keel configurations

Hi there,
I will do some towing tank tests with a model of a Sailingboat . (L=16 m)
I want to compare three keel configurations. A "racing-keel" 3.5m deap, a cruising keel with 2.25m and a centerboard inside wich can be moved out to 3.5m. And at least a tandem keel, 2.25m.
My question is, has anyone experiance with the tandem keel configuration,knows some publications or did towing tests with it ?
I would be very pleased if you could help.

Ciao Max

 

I presume you've checked out the IACC tandem keel yachts. I don't think you'll find any advantage with the tandem keel. Unless, there is a good reason for it. For example, the IACC rules permit only two movable surfaces below the waterline. Therefore, a tandem keel with flaps can produce more righting moment for a specific size and weight than a single-fin keel it also can be made to double as a rudder too. Thus there is good reason. If righting moment is not an issue I don't think you'll see any gain whatsoever. In fact, drag due to surface area will be excessive over the other two configurations. So don't expect any amazing results from it, sorry. Do share the results with us, I'm sure that a lot of people on this forum would be interested.

Cheers,

Tim B.

 

I'm with Tim. I presume each of the three keels has the same planform area so that you're looking at the difference between span and single vs twin. I'll bet on configuration #3, and both of the single keels to beat the tandem.

You might take a look at http://naca.larc.nasa.gov/reports/1923/naca-report-121/naca-report-121.pdf This is Max Munk's original paper in which he establishes the Munk Stagger Theorem. This says that the induced drag of lifting surfaces is independent of their streamwise location, provided that the loading on each surface is kept constant. Which it never is in practice. But if you ignore all the massive integrals and just pay attention to the text, there's some really valuable information in there.

 

Thanks you very much for your replies !
The point is that the guy who asked me to develop a boat for him, will sail mostly in germany and denmark in the baltic sea. He wishes to have high sailing performance and also the possibility to enter small harbours with 2.25 m. So I want to find the best compromise for him compared to a conventional racing keel.

Ciao

Max

 

In that case, I think the first configuration with the telecoping dagger board is the way to go.

Another possibility might be to use a variation of the canting bulb twin foil (CBTF) concept. The canting bulb would be limited by the depth restriction, but would provide additional roll trim compared to a fixed keel. Either one or two dagger boards would provide the principal leeway resistance with large span for low drag. They could then be retracted to the same depth as the keel for shallow draft. This would be very much like some of the Mini-Transat arrangements (see other thread), but adapated for shallower draft.

The biggest drawback of the canted keel is the room it takes inside the hull. Unacceptable if this is a cruising boat, but for a racer it might be worth it.

 

what a bow..!!!

Why do you desing these bow forms...??

 

Yes it's a strange bow, I can't see apparently any design reason for that...

 

My suggestion is that you make the aft part of the bulb rotatable and put a high aspect foil on it, so that you have a single adjustable winglet. I hold the U.S. patent on this idea. Have a look at www.delphion.com/details?pn=US06453836__
Optionally the secondary foil can be made to kick up should it hit bottom and/or it can have a flap with which various effects can be achieved.

I'd be interested in working with you on it. There's a German patent that I cite as a prior art reference, but I don't think it claims the same single winglet configuration.

 

Here's an illustration of my keel with a flap on the secondary foil. Without the flap it would still be pretty effective, and would be mechanically simpler, so that's also a good option. Engineer Finian Moore did some CFD analysis on a version with no flap, and the results were encouraging.