Seattle Lasers,
All the light wind lately has made me think of the Centerboard. What is its purpose? How does it work? How can it be made to work more effectively? What is a jybing centerboard?
Purpose
The centerboard simply prevents the boat from sliding sideways when trying to
sail upwind. Lift your board half way out of the trunk and try to sail
upwind. You will notice the boat will move both forward and sideways and the helm
will feel very different. The term for the sideways movement is "leeway".
The purpose of the centerboard is to minimize leeway and maximize fwd motion.
How it works
Think first above the water. While sailing upwind, wind hits the sail at
about a 40 degree angle from the boats centerline. As wind flows around the
asymmetric sail, low pressure builds on the lee side and high pressure builds on
the windward side. This pressure delta provides the lift that allows you to
sail close to the wind. It is important to realize this pressure delta happens
because the wind has to travel a slightly longer path on the lee side as
compared to the windward side.
Now think below the water. The leeway (discussed earlier) creates a situation where the water flows over the centerboard similar to the wind flowing over the sail. A big difference is that the water flows from an angle to leeward of your boat centerline. Therefore the water flows over the centerboard so that the low pressure (longer path) happens on the windward side and the high pressure happens on the lee side. This situation causes the lift generated by the centerboard to nicely balance the lift generated by the sail and allow the boat to sail efficiently to weather.
How can it be made to work more effective? If you have chips, bends, or cracks in your centerboard it is equivalent to having a torn or blown out sail. Sighting down the trailing and leading edge of your board you should see a straight line. High heat can warp a blade. If you are having problems keeping a lane, check this. If heat can warp it, it can also straighten it, however you must setup a rigid clamp. I've done this once by wrapping a black hefty bag around the board while in the clamp. I let it sit through a couple of heat / cool cycles in my back yard and after a couple days it was straight as an arrow.
While sailing to windward, if you allow your board to rise up 3 to 4 inches you are losing that much lift that the board is providing to keep you from sliding to leeward. Would you reef your sail in light winds? Take advantage of every square inch the blade provides.
What is a jybing centerboard?
This is always something I've heard people say, but never really understood.
The class rules for boats I've sailed do not allow a jybing board so I've
never taken the time to learn. I think it has something to do with the way the
board sits in the trunk. In the Laser, the leading edge is forced against the
windward side of the trunk and the trailing edge is forced against the lee
side. The slop in the trunk along with the leeway angel forces this to happen.
With a jybing board, I believe they put shims in the aft part of the trunk to
keep the trailing edge always on centerline. The leading edge has some kind
of adjustable shim that allows control of how much it is allowed to "gybe"
every time you tack. In light wind you would want a different trim angle on the
board than higher wind just as is true while trimming the sail. The sail is a
"gybing" foil in reverse in that the mast is stationary and the leech angle is
controlled with the main sheet. If my theory is off base, and you understand
it, please let us all know.
Conclusion
When trying to hold a lane, the centerboard plays an equal role in your
success as your sail. You must balance the flow angles over the sail and
centerboard. To get a better feel for the importance of the board, try sailing upwind
with it up 12 inches. Note the helm feels very strange and the boat slides
sideways. Put the board down two inches and sail for another while. Note how
the feel in the helm changes for each 2 inch increment until it is all the way
down. It is very informative to do this with another boat while lined up for
upwind sailing.
Mike Johnson
161978
Received on Thu Jun 5 08:46:16 2003