Post by colin on Nov 6, 2013 10:32:34 GMT
As the breeze gets up there is an inherent problem with rudder rake angle. On the beat, raking the rudder makes the steering very heavy. However, due to the aggressive rocker on the hull we have to keep the bow up to plane. I have estimated this to be in the order of 20 degrees. Therefore, if you have a vertical rudder blade on the beat it will rake forward by 20 degrees on the plane. This moves the blade centre of lift way too close to the stock pivot axis resulting in a boat that is very sensitive to rudder movement.
Taking the Skinner GRP blade as benchmark (shown on the right) I found that splitting the difference gave a serviceable compromise between steering heaviness beating and stability on the plane. If you run something like the Skinner profile I would suggest something like 10 degree rake as a starting point, (the picture shows the blade at this rake).
The new blade is a full quarter ellipse profile with a vertical trailing edge. Even at high levels of planing angle, the centre of lift stays reasonably constant. The results in a very light steering load[img src="[/img]"] on the beat and a relative level of calm when high speed planing.
The blade in the middle was an experiment to look at reducing wetted area by taking a bit of a hit on the other drag components (the section is NACA 00 15). I have included it as it shows a simpler profile than the quarter ellipse that is a complicated shape to make accurately. If the chord at the tip of the blade is around 50% of the chord at the top it approximates to an elliptical profile characteristic (remember it is the loading distribution that is elliptical not the profile).
[/img]
Taking the Skinner GRP blade as benchmark (shown on the right) I found that splitting the difference gave a serviceable compromise between steering heaviness beating and stability on the plane. If you run something like the Skinner profile I would suggest something like 10 degree rake as a starting point, (the picture shows the blade at this rake).
The new blade is a full quarter ellipse profile with a vertical trailing edge. Even at high levels of planing angle, the centre of lift stays reasonably constant. The results in a very light steering load[img src="[/img]"] on the beat and a relative level of calm when high speed planing.
The blade in the middle was an experiment to look at reducing wetted area by taking a bit of a hit on the other drag components (the section is NACA 00 15). I have included it as it shows a simpler profile than the quarter ellipse that is a complicated shape to make accurately. If the chord at the tip of the blade is around 50% of the chord at the top it approximates to an elliptical profile characteristic (remember it is the loading distribution that is elliptical not the profile).
[/img]