Subject: [harryproa] +Re: BD sailing
From: "cateran1949" <cateran1949@yahoo.co.uk>
Date: 10/11/2009, 9:19 PM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

Adding the jib gives more drive and shifts the centre of leeway resistance forward, so the boat starts to pivot around the lee bow. When you consider the leeway resistance offered by the lee bow, one can see the slight shortening of the lever arm by bringing the rudders forward is not such a big deal. The fact that putting the front rudder down makes things worse suggests that the rudders being further away from the lw hull are making things worse.

I agree that an ogive section fore/aft symmetrical leeboard could work providing there are no other issues we haven't thought of.

--- In harryproa@yahoogroups.com.au, "k_s_oneill" <K_S_ONeill@...> wrote:
>
> Hi
>
> Starting off with a caveat here too; I'm not a Harryproa sailor, and they're a bit different than my boat or the other local Texas proas I have sailed on. So all the following could be wrong. Onwards:
>
> I would think too that moving the rudders forward is the problem; less lever arm means more angle to get the same 'torque', as you say.
>
> I don't think the CLR of the lee hull is at the middle of the hull, as I think you are saying. The CLR of any symmetric foil, even a very shallow foil like a Harry lee hull, is well forward of the center of area, more likely 25% to 30% of the way back from the bow. Harryproas are so round-hulled that the hull lift may be easily dominated by the rudder lift, but I think the CLR of the hull is still where it is in most proas, way way forward of the center of the submerged area.
>
> The only thing that confuses me about this story is the jib making things worse (!!) rather than better. That's just weird. I guess since it's a balestron rig you can't sheet the jib by itself to get going, so the effect of raising the jib is to make the sail area bigger and to move the CE forward a little, and I guess the added power of the former overwhelms the improvement of the latter.
>
> How hard would it be to arrange a leeboard on the lee side of the lee hull, using one of Tom Speer's symmetric sections? I bet that would fix it, it's just a question of if you want to put up with the trouble of handling it, and the added complication on the boat.
>
> Kevin
>
> --- In harryproa@yahoogroups.com.au, Luca Antara <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.
> >
>

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