If everything less complicated fails - how about building the rudders as articulated wings which make very effective rudders. on a new tack/shunt lock each rudder at the angle appropiate for wind and lateral trim and steer only with small flaps at their trailing edges.
Heinrich
--- In harryproa@yahoogrou
>
> Hi Robin,
> These are things I have been nattering on about for quite a while now. I tend to agree with your concepts. I did an analysis of the torque generated by the various sails and the bits of lateral resistance. I resolved the torques around the lee bow, the ww bow, and the centre of drive of the rig using the centre of drive of the Ballestrom rig to be just behind the mast. The different resolutions gave the same answer but different perspectives
> What it came down to is how much load is on the rear rudder and the importance of keeping the ww hull windage down (as Rob keeps emphasising)
>
> My argument for the importance of foils being closer to the lw hull is supported by:
> 1/ Putting the froward rudder down did not improve things. If there wasn't significant luffing torque from the drag of the forward rudder being away from the centreline, it should have reduced by at least 20% the load that the rear rudder needed to handle.
>
> 2/ The difference of 800mm fore and aft is very little in the scheme of things when you consider the distance from the rudder to the centre of lateral resistance of the bow. It can't be more than 10% and is probably about 8% extra load on the rudder as far as this distance is concerned.
>
> I see the problem as: the induced luffing torque from the rudder being further from the lw hull increases the restoring force needed, further increasing the induced drag and the therefore the luffing torque and you have a nasty feedback till the rudder stalls.
>
> Putting a decent centreboard close to the centre of the drive of the rig, means the lever arm of the rear rudder becomes much longer relative to the lever arm of the drive of the rig, but it has to be a significant size to completely dominate the bow. This would require either two centreboards or a single ogive section that pivots fore and aft. This could have ventilation problems. A good compromise is to have the centreboard central or even a mini keel providing the rudders are close to the lw hull. Whether the extra drag of the extra foil outweighs the reduced induced drag of the bows and rudders is still a question but would give an extra margin in nasty conditions. Having the rudders slightly further back would be an advantage with a mini keel or centreboard.
>
> In the case of two rudders only, if the rudders were simply placed right next to the lw hull, then it is probably an advantage to have them off the crossbeams as the forward rudder would be closer to the centre of drive and the lever arm ratios would be better but would be slightly less stable in steering.
>
> The problem is exacerbated as the wind builds and the drag of the ww hull adds to the luffing moment. Therefore, I feel the rudder needs to come as close as possible to the lw hull, it only needs to have a clearance that allows 30 degrees of movement from the centreline. Even better would be to have it on the lw side but there may be other complications.
>
> My Harry concept is a stretched Harry fore and aft and so relatively skinny thus reducing the luffing torque of windage of the ww hull in strong winds. I have two masts which allows the centre of drive to be brought well forward in the case wanting to point in skinny waters or sixty knots of wind. This has many disadvantages compared with a ballestrom rig but is a compromise for the waters I plan to sail in. I feel an optimum distance between the rudders would be about 8m as it would bring the lateral resistance of the forward rudder back and improve the lever arm ratios but will be about 9.2 due to other restrictions in design
>
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>
> --- In harryproa@yahoogrou
> >
> > 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.
> >
>
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