Do Keels need to be aerofoils?

[upchurchmr]

Interesting to see the different attitudes
1. Looking for an answer
2. Trying to answer with some scientific/ engineering justification
3. Wild eyed questions supposedly implying something.
4. The ever present "why isn't a poor boat good enough, it was for my grandad"

Even the fishermen wanted a fast boat, to get to the fishing quicker, and back to the buyers before everyone else sold their catch.

Cargo vessels, ferries, and especially warships all want to be able to go fast - the first two for economics and the last to survive.

Yes there are square wings, look at Cessna (actually that's not fair, but what the heck)

Daquari, if we look at your illustration shouldn't the bottom of the keel be sharp? Actually I've see that exact illustration before at work and it depended upon an angle of attack being large >20 degrees. Does it apply where the angle would be 3-5 degrees? Especially if the end is rounded?

[BATAAN]

Originally Posted by tunnels

Thats the perfect place to try a square keel and a foil shaped keel !!
USE A GPS TO CHECK YOUR SPEED !!
Racing is where inventive and creative people dare to venture and prove there ideas are better than others because they go faster . if we didnt have racing we would still be walking in bare feet dragging a cub and eating taw meat .

Here's "dragging a cub and eating taw meat", not racing, just doing a job.

[BATAAN]

Small AOA benefits from hydrodynamic lift of foil section. Most boats with larger AOA get the lift from "impact" of lee side of keel and immersed topsides pushing against the water. Careful fairing of surfaces and keel entrance/exit will always help flow and efficiency, but in this case there is little effect.
A light displacement 10 footer with long dagger board and comparable rudder and rig would benefit most from careful foil analysis.

[daiquiri]

Originally Posted by upchurchmr

Daquari, if we look at your illustration shouldn't the bottom of the keel be sharp? Actually I've see that exact illustration before at work and it depended upon an angle of attack being large >20 degrees. Does it apply where the angle would be 3-5 degrees? Especially if the end is rounded?

The illustration is not mine, of course. I have taken it from J.D. Anderson's "Fundamentals of Aerodynamics" (it is used in several other books and research papers), but the credit for the original image goes to mr. John Stollery of Cranfield Institute of Technology. I have just added the colors and eliminated the right semi-wing, to make it resemble a keel attached to a hull (represented by the solid block). This is the original image:


One significant characteristics of low-AR keels is that they produce much less lift for a given angle of attack than a high-AR ones - about half or even less. It means that a low-AR keel has to work at twice (or more) higher angle of attack in order to produce the same lift as a high-AR keel. So, while 3-5 degrees which you have mentioned might be a typical leeway angle for a fin-keel boat, it will be twice that value in case of very low-AR keel which has to produce the same lateral force. In other words, 3-5 degrees angle of attack will not really be of interest in this particular case - around 8-10 degrees will be more likely (everything else being equal - keel area, lateral force).

As about sharp leading edge vs. round leading edge delta wings, you are correct when you say that the above picture is valid for sharp-LE wings. In this case the flow separation occurs right at the leading edge, and the strong primary vortex is created along the whole leading edge, visible in the picture above.

If the leading edge is rounded, the flow will not separate immediately. The flow will remain attached as it moves over the leading-edge curvature, and will detach to form a vortex later along the streamline. The primary vortex will still be there, but will be shifted more backwards and will be less strong. Another vortex will appear towards central portions of the keel/wing, and it is called inner vortex:

(picture taken from Ph.D Thesis of Shabudin Bin: http://theses.gla.ac.uk/2387/ - hope he or Glasgow University wouldn't mind)

The overall strength of this vortex tandem on a round-LE keel will be roughly the same as the strength of the single primary vortex on a sharp-LE keel but (since the vorticity is spread over two vortices in former case) the dissipation of energy (drag force) will be smaller for a keel with a rounded leading-edge.

This is reflected in the following graph, which shows the difference between L/D curves of delta wings with sharp and rounded leading-edges:

(From "Fundamentals of Aerodynamics" by J.D. Anderson Jr.)

Cheers

[tunnels]

Originally Posted by upchurchmr

Interesting to see the different attitudes
1. Looking for an answer
2. Trying to answer with some scientific/ engineering justification
3. Wild eyed questions supposedly implying something.
4. The ever present "why isn't a poor boat good enough, it was for my grandad"

Even the fishermen wanted a fast boat, to get to the fishing quicker, and back to the buyers before everyone else sold their catch.

Cargo vessels, ferries, and especially warships all want to be able to go fast - the first two for economics and the last to survive.

Yes there are square wings, look at Cessna (actually that's not fair, but what the heck)

Daquari, if we look at your illustration shouldn't the bottom of the keel be sharp? Actually I've see that exact illustration before at work and it depended upon an angle of attack being large >20 degrees. Does it apply where the angle would be 3-5 degrees? Especially if the end is rounded?

Especially if the end is rounded?[/
If the ends are rounded then you are begining to create a foil !! to better the flow of water .

Yes there are square wings, look at Cessna the cessne wing is not square !! it has shape it has a foil !!

Square means just that !!!,90 degrees flats !! square !!!as in sharp corners as in box shaped !!

[stonedpirate]

I never said square ends.

Just a flat board with a round nose.

Its a 10 footer so its speed is limited by its waterline, it wont go fast regardless of the keel, or hull or whatever.

Was more interested in windward performance.

Would having a shaped keel give me a few extra degrees to windward?

It is a round bilge hull with keel stuck on the bottom, not molded onto the hull.

[Chase_B]

Just curious has anyone ever attempted to try vorticy generators on any part of a boat hull?

[BATAAN]

Shaped keel in this case would reduce drag slightly, giving possible apparent better windward performance, but won't point higher due to "lift".

[Chase_B]

I was thinking for speed, but I wasn't taking into consideration the VG causing seperation which would not be good

[BATAAN]

Anything reducing drag helps speed.

[Chase_B]

Well what I was thinking was a grid of micro vorticy generators placed on a hull to create air between the hull and the water,
With aircraft vorticy generators help the wing travel at a slower speed before the wing stalls, but they do not change the maximum speed the wing can travel before reaching high speed stall, on a boat I'm thinking micro VG's would allow a 20 knot hull to achieve a faster speed by decreasing drag, because the VG' s would put a layer of air between the hull and the water, the trick would be conforming the VG's so they themselves would not create more drag that would null the advantage