keel and rudder design..

[sailingdaniel]

While reading and trynging to understand keel and rudder I get confused. The 4 didget seem to be the standard answer for a crusingboat when I browes the net.. And that was the profil that was chosen as rudder in the exampelboat in principals of yachtdesig. When I try to look at the graphs and read aboute the different profiles in my mind something like the 63 ore 64 series seem to do a better job. U can have a thicker profile whit less drag, good for ballast in a keel and strength in a ruddershaft. And if the maximum lift is less couldent u make the fin slightly bigger whith out getting mere drag than a smaller 4 didget fin ? Is it just that the surfes have to be smoother that’s opt for the 4 diget.? Having a thicker profil whith less drag would make the “dragbucket” wider as well. I cant do the math wery good and understand it that way, but from reading and looking at graps it seems strange to me.? What am I missing? Hope u understand my English and what I meen, why is the 4 didget profile often the standard answer?
Daniel

[jehardiman]

Simply put, because of wave orbitals and vessel motion, the inflow AoA and velocity into the keel and rudder of a normal vessel is too unstable to allow the formation of the "drag bucket". While there may be limited specific circumstances to support the use of 63xxx or 64xxx NACA profiles, they should not be the first profile considered. YMMV, DNBEYROTI.

[sailingdaniel]

Thanks jehardiman.
I think i get what u are saying. i diddent get "AoA" ( maybee my english). Would it help of the 63 , 63 profil was of a higher aspectratio. ie shorter an deeper and threre fore traveling at a higher speed (reynolds number).
Ore if the boats prim thing wasent upwindsailing but 95 persent reaching ore running as a crusing boat does , beacause it seems to me to be a big different in drag?

[jehardiman]

AoA is angle of attack. Unless the vessel is of very high speed (30+ knts) the normal AoA of the keel due to roll or wave orbitals would be outside of the drag bucket. It is not a matter of being on or off the wind. Actually, higher aspect keels are even worse for 63/64 series foils because the tip velocities due to roll and pitch are greater. A high aspect board would be better served with a 00XX section.

[sailingdaniel]

, I need to read more for sure.. I also read in this book that for the keel they have chosen a 10.5% 63 profil that goes to a 17.5% 65 section at the tip, (thats the 40 fot cruser/racer exampel.. Do u have any tips foor webpages where i can get usful info, im in a place where i cant get any other book at this time.. But , thansk for youre answers.