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post #824 of Old 05-21-2013
grumpy old man
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Re: Full or fin keel?

I asked a very simple question of Paulo when he claimed the tandem keels were designed using CFD studies. "How do you know that CFD was used to design those tandem keels?"
Getting a succinct answer was anything but easy.

My point was this:
The tandem keel (if we are going to call it that) Has been around for about 20 years now. It was tried on an AC boat with some success but other AC boats did not use it. There's probably a very good reason for that. In my consultations and work with Laurie Davidson and his AC effort I was exposed to some experimental AC design ideas. Most, the "hula" for example, were abandoned along with the tandem keel and the forward rudder. And certainly by any criteria the AC boats had restricted draft. It was a rule. One of the things they found with the tandem keel is the distance between fins was critical. The aft fin is operating in the "bad air" of the forward fin, i.e. turbulence off the forward fin and an increased angle of attack. Think about racing and trying to sail straight up the stern of the boat ahead. It NEVER works. So in order to get some clean flow of water with a reasonable angle of attack on the aft fin the two fins have to be separated. The basic rule, as I recall, for the aft fin to have "clean water" was 7 times the tip chord of the fin. You can clearly see that in the photo Paulo posted of the AC tandem keel.

But in the new, tandem keels the two fins are squeezed together with barely one chord length separating the two fins. With the two fins this close together I can't See how the aft fin can have any clean flow over it. It has to be operating in turbulence and with an increased angle of attack, i.e. increased drag. And, unlike a sail, you cannot trim this fin. You cannot change the angle of attack.

So if these new "compacted" tandem keels are working as well as the promotional material claims, then I would like to learn more about them.

Paulo said the tandem keels were the product of CFD studies but he was assuming that. I'd like to know if they were and I'd sure like to see the evidence so I can learn something. I am always looking for ways to reduce draft while preserving windward ability.

"Breakthrough" keels come along regularly. The Scheel keel was an attempt as was the Reijo Salminen wing keel to name two. Both worked, sort of and mostly because they lowered the VCG. But they are clunky keels and not good for boat speed. If the tandem keel preserves stability and windward ability on an equal level with a keel of reasonably deep draft then I would like to know more about it.

I am not going to buy into every piece of promotional material I read from any designer or any builder, even if I do read it on the internet.

I am waiting this morning for a new custom design client. He wants a variation of the 62 SLIVER project I have going now. Undoubtedly the question of draft will come up. I need to be prepared to present him with workable, efficient options. Talking the talk won't work for me. I have to be be ready to walk the walk.

Right now I believe I'll go walk my dogs.

Please visit my blog. It's fun to read.

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