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Weather Helm Demystified
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- Опубликовано: 11 мар 2008
- This is a clip from a Tech Dinghy "B" classroom lesson at the University of Wisconsin - Madison Hoofers Sailing Club. The rest of the video is forthcoming.
Thanks!...I've been having some issues with my sailing canoe and understanding proper balance....This video made sense..
Good points, Fred. One needs to be careful about equating the centroid of the sail cloth with the center of effort. The CE is actually the centroid of the integrated distribution of lift. Since most of the lift occurs near the leading edge of any foil, the CE will be forward of the centroid of the sail cloth.
Consider the wing shapes of aircraft that are designed for efficiency. They have very long, high-aspect wings to maximize the leading edge relative to the total wing area.
@dressmeister OK, it doesn't look like you understood what I said. Here, the sails do NOT push the boat, the mast pushes the boat, the sails push the mast. Therefore, the boat only feels the force generated by the sails AT the mast.
If you're talking about torque, then it might make sense, as a mainsheet with a centroid further sternward would generate more torque and twist the boat, but comparing the centroid of the hull to the centroid of the mainsheet effectively does nothing without torque
When you let the sail out it reduces the rounding up force "weather helm" and move the CE forward. Even though the front of the sail luffs the tip of the boom is much farther out and more of the power of the leach is spilled out. When you ease the sail more of the force from the sail is directed forward into "speed" vector rather than the "side force" vector, This is especially true at the tip of the boom is farther out. Weather Helm is also developed because of the healing the boat, the form shape of the hull wants to turn the boat into the wind. Most boats have very little weather helm designed into them. If you sail flat and ease the sail slightly in a puff there should be very little weather helm. Sail it Flat, Ease, Hike and Trim (3:54 in this video)
More presicely, and in all examples above, using the square sail example, the CE would be located at the center of the square or rectangular sail with the four corners defined as being from the tack to clew and head to leach. The clew is rarely located at the end of the boom so the centerpoints are actually the centerpoints between tack and clew and tack and head.
I have to agree with Kris Dressler. The CE is NOT at the mast it is somewhere alone the boom proportional to the amount of sail cloth above it. If the single mainsail was square and framed to stay that way the CE would be at the center of the boom considering it is held fast by the mast in front and the mainsheet at rear. Since there is more sail cloth between the center of the boom and the mast the CE is always somwhere between the two. Add a jib and the CE will be even farther forward.
@sushimoustache Don't forget the mainsheet! Last time I got my finger stuck in the main block, I experienced quite a bit of force. The mast, stays and mainsheet will add up to transfer the force from the sail directly under the CE.
@Raxarax I think there is significant force transmitted via the mainsheet.
@sushimoustache There is absolutely a hull-trim aspect to weather helm. I always address this with the "imagine you are a fish looking up at the bottom of the hull" approach. I thought it is in this clip, perhaps it isn't. Good point though.
@sushimoustache Check the part right around 3:58 or so. I think I address weather helm pretty well. However, if you have anything to add.. PLEASE let me know. I'm always looking for better explanations.
@sushimoustache Don't forget the mainsheet. Quite a bit of force coming from that little rope.
To clarify I don't mean the CE to be AT a point ON the boom I mean a point above the center of the boom. In the case of a triangular sail the distance between the end of the boom and the top of the mast must be considered in the calculations. Visualize it as the boom being between two poles (masts) the mainsheet block is the invisable one.
@Raxarax "the boat only feels the force generated by the sails AT the mast?" So where does the force in the mainsheet come from?
Farthermore, if all the energy (CE) was concentrated at the mast attachment to the hull it would be twisted off at the base in a very light breeze.
Whoah whoah whoooooah, his CE thing is completely wrong. CLR is fine, but the force of the sails is transferred through the mast. The mast is actually where the boat feels the force, and that is well ahead of the CLR. So, even if he had calculated the centroid of the sail areas, the area where all the forces are equal distance from the edges, that means nothing, because all that force is being transferred through the mast.