Does size matter?

[colvic]

Is there a formula or method for calculating the correct size of rudder for a boat?

Is there a performance difference between flat sheet steel rudders and the shaped hollow one?


much obliged


Phil

 

[oldsaltoz]

G’day Phil,

Don’t know if a rudder formula is available, but they get smaller as the boats get faster.

The shaped rudders reduce drag so you go faster(under sail) of use less fuel(Stinkpotting).

Avagreatnewyear Old Salt Oz…/forums/images/icons/cool.gif

 

[Anchorite]

Re: Shape matters too

Assuming it's a raggie: from a side view of the underwater shape measure
the (flat) area. Rudder should be about 10% of this area for displacement cruiser.
Shape is also important, a rudder is a wing so high aspect (height compared
to length) is an advantage. The height factor is usually imposed by the boat
shape. Square-ish corners are better than round shapes. And a fat rudder is
better than a plate: at 15° angle of attack a fat rudder has 50% more lift than
a plate. Fat means a width of up to 20% of length: the fattest part is 30%
back from the leading edge, then it tapers to (nearly) nothing. This wing shape
is called NACA 0020 with very precise coordinates: for our purposes anything
near (i;e; flare + taper) will give a satisfactory result.You can try Google (for Naca) but it's heavy going.

 

[colvic]

Re: Shape matters too

Thanks for that, just what I was after.