foil shapes

will asymmetrical foils produce more lift than a comparable symmetrical one? say a daggerboard or keel fin for an example. Everything i have read and done suggests that they do.

 

asy /sym

The answer is yes,generally. If you compare a 63412 asymetrical section to a similar(t/c ratio) foil that is symetrical the 63412 first of all develops lift at 0° where the symetrical foil does not and in fact has about the same Cl(lift coefficient) at 0° as the symetrical foil does at around 2.5°angle of attack. The 63412 section in particular has a much higher maximum Cl before stall than many comparable symetrical sections.This info is related directly to some current sailing hydrofoils: the Rave uses a symetrical section and the Moth uses a 63412 section. The Race mainfoil is mounted with a + angle of incidence of 2.5° where the Moth foil is mounted at 0° angle of incidence but sails from probably around +5°(angle of attack) down to close to zero(angle of attack) as the wind picks up.The angle of incidence is usually referenced to something like the static waterline; the angle of attack is the angle the foil actually makes to the oncoming flow.
The symetrical section can have a "drag bucket"(and so can asy sections-just in a different location) which you can see represented for many symetrical sections in Abbott and Doenhoff where the drag for the section takes a sudden leap downward within around 2 or 3 degrees of zero.
If you have access to Steve Killings book"Yacht Design Explained" you can find a good illustration of the properties of two symetrical foils:0010 with no "drag bucket" but high stall angle and 64A010 that has a drag bucket but stalls substantially earlier.Killing thinks the 64A010 is the better section for a keel because of the lower drag at small angles.

 

Only in one direction. I assume you mean lift/drag or stall angle as Doug said. I've never understood the small angle arument. Everything I've read says that even the keel usually operates at a minimum angle of about 3-4 degs except on a dead run, and those buckets all look like they're maybe 2 degs wide at the most.

 

drag buckett

Killings argument was that the drag bucket could produce a significant gain downwind as compared to a foil w/o one...
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There are patents on systems to make keel fins develop an asymetrical section from tack to tack
-not just flap/trim tabs.

 

Is there anywhere to get more information on these patented designs that alter keel symmetry from tack - tack?

Also how effective are the keel trim tabs? I imagine they would induce more drag, but would the extra lift outweigh the drag?


Thanks

Mike

 

Problem with asymmetrical foils is that it only work on one tack......unless it is applied to twin keels to gain the advantage

 

patents

You'd have to research the patents; one can be found by looking under one of the co-inventor's: Will Gorgen.
I think there are discussions about this in Marchaj and Gutelle.

 

thanks everyone for your responses.

 

open 60's etc use asymmetric section foils on twin daggerboards. ( with a canting keel for RM )

the windward foil is retracted, so only one foil is working the 'right' way,

the hull does not track parallel to the centreline due to the very wide flat shape sailed heeled- the foils are angled to the centreline to get the required angle of attack.

 

the reason i was wondering this is because i was looking into forward foil shapes for a canting keel design and wondered whether the retractable daggerboard with trim tab was better than a retractable rudder-type.

 

Canting keel

2, look at it this way: the reason CBTF uses collective control is to remove the "work"of leeway producing from the keel fin so as to allow it to be a shorter chord thicker lower wetted surface section than it would have to be otherwise. If the forward foil does not do this then the keel fin, bulb and hull will produce induced drag and you will have "Triple Draggy Foils".
So whether you turn the foil(and get sued / pay the patent fee) or use a trim tab for partial results or use wings you have to do something or the boat will be slow upwind.Maybe a gybing board is legal but I doubt it in CBTFco's eyes.
Why don't you contact CBTFco and at least talk to them?

 

thats actually a very good idea.I think i may contact them and ask them a few questions. A CBTF keel section is no different from a TMF keel section as they are both removed from the job of producing lift. A gybing board would probably be legal as long as one does not have any control of its angle.

 

Gybing boards have been used for years, since I was a kid if not before. I remember them in the International Cadet class in the '60s. Prior Art, even if unpatented....

 

gybing boards

Gybing boards like you say have been around for years; so have twin rudders.
As I understand it they are only a problem with infringement when used in conjunction with a canting keel. I'm no patent expert but I believe that is the position of CBTFco.

 

i know that the twin rudders produce the zero leeway effect and sometimes negative leeway, but an asymmetrical board (in this case one with a trim tab) would create better lift even with the induced drag created by the board. in either case (rudder vs daggerboard) your going to get some form of induced drag.