Loop Keel

[higgs]

Last month's Sail had a short article on the loop keel. Interesting idea. One can have it retrofitted. With a full keeler i'm not considering it. Here is what the manufactures say:

The 'Loop Keel' is a new form of keel that creates a faster, safer, more seaworthy yacht, but with the added advantages of a kinder motion and greater comfort at sea.
We have spent three years developing the loop keel from the initial concept. The process has involved building seven different prototypes and conducting two sets of tank tests at the Wolfson Unit in Southampton. These, in turn, have been followed up with successful physical towing and sailing tests at Grafham Water Sailing Club.
By applying such rigorous testing methods, we have developed a product for the marine market, which offers significant benefits in terms of both speed and seaworthiness. As well as fine-tuning the loop keel, our tests have proved that it will outperform a fin keel of the same area and mass, making it a compelling case for every new cruising yacht to be fitted with our product.

 

[Maine Sail]

How..

And how do you sail with that in Maine??? It looks like a giant seine net for lobster pots....

 

[Tim R.]

Interesting Idea. I wonder if they have engineered it for storage on the hard. That's a lot of weight sitting on that loop. Maybe you need to insert a support between the keel and hull to help support the boat's weight.

 

[sailingdog]

Maine Sail—

I was just thinking the same thing when I saw it... you'd be cutting pot lines every trip out.

I'd also point out that stuff that seems to work great in the test tank, often fails due to unforeseen conditions in the real world. The lobster pot trapping potential of a loop keel makes it pretty impractical for many areas.

For example, what happens if you hit a deadhead or other semi-floating object, and it gets jammed in the loop??? That would kind of suck—especially if the object were just large enough to wedge there and damage the loop keel's connection points on either side of the boat.

In general, I'm not a big fan of having a boat design that invites foreign object damage so readily.

 

[Tim R.]

Good idea dog. We can sharpen the leading edge so we never get caught again. Of course we may get shot at instead!

 

[higgs]

The keel is attached to the boat's chainplates. The article in sail says that water actually piles up on the windward side of the keel, kind of like having water ballest. This means a stiffer ride so you can carry more sail than a normal keel boat.

 

[Ian4781]

Maine Sail.. I would think that with little modification to the leading edge on the bulb and arking on each side to deflect any lines,that this design might well be more friendly than a fin keel in your area,I might also question that there are design features that would make this a stronger,safer keel than a fin.I love to see new ideas and hope that we can all add to positive development of new ideas.Merry Christmas to all Ian

 

[Ross]

I don't think that I would be very comfortable with that if I ran aground just after high tide and had to lay on that for 12 hours. But for a purpose made boat it might work rather well. Let's see if any of the cup races pick up on the concept. For the coastal sailing crowd I don't see any great gain.

 

[DanW9494]

The power for sailing

comes from the coordination of the foils above and below the water.

This is the opposite direction from Hydroptere, but viva la differance!

I think that when we eventually figure out how to use more efficient ridgid airfoils instead of sails, we will probably be using dynamic hydrofoils underneath. More things to optimize for more potential speed in more seastates.

 

[DanW9494]

These are the training wheels

for the monohullers. Look how well they would fit into the loop keel design hull attachments.

http://www.harken.com/Interviews/DynamicStability.php

I would think that anything that increases stability at higher speeds, loop keel and other ideas, will increase the popularity of sailing. Especially with our wives.

 

[Franklin]

What I question is it's pointing ability. The fin keel is great at pointing because of the fin creating windward pressure. I don't see how this can produce that. Somebody educate me.

 

[BobM]

I am waiting...

For Roger to provide his opinion. It should be interesting and insightful, as always.

I was looking at the juncture between the hull and the attachment points...looks like a nice home for a barnacle.

Maybe we can rig a net on it and fish while we sail.

 

[DanW9494]

Probably asymmetric foils

A fin keel is a symmetric foil- has to work equally for windward lift to either starboard or port. That means it is not as efficient as a good asymmetric foil that always lifts to the same side.
An aerobatic airplane wing that has to fly upside down is symmetric and doesn't have as much lift as a regular airplane asymmetric wing that only needs lift upward. So the Cessna has a slower stall speed and can use a smaller engine because the wing has more lift for the same size wing.
When the loop keel boat is at zero heel, the opposing asymmetric foils cancel each other out. When the boat is heeled 45' then only the windward side would have windward lift (which could be more forceful than a good symmetric fin keel with less wetted surface area), while the leeward foil would be pulling the boat down. At more common heeling angles it would be a mixture. And that could be optimized by having different amounts of lift at different distances from the bulb.
Asymmetric foils (like a sailplane) don't have to have more drag than symmetric foils at slow speeds, but are so much more efficient at lift, that the wetted surface area could be less by making them smaller. And without much drag, it doesn't slow the boat, except by pulling the boat lower at high angles of heel when there is very high wind forces to compensate.
But, I don't doubt there are other explanations

addendum: the fin keel loses windward lift as the heeling angle increases while the the loop keel would increase windward lift as the heeling angle increases up to 45'.

 

[Hermit Scott]

Since it pulls the boat down doesn't that increase the wetted drag area? Dan are you guessing at that analysis or do you work for them or something? Cause, it seems to me if there are forces cancelling each other out then there is a component of force that is slowing the boat down. I haven't studied this loop keel, but that seems unlikely. Maybe it's like the bumblebee.?
I never really understood the idea that a symetrical fin keel acts like an airfoil or airplane wing. The reason why I don't see it is because the air foil creates a pressure differential by causeing air to speed up over the wing becasue the air has farther to travel over than underneath. The point where the air splits on the front of the wing must meet at the back of the wing. I don't know why wind feels it MUST do this but nevertheless it does. Fast air on top, lower pressure. Slow air underneath higher pressure, so the wing is pushed up. Everybody knows that, I know.
How does this apply to a symetrical keel moving through an incompressable fluid? I can see how a keel resists leeward slide, but I don't see anyway that it could create a lift to windward. Otherwise if you had a keel with enough lift you could sail into the wind.
Someone set me straight on this.

 

[sailingdog]

I never really understood the idea that a symetrical fin keel acts like an airfoil or airplane wing. The reason why I don't see it is because the air foil creates a pressure differential by causeing air to speed up over the wing becasue the air has farther to travel over than underneath. The point where the air splits on the front of the wing must meet at the back of the wing. I don't know why wind feels it MUST do this but nevertheless it does.

It has to meet back up otherwise there would be an empty space... and nature abhors a vacuum. However, if particles A and B hit the foil at the same time and one goes to the left of the foil and one to the right, there is no guarantee that they'll reach the aft edge of the foil at the same time.

Fast air on top, lower pressure. Slow air underneath higher pressure, so the wing is pushed up. Everybody knows that, I know.
How does this apply to a symetrical keel moving through an incompressable fluid? I can see how a keel resists leeward slide, but I don't see anyway that it could create a lift to windward. Otherwise if you had a keel with enough lift you could sail into the wind.
Someone set me straight on this.

IIRC, with a symmetrical fin keel, the lift is generated by the angle of attack of the keel to the water moving by it, since the boat isn't moving straight forward, but at an angle. Just like a rudder can generate a bit of lift if your boat has a slight bit of weather helm, the keel can generate lift.

Of course, a lot of this depends on the speed the boat is moving at and the design of the keel, with the more radical bulb keels on thin foil-shaped struts only working at significant boat speeds—where fuller keels will work at lower speeds—not by generating lift, but by resisting the sideways slippage.

 

[Roger Long]

I never really understood the idea that a symetrical fin keel acts like an airfoil or airplane wing. The reason why I don't see it is because the air foil creates a pressure differential by causeing air to speed up over the wing becasue the air has farther to travel over than underneath.

Oh groan (not your fault) You don't understand because that almost universally repeated chestnut about lift is wrong. You wouldn't believe the hours I've spent arguing about it on the Internet. That led to my writing two articles for an aviation magazine with a simple, non-mathematical, description of how lift really happens. These were, and probably still are, used by at least one aeronautics professor in his introductory college classes. My quote, "Eggs don't cause chickens." comes from these articles.

North Sails was once going to have me do something on their web site along the same lines but the idea was an enthusiasm of the boss. His staff wasn't really behind it and I was busy with a lot of other stuff so it just slipped through the cracks. I did get a fascinating tour of their main facility though.

If anyone is interested in a simple but true explanation of how air and hydrofoil lift works, ask me after Christmas. I have an animation, created from wind tunnel photographs, which changes the reference point to that of watching a wing move through still air. Once you start seeing the motion of previously still air, as in the case of an airplane, it all becomes very clear. It’s a little more complicated with a boat because you have to mentally put yourself in a hot air balloon drifting with the wind but, once you understand wings, it’s easy.

Everyone have a Merry Christmas in the meantime.

 

[dmc]

Check out u-tube

http://www.youtube.com/watch?v=eebE-4hWzo4

Peace

 

[Todd Smith]

I never really understood the idea that a symetrical fin keel acts like an airfoil or airplane wing.

Arrgg...

But it's ok, a lot of pilot's don't understand either. As both a pilot and aero engineer, I hear a lot of wrong thinking from people who don't really understand how an airfoil works.

See this link for a very thorough explanation.
http://www.av8n.com/how/htm/airfoils.html

I will summarize my important thoughts.

1) airfoils ( wings, keels and sails) produce lift because of Angle-of-Attack
NOT because of shape.

Angle-of-Attack is the angle between the velocity through air (or water) and the centerline of the airfoil.

Symmetrical airfoils work just fine, thank you.

2) The 2 air molecules that split at the leading edge DO NOT meet up at the trailing edge.

Todd Smith

 

[Roger Long]

Don't forget the most important point:

A quantity of fluid must be displaced from where it would have been if the wing/sail/keel had not come along. Every movement of a mass having an opposite reaction, it is the movement of this mass of fluid that results in the force of lift. Most discussions focus on the complex flow and pressure patterns around the foil and forget to make clear this basic principle.

I'm a now inactive pilot. Is Avweb.com still around? Last I looked, they still had my articles on lift archived but you had to be a member to look at them. I let my membership lapse because it's still painful to thing about flying which I loved as much as sailing.

 

[Todd Smith]

Foils and lift

Don't forget the most important point:

A quantity of fluid must be displaced from where it would have been if the wing/sail/keel had not come along. ...

As I see it, that's the least important part.

Displacement of the fluid is a result, not a cause.

You can calculate the lift on a foil from fundamental fluid flow, without ever considering displacement of the fluid. In fact, for a wing of great span, there is practically NO displacement. That's why deep keels and wide aircraft are the most efficient (in terms of lift versus drag).

Todd