Unusual Keel design

[SURV69]

I am not a nautical engineer, but I once stayed at a Holiday Inn ... back in 1980 ... SO, I have a lot of experience ...

From a racing standpoint I would think it would make tighter turns with the gap and on broad runs, I would think the disturbed water over the forward fin, would make the water passing by it as something akin to a keel existing in between the twos(acting like one longer keel), and there would be minimal friction between the two keels and since the water is disturbed, I would say the water that passes the second keel, being already disturbed would be less friction that if the keel had been complete.

I know this is a little wordy and I know I'm probably reaching in consensus, but the fact is ... I got a really nice room and slept in late ...

[darylat8750]

Originally Posted by Jim Cate

I can understand the possible structural advantages, but as yet, no one has addressed the question of why there would be a better lift/drag ratio with this configuration. I'm still wondering! The one with the saildrive between the two foils is really odd to my eye... but it would give good prop protection!

Jim

My thought is that additional leading edge for a given surface area could possibly be one +. Then the bulb connecting them could reduce tip loses as well.

[RaymondR]

It would appear to have structural advantages in that the keel loads can be carried up into the hull and tied into frames at two points.


If hydrodynamic drag is the sum of skin friction and form drag one would reduce the skin friction and double the form drag at zero angle of attack but what happens when leeway comes into the equation and the angles of attack increase?

[SURV69]

I only stayed at the Holiday Inn one night, but, in my simplistic thinking, wouldn't the flow of water off the first keel toward the back of the boat(and the next keel)help reduce leeway in the gap by disturbing the forces of leeway trying to move laterally through that gap?

[UNCIVILIZED]

Originally Posted by Jim Cate

I can understand the possible structural advantages, but as yet, no one has addressed the question of why there would be a better lift/drag ratio with this configuration. I'm still wondering! The one with the saildrive between the two foils is really odd to my eye... but it would give good prop protection!

Jim

Jim,
There are some explanations over on Boat Design.net. Along with a bunch of the math that goes with. Which I'm too tired to dig through at the moment. Tandem Keel...? - Boat Design Forums
And of course, there's -> warwick collins tandem keel - Bing

But, bottom line, I'm thinking that if such keels were a really big advantage, then they'd be fairly common on racing boats... Unless, they got Hated out of existence by the YC rule maker types.
Though, that still wouldn't explain why these keels weren't common on Open type Class boats, prior to canting keels becoming the in thing.

My guess is, is that there's just too much surface area on them, causing drag, for them to be as efficient as a well designed deep fin. Or a deep fin/foil with a heavy bulb on it's end.

But again, engineering a boat to have a Heavy bulb on a deep, thin foil, wasn't really easily possible, until the 90's or so. And they still have "teething problems", in that, when a boat which has a bulb that (often significantly) outweighs the sum total of the rest of the boat combined, it makes for some "interesting" engineering.

As when you hit a wave, the bulb wants to keep going in a straight line more than the rest of the boat does (thanks to physics). So the foil flexes, & the bulb gets it's way. Which makes things a bit tougher on the boat, & the keel's support structure, from an engineering perspective.
I had to help sort this exact thing out on a recer with such a configuration, a few months back. Even after they'd brought in an "engineer".

Plus, when you're crewing on such a boat, it's an odd feeling, passing through waves with such a keel. And it was definitely a bit trippy at first, with the IACC boats, to say the least. As on them, the bulb weight to hull weight ratio is about 10:1

Still, the theories on the Tandem Keels, over on Boat Design.net are fairly interesting. And if one's truly curious, I'm sure there's more info out there online, as well as at AYRS.org (the Amateur Yacht Research Society). A truly interesting info source.

[Polux]

Originally Posted by UNCIVILIZED

...

But, bottom line, I'm thinking that if such keels were a really big advantage, then they'd be fairly common on racing boats... Unless, they got Hated out of existence by the YC rule maker types.
Though, that still wouldn't explain why these keels weren't common on Open type Class boats, prior to canting keels becoming the in thing.
....

As Martin Defline says (the NA that studied them extensively and uses them in several boats) they are not an alternative to absolute performance but a very good alternative in what regards swallow draft keels performance.

He points also that they are a valid alternative to modern twin keels in terms of swallow draft performance and that both are the best solutions to that (in what regards performance), considering only fixed keels.

Regarding absolute performance there is nothing like a very deep torpedo keel on a fin.