torque = Force x Distance (T= F x D) where force is the force generated by the rudder and distance is the perpendicular distance of the rudder force vector from the CLR. Rudolf has moved his rudders forward 80 cm and 80 cm to the windward, both causing a reduction in D. To maintain the same torque opposing the CofE the rudder can either be made larger or the angle of attack can be increased. Increasing the angle of attack might induce stalling. 

Going the other way, moving the rudders more to the stern will increase D and therefore force (and angle of attack) can be reduced. 

On the subject of where the CLR is, I had drawn it for Rob as being somewhere around the centre of the boat and along the fore and aft axis of the lee hull. On further thought it occurs to me that while it is somewhere near the symmetrical axis of the boat, it is may actually be somewhere in between the two hulls as they both resist lateral motion. Assuming the windward hull is 2/3rd the length of the lee hull then the CLR should be approximately 1/3rd (or is it 2/5th?) the distance between the lee hull and the windward hull. This gives a larger value for D, and a so larger torque for the same force F. Up until now I have been unable to get my head around why Harryproas seem to steer OK in spite of the large hull to windward; could this be a reason?

Cheers,

Robin.

•  1
  New Members
•  4
  New Photos
•  2
  New Files