Canting Keels In Production Yachts

The planform is the span and area. Since the neither the area or the span changes, the planform does not change.

Correct, the total lift = Cl x Area x Density x Velocity^2

Since all 5 components contribute lift, the area of all 5 components is included in the area.

 

Two equal area foils of span "x" will have the same lift and induced drag as a single foil of the same area that also has span "x"

If the two foils are combined into one foil that has eqaul area and span = 2X the AR will be higher so the induced drag for the same lift will be lower.

2x10=20 20x2foils = 40 each foil has A:R = 5:1 The combined foil of area 40 and span 10 has A:R = 2.5:1

A single 2x20 foil has area = 40 and A:R = 10:1

For the same lift the higher AR foil has less induced drag for the same surface area.

 

If you use leeway to create the lift, then you have induced drag also from the bulb and it's strut. With minimized leeway this drag should be very small. I think this is the most important difference. Sometimes amateurs building boats with rotating keels claim that they will sail higher. That is probably not the case, but they will sail (a little) faster

 

Correct, they will sail no higher. They may sail at different leeway angles (point higher).

The total induced drag does not change, since the lift requirement does not change.

The major underwater drag producers are Induced drag from lift, Parasite drag from skin friction at speed/length ratios below 1, wave making drag at S/L ratios above 1, and the smallest drag producer is profile or form drag.

I suspect that wave making resistance of a hull is higher when it is moving through the water at a yaw angle to the direction of travel. I don't have any experience trying to quantify the effect of yaw on wave so I won't post an opinion on the possible magnitude of drag reduction available.

 

Back to Canting keels, it looks that the VOR70 have shown a set-back of the system (not mentioning reliability). It looks to me that when the boat is surfing down a wave, the almost horizontal foil of the keel will produce an enormous hydrodynamic force, pushing the bow down. That's dangerous.

The obvious way to solve that problem would be by using a pivoting keel which would allow to control horizontally that foil (the keel). That way you could have a very good control of the boat when surfing big waves.

Of course....that will be even more complicated....more electric engines, more power to run the boat..

 

Obviously, nobody thought of the bycoming effects just as Paulo just mentioned. En plus, I was thinking of another event that might be a source of problems within the construction of the canting keel suspension. ( Do we call it a suspension?)

The struts are designed and engineered in such a way that they have a minimal wetted surface. The pivot (hinge) is very high up leaving a long and tremendous heavy arm working against a short arm that is hydraulically powered to move the keel to the two boards.

The boat is without interruption subject to the violent movements of the sea and also the keel must transmit heavy vibrations caused by the thin fin (strut) finally causing damages to the hydraulic system.

Vibrations that also influenced negative impacts on the attachment of the webframe monted in the hull, with all the stresspoints located on 1,5 sq. metres; without an equal spread of all those forces along the hull.

 

CBTF and Induced drag

This discussion started to illustrate why a CBTF boat performs better upwind than a canting keel boat with a single fixed daggerboard. To illustrate this I chose the example of one CBTF boat with both twin foils on the centerline, the 0° case. The other example was the same CBTF boat with the twin foils turned to an approximate +3° angle of incindence. In the 0° case the whole boat contributes to the development of lateral resistance(lift) and consists of these elements:1) the forward foil, 2) the hull,3) the strut,4) the bulb and 4) the
aft foil. In the +3° case only the twin foils contribute to lateral resistance(lift).
Just for the hell of it lets give each of these components some numbers corresponding to their planform area:1)the forward foil=12 sq.ft.,
2)the hull=120sq. ft.,3) the strut 21.6'sq.ft.,4) the bulb 18 sq.ft. and the aft foil 12 sq.ft.. Total area =183.6sq.ft.
According to Munks theorem if you have multiple LIFTING surfaces lined up in the streamwise direction, the drag due to lift(induced drag) is the SAME as a single surface of the same span with it's lift distributed the same as the total of all multiple surfaces. Doesn't matter how the lift is distributed between the multiple surfaces or how far they are separated.(paraphrased from Tom Speer)
Effective aspect ratio is the max span(depth in both cases) squared divided by the total area.
---------------------------
0° case(hull+ foils +strut+bulb lifting) : total area 183.6 sq.ft; max span(wl to bottom of bulb)=16'. Span squared(256) divided by total area(183.6)= an effective aspect ratio of 1.39 .
-----------------------------
+3° case-(only the twin foils lifting) : total area=24sq.ft.; max span=10'. Span squared(100) divided by total area(24)= an effective aspect ratio of 4.16/1 .
==============
This shows ,again, that the 0° case has lots more induced drag than the +3° case since the higher the effective aspect ratio the LOWER the induced drag. This is an illustration of what actually happens in the real world between two identical CBTF boats going upwind ,one with the foils on the centerline and one with the angle of incidence adjusted so that the foils develop all the lateral resistance. It illustrates why a fixed symetrical, forward foil at 0° angle of incidence with respect to the centerline of the hull on a canting keel boat is the worst possible solution.

 

To continue ...

Assume:

The total Lift required is 1000 pounds
The boat speed is 25 ft/sec (about 15 knots)
The span effeciency is 100% so e=1
The density of water is 1

Case 1

CL = 1000 (lift)/(183.9 (area) x 1 (density) 25^2 (velocity squared) = .00814

CDi (Coeffienct of induced drag) = CL^2/(pi x AR x e) or in Case 1

CDi = .00814^2 / ( pi x 1.39 (AR) x 1 (100% span effeciency) = 0.0000173

Thus induces drag = 0.0000173 x 183.9 (area) x 1 (density) x 25^2 (velocity squared)

Drag = 1.989 pounds

Lift = 1000
Area = 24
AR = 4.16

CL = .0667
CDi = 0.000339
Drag = 5.093 Pounds

Since the 0 degree case using your numbers and known formulas has 2 pounds drag and the 3 degree case has 5 pounds drag what is shown?

That twin foils more than double the drag?

Please check my math.

 

Cbtf

Randy, thanks for your help in understanding this. I was certain that the induced drag between the two cases was different ; I just expected it to go in the opposite direction! Thats ok because I find this fascinating and will learn (soon) what the missing factor is. I know there is one because when two identical CBTF boats sail upwind together the one using collective is faster(+3° case). I just don't know how to show that yet but I'm learning.
I'm fairly confident that the Oswald span efficiency factor"e" is the missing link and I suspect that e for the 0° case would not be 1 but something between .5 and .7 since lift distribution for the majority of the area is poor.For the +3° case, at this point it appears that the lift distribution would allow an e of 1. May have to increase foil area a bit too I don't know yet. But the facts appear to be that induced drag can be different for two different systems producing the same lift which I've thought I understood for some time. But I've got a ways to go to find this missing factor so that the formula's results correspond to the facts on the water but I'm looking....

 

My guess is that at lower speeds the difference is not great. But as Wave making drag starts to dominate, the drag reduction comes from a combination of the hull making smaller waves and the change in pressure at the forward foil station reducing the size of the bow wave further.

Relatively simple Aerodynamics does not predict any change in the induced drag from lift. Altering the size and shape of the bow wave would have a huge effect. Since these boats are sailing upwind at S/L ratios of over 1.1:1, they are starting to push into the forced mode.

The whole hullform vs wave drag thing is WAY beyond my knowledge level. I know enough Aero to design planes that fly, but as someone stated in another thread, "Designing airplanes is easy compared to designing sailboats, airplanes only have to fly."

I do know that the ideal shape for a glider fuselage "looks" wrong and the nose should droop to align with the streamline of the upwash ahead of the wing, rather than align with the flight path. It could be that giving the forward foil an angle of attack puts the forward part of the hull more inline with the upwash ahead of the foil?

I'm stumpped.

 

Cbtf

It's definitely complex but in the example above we're talking about two identical boats(and ,to some extent,about a fixed forward foil on a"normal" canting keel boat.) With two identical CBTF boats the foils are placed in the high pressure area of the hull(according to CBTFco) and the benefits of wavemaking drag reduction should, I would guess, accrue to both boats. Whereas only the +3° case eliminates leeway. I'm going to keep working on it....
edit: I tried running the numbers above with a different(.52 and .7) "e" in the 0° case and it makes the difference(also increased foil area ) . I have no idea what an accurate "e" factor for the 0° case would be-maybe CBTFco will help me with it.....

 

Hey Doug,

We're way off topic. I sent you a PM.

Randy

 

Mf 35

Randy, the topic is a discussion of "Canting Keels in Production Yachts" and our exchange began more or less with the MF 35 when I wondered why they would have used a fixed forward foil -the slowest choice for a canting keel boat. We may be boring some but it just takes one person to bite into this and help explain the situation.The CBTF guys may even look in here so I think we're "on topic" just kinda heavy into the technical aspects. I think your PM has relevance- as does this discussion- to the original topic .

 

Although your calculations require some attention while reading, I agree that it is not off topic. Since the CK is a nice way of enhancing performance and ballast distribution the techniques around it might be optimised as well. Any arguments are welcome here, and numbers are harder then opinions.

But in smaller production yachts in the (god save me...) cruiser/racer group, interior space is of some importance. The CK-mechanism is a problem, but might become quite flat so a slightly higher floor could solve that. Any lifting (daggerboard) mechanism will be far more costly when it comes to interior space.

Therefore I would recon that there is little future for a PRODUCTION YACHT with canting keel and movable daggerboard(s). I reason that a yacht big enough to accommodate such a mechanism is (semi-)custom and cannot be considered a "production yacht".
Any mechanism with daggerboard(s) might find it's way into smaller OD-classes, where interiour space is less important.
But hey, this is just what I'm reasoning here.

ps. The MF35 has a canting keel and a retractable (carbon fibre whoho!) daggerboard, I'm not sure if that was mentioned...

 

kFOIL™

I'm 100% convinced -at least for the time being- that CBTF is the fastest canting keel system. But for prodution yachts the kFOIL may offer the best solution: it using a retractable foil mounted in the bulb and requires no invasion of space inside the boat for an extra foil. Another solution (though not as good as the kFOIL in my humble opinion) is Andy Dovell's use of fixed wings on the canting keel bulb. Julian Bethwaite has recently changed the keel design on the 79er from a bulb molded in the shape of a wing(like Procyon years ago) to a bulb with fixed wings similar to Dovells pioneering work.Unfortunately neither Dovells nor Bethwaites wings retract but
and they can break off-it happened with Dovell.The new Backman 29 has foils on the bulb "that are controlled by hydraulics" but there is no info yet on WHAT (angle of incidence,sweep angle, retractability??) is controlled.
The kFOIL will retract if it hits something so it has current lateral resistance solutions beaten on room inside, safety, and retractability.
kFOIL™ :
http://www.sailinganarchy.com/editor/2004/edword_october2004.htm