Friday, November 11, 2011

Winged keels?

If you didn't read that title carefully, you saw "winged heels" and are probably visualizing Mercury's boots.

But that's not what I want to talk about.  Instead I want to discuss winged keels.  I mean what is the big deal, after all?  Sure, they're modern, sure they're cool, and "every boat must have one".  But why, exactly?

It's all about end effects.

Huh?

OK, now I have to build something in your mind here.  Please give me some latitude and play along... Imagine an airplane wing.  Regardless of which theory you consider to describe how it holds the airplane up in the air, all the theories condense to the singular fact that there is lower pressure on top of the wing than on the bottom.  The airplane is held up by suction.  So what keeps the high pressure air under the wing from just flowing up into the low pressure area above the wing?  Well, on the leading and trailing edges of the wing, it is momentum - that wing is moving right along, and the air at the front and rear of the wing just doesn't have a chance to flow "upstream" to the low pressure area.

But what about at the ends of the wing?  Well, at the inboard end, there is the fuselage in the way.  Ah, but at the outboard end, there is... nothing.  And air does indeed flow up from under the wing around the end to the top.  This is the source of the "wingtip vortices" that cause the spacing out of flights at airports.

So, if you were an aeronautical engineer, how might you stop this flow around the end of the wing?  Well, you might put up a fence.  On the end of the wing.  In point of fact, this is exactly what is done on modern airliners.

OK.  So enough about airplanes... but hold that thought.  The keel of a sailboat is amazingly similar to an airplane wing.  It is "flying" thru the water, and is required to provide lift, to keep the boat from sliding downwind.  The analogy is really very good.  So then, what keeps the water from flowing over the bottom end of the keel, from the high pressure side to the low pressure side?  Nothing...

Thus enter the winged keel... put up a fence.  However, since each side of the keel is alternately the high pressure side and then the low pressure side as you tack, the keel has to have a fence on both sides,  explaining the now-familiar shape.

The design is so effective that significantly less ballast is required, the force being replaced with more effective lift.  Thus the weight of the keel can be reduced.  And the wings serve as a good place to stash lead, down there at the very bottom of the keel, so the draft of the keel can be reduced.  And then, since everything in nautical design is connected to everything else,  the wetted surface of the boat is decreased because less total weight is being carried.  And the boat goes faster.

It really is a good idea.

So then why then did winged keels take so long to appear?

See the First and Second Corollaries of Salnick's First Law.
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6 comments:

RLW said...

Ummm yes and no...

Wings on keels and rudders make them much more effective as you have described quite well. So they do allow you to sail closer to windward.

On the other hand they don't add anything to the righting moment of the hull/rig so won't affect the amount of ballast needful to keep you mast pointing up.

bob said...

RLW -

Oh, but it does indeed. I failed to make the point clearly enough above.

By their design, the wings are not thin, filmy structures. Instead, they are thick, containing a large volume of lead. In fact, strictly from a ballast point of view, the wings are a bulb keel. So, like with a bulb keel, you can have shallower draft with the same stability, or more righting moment with unchanged draft. You pays your money and you takes your choice.

Bob

Drew Frye said...

But there is a serious down side, sufficient to have prevented me from adding these to my LAR keels....

They are HELL if you plow them into mud. They look like an anchor and act like an anchor. As a result, in the very shallow Chesapeake, they have not cought on. Folks just use shoal keels.

bob said...

Drew -
I never thought of that! A wing keel does look like a Bruce anchor!

Bob

jap said...

We have found that in the modern 6 meter class where we are forced to deal with a low aspect ratio vertical keel by the rule, a conventional bulb with thin, high aspect ratio wide wings that are flat on top and pitched down slightly work the best upwind and don't drag as much downwind.

Lead winglets may be an acceptable minimum draft solution where a deep keel is not practical, but lead winglets low aspect ratio, high wetted surface, form drag, and low angle of attack make them inferior to conventional keels just a little bit deeper. Otherwise, I think you would see more of them on the market. A better solution is a dagger board or centerboard to augument the shallow keel.

The pictured designs stem from the early 80's 12 meter attempts and were obsolete shortly after their introduction by Australia II in 1983. The fashion stuck in some production boats.

bob said...

jap -

I am not clear about the point you are making... but if you are saying that wing keel design is evolving and that the randomly chosen picture above does not represent the state of the art (at least as far as race boat design is concerned), you will get no argument from me.

bob

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