David, Aquarius Sails builds production boats as advertised and shown on their website. Aquarius will also build whatever size boat you want as long as it can be built with production tooling. In response to questions about a 20ft ARC boat from sailors considering a distance racing circuit, I pointed out to them the capability built into the original 22 tooling. A 20ft boat can be built in the ARC22 mold by simply placing the transon immediately after the rear beam. This boat is a true 20ft ocean racing catamaran with the mast 1ft aft of half hull length and the tall eliptical hull forward for sailing in big waves. This forward hull shape allows for the maximum forward migration of the instantaneous center of bouyancy and the rear beam being on the transom allows the crew to move further aft freely; no rear beam and tillers in the way. This maximises the distance betweem the CB and CG for maximum pitchpole resistance. This increase in forward stability leads to more reaching speed and downwind speed with the spinnaker especially in big waves. Bill
To see if I could learn something from your reply I did a rough calculation of the size the rudders would need to be to operate in the bucket. I considered a catamaran of similar dimensions to the SC-17 sailing upwind at 6 meters per second, and looked at the effect of rudder area on resistance (assumed NACA 63-009 section).
I learned that while increasing the area (and span) of the rudders would reduce resistance, the effect of operating in the low drag bucket was minimal (~ 10 N) compared to the total resistance. The rudders also would need to be impractically large to provide the required lift at a Cl of only .20. Did not see any obvious minima to optimize around, but I did not (could not) consider the effect of leeway angle on hull residuary resistance.
So, my rudder question was a rather silly one.
The 63-0xxx are nice sections for avoiding ventilation at high speed and load, which is a much more important consideration in this case (guessing that's why it was chosen).
Thank you for indulging an amateur with your explanation.
Hi Colin, Your question was not silly. Look at what you learned. That is very worthwhile. To windward the best we can do is have the daggerboard and rudder operating at leak lift to drag ratios. To calculate L/D, simply divide Cl by Cd at points along the drag polar. I think you will find that peak L/D occurrs at or very close to a lift coefficient value of one, Cl = 1.0. Read several points along the polar. Calculate L/D. Plot L/D vs Cl and against angle of attack. This will tell you some interesting things. Good luck, Bill
Bill…could you briefly explain the “Drag bucket” concept? You always seem to have explanations that are technical enough to give a good grasp of a concept without making it necessary to have a graduate level of Engineering knowledge in order to comprehend. I feel in this case like I missed the foundation that the rest of the discussion was based upon...as if coming into the middle of a conversation already in progress.Thanks...
Section peak lift to drag ratio occurs - just as you said - at a lift coefficient close to 1.
Then I included a term for the hull total resistance (using Dr. Norwood's estimation method) and the induced drag of the rudder (since it has finite span). The optimum lift to drag ratio for the hull and its rudder occurs when the induced drag is equal to the sum of the hull total resistance and section drag - which makes sense. The induced drag is decreasing with speed while the other drag terms are increasing so there is a local minimum in the curve of resistance versus speed when these terms are equal.
The more rudder area I gave it the lower the speed at which the optimum lift to drag ratio occurred. The lower the speed at which optimum lift to drag ratio occurred the better the optimum lift to drag ratio got.
The rudder really is working hard no matter what. The optimum lift to drag ratio for the whole boat happened when the rudder was just about to stall no matter how large I made the rudder. That happens at a lift coefficient - as you said - of about 1.
At the high angle of attack the rudders operate it is really nice they can be angled relative the hull.
Thank you for your suggestion and encouragement. -colin pitts
I just meant the low drag portion of the lift vs drag plot for the section.
The 6 digit series airfoil sections have pressure distributions that encourage laminar flow over a good portion of their surface at low angles of attack. Laminar flow reduces the skin friction so they have lower drag when it is present. It shows up as a "bucket" on the lift drag plot for the airfoil section.
This turns out to be not so important in the overall lift to drag ratio of a catamaran. I could not see the forest because there was a tree in the way.
