[matthewwhill]

I have read that the keel is a foil that creates lift. But I am confused.

So it does more than resist leeway (which seems like drag against leeway)?

What causes the lift? Water flow? Keels aren't shaped like a sail or a wing with one side rounded and one flatter.

What direction does this lift vector have? Straight ahead?

If water flow is perpendicular to the direction of the boat, which direction is the lift? Forward?

Thanks in advance. I am ready for the deluge of replies.

 

[Fstbttms]

Have a look:

Keel Lift- a misnomer | CNNews

 

[Puddin'_Tain]

A keel is a symmetric foil, rather than an asymmetric foil. Lift is produced because the keel has a certain angle of attack relative to the flow of the water. As your boat moves forward, it is also moving a bit downwind. Sailors usually refer to this "sideways" movement as leeway. This produces an asymmetric flow, with the apparent direction of the flow coming from slightly to the downwind side. If the flow were coming from directly in front of the keel the pressure distribution around the keel would be symmetric and there would be no net lift (left-hand figure, below). But, since the flow is asymmetric the pressure distribution is also asymmetric and there is net lift on the windward side of the keel/boat (right-hand figure, below).

Original figure legend:
Pressure Distribution
The green lines indicate a negative relative pressure (suction) and red lines positive, and the arrows indicate the resultant forces associated with the pressure distribution. It should be noted that as the angle of incidence increases, the bottom surface may in fact experience a positive relative pressure. The green arrows indicate the lift associated with each side of the foil, and the red arrow indicates the drag associated with the pressures. It should be noted that as pressure forces act normal to surfaces that these arrows are the cumulative forces of the pressure distribution integrated over the foil. That this foil shown is symmetric is to indicate that the pressure forces themselves are symmetric when the flow conditions are symmetric, and the foil only generates "lift" in one direction when it is cocked in that direction to the flow.

 

[capecodda]

Even at flat board will create lift at reasonable angles of attack. Stick your board out the window of your car and hold on...let someone else drive

 

[Frogwatch]

Normal keels produce almost no net vertical lift. A "winged " keel maybe but not a normal keel.

 

[christian.hess]

I forget where I read about the common missunderstadnings of keel and lift and whatnot and basically the above was "kind" of the end result....or findings if you will

a lot of hooplah surrounding how much lift, resistance, weight etc...flow...

it was an interesting article....

 

[denniscloutier]

What Puddin Tain said. The keel produces horizontal lift, which opposes the sideways forces on the sail and keeps the boat from just being pushed leeward. The angle of attack of the keel is the difference between where the bow is pointed and your actual direction of travel.

 

[christian.hess]

however its the actual ballast that is more important...

the importance of the lift cannot be denied but for sailing purposes on normal fin keel(no wings bulbs etc) its actual ballast in relation to sail area and wind strength that determines say pointing ability, heel angle etc...

man I wish I could find this article

basically people would focus too much on the lift part of the equation when in reality the biggest factors on any sailboat is the actual shape of the keel and its ballast in relation to sail area and mast height, forces etc...

 

[denniscloutier]

The lift and the weight are working together. The lift on the keel is in the opposite direction from the sail's sideways force, so it opposes leeway. But since these two forces are opposite they create a moment which wants to roll the boat over on its side. Gravity force downwards on the keel opposes this moment when the boat heels because the keel is moved to the windward side of the boat's centre of buoyancy.

A dinghy's daggerboard does the first part of the keel's job, but since it doesn't weigh anything there is no force counteracting the moment. You need to use your body weight to do that.

By the way, this is why a longer keel can weigh less. The further the keel weight is from the centre of buoyancy the lighter it can be and still provide the same moment.

 

[Puddin'_Tain]

however its the actual ballast that is more important...

the importance of the lift cannot be denied but for sailing purposes on normal fin keel(no wings bulbs etc) its actual ballast in relation to sail area and wind strength that determines say pointing ability, heel angle etc...

man I wish I could find this article

basically people would focus too much on the lift part of the equation when in reality the biggest factors on any sailboat is the actual shape of the keel and its ballast in relation to sail area and mast height, forces etc...

The importance of the lift produced by the keel is not the vertical component of that lift, it is the horizontal component. Keel lift does not resist heeling; in fact, it contributes to heeling (at least to a small extent; even at fairly high heel angles most of the lift will still be horizontal). The importance of keel lift is that it keeps the boat from simply slipping sideways as the wind impacts the sails. If you were to hang a huge lead ball under a boat at the end of a stiff cylinder heel would be minimized, but there would be no net left and the boat couldn't go upwind. Conversely, a centerboard dinghy was virtually no ballast in its "keel" (it resists heeling via the buoyancy of its hull and internal ballast; i.e., the crew), but it resists moving sideways due to the lift generated by the centerboard.

Remember, the lift produced by most keels is NOT vertical, it is horizontal (winged keels being the exception; but even then MOST of the lift is horizontal).

EDIT: Doh! Dennis beat me to it.

 

[christian.hess]

yeah yeah Im aware...im just saying that people focus too much on one aspect of the "keel" and its role in moving forward...

cheers

 

[christian.hess]

The lift and the weight are working together. The lift on the keel is in the opposite direction from the sail's sideways force, so it opposes leeway. But since these two forces are opposite they create a moment which wants to roll the boat over on its side. Gravity force downwards on the keel opposes this moment when the boat heels because the keel is moved to the windward side of the boat's centre of buoyancy.

A dinghy's daggerboard does the first part of the keel's job, but since it doesn't weigh anything there is no force counteracting the moment. You need to use your body weight to do that.

By the way, this is why a longer keel can weigh less. The further the keel weight is from the centre of buoyancy the lighter it can be and still provide the same moment.

completely aware of that...

 

[jackdale]

yeah yeah Im aware...im just saying that people focus too much on one aspect of the "keel" and its role in moving forward...

cheers

Lift the keel and then try sailing to weather.

Do not ask how I know.

 

[christian.hess]

geeze I shoulnt of even commented...feel like Im in school again jajaja

Im not arguing with you guys...Im arguing with the focus on one sole aspect of keel performance

notice how much keels shape changes on similar fins? you can have a flat j fin or a bulby older design fin, then a nacra type foil etc...

basically any shape out there and they will all work...

ballast dependant

with that im out!

take care guys

 

[christian.hess]

Lift the keel and then try sailing to weather.

Do not ask how I know.

what sort of lifting keel keel boat do you have?

 

[bobperry]

Ballast amount/location and lift are two separate but equal parts of any keel design.I am always very concerned with the Lift aspect as I am with the ballasting aspect. No designer interested in performance would priorities one aspect of keel design over another. There are often compromises involved and that's where designer skill comes into play.

I wrote an article exploring keel design for GOOD OLD BOAT.

 

[christian.hess]

bingo Bob

thats what I was trying to stress...jajaja

now wheres my lamb shanks???

 

[christian.hess]

Ballast amount/location and lift are two separate but equal parts of any keel design.I am always very concerned with the Lift aspect as I am with the ballasting aspect. No designer interested in performance would priorities one aspect of keel design over another. There are often compromises involved and that's where designer skill comes into play.

I wrote an article exploring keel design for GOOD OLD BOAT.

maybe we can post that link here? jeje

 

[bobperry]

Christian:
We ate your lamb shanks on Monday night. My neighbor is moving and I made him a going away meal. He requested lamb shanks in a Bob sauce. They were superb. I ate two.

But they are on sale now at my grocery store $4.99 lb. I can do them again. All you have to do is to get here!

 

[christian.hess]

good stuff...! someday

take care