Centerboard performance

[Quoddy]

I've heard it said, in regards to centerboard position.
“Close reach down. Beam reach halfway down. Broad reach up.”
As I understand it, the centerboard creates lift and drag. It seems the only time you want lift is when sailing to windward. So I would think for best performance you would put the centerboard fully down when sailing windward (best lift to drag ratio) but keep it up otherwise.
Can anyone elaborate on how to use the centerboard for maximum performance?

 

[JerryA]

Quoddy,
In general you're on track. With the board up you will slide more sideways when going upwind. I will sometimes raise my centerboard to reduce heal too. I have a shoal keel on my P23 even when the board is up, so I don't know how well that will work on your Hunter. Play with it in different situations. Downwind it's just drag generally.
JerryA

 

[Joseph Shirley]

I believe that you get very little lift off off of a centerboard because it is symmetrical, especially in a horizontal plane. There is a lift off of a 26 foot Thunderbird keel however, because it is a tall triangle which is thin at the top at the hull to the max width at the bottom; so that when the boat is heeled, the horizontal chord on the windward side is greater than the chord on the leeward, like an airplane wing.

What your centerboard does provide however is resistance against both forward speed and side slip, which allows you to go to winward. The closer you sail to the wind the more important resistance to side slip or leeway becomes. As you get further off the wind the less important it becomes until with the wind abeam you can compensate for the leeway with the rudder, and with the wind aft there is no leeway other than your course.

Obviously dead down wind you want to reduce your drag so your board is up. That means that everything between full and bye (close reach), and dead down is a trade off. A serious racer will play the board when allowed by the rules. PHRF in my area when I was racing didn't allow it. I found out the hard way by playing the board, winning, and then being disqualified. Such is life.

Have fun

Joe S

 

[Roger Long]

I believe that you get very little lift off off of a centerboard because it is symmetrical, especially in a horizontal plane. There is a lift off of a 26 foot Thunderbird keel however, because it is a tall triangle which is thin at the top at the hull to the max width at the bottom; so that when the boat is heeled, the horizontal chord on the windward side is greater than the chord on the leeward, like an airplane wing.

I think this gets the award for the most mis-information packed into a single post since I started participating in this forum.

Lift is not dependent upon the difference in curvature between the two sides of an air or hydrofoil although the lift drag and stall characteristics are generally improved by the section being asymetric. Many aircraft have symetrical wings. Centerboards provide plenty of lift. Otherwise, centerboard boats wouldn't go to windward. 99.99% of sailboats have symetrical underwater sections. The asymetry that creates lift is provided by the leeway creating an angle of attack in the water flow.

There was a racing fad in the 70's to produce asymetric keels by adding a trim tap (like a second rudder) the the aft side of fin keels. "Jibing boards" that can twist slightly in the case to produce angle of attack while the hull goes straight through the water have been tried on dinghies. Neither of these attempts to produce hydrofoil asymetry from tack to tack produced results sufficient to move the concepts into the main stream.

 

[tsmwebb]

I think this gets the award for the most mis-information packed into a single post since I started participating in this forum.

It was special...

For the OP: On every point of sail except exactly down wind you need some lift. Your hull will provide some lift even with the centerboard up. But generally speaking as you head more upwind you will want to put more board down. When you are overpowered or moving very fast you might want to use less board. With the board up most centerboard boats will be hard to steer. So, you might not want to sail even directly down wind with the board fully up...

There was a racing fad in the 70's to produce asymetric keels by adding a trim tap (like a second rudder) the the aft side of fin keels. "Jibing boards" that can twist slightly in the case to produce angle of attack while the hull goes straight through the water have been tried on dinghies. Neither of these attempts to produce hydrofoil asymetry from tack to tack produced results sufficient to move the concepts into the main stream.

Trim tabs were quickly outlawed. That probably had more to do with their lack of use than their efficiency. Though they are hard to set correctly. They are still used where allowed (eg IACC -- should they ever sail again). Asymmetric bilge boards are now common additions canting keel yachts. And foiling moths use asymmetric lifting foils... And... Well, you're right 99% of boats don't use any asymmetric hydrofoils but still depend on lift. But IMO, the reasons that they are uncommon have more to do with simplicty, conservatism and a sense of fair play in the yachting community than with strict performace results. Of course, in the context of a simple, slow, heavy, cruising sailboat a trim tab or jibing board would be silly.

--Tom.

 

[Todd Smith]

Also helm balance

As others have said, the centerboard produces lift (also called lateral resitance). Sailing to windward requires the most lift, reaching less and downwind even less.

Assuming a swinging centerboard, not a daggerboard. The other effect of centerboard position is helm balance. Moving the centerboard aft changes the center of lateral resistance and changes the load on the helm. Less load on the helm means less drag on the rudder and faster sailing.

Todd

 

[Joseph Shirley]

Roger, thank you for you insight but you and I evidently studied the subject at very different times. I think however that it would have been more beneficial if you had responded more to the question on the use of the centerboard and less on the semantics of lift.

You did say: "The asymetry that creates lift is provided by the leeway creating an angle of attack in the water flow." and if that is truly what you are referring to as lift it would increase as your leeway increased. So the poorer you sail and the slower you go, the more leeway you develop and the greater your lift. That seems like a negative to me.

Now when I was learning about it, we acknowledged that there was indeed a force caused by the angle of attack by the water flow, which we resolved into two vector forces. One directly opposite to your course which we called drag, and one perpendicular to your course and toward the wind, which we called resistance (to side slip).
On the driving side we resolved the sailpower into two similar vectors of heeling or side force, and the forward drive force. Since the side force was greater than the resistance we would always have leeway, and if the drive was greater than the drag the boat made forward speed. That my friend, is why sailboats go to windward. Not as you say; "Centerboards provide plenty of lift. Otherwise, centerboard boats wouldn't go to windward".
Lift, as you and many others use the term today, is a misnomer.
A boat goes to windward through a resolution of forces like a tidliwink being squeezed between a chip and a table. Not because because of lift.
A centerboard is down when going to windward to maximize the resistance to side slip and reduce the leeway. Off the wind where leeway isn't critical it creates mostly drag and should be pulled up.

Have fun out there

Joe S

 

[tsmwebb]

...
Lift, as you and many others use the term today, is a misnomer.
A boat goes to windward through a resolution of forces like a tidliwink being squeezed between a chip and a table. Not because because of lift.

We had a long discussion on this board:
http://forums.beneteau.sailboatowners.com/showthread.php?t=104149

It is because of lift forces that a boat is "squeezed" and goes to weather.

--Tom.

 

[Todd Smith]

Lift IS lateral resistance

...
Now when I was learning about it, we acknowledged that there was indeed a force caused by the angle of attack by the water flow, which we resolved into two vector forces. One directly opposite to your course which we called drag, and one perpendicular to your course and toward the wind, which we called resistance (to side slip).
...
Joe S

Joe,

What you are calling "resistance (to side slip)" is LIFT as defined by fluid flow texts and aerodynamicists everywhere. Lift does not require an asymetric foil shape, nor even a very rounded one. A flat board can provide lift, just with more leeway and more drag.

The rest of your analysis seems correct ( now that we have identifed the difference in your terminology ). This part:

...
You did say: "The asymetry that creates lift is provided by the leeway creating an angle of attack in the water flow." and if that is truly what you are referring to as lift it would increase as your leeway increased. So the poorer you sail and the slower you go, the more leeway you develop and the greater your lift. That seems like a negative to me.

This might be counter-intuitive but is correct. For a given wind condition and sail trim, there will be a certain side force on the sails. Leeway will increase until the lift ( lateral resistance ) matches the sail's side force.

Remember also the the total lift force will be a product of the angle of attack ( angle of leeway ) and forward speed through the water (squared).

Lift = ( several constants) * AOA * V^2

So the faster you are going, the smaller the angle of leeway is required to balance the force from the sails.

Todd

 

[Bill Roosa]

Check your variables Joseph

If you increase the side slip (Angle of attack) you increase lift. if speed is constant
if you decrease speed you decrease lift for a given angle of attack (side slip)

If you vary both (go slower and increase side slip) you can't tell what you will get.

 

[Quoddy]

How to?

As the forward hinged CB is raised it becomes sweptback. Most of its surface is still exposed until the most retracted position. Doesn't the sweptback effect change the lift -drag-speed relationship as well as moving the CLR aft?
With the H260 a broad reach is the lowest you sail so it appears I should always have some board on any point of sail.
How do I determine how much? Helm /Speed /Leeway. I suppose all of the above but were should I start?

 

[Todd Smith]

Yes, the sweep will effect the lift-drag-speed relationship, but not too much for the first 30 degrees or so.

When sailing on a reach or broad reach, you don't really care how much leeway you are making. That's most important beating to windward.

I would think that balancing the helm would get you pretty close to the fastest speed. If you want then use your knotmeter ( or gps ) to tweak for the fastest. Or you might just want to balance the helm to make tracking easier and ignore speed.

Todd