Tandem Keel...?

[tspeer]

Originally posted by Guest
...
-the bulb/ wing is really big and works as a fence/ endplate (dont know the right expression in English), thereby creating an almost 2D- flow with little vortex around the keel "tip"/ endplate. ...

It's not that easy to get rid of the induced drag. There is still some vorticity shed at the junction and additional vortices shed at the tips of the wings. It's a misconception that the trailing vortices come just from the fluid spilling around the tip. In fact, the vorticity is shed continuously along the span, with a greater concentration at the tip.

Wings don't get rid of the trailing vortices, they just move them laterally away from the rest of the keel so that they don't have as great an effect. But for the same addition of wetted area, you can get a greater gain by making the wing an extension of the keel than you can by mounting the same surface as a wing.

Wings only make sense if there is some constraint on the depth of the keel. The constraint could come from rating rules, or from the need to sail in shoal waters, or even for easier carriage on a trailer. Wings may also be a more productive way of packaging the volume of ballast. But for pure hydrodynamic efficiency, go for span every time.

... But where are the hard facts? Does anybody know about reports/ proceedings about the Collins keel? Are there test reports with the same hull design, but different keel configurations?...

Exactly. There are lots of claims made for various keel configurations, but the explanations usually sound like one of Kipling's "Just-So Stories".

It's always interesting to ask, "So, may I see your data?" If you get a lot of mumbling and handwaving and statements like, "Well, it's all due to the collateral thermal end-plate effect, and you really can't compare it with a conventional configuration," then I'd be inclined to walk away. And if he claims to have discovered some revolutionary new physics then I'd not walk away, I'd run.

But if he says, "Here are some estimates of a comparable conventional configuration compared with published test data so you can see how well the estimation technique works, here are the estimates of the new configuration showing the improved performance, and here are our test data on the new keel validating the predicted performance," then I'd be inclined to plunk my money down. It's perfectly reasonable to expect a minimum of two out of these three.

[Fco.Lopez]

If tandem keel really works spectacular...

All of IACC yachts will have one...

I don't like the tandem keel posted on page 1... i think for these boat, will work better a semi-long keel...

[SailDesign]

THe Collins keel seems to work really well. tRodger Martin and I put one on a 43' cruiser/racer (Quadrille) for a client who wanted low draft and high performance. There was no appreciable down-side to the thing, unless you like being bounced around on the mooring ;-)
Performance was just right, and it had the added benefit that at launching time one of the yard crew rode it down "Dr. Strangelove" style
Steve

[yipster]

"Dr. Strangelove" style

sitting backward on the bow waving a cowboy head?

[SailDesign]

Yipster - sitting between the two foils, on the bulb, waving a baseball hat
;-)

[Tohbi]

well, i'm dizzy from trying [unsuccessfully] to figure out the numbers, but i seem to remember an america's cup 12 meter that had twin keels. wasn't the designer or owner named blackaller, or something similar?

it seems that boat pointed high but the widely separated keels caused some handling problems. america's cup rules only allowed for two underwater appendages so the keels had to act as rudders, as well.

how about a trim tab on an existing keel design? how efficient is that addition?

[Fco.Lopez]

Originally posted by Tohbi


how about a trim tab on an existing keel design? how efficient is that addition?

Good question...!!!

Somebody sailed a boat with trim tab? Or design, build...

[Stephen Ditmore]

If anyone would like to comment on my keel idea, check it out at
http://boatdesign.net/forums/showthr...5&pagenumber=2

BTW the first tandem keel I'm aware of was on a boat designed by Art Paine, identical twin brother of Chuck Paine and built in the late 1960s (or perhaps 1970) to compete with the Soling and the Etchells to be the new 3 man olympic keelboat. Art built the boat with the assistance of a young fella named Eric Goetz, but they didn't have it ready in time for the competition (Eric's been lothe to miss a deadline since!).

[SuperPiper]

Recently posted on "Hull Is Slower Without The Keel":

I sail a micro-cruiser. It is a Sandpiper 565, 18'6" LOA with a 15'0" LWL. This little boat has 300 lbs of lead ballast in a keel that raises vertically into the cabin table. When completely retracted, the bottom of the keel is flush with the bottom of the hull.

Recently, while sailing straight downwind wing-on-wing, the keel was cranked up to reduce wetted surface and to reduce drag. To our amazement, the boat speed appeared to decrease! So the experiment was repeated a 2nd time and the result was inconclusive. But, the speed did not increase as expected.

The experiment was later repeated by a different boat using its outboard motor. Again, there was no detectable change in speed.

The boat speed was being measured using GPS with resolution to 0.1 knot. The sailing experiment was conducted at about 2.8 - 3.1 knots. The motoring experiment was conducted at 4.7 - 4.8 knots. Hull speed is estimated at 5.3 - 5.7 knots.

Is there an explanation why withdrawing the keel should not increase the boat speed? Especially while travelling below hull speed? Does the keel somehow change the boat's wave-making ability to give the illusion of a longer boat? Does the appendage sticking out the bottom of the hull somehow mimic the winglet on a bulb keel? Does it provide less drag despite its increased wetted and frontal areas?

Originally Posted by tspeer

It's a common misconception that induced drag depends on aspect ratio. It doesn't. Induced drag depends on span and is independent of aspect ratio if you fix the span. Here's why:

Let's assume a parabolic drag polar, typical of wings and keels when wave drag is not a factor. The drag coefficient can be written as

AR = b^2/S

CD = CDo + CL^2/(pi * AR * e)

AR = aspect ratio
b = span (depth of keel)
CD = total drag coefficient
CDo = parasite drag coefficient
CL = lift coefficient
e = efficiency factor (approx. 1.0)
pi = 3.142...
S = reference area

This makes it look like the induced drag (second term in the equation above) depends on the aspect ratio. But this is a nondimensional drag coefficient - it's been divided by the really big influences; the velocity, fluid density, and reference area.

Now take a look at what the drag really is, in pounds or newtons:

D = total drag
L = lift
rho = fluid density

D = CD * 1/2 * rho * V^2 * S
L = CL * 1/2 * rho * V^2 * S

D = CDo*1/2*rho*V^2*S
+ [L/(1/2*rho*V^2*S)]^2 / (pi*b^2/S*e) * (1/2*rho*V^2*S)

D = CDo*1/2*rho*V^2*S
+ L^2 / (1/2*rho*V^2*pi*b^2*e)

Aspect ratio has totally disappeared! The parasite drag depends on the reference area, and if you picked the wetted area as the reference area, CDo would be close to the skin friction coefficient.

The induced drag depends on span-squared, not aspect ratio, and e, which depends on the planform shape. Winglets get into the act by increasing e, and improving b when the boat heels. If you keep the span, b, the same and increase the chord, the drag goes up because you're adding wetted area to the parasite drag. The aspect ratio is going down, too, but the induced drag isn't changing (assuming e stays constant). Finally, the induced drag goes down with speed if lift is held constant. So if you have a short span (like a boardless catamaran) you can make up with speed what you're lacking in depth. This is all completely opposite of what you'd expect from looking at the drag coefficient instead of the actual drag.

Instead of thinking of aspect ratio as being a "skinny-ness" factor, think of it as the nondimensional form of span (squared). When nondimensionalizing an equation, anywhere you have a length^2 component, you divide it by reference area. So b^2 becomes b^2/S and you have aspect ratio. Long skinny glider wings have low drag because they have a lot of span for their area - this is what aspect ratio is really saying. Or because they have small area for their span, which would be another valid way of looking at it.

If you look at it in this light, then the nondimensional equation starts to make more sense. The induced drag is inversely proportional to the nondimensional span-squared (aspect ratio), just like it is in the dimensional equation. For a fixed lift, the lift coefficient drops with speed, and this accounts for the reduction in induced drag with speed. So the two approaches are consistent.

This old post by Tom Speer may include some revelation about my retracting keel. Tom's logic held the span constant. In my Sandpiper example, the span and the area are both changing - each countering the advantage of the other. Reducing the span causes a marginal increase in drag but reducing the wetted area causes a marginal decrease in drag.

So, ain't that cool ?

[Vega]

Harmony is a French big prduction sailboat manufacturer directed and owned by Mr Poncin, the man that was for many years the Director of Dufour yachts (that he has sold to the Italians some years ago). The Harmony are on the rise and that in a country that is the home of Beneteau, Jeanneau and Dufour (that made boats for the same market segment) is quite an accomplishment, considering that Harmony has no tradition and is a completely new company.

One of the most distinctive features of the Harmony is the tandem Keel that is offered as an option. The Tandem is the keel choice for cruisers and has proven very well.

Harmonies are designed by Mavrikios and Mortain and they say about the tandem keels on the Harmonys:

“Those looking for small draughts will be delighted to know that cast-iron tandem keels …offer almost the same sail stiffness and the same ability to go close winded as lead keels with far deeper bulbs”.

http://www.harmony-yachts.com/public...%20Harmony.pdf

http://www.eurobateaux.com/accueilpy.htm

http://www.mortain-mavrikios.com/

They also do them for the Etaps.

Etap says about them:

"After thorough investigation and numerous tests, ETAP Yachting N.V. is pleased to introduce its ETAP tandem keel. The most important advantages of this keel are the excellent sailing qualities at a considerably reduced draft. This new design is the result of a co-operation with the architects' bureau Mortain-Mavrikios.

The two most important features to reduce drift, are the size of the lateral plan and its efficiency. The efficiency is defined by the proportion between the depth of the keel and the length. Also a wing section is a classic aid to improve the efficiency.

For a strong reduction of the draft neither a wing keel or a bulb keel were sufficient. The solution was found in placing two shorter keels behind one another, linked by a wing-bulb profile : the ETAP tandem keel.

The ETAP tandem keel gives a better aspect ratio, thus generating more lift.
In addition to increased stability, the wing-bulb also provides better hydrodynamic characteristics. "

http://www.etapyachting.com/index.php?pageID=37

[Paul Scott]

n+1 > n just turned into EMO's bad dream? Say it ain't so! Must be those pesky details Tom's mentioning. I must admit, though, when those slotted windsurfer skegs spun out, they REALLY spun out. Usually when goofyfooting on the starboard tack. Yikes.

Exit, stage L, right ankle (and L hemisphere) throbbing,

Paul

[naval ark]

Originally Posted by SuperPiper

Is there an explanation why withdrawing the keel should not increase the boat speed? Especially while travelling below hull speed?

This old post by Tom Speer may include some revelation about my retracting keel. Tom's logic held the span constant. In my Sandpiper example, the span and the area are both changing - each countering the advantage of the other. Reducing the span causes a marginal increase in drag but reducing the wetted area causes a marginal decrease in drag.

Yes, of course the presence of an appendage alters the hull-generated wave pattern, whether this is in a positive or negative manner depends on the length/speed and number of wave crests between bow and stern.

Might I suggest that your initial findings are a little on the crude side, but as you found out, in most cases the difference in viscous resistance between centerboard down/retracted is marginal when compared to the wetted surface and wave-making capacity of the hull.

Tspeer makes some very good points, and he is entirely correct, but two and three surface interactions can be very difficult to predict - possibly for the reason that obvious definitions in the aerospace industry become muddied rather quickly when applied to sailing yachts. For example, in the above example the aspect ratio term drops out entirely because he is defining AR as b^2 over S, where S is the total keel area (more accurately could be total underwater planform area). This is valid for closely spaced foils but if stagger increases to around 3 or 4 (distance normalized against chord length) then considering the foils separately and each having their own AR (b^2/S) would be valid. This is what the America's Cup yachts were doing, although they were hampered by the two movable appendage rule, which is something a tandem can really use to advantage...

[SuperPiper]

Naval Ark:

Great explanation. Even I comprehend. Yes, the wetted surface of the keel is only a fraction of the hull's wetted surface. And it is the hull that is making waves.

Thanks for the insight.