Full or modified full keel rudder attached...why not vertical rudder?

[Stumble]

Originally Posted by roland stockham

I may be completely wrong on this. My understanding is that an elliptical shape on any wing reduces turbulence and drag produced by tip vortexes. Long keel boats are designed to sit on the keel alongside so can't have this shape but some taper helps. Also if done right it can produce significant lift when going to windward heeled. This is definitely true on the Folkboat, I have occasionally seen positive numbers for leeway from them. Remember to think of the keel as a second wing flying through the water and balancing the wing flying in air rather tha a static counterbalance.

You are right.... But.

Elliptical foils are more efficient (more lifts less drag) for the same size, but they have other issues. When an elliptical foil stalls it stalls all at once, there's is no progressive loss of attachment as the rudder stalls from the bottom up.

In a verticle rectangular rudder the tip stalls first as water flow is directed under the bottom, the closer to stalling the rudder gets the more water flows under instead of across. So the tip stalls first and the stall progresses up the rudder slowly decreasing rudder effiectivness.

On an elliptical rudder the elliptically shape ensures that it isn't always faster for water to flow past instead of under the tip. This means that the entire rudder stalls at exactly the same time. There is no progressive loss, no slow and stead shudder, you are just sailing along and bam the rudder is gone.

So yes elliptical rudders from a pure efficiency standpoint are better, but they may not be suitable for every boat. You really need a better driver who is paying more attention to justify them, because they can lead to unexpected broaches and loss of control.

[Benz]

This is a feature on many full-keel boats that have no reference to any racing rules. But the simple answer is two-fold: Fist, a transom with a slight slope is more attractive than a perfectly plumb one, and a stern-hung rudder (the only proper sort) has to follow the sternpost.
Second, a rudder hung that way will naturally try to find the midships position all on it's own due to gravity. It doesn't need the extreme slope of an Alberg or an Allied Seawind to do this: just a little will do.
Another neat feature of these rudders, rightly done, is that they're biggest at the very bottom, where the cleanest flow of water (furthest from surface turbulence , etc) exists.
I don't think reducing wetted surface had anything to do with this originally, since full-keelers don't have the superstitious fear of wetted surface that came later, when designers were doing all sorts of ridiculous things to try and innovate.
Ben
zartmancruising.com