Daggerboard question

[Christine DeMerchant]

I've just ordered a new sail kit for my Skerry.
Its a balanced lug quite a lot larger than the original sprit. (78 sq feet vs 56 sq ft.)

The balance of the rig is all right and will not change very much with reefing.

I am wondering if I need to also adjust the size of the daggerboard. I imagine that a bigger sail needs to push against a bigger daggerboard.

Maybe it doesn't make much difference in that size range.

Does anyone know?
Christine

[Hwyl]

Probably not, the daggerboard is in a much thicker fluid and will peform just the same.

[paladin]

If it's a daggerboard and you think you need a new one, or just want to try a longer one, make it 20% longer then the one you have and the same cross section, leave the end square, but round off the leading and trailing edges about 1 inch in diameter. You will probably see a difference.

[johnw]

Actually, with the larger sail you may find that you are moving faster in light winds, which means the board will be less likely to stall. I'd say don't worry about it.

[Woxbox]

Right -- the faster you go, the less board you need. Just as faster airplanes have shorter wings.

[Todd Bradshaw]

I've built lugs up to 68 sq. ft. for Skerrys and nobody has mentioned their boards needing more area to compensate for the sail area change, so I suspect you're probably OK. The determining factor is likely going to be your ability to keep the heel angle within reason and the board working.

[Killick]

Speaking from my experience in sailing Lasers and other small dagger/centerboard boats, it's not uncommon to retract the board to present less board surface to the water when the wind conditions get gnarly. It makes for a bit more leeway, but can prevent you from an unintentional swim. :-)

If you increase the size of the daggerboard you'll be increasing the boat's lateral resistance which will in turn increase the heeling moment, which may make it more difficult to maintain an acceptable angle of heel, particularly in stronger winds.

[johnw]

Plus, the water's leverage will be lower, giving it a longer heeling arm.

[Christine DeMerchant]

I am puzzled now. I always thought that the push of the sail was resisted by the centerboard which then permitted the air to flow across the sail and the differential in air pressure moved the boat. The heeling motion being resisted by the weight of the sailor.
How is it that when there is alot of wind you might want to reduce the daggerboard when the pressure on the sail is larger?
(I am quite a beginner)
This sail is intended to be used full only when the wind is quite moderate, or I'm feeling particularly brave.
I have been thinking of making a slightly thicker board since mine is a bit sloppy. It would be simple to make it a bit longer as well.
I don't understand the meaning of "stalling" when referred to a board.
Thank you for your help
Christine

[dredbob]

In the same manner that the sail interacts aerodynamically with the wind to be able to move the boat to windward, the daggerboard (or centerboard, or keel) interacts hydrodynamically with the water to produce a lift force which resists the sideslip of the hull. The faster the boat is moving, the more lift is produced. Just like an airplane wing, if the boat moves too slow, the board "stalls", which just means ceases to produce lift. An airplane falls out of the sky. A sailboat just starts to make more sideslip, or leeway. That's the explanation in a nutshell. A good basic sailing text, like David Seidman's _The Complete Sailor_, will explain this and a lot more.

Bob

[johnw]

I am puzzled now. I always thought that the push of the sail was resisted by the centerboard which then permitted the air to flow across the sail and the differential in air pressure moved the boat. The heeling motion being resisted by the weight of the sailor.
How is it that when there is alot of wind you might want to reduce the daggerboard when the pressure on the sail is larger?
(I am quite a beginner)
This sail is intended to be used full only when the wind is quite moderate, or I'm feeling particularly brave.
I have been thinking of making a slightly thicker board since mine is a bit sloppy. It would be simple to make it a bit longer as well.
I don't understand the meaning of "stalling" when referred to a board.
Thank you for your help
Christine

The heeling force is created by the rig's leverage above the water and the keel or centerboard's leverage below the water, with the whole arrangement basically pivoting at the waterline. In a keelboat, a deep keel helps keep the boat upright because of its ballast. The deeper the keel, the farther the ballast moves off the centerline as you heel any given angle.

In a centerboard boat, the centerboard doesn't create stability as you heel unless it's very heavy, it's just the weight of the crew and the shift in center of buoyancy as the boat heels that gives you stability.

In a strong wind, you can reef the sail to reduce heeling force, sit out farther to increase stability, or pull the board part of the way up so that part of the energy that would heel the boat instead goes into leeway.

[Christine DeMerchant]

Thank You!
I get it!! I love a nice ah! ha! moment.
I never thought of the centerboard as having lift. Its obvious now. It explains a few things including the importance of the shaping.
Funny you should mention the Complete Sailor. I've just started reading it a second time. It seems so much more informative than the first time I read it. I guess I'm not so quick in understanding all the new concepts as I would like to be.
OK, tomorrow I will sail and I will watch for the effect of the board with new eyes.
Thanks guys.
Christine

[dredbob]

=dredbob; A good basic sailing text, like David Seidman's _The Complete Sailor_, will explain this and a lot more.

After thumbing though my copy of this book this morning, I see that he does not really explain this. A glance through several other similar books turns up a general lack of detail on the interplay between board and water. No wonder folks don't get it. I guess you have to read some of the boat design oriented books to get detailed information.

Bob

[Christine DeMerchant]

Even if The Complete Sailor had the information I'm not sure I would have understood. Understanding only comes after the basic information is digested. If the framework is not there NO CAPISH.
I'm not sure that many sailors understand the theory very well. They might be able to react instinctively because of experience but not why.
Off the the big box to get spar material for the new sail.

Christine

[Killick]

A simple diagram to help illustrate the concept.
The wind acting on the sails produces a sideways force above the waterline.
The daggerboard resists this by a (nearly) equal force in the opposite direction below the waterline, making the boat want to pivot (heel). (Think of a teeter-totter rotated 90 degress with people pushing on the ends from opposite sides -- it'll want to rotate.)

This heeling moment must be counteracted by weight on the windward side (or a heavy keel will do the same trick).

A shorter daggerboard (or lifting the 'board somewhat) will reduce the lateral resistance force and also raise it a bit, resulting in a lower overall heeling moment. Thus less weight/effort is required to keep the boat at an acceptable angle of heel. The downside is a bit more sideways drift (leeway), but this is often an acceptable compromise.

Tony
p.s. Sorry for the attachment. I don't currently have a service where I can upload the image and post a link to it.

[johnw]

Killick, try Picasa. It's free, and it's Google, so it's unlikely to go away.