Tandem? Keel on Bavaria

[Andrew Adams]

I attended the Strictly Sail Chicago show this weekend and the strangest thing I saw there was the keel on a Bavaria 33. This keel had two foils on the centerline separated by about one chordlength and joined at the tips by a flattened bulb. It looked from the side kind of like this: LL

I didn't get an official name for the thing, but the salesman did say that the keel was an improved shoal draft design. The available write up on the keel said that it had a lower keel CG than a single foil with a bulb. It pointed out that each foil had a higher aspect ratio.

I wonder how clean the flow is for the second foil? How far apart would two foils have to be to act independently? How would you characterize this? - Reynolds Number, Angle of Attack (leeway angle), Downwash angle, number of mean chords of separations?

Or is this a slat situation where the trailing foil greatly improves the performance of the leading foil at some decrease in its own? What separation optimises slat action? How much would the CLP shift as speed increases?

As for lower CG, why not core a single foil and move the core volume of lead to the bulb? You could even leave the sand core in place. It's less dense than lead.

Has anyone run into any papers on this keel type? For that matter, does it have a proper name?

[Windvang]

The tandem keel has been tested in the Delft University tow tank. The results were not encouraging. Mainly used as a marketing tool now.

[Tim B]

The major problem is that the flow over the rear foil (even assuming that the forward foil isn't stalled) is not at the same angle of attack as the forward foil, therefore, you can optimise the set-up for sailing in one condition only.
Also, you tend to find that the added drag due to the presence of a second foil isn't worth the increase in side-force.

Tim B.

[Andrew Adams]

Thanks for confirming that Tandem is the right term. I googled tandem keels and got among other things, this earlier thread.
http://forums.boatdesign.net/showthread.php?t=1058

[Vega]

Originally Posted by Windvang

The results were not encouraging. .

Can not be that bad. Harmony is using them too, for shoal draft .

http://www.poncinyachts.com/Poncin/Poncin.nsf/c9c4421660fb3277c1256a95004a20a9/49508413fc365f6980256f2e004d1462/$FILE/GAMME%20HARMONY.pdf

[tspeer]

Originally Posted by Andrew Adams

... It pointed out that each foil had a higher aspect ratio....

This is a misunderstanding of what aspect ratio is. Aspect ratio is not a measure of slenderness, it's really a measure of span (depth). To compute the aspect ratio of the keel, you need to divide the depth^2 by the total area. Dividing the area into strips does not change things.

The reason for considering aspect ratio comes from the two main sources of drag:

D = 1/2 rho V^2 Cd S + L^2 / (pi d_eff^2 1/2 rho V^2) + wave drag, etc.
rho = fluid density
V = freestream velocity
Cd = parasite drag coefficient
S = reference area
d_eff = effective depth
L = lift on keel

Since area increases drag and span (squared) decreases it, a good figure of merit is span^2 / area. This is where aspect ratio comes from. When you decrease the area, you're cutting the skin friction. When you increase the depth, you're cutting the drag due to lift.

Probably the best way to compare the tandem keel is to say it has less wetted area than a keel of the same total length and depth. It would be a much stiffer way to support a long bulb than a single fin. It may have an effective depth that is closer to its physical depth, but I doubt there's much difference there compared to a conventional keel.

[Shife]

With their recent Match 42 debacle, maybe Bavaria figured two is better than one? That company churns out some shady build quality. Perhaps using gimmicks will take the focus off the companys recent bad publicity.

[Skippy]

That did get me thinking twice about the idea once I saw who was behind it.

Is there a significant risk of something getting fouled in the opening between the keels? Tasteless pranksters chaining it to a dock or buoy? Giant octopi grabbing the bulb and dragging the boat down? Is that kind of thing a realistic danger, or is it safe enough if you avoid being excessively careless?

[water addict]

if I remember my hydro theory right (I've focused on the structures aspect of nav.arch.) in order for the 2 in-line foils to work to their potential, the gap between them should be at least one span length. Looking at the photo of New Zealand earlier in this thread, that looks like about what they did on that boat. If the gap is less than the span, then the rear foil is in the downwash of the fwd foil and the induced drags from the 2 foils just add up and you get roughly the same situation as if you had both foils connected. The stubby tandem keel with the slot is most likely as someone stated earlier "marketing".

I'm rusty on my hydro, so maybe someone more proficient should comment though.

[tspeer]

The tandem lifting surface problem was solved by Max Munk in 1923 - see The Minimum Induced Drag of Aerofoils, NACA Report 121. He showed that the drag of two or more lifting surfaces lined up in the streamwise direction is the same as a single surface that has the same total lift distribution. It doesn't matter how the lift is distributed between the two surfaces or how far apart they are located. This is known as, "Munk's Stagger Theorem."

The reason some designers think a tandem keel would have an advantage comes from a misguided assumption that the two panels act like independent surfaces. Since induced drag is proportional to the square of the lift, it would seem that having two surfaces carrying half the lift would cut the induced drag in half. But this ignores the mutual interference between the two surfaces. Induced drag really comes from the whole wake, not just the flow around the tip. The downstream panel causes additional drag on the upstream panel, and vice versa, in equal amounts.

For a tandem keel to have any hydrodynamic advantage over a single keel, it must have a greater effective span than the single keel. In order for this to happen, it has to have a greater physical depth or the connecting bulb has to act like a winglet. Either might be facilitated by the structural differences between the tandem and single keels. But there's no reduction in drag simply because there are tandem surfaces.

[Vega]

On these days of cheaply fixed keels and fragile boats, the tandem keel has an advantage for a cruising boat: It offers a much bigger contact area with the hull, permiting a better distribuition of the tensions by a larger part of the hull .

Take a look at the harmony keels, the standard tandem and the optional fin.



By the way, I have seen that keel this weekend and it looks good. Two very thin supports take the weight of a really big torpedo, permiting a low center of gravity and a shalow draft (and that's almost the only thing that I like in that boat, the interior is really ugly).

[Skippy]

What about an anchor rode or whatever getting fouled in the hole, or moronic pranksters (or the octopus )? Is that something worth worrying about for a cruiser?