Keel design issues
 

In perusing the forum I’ve stumbled across much about foil shape, though I’ve yet to find any discussion concerning:

1) The fin keel - hull interface (root fillet or radius & root leading edge).
2) Fin keel tip shape & tip trailing edge.

What are the present approaches to the above issues?

Regards, RW

One opinion:

The real answer is that you do what you can for the root fillet given other design considerations (space and shape) and that there is not much you can do about the tip because hydrodynamic effects there are not primarily dependent on tip shape (more dependent on pitch, roll, yaw, heave, wave orbital flow effects). Most CFD codes cannot address these problems, which leads to improper anaylsis of what is the "best" for a keel shape.

From my reading Helodriver, best tip shape is straight and level looking from side, and a blunt rounded V section at the tip, from the front elevation. Filleting is advantageous at the root, but rudder root / hull clearance is crucial. Minimum gap or all the good work will come to nothing.

I hope this answers your question.

Keel root
 

I have noticed the leading edge area of root of many fin keels
-to angle forward from the leading edge of the keel a good amount, and have noticed
-that the leading edge of the root area to be quite sharp in comparison to the leading of the over all keel itself.

Any thoughts?

This is what I meant when I said the root fillet was constrained by space and shape. As an exercise, try to lay out a 3D expanding section.

Keel Shapes!
 

I am not going to approach this from strictly a technical viewpoint.Suffice to say I did the compartive tnak testing for Intreepid, 2 versions, and Courageous. On the weekends we took the models off the force balance and with my mentor, Dr. Al Acosta formerly the head of Fluid
Dynamics and Chair Holder at Cal Tech.We worked on a new keel shape by N.A. Morgan Embroden. He used a modified NACA foil shape that was very low and squat to keep the draft to a minimum. We worked at developing this shape to minimize the Von Karman effect of the edeges and to develope all the lift we could from the shape its self. we ended up with a kell for a 32 foot boat with a 28" wide keel tapered from forward to aft and almost flat across the bottom, the keel developed lift on its own above 9.5 knots.A boat was built around this keel design and at 32' loa and beam of 10'8" with draft of 3'4".
It was the first small boat ever allowed in a Transpacific y.c. race. For the first 1800 miles of the race it was the first boat in the fleet of down wind flyers.Going down the channel, she was clocked at 16 knots and throwing a roster tail of 30'. The American's never caught on to the design, butr over 355 in aluminum,fiberglass balsacore,etc. were built in europe. Dr. Acosta was so impresswed that he had one built for his own use. I own the prototype, Teacher's Pet III. U can verify myinformation by going back to the LA Times for the 1971 TransPac.
The reason the design never caught on was simple, my roomate of the period, one Bill,Lee designed his first ultra light and set the sailing community agog with the results of his SC series of boats including Merlin.
For an actual fast cruising boat the Teacher's Pet III series were much better boats.

3D expanding section?
 

You are going to have to "expand" on that statement for my understanding!

I was looking at a ID35 the other day in the lift. It had no more than a one inch radius all the way around the joint, except for the leading edge which was angled forward at approx 45 degrees and very sharp on the leading edge compared to the remainder of the keel's leading edge.

Regards, RW

OK, RW here it is;

I assume you know why you have a root fillet? Root fillets are to minimize the horseshoe vortex at the root. This vortex is formed by the curvature of the keel section causing a low pressure area above the flow along the hull. Pressure then drives the flow away from the hull into the low pressure area. This does two things; 1) it decreases the effective span of the keel and; 2) increase the drag on the hull.

In order to prevent the vortex, the instantaneous change of curvature of the foil must match that of the hull. If you try to do this absolutely then you get something that looks like an old plank-on-edge hull because of the large radius of most hulls rocker. For modern canoe hulls a different approach is used. In modern hulls the idea is not to eliminate the vortex, but to minimize it. We do this by gradually decreasing the radius of curvature of the fillet from the hull to the foil (which also decreases the cord) and fair the rocker to the foil.

How much fairing and at what rate? That's what you pay the designer to stick out his neck on! :D Note that with the above requirements, a large leading edge radius on the fillet would cause a large pressure difference and therefore a large vortex. So the leading edge is eliminated as much as possible and the flow is expected to split on to either side with minimal disturbance at low angles of attack (which is the operational condition).

Finally, two additional points must be considered. On very thin highly loaded keels, the fillet may also be required for structural reasons. And on small, high aspect, keels installed on overpowered dingeys and “sport boats” you’ve got so much power you just don’t care about the vortex unless it interferes with the rudder.

Root fillets
 

I'm slowly digesting all you written and it has begun to make sense.

Thanks much, RW

The latest approach to wing/body fairing is called a "dillet". It's a small, rounded leading edge extension that is faired into the hull.

What's happening is outside the boundary layer, there is high pressure along the stagnation line on the leading edge. But the dynamic pressure in the boundary layer is less than it is outside the boundary layer because the speed of the flow is reduced inside the BL. As a result, when the flow in the boundary layer at a given keel station meets the leading edge, the stagnation pressure is less there than it is at a keel station farther down.

So at the stagnation line, the flow is brought to a stop and there would be less pressure in the direction of the hull. As a result, the flow at the leading edge in the viscinity of the boundary layer wants to flow toward the hull. When it meets the hull, this secondary flow is forced to turn away and spread out. It can't penetrate very far upstream inside the boundary layer against the oncoming flow, so it finally rolls up into a "necklace" vortex that's wrapped around the root of the keel. This is the same as the horseshoe vortex jehardiman mentioned. Here's a picture of the velocity vectors in the plane of a wing/body junction at the leading edge, and with no lift:

(Ref. 1.)
Note how the flow turns down at the right side of the plot, which is up against the leading edge.

The idea of the dillet is to sort of build a ramp out ahead of the leading edge that channels the flow away from the body and opposes the flow moving toward the body along the leading edge. Ref. 4 shows a large fillet geometry all around the wing root that's consistent with modern thinking. Ref. 7 also shows another modern leading edge fillet shape.

The traditional approach to designing fillets for the trailing edge is to start near the maximum thickness and use a constantly increasing radius for the fillet. This results in a sharp trailing edge extension as the fillets on both sides intersect.

See:
1. http://cfd.me.umist.ac.uk/ercofold/d...08/test08.html
2. http://books.nap.edu/books/030904575...5.html#pagetop
3. http://www.diiar.polimi.it/franz/fra...i/montreal.PDF
4. http://www.as.go.dlr.de/~helmut/h85www/H85.pdf
5. http://www.ssa.org/Johnson/87-1997-06.pdf
6. http://university.fluent.com/2003con...7Kpn_paper.pdf
7. http://www.as.go.dlr.de/~helmut/h150www/H150.pdf
8. http://www.as.go.dlr.de/~helmut/h176www/H176.pdf

Tom:

I am always amazed in what you respond AND how quickly and professionally you present your information.

So, here is a new challenge for you. I have a cockpit drain about 40mm in diameter more or less perpendicular to the hull (about 10degrees off vertical in the rocker of the hull). What would be the ideal exit shape or hull intersection shape for this outlet? I am guessing that the leading edge of the hole should remain somewhat sharp while the trailing edge should have some kind of radius. If correct, this would produce a tear-drop shape at the hull.

What is your experience?

Tom,

Your detailed response is greatly appreciated along with the links.

RW

Just one caution about extracting design parameters from aero work. Assure yourself that incompressability issues will not come back to haunt you. In my office we have found that some aero analysis turned out to be not suitable for hydro work.

Simple? Now do you understand? There will be questions after (at the bar :~)

whoa, that's a lot to take in!

Jehardiman...

You room mate was Bill Lee? Thats to kewl! I met Bill back in the 80's when everyone was pushing IOR, and he was saying, lets just see how we can make them faster.

small world.

best, Phil

www.theartofsailing.com

No... I think that was ginotworivers who mentioned Bill Lee. I've only seen him in the yacht club bars and ahead of me around the course in one of those "chicken coop sleds". :D