Arttu
The rudders and keels have much greater influence on the CLR than the hull. This is particularly the case with a long, shallow round chine hull like the lw hull on a harryproa. With rudders perpendicular to the hull the 18m proa will sail sideways at 2kts in 10kts of wind. Side drift is much less when the rudders are in line with the hull.
We have found that steering on the 18m proa is easier with the leading rudder than the trailing rudder up to the point where the position indicator is not visible due to the spray off the flat reverse bow.
There is one little quirk that we have noticed; particularly with the leading rudder. It has a slight instability about 5 degrees either side of centre that is exaggerated by any slack in the linkage. I reason this is due to a change in induced drag as the top edge of the blade starts to vortex shed when the top of the rudder is not against the hull - we have seen the vortex being shed off the top edge. If the hull bottom was flat rather than round there would be no gap opening up as the blade is turned and no dramatic change in rudder efficiency that causes the instability.
Above 12kts, moving the leading rudder quickly through the unstable angle can take your feet out if you are not braced. So steering from the leading rudder is very aggressive. The trailing rudder does not seem to be quite as aggressive through this unstable region - possibly because the blade has a reduced end plate from the hull when trailing.
If the CLR was only 1/3rd of the distance from the bow then the boat would still have weather helm when sailed with ww hull to lee. That is not the case. Under that condition the boat has strong lee helm.
Rick
On 28/06/2012, at 7:21 PM, Arttu Heinonen wrote:
Hi,
I learned something that explains why steering with fwd rudder is so hard.
The reason is the that the COE of a hull is at 1/3 of the hulls length. So the arm of the fwd rudder is half of the arm of the rear rudder, IF there are no other foils in the water.
Rocker does not change this fact, except if the situation is that the stern is lifting up and there was rocker.
A hull behaves like a foil in a water. The same works in big ships too, even if they are box shaped.
Now that there is "easyrig", it is impossible to make a harryproa neutral to steer ( rudders ponting ahead, still pointing straight) without any extra sails sheeted to a hull.
A friend from http://www.napa.fi/ gave me this lesson.
Well, a B2 stealth bomber does fly, why not a harryproa couldnt sail ;)