Hi Mike,
where we disagree is the possibility of the lateral resistance of the lee bow being so far forward that the front rudder is actually bringing the lateral resistance back. When the jib comes in, in spite of the COE coming forward, the extra loading on the bow shifts the CLR more. The boat is OK balanced by the two rudders when the boat is sailing flat and therefore the distance between the rudders is adequate. ie the moment arm from the front rudder to the aft rudder is enough to balance the moment arm from the COE to the front rudder.IN fact the difference in ratio of the moment arm of the rear rudder to the front rudder and the COE of the rig to the front rudder is actually improve with the closer together rudders. as the front rudder is brought proportionally closer to the COE compared with the rear rudder. As the load on the bow increases the relative moment arm from the aft rudder to the lateral resistance forward compared to the moment arm of the COE to the forward lateral resistance decreases. The difference in 800mm aft is not the 20% if the bows are taking up so much of the forward lateral resistance but more like 8 to 10%.
The boat manages to sail without the front rudder down. Is this because there is more sideways drift and the rear rudder can keep up, or is it that the induced drag of the front rudder creates that little bit extra luffing torque which more than compensates for bring the lateral resistance aft? Or is it a bit of both?
I notice a slight difference in tacks on my cat between having the ww or lw CB down but the CBs are much further aft, the COE is further forward and further over. When the vectors are drawn up it is quite significant the difference compared with the Harry as the CBs are only just forward of the COE. In the case of your tri, tilting it back just a tad may take it to a neutral position and cause all sorts of confusion as it oscillates between weather and lee helm. In the Harry the rudder has to handle a fair bit of weather helm, and only does so if well balanced. This is the reason for the change of rudders stylae as the rudders weren't balanced if partially raised and the weather helm became too apparent. ANY extra luffing torque is significant
The 800mm inwards of the rear rudder is even more significant as it is working much harder and therefore causing significant induced drag
We agree that a central CB should make a difference. I surmise that a leeboard will be marginally better than a lee hull centreboard or lee hull ww board and could be the difference between working or not unless the board is made extra large to completely dominate the lw resistance of the bow. In my Harry the rudders are both close to the lw hull and further out towards the ends so we can't really find out with this what is really going on, but it would be interesting to lash the rudders to the crossbeams next to the lw hull and find out. I will try and organise some lugs to attach some arrangements to allow this.
Also the 800mm across
--- In harryproa@yahoogrou
>
>
> 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/
>
> ---
>
> 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.
> >>
> >>
> >>
> >>
> >
> >
> >
> >
> >
> >
> >
> >
> >
>
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