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!
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
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.