Double keel or single keel ?

[Fanie]

If one uses two small keels a distance apart, would they have the same effect / functionality as one with double the area ?

[Petros]

They theoretically would be the same. But it seems to me that one larger one would be more efficient and have less wetted area, and only one tip loss, less complex too (fewer parts to build). One fewer keel to raise when in the shallows.

However, on a cat having two small ones has the advantage of always having one in the water in chop or when flying a hull. There is also some advantage to redundancy in case of failure, two smaller ones would have less draft too.

I have thought this back and forth on a cat I want to design and build, but I have not reached any conclusions. It seems there are benefits to both, so take your pick.

[PAR]

Tandem appendages have been tried, but you have more drag, even if the combined total area is the same, the additional leading, trailing edges and vortices mostly counteract any advantage.

Tandem dagger or centerboards has a lot to offer, especially in helm balance and a fair number of yachts have been fitted this way.

[masalai]

Fanie, On your new toy, Go fast & you will never experience any lee worries - having too much fun. Fly both windward & centre hull for more drive (lift) and don't worry too much about that (pointing higher) yet.

[Fanie]

I apologize, I should have mentioned this is for the centre hull of a trimaran. One consideration to using two shallow fixed keels would be so the centre hull could stand upright by itself. Another would be shallow draft.

Thanks Masalai, but I doubt the cat is going to be fast, maybe if I redo the ally mast with GPR and make it longer and fly a larger sail. At the moment I'm just trying to figure out what is functional and what not.

[masalai]

My cheeky answer still applies, remember KISS. In that vein, single dagger-board is more than adequate & easier to control. The leading edge of the case can be part of the mast step.


If you must then extend the case & make it swing up, held forward by a rope and your magic cleat. (I suggest against the cleat - rather be aware of the depth than unexpected broach as the keel swings up)

[Guest625101138]

I apologize, I should have mentioned this is for the centre hull of a trimaran. One consideration to using two shallow fixed keels would be so the centre hull could stand upright by itself. Another would be shallow draft.

Thanks Masalai, but I doubt the cat is going to be fast, maybe if I redo the ally mast with GPR and make it longer and fly a larger sail. At the moment I'm just trying to figure out what is functional and what not.

If you have a single board that is 2ft deep and 1ft long replaced by two 1ft square boards then the drag on the boards to achieve the same lift is the single board is close to twice as much. This is based on a NACA2412 foil. Will be similar for others.

The reason for the dramatically reduced performance is the change in aspect ratio. This increases the induced drag for the low aspect foils.

If you keep the same aspect ratio then the loss in performance is not so great. You are changing the Re# so the impact will be less significant as the speed goes up.

Rick W.

[Brent Swain]

Yes. if they were canted out 25 degrees making the leeward one upright when heeled 25 degrees, increasing it's lateral resistance 100% from that of an equal sized one heeled 25 degrees.
Brent

[rwatson]

Yes. if they were canted out 25 degrees making the leeward one upright when heeled 25 degrees, increasing it's lateral resistance 100% from that of an equal sized one heeled 25 degrees.
Brent

To complicate that effect, the windward side dagger now totally loses resistance because it is 50. to the vertical, so the net gain is negated, but maybe its better to have one board doing all the work under water instead of up in the turbulent windward side?

Maybe the 25. angle is a bit too big for a MacGregor style craft that have to be sailed like a Laser, (not recomended beyond 10). heel.

Also, the bigger angles make inroads into the accomodation area.

My next boat is planned to have dual daggers, so I am interested in the logic here. I have specified them to be vertical.

I am also interested in the "toe-in" geometry calculations and how performance is affected.

[Fanie]

How about more than one dagger board per hull - one more foreward than the other... you use one at a time only and could use one in one hull at a time. High speed sailing you use the foreward dagger board to improve the rudder leverage, switch to the aft daggerboard to tack faster and decrease the turning circle diameter.

[Fanie]

In my mind toe-in (if any) should be very small. Half a degree. The toe-in angle will increase with speed without adjusting the angle and only appy if more than one dagger board is used. Streight daggerboards should have a sideways bobbing action side to side, but only if the hulls are as streight as well. The slightest deviation from 100% everything in line should veer slightly to one side which is corrected with the rudder without knowing. Assuming still water and no wind.

[Brent Swain]

Want toe in? Sheet her closer and point higher. Same effect.
Brent

[rwatson]

Its not that I would want toe-in just to say "Look Ma - I have toe in!"
Boats on a beat dont sail straight - they crab across the desired direction of travel.
Many modern day racers have variable attack angles for their boards to customise gain v speed for their boats
I have been told that there are optimal angles for dual keeled boats.
These angles usually result in a "toe in" angle to increase lift on a beat, but without creating too much drag under other conditions.
I have yet to read any theory on this - but someone may know a source.

[PAR]

Fin mounted trim tabs . . .

[Brent Swain]

The toe in theory was developed by tank tests , which propel a boat from a fixed point rather than by sails. The relevant angle is the angle between the keel and the sails, something that propeling a model in a tank from a fixed point doesn't relate to.
Brent

[rwatson]

Oh for goodness sake - are you trying to tell me that the angle of centreboards to the hull and direction of travel should be determined by the sailed course only?
Thats like saying a pilot should control lift only with the tail plane, and not use the ailerons on the main wings.
And to top it off, by implication you are saying that those boats that are using variable angle centreboards, or trimtabs (thank you Par), are not getting any benefit!

[Brent Swain]

No, they are probably not getting any benefit compared to simply sheeting in more and pointing higher. Its still the angle between the keel and the sails no matter what the pushing from a fixed point in a tank tels you .That model is beiing pushed form a fixed point, not by sails.
Boat are boats , planes are planes . They have much in common , but not everything.
Brent

[rwatson]

Brent, if you dont have any understanding of hydro or aero dynamic forces, and cant put those concepts into a readable form, you dont have to contribute to the discussion to fill in space.
If there is anyone else that has an interest and can contribute something, I look forward to reading it.

[Guest625101138]

.........
I am also interested in the "toe-in" geometry calculations and how performance is affected....

From an analysis perspective you could do a first order calculation treating the hull as a low aspect foil and the keel as a high aspect foil. I expect this approximation would be good for a long hull and short, high aspect keel. You will find the keel is substantially more efficient (L/D) than the hull simply by the higher aspect. So if the keel is given some toe-in such that the hull tracks true, resulting in no induced drag from the hull, while the keel develops the entire lift to ballance the sail force there should be a substantial gain.

The best L/D for the hull may be around 4 while the keel could easily be 20 or more. So makes much more sense to generate the lift from the keel. You see most modern aircraft have asymetric wings that provide lift when the fuselage is in line with the flow. I expect the majority of the lift comes from the wings not the fuselage.

The same thing could be achieved using two asymetric dagger boards. Also twin fixed keels angled out at say 30 degrees. The latter set up would tend to create a righting force with a downward componment that would increase hull drag. You are also contending with extra drag when running.

I can recall seeing a design with what looked like two large rudders instead of a keel. One well forwadr of centre as well as the stern. I am not sure if both could be rotated.

As I have said on other threads you can use JavaFoil to quickly get a quantitative insight into operation of foils.

Rick W.

[rwatson]

Some good points Rick. I think my line of thought as this stage is more the geometry of the control surfaces, than their lifting co-efficient.
The hull shape I have specified will probably produce very low co-efficient of lift, as it is designed to be a very boxy shape for maximum accomodation and power performance (a la macgregor etc)
From what little I can understand, the geometry of the fins will be more about pure resistance because of the relatively low sailing speed (7 knot average), rather than any induced lift factor.
Like a jet gets more of its lift from the sharp attack angle on takeoff at low speed, (like a kite), then at cruising speeds the co-effient of lift from curved surfaces is enough to hold it up in the air.
The attached picture is how I understand it works - angling the leeward fin to reduce the actual boat vector, while the windard fin ends up being in line with the improved vector, thus not producing drag.
I am tempted to make the fin casings oversize, and put some mechanism inside to adjust the angles while sailing. I suspect that varying sailing conditions (like running and reaching) might make this a usefull (and fun) feature.

[Guest625101138]

I have attached some screen dumps from JavaFoil with your drawing in mind.

I am modelling the hull as a NACA0030 foil with AR of 0.2.

I have modelled a single keel as NACA0010 with AR 4.

If you assume the boat is designed with a single keel and design leeway is 5 degrees then the L/D for the hull is 1.9 while it is 18 for the keel. This means the keel will achieve roughly 10 times more lift for its cost in drag as compared with the hull. You would need to compare actual areas to determine the relative contribution.

With an asymetric keel you can get even higher L/D with the same AR. The third set of data is for a NACA3410-43 with AR of 4. It has a maximum L/D of 23 at 0 degrees AoA. So this could be set up to provide the entire lift without any leeway. You would need one on either side and be able to lift the windward one - a bit of a pain when tacking. Angle them outward so they get maximum draft when at design heal.

The point I am making is that it makes sense to generate all the lift from a high aspect foil than relying on any contribution from the hull. Any leeway is costing you in induced drag on the hull.

Also keep in mind that any wetted surface will add drag even if it is not lifting. You can see the Cd for the NACA 0010 at 0 degrees is .0055. Roughly 1/4 of the drag when it is working hard at AoA of 5 degrees. A big penalty to be dragging around all the time when not actually working.

One of the reasons I like small, fat, bottom hung rudders is that they will generate plenty of lift without the penalty of dragging a much larger blade through the water as in large, thin transom mounted rudders.

Rick W.

[rwatson]

I couldnt follow much of what you were trying to get at. I think I already said that the hull for my purposes can be largely ignored as far as lift goes, and I dont have to be convinced that the foils will provide much more significant lift.
The last sentence puzzled me even more.
Small fat rudders dont generate much lift, not "plenty of lift" as you wrote!
I know the bigger the appendage, the more drag, but if you dont have a decent length to the foil, you dont get much lift because there isnt much leading edge, and also the laminar flow just drops off the ends of a shorter rudder as lost energy.
Thats why slow moving gliders have longer wings than fast moving powered planes - they need maximum lift at slower speeds, while powered planes can reduce wing size, and thus drag, because the high speed fluid flow can produce plenty of lift across a shorter leading edge.
At slow sailing speeds, short rudders or foils just wouldnt create any viable lift. And you do want lift in the rudders - its more efficient to produce steering pressure from laminar flow than from mere angle of attack, where often caviation occurs with resultant heavy drag penalties.
Thats why you never see anything but long foil rudders on sailing boats. So I dont know what you are getting at with the "small fat" rudder concept.

[Guest625101138]

If you have leeway then the hull is producing lift and there will be induced drag as a result.

I think you are confusing length with depth. In my reference frame, length is measured along the logitudinal axis of the hull. Depth is measured below the hull. So a short rudder has a short chord length. The higher the aspect foil (deeper relative to length) the more efficient.

Once you get Re# over about 50,000 you get reasonable foil performance. A 150mm long rudder at 6kts is well into effective regime. I also think you are confusing streamline flow and laminar flow. A fat section rudder like NACA0020 will carry streamline flow up to about 20 degrees. I have attached JavaFoil polar curve for a rudder with AR of 4. You will see that the Cl peaks just under 1.1. Using a NACA0010 section of AR of 2 the Cl peaks at 0.4. So for a draft constrained rudder the fatter rudder only needs to be about 1/3 the area of a thin section rudder like NACA0010 to generate the same lift. The in-line Cd for the fatter section is less than twice the thinner section so there is an overall reduction in drag by using the fatter section. I have never taken the time to determine what the best rudder section is for a given condition but NACA0020 is not far off. It might be as high as NACA0025.

If you are thinking about toe-in or asymetric keels a similar sort of analysis is required to get the optimum result. The aim should be to eliminate leeway so there is no induced drag on the hull. I expect you need a symetrical steerable keel or two drop keels with asymetric section to get the optimum.

Rick W.

[Richard Atkin]

rwatson
the type of rudder that Rick refers to is not 'short'. It is small but still high aspect, and fat when you look at it from above. It produces a lot of lift and must be protected from cavitation/ventilation.

[Richard Atkin]

Just been reading the thread properly. Rwatson....you were a bit harsh on Brent don't you think? I wouldn't like it if someone brushed me off like that...and I'm sure I have said things that don't make sense! Be nice :D

[Richard Atkin]

geeez I sound like a woman

[rwatson]

Its not my fault Richard - Honest Darling !!!
Its only because people keep trying to confuse me - like
I know what Laminar Flow is - its "streamline flow" when people are trying to confuse me!
"Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers" according to google definitions.
And "small, fat" means I am "confusing length with depth"
In my school a rudder would be "small" as in short, not small as in length, so of course I assumed "fat" was the other axis.
But now thats all sorted out, Rick really wasnt incorrect - He just wasnt talking about my area of interest - which is twin keel geometry, not foil sections - which was another attempt to confuse me!
But for all that, I am going to have to think about foil sections as well I guess - but that might be too confusing. The suggestion of having asymetric sections will probably result in more confusion.
I just need the web address for the "Twin Fin Geometry Corporation", and then I can go and buy a couple of kilos of "Twin Fin Configurator" to splash in the water around the boat. is that too much to ask??????

[Guest625101138]

To ease the confusion I should be refering to attached flow rather than streamline flow.

Foils of the scale used on a full size sailing boats operate in the turbulent boundary layer regime with "attached" flow over most of the surface. The flow over the majority of the surface is NOT laminar as there is a turbulant boundary layer but the flow remains "attached" to the surface. At high angles of attack the flow becomes "detached" and the foil stalls.

This article might help the understanding:
http://www.sportpilot.org/magazine/feature/2006%20-%2007%20July%20-%20In%20Pursuit%20of%20Airflow.pdf
It covers streamlines, laminar flow, turbulent flow, attached flow and flow separation. You can see how turbulent flow is induced to improve performance of the foil in certain circumstances.

I have attached an output screen from JavaFoil that shows the flow transition points and the flow separation points for a NACA0020 at 5 degrees AoA and Re# of 300000. You can see the transition point on the upper surface occurs at 20% along the surface. Flow separation is only occuring near the trailing edge.

Keels, rudders, sails, propellers and wings are all foils and JavaFoil provides a wealth of insight into their operation and good numbers for design purposes. With a little effort it will be your Twin Fin Geometry Corporation and provide the necessary insight.

Rick W.

[tspeer]

Want toe in? Sheet her closer and point higher. Same effect.

Not quite. The total side force, of course, has nothing to do with the keel design or toe-in. The total side force from the keels and hull will be a function of the sail trim. The leeway angle will adust to match the force from the sails, so you're right about that.

Toe in has the effect of shifting the load from the windward keel to the leeward keel. At a given leeway angle, the leeward keel will have a greater angle of attack than the windward keel.

Since the windward keel operates much closer to the surface, the induced drag due to lift is significantly higher for the windward keel. Plus, the normal force from the windward keel is not oriented to oppose leeway nearly as well as the leeward keel. So you want the leeward keel doing all the heavy lifting and the windward keel along for the ride. Ideally, the zero lift line of the windward keel would be aligned with the incident flow at the leeway angle.

Then when you tack, the load shifts onto the other keel. So, although the wetted area is not reduced, the effect on induced drag is like raising and lowering bilge boards on every tack.

[rwatson]

Thanks Tom, that confirms my understanding, and I have modified my working diagram to clarify these concepts in my own mind.

I am leaning towards Ricks hint at using asymetric foil sections, and angled boards as Brent was talking about. (In fact the angled boards could be incorporated in the angle of the cabin sides, making it a nice neat building solution).
If a chord of 150mm in a fin can produce lift at 6 knots using Ricks example, I would expect a slight attack angle would help keep the flow attached, and also provide directional assistance, thereby reducing the leeway angle.

And I think your sentence "the zero lift line of the windward keel would be aligned with the incident flow at the leeway angle" means that one should try to keep the angle of the windward fin as close to the boats true heading (not steered heading) as possible.

That is probably the answer to the mystery of the optional toe-in angle for a boat, and it sounds like it would vary a fair bit from one boat to another depending on hull/sail/size factors.
If I dont master the math, I might just incorporate adjustable foils and experiment in real life conditions.

[tspeer]

...I am leaning towards Ricks hint at using asymetric foil sections, and angled boards as Brent was talking about.
For a symmetrical section, the angle of attack for zero lift is zero. For a cambered section, the angle of attack for zero lift is negative, and typically on the order of minus three degrees. If you reference the alignment of the keels to their zero lift lines instead of the chord, there's no difference between a symmetrical section and a cambered section in this context.

Because of the orientation of the zero lift lines, when you mount a cambered section so the chord is parallel to the boat's center plane, it is effectively toed in already by the amount of the zero lift angle of attack.
...If a chord of 150mm in a fin can produce lift at 6 knots using Ricks example, I would expect a slight attack angle would help keep the flow attached, and also provide directional assistance, thereby reducing the leeway angle.
The leeway angle will be reduced, but the boat's heading will also change. If there were no difference in drag, then the boat's path through the water would be the same, but the bow would be rotated more off the wind to align with the direction of travel. So the most immediate effect of toe-in or camber has as much to do with changing the angle of the apparent wind to the boat's centerline as it does to changing the leeway angle relative to the hull.

Of course, the whole point of the exercise is not to keep the drag constant, but to reduce it. This is really the difference between the cambered section and the symmetrical section. Each section has a range of angles of attack at which its profile drag is minimized. A designer can make the low-drag region deep and narrow, or shallow and wide. For a symmetrical section, the low-drag region is necessarily centered on zero lift, by symmetry, and the maximum lift at which the section operates in the low-drag region is only half the width of the low-drag region. It's not only easy but probable that the lift coefficient going to windward will be outside the low-drag region for a symmetrical section. For a cambered section, the low-drag region can be centered on, or encompass, the intended operating condition. So camber should be viewed from the point of optimizing the profile drag rather than adding lift at the operating point.

For a twin-keel configuration, it would be advantageous if the low-drag region encompassed zero lift so the windward keel's drag was minimized, and the operating lift coefficient of the leeward keel when sailing to windward.
And I think your sentence "the zero lift line of the windward keel would be aligned with the incident flow at the leeway angle" means that one should try to keep the angle of the windward fin as close to the boats true heading (not steered heading) as possible.
A designer can come at this backwards, especially if a velocity prediction program (VPP) is available. A first cut using a non-toed in symmetrical keel will provide an estimate of the leeway angle. The zero lift line of the selected section (symmetrical or asymmetrical) can then be toed in by the amount of the leeway angle. Then the VPP rerun to see the difference in performance due to the better optimization of profile drag and induced drag (due to loading the deeper keel).
That is probably the answer to the mystery of the optional toe-in angle for a boat, and it sounds like it would vary a fair bit from one boat to another depending on hull/sail/size factors.
If I dont master the math, I might just incorporate adjustable foils and experiment in real life conditions.

A flap or trim tab on the keels would allow you to change the zero lift line very easily and home in on the best effective toe-in angle. It wouldn't even have to be adjustable from inside the boat, although that would be the most convenient. One of the main reasons for twin keels it to allow the boat to dry out, so the flaps could be adjusted at low tide.

[Brent Swain]

The hydro - aero dynamic forces are forces between the leeward keel and the sail. The position of the hull relative to either is somewhat irrelleveant, in the small anges we are talking about. I've built boats with assymetrical keels and symetrical ones . There was no noticeable difference in performance on any point of sail.
I can see problems with keels toed inward when running downwind in rough conditions. When the boat rolls ,the deepest keel points inward, steering the bow in that direction. Roll the other way and the other keel steers the boat in the opposite direction. Thus keels toed inward, while having no net benefit sailing to windward , can drastically reduce directional stability downwind ,and make a boat far more prone to broaching in rough seas.
Brent