Rudder Trailing Edge.

[785boats]

I came across the concept in a yachting forum some time ago & was wondering if anyone had any theories on the concept. ( see pic) Would the wedge shaped rudders on power boats benefit ?
The theory is that the trailing edge is angled at 45 degrees so that the vortices coming off the trailing edges mesh with each other instead of hitting each other.
This creates a more narrow turbulence behind the rudder which means less water being dragged by the rudder. So with less drag the boat goes faster.
As the drag on anything through the water is an exponential curve in relation to speed my thoughts were that on a yacht with a fairly low drag co-efficient at say 10mph to a powerboat with a much higher drag at 80mph any drag reduction would result in higher speeds.
Any one ever tried it?
Cheers.
Paul.

[Ub Hauled]

I have been playing with it but I have no solid numbers yet... but your theory seems to have some logic behind it.

[NitroVal]

I think the reason powerboats use a a more blunt trailing edge is the force needed to turn something cutting through the water at high speeds is greater than that on a sail boat. Its hard to explain , I'll try to draw something up to prove the point..

[NitroVal]

Here's my rendering of how I understand the theory. Obviously the name of the game in sailing is efficiency and streamlining, however sailboats dont need to take sharp turns at 100mph, plus they have the added steering of tacking and using the sails- one of the reasons a sailboat's rudder is small in comparison to hull length.
I shot an email off to one of my father's friends who holds a PhD in fluid dynamics and works for North Sails, so maybe he can shed some light on the subject

[Ub Hauled]

another question, what's wrong with using the "wing shape" (sharp trailing edge) in a rudder?

[Ub Hauled]

Is this why or am I way off?

[Ub Hauled]

anybody?

[domwilson]

Been thinking about it myself. I had one made up. Haven't tested it yet.

[Ub Hauled]

Dom, would you let me know the results once you're done testing please? I am curious...

[domwilson]

A couple more of the edge.

[NitroVal]

You need a blunt trailing edge or else you wont be creating enough high pressure to turn a large hull at speed

[domwilson]

The blunt end is the trailing edge. The sharper edge is the leading edge.

[785boats]

I was under the impression that the wedge shape creates less drag in the turns on the lee side of the blade unlike the foil shape which creates a lot of turbulence, lift, & drag ,as in your sketch Ub Hauled. I think a thin foil shape for SAW runs would be ok.
I'm sure the trailing edge of a wedge must have more drag than a thin rudder in a straight line & I just thought that by using the 45degree angle at the trailing edge the speed might increase in the straights but still have the benefits of the wedge in the turns.
Bear in mind that drag quadruples when the speed is doubled.
The amount of power required to double the speed is the cube of the speed i.e. the the amount of power required to double the speed goes up by a factor of eight to overcome the drag.
That's why every little reduction in drag we can find results in more speed.
Cheers.
Paul.

[sailr]

yew gize kep cumin up wit some weeeerd stuff! whey abuv my eyeque!

[Ub Hauled]

yew gize kep cumin up wit some weeeerd stuff! whey abuv my eyeque!

funny...