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Old 17-06-2008, 03:42   #16
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Originally Posted by Nordic cat View Post
I see 2 levels of redundancy with boards, one where you can actually move them from one side to the other, the second is that you can always sail on one board regardless which side its on...


I can understand that having 2 boards exactly the same, is an easier option, but do you think there are any benefits to asymmetrical boards perofrmance wise or otherwise. e.g. could I go with lighter and smaller asymmetric boards, compared to symmetric? (Easier handling)


Regards

Alan
For a performance cat I would chose asymetric boards that can be identical if the boards are changed over by lifting them out of the casing and dropping them in upside down if ever necessary however this will be seldom if ever so just design the ideal shape .
I would normally sail with the windward board down in strong winds adding to safety since if the hull starts to lift the boat will slide away .
In normal conditions that is no issue.

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Old 17-06-2008, 08:05   #17
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I don't believe a modern cat will "trip" over the leeward board. Wind induced flips are caused by carrying too much sail, and are in fact seldom on modern wide beam cats with a beam/length ratio over 0.5.

Wave induced flips are practically unheard of, as the wave passing under a cat will be moving at such a high speed that it passes the beam of a cat in less than a second. I can't remember where this was proven, someone had it on a link - maybe Richard Woods site.

Pitchpoling is a bigger danger and not affected by daggerboards.

Daggerboard sizing is based on the required lift you need at a given angle of attack (leeway). The next step is to decide what profile you want, and the required strength needed. Aymmetric boards can be made with a shorter chord but need to be relatively stronger as they are handling a larger load per unit area, and probably require the use of more exotic materials.

As in all design it's a compromise....

Symmetrical boards are my choice in this case - no need to complicate things without a major benefit for a cruiser.

Regards

Alan
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Old 18-06-2008, 11:15   #18
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Hallo Alan

regarding your remark pitchpoling is not affected by daggerboards I do not agree.
Any resistance under the water will increase the chance of pitchpoling
It gives extra drag below the centre of gravity so it will increase the chance of pitchpoling.
Having symmetrical boards is good for keels but not for Daggerboards since you gain no lift from them at all while you increase drag with symmetrical boards over asymetrical boards .
You are already going all the way with your fantastic looking cat.
Why stop short of making the boards the ideal types ?

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Gideon
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Old 18-06-2008, 11:17   #19
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Originally Posted by 2Hulls View Post
Not entirely off topic > I recall the old MacGregor 36 had gybing boards. The forward portion of the trunk was intentionally too wide for a snug fit enabling the symmetrical board to "gybe" depending on which tack you were on, thereby angling the board. This supposedly resulted in lift to weather, mimicing an asymmetrical board. I have no idea if this worked as intended or just slowed you down.

Dave
You are right Dave
the mac gregor 36 did have jibing boards and it did give more lift but also slowed the cat down, they where only in the right position going upwind.
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Old 18-06-2008, 14:50   #20
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Having symmetrical boards is good for keels but not for Daggerboards since you gain no lift from them at all while you increase drag with symmetrical boards over asymetrical boards .
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Gideon
Symmetrical boards give no lift? Could you explain that for me please Gideon? I thought that they generated lift - maybe something I misunderstood??

Regards

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Old 18-06-2008, 19:57   #21
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The MacGregor 36 boards rattled something awful if you were below a tight reach, and there was a lot of turbulence from the wide open slot sucking air down the slot and probably destroying some of the effective lift.

The Stiletto 27's had fold-down boards in the middle of the boat, and they suffered from deep cavitation because they penetrated the water surface and allowed air to be pulled down the low pressure area on the board. For smooth flow (and no cavitation) keels need the end-plate effect of the hull, or a complicated end plate that can stay parallel to the water flow. The keel to hull join is very important and receives considerable testing on the AC boats.

I understand that cavitation is not strictly the right term for what goes on when a foil penetrates water, but it seems to convey the concept better that "airation". You could really see it on the stiletto, and hear it on the McGregor. My Stiletto 30 had pivoting centerboards, with a thick rubber seal to close the slot behind the boards when they were down. It worked to make the boat faster, but was a RPITA to maintain. Fun boat, though!
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Old 19-06-2008, 15:05   #22
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Originally Posted by Nordic cat View Post
Symmetrical boards give no lift? Could you explain that for me please Gideon? I thought that they generated lift - maybe something I misunderstood??

Regards

Alan
Hallo Alan

When the cord lenght is exactly the same on each side the fluid or air will travel at the same speed so no lift is generated. Only if one side of the cord is longer will the air or water have to travel a longer route and lift is generated on the longer cord side.
Just look at any aircrafts wing the top has the curve while the bottom is ( always ) flat to keep the distance traveled on the bottom minimal.
A slow airplane will have a fatter wing to create enough lift.

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Old 19-06-2008, 15:30   #23
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My previous post was meant to be a bit "tongue in cheek", it seems that a better understanding of foils would be in order before making categoric statements

All foils generate lift at a certain angle of attack. A symmmetrical foil generates lift due to the leeway created by the force of the wind on the sails, rigging and hulls.

Lift is dependent on speed, foil shape and size as well as angle of attack.

One can select a foil with the desired characteristics for a given set of parameters, but as in all design issues they will be a compromise.

According to your statement above Gideon, a keel on a monohull will not generate lift as it is symmetrical in shape!!!!

For some very good information on foils have a look here: Tom Speer - Home -

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Alan
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Old 19-06-2008, 22:01   #24
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Open 60 Daggerboard

Here is the hi-tech solution on the Open 60's:

This is looking down at the upper (deck level) dagger board bearing. The keel is there for stability, while the dagger boards carry side forces. These are angled to the center so the hull can go straight through the water. Note the asymmetric foil shape. Just guessing now, but they look a lot like a 68 series laminar section. Very efficient. Of course, you have to tack the boards - only the leeward board can be down at one time. So there are two control lines led aft for each board. One forces the board down, while the other leads to a block on the mast for raising the board
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Old 20-06-2008, 05:29   #25
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Thanks for the picture Brian.

There are advantages to using asymmetric boards, especially on monohulls, where you have a symmetric keel as well.

For downwind work, and better tracking, it might be necessary to have the boards slightly immersed, and 2 asymmetrics will be working against each other..

The optimal solution for a cat woulod probably be boards that are symmetric at the bottom and morphing to asymmetric for the top 75%.

The foil in the picture above looks like it has a limited envelope of use as befits a racer. A cruiser needs a more forgiving leading edge shape (rounder) so that you don't lose lift when the smaller boat yaws. Longer boats at high speed don't yaw as much and can better keep the flow attached.

These Open 60's are fully powered up in less than 10 knots of wind, whereas a cruising cat is first fully powered up at say 20-22 knots of true wind.

Horses for courses again, but really interesting to see real life pictures...


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Old 20-06-2008, 08:25   #26
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Quote:
There are advantages to using asymmetric boards, especially on monohulls, where you have a symmetric keel as well.

And why not catamarans also??

Quote:
For downwind work, and better tracking, it might be necessary to have the boards slightly immersed, and 2 asymmetrics will be working against each other.

For downwind sailing on a cat you will likely have to use very little board at all, and its asymmetry will hardly be noticed at all

Quote:
The optimal solution for a cat would probably be boards that are symmetric at the bottom and morphing to asymmetric for the top 75%.

Donít know that I understand this? And would surely make things very complicated.

Quote:
The foil in the picture above looks like it has a limited envelope of use as befits a racer. A cruiser needs a more forgiving leading edge shape (rounder) so that you don't lose lift when the smaller boat yaws. Longer boats at high speed don't yaw as much and can better keep the flow attached

Many different foil sections will work just fine, just some may be slightly more efficient than others. These open 60ís certainly present the leading edges of their foils to very demanding variations of incoming flow, so the choice of this particular foil section was not considered lightly. It really is only very slender foil sections with Ďpointy endsí that present the real possibility of stalling.



Here is a contribution by Malcolm Tennant

If you go to Abbott & Von Doenhoff "Theory Of Wing Sections" you will find out more about wing sections [plan forms are another matter] than you probably ever wanted to know. You will find it is obvious as you look at the data for the airfoils that the asymmetric sections have a better lift to drag ratio ie: for foils of a given area, aspect ratio and percentage thickness the asymmetrical section may develop as much as 50% more lift for the same amount of drag. It may also do this over a greater range of angles of attack. So why aren't asymmetric section foils more widely used? I was glad to hear that the 20% foil was a typo. It seemed more like a lifting body! I have used a number of different foil sections over 40 odd years and kept coming back to the good old traditional NACA0012 [I actually use a slightly thicker 13% section] for the rudders and a more laminar flow section, the 65-013 for the symmetrical dagger boards. Asymmetrical foils are usually 64A413 sections, or as close as I can get them without CNC 5 axis milling.

Abbot & Von Doenhoff is one of the most heavily thumbed volumes in my reference library.

Regards,
Malcolm Tennant
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