Something happened to me Saturday that I still don't fully understand. We had a beautiful sailing day, 60 degrees, sunny 12-15 knots of wind. I was sailing on a broad reach with the main out quite a bit and was also flying my assymetrical spinnaker. We got hit with a pretty good gust of wind and my boat headed up on me despite putting the helm hard over. The gust passed and we got the boat back under control. What I don't understand is this. Weather helm occurs when the center of effort of the sails is aft of the hull's center of resistance. With a big spinnaker drawing up front, how can the center of effort of the sails be aft of the center of resistance? There's an article in this month's Cruising World about a new Code Zero design foresail. It mentions the heading up problem with a cruising spinnaker, and refers to it as a free-luff (no that's not the same as free-love) sail, i.e. the luff is neither hanked on nor fed into a roller furler. Because it's a free luff sail it has less lee-helm effect and consequently the boat can head up. Is this the whole story? Any other thoughts on this? Thanks for your help. Gary Wyngarden S/V Shibumi H335
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That happened to me once
I was beam-reaching a Catalina 30 under just the partially reefed genoa in about 25 kts of wind. An unusually hard and sustained gust put the lee rail in the water and the boat rounded up despite my best effort at the wheel. The only conclusion I could draw was that the excessive heel led to an asymmetrical hull shape underwater, which caused the severe weather helm. I remembered from my dinghy sailing days that weather helm can be reduced by hiking out hard to keep the boat flat. A small sailboat can be steered witout using the rudder by moving weight from one side to the other and creating heel. I would imagine that weather helm due to an unbalanced hull shape underwater could probably override a balanced sailplan, because water is more dense than air. Just a guess... Peter H23 "Raven"
I was beam-reaching a Catalina 30 under just the partially reefed genoa in about 25 kts of wind. An unusually hard and sustained gust put the lee rail in the water and the boat rounded up despite my best effort at the wheel. The only conclusion I could draw was that the excessive heel led to an asymmetrical hull shape underwater, which caused the severe weather helm. I remembered from my dinghy sailing days that weather helm can be reduced by hiking out hard to keep the boat flat. A small sailboat can be steered witout using the rudder by moving weight from one side to the other and creating heel. I would imagine that weather helm due to an unbalanced hull shape underwater could probably override a balanced sailplan, because water is more dense than air. Just a guess... Peter H23 "Raven"
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Rodney Kidd
Center of Effort
Gary, Think about where the center of effort is when the boat is hit by a puff and heels over. The center of effort is out over the leeward side of the boat, maybe 1/4 the rig height away. The CE is not aft of the center of effort but along side. Having all of that power alongside will cause the boat to round up. The above effect coupled with the fact that our modern beamy boats will roll some of the rudder out of the water when they heel too much is a pretty good recipe for a round-up. Rodney Kidd C-38 #297, Flying Bear
Gary, Think about where the center of effort is when the boat is hit by a puff and heels over. The center of effort is out over the leeward side of the boat, maybe 1/4 the rig height away. The CE is not aft of the center of effort but along side. Having all of that power alongside will cause the boat to round up. The above effect coupled with the fact that our modern beamy boats will roll some of the rudder out of the water when they heel too much is a pretty good recipe for a round-up. Rodney Kidd C-38 #297, Flying Bear
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Carl and Juliana Dupre
Heeled Hull Shape
Sounds like a bit of an exciting ride, Gary. Question; did the gust heel the boat significantly when it hit? I have read that once a boat is heeled the hull shape that is actually in the water is decidedly non-symmetric and the heeled wetted hull shape will drive the boat to round up. Several people have said that this is a much more significant force than the center-of-effort of the sails, especially with the wide beam brought all the way back to the stern that typifies modern boats; one of the reasons that modern cruising boats really need to be sailed upright. Does this make any sense? Carl and Jule s/v 'Syzygy'
Sounds like a bit of an exciting ride, Gary. Question; did the gust heel the boat significantly when it hit? I have read that once a boat is heeled the hull shape that is actually in the water is decidedly non-symmetric and the heeled wetted hull shape will drive the boat to round up. Several people have said that this is a much more significant force than the center-of-effort of the sails, especially with the wide beam brought all the way back to the stern that typifies modern boats; one of the reasons that modern cruising boats really need to be sailed upright. Does this make any sense? Carl and Jule s/v 'Syzygy'
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Rob Rich
Center of Effort....
I believe Rodney had it right. From my flying days, I recall that greater lift can be achieved on a wing by introducing a leading-edge slat. Your asym spinnaker was acting as a slat, accelerating the relative wind over the top (arched side) of the mainsail. In effect, your mainsail was probably where the majority of your lift vector was being generated, as it had a greater pressure difference between its top side and bottom (concave) side. Kind of counter-intuitive, considering the size of that big chute... Kinda make sense? Well, its a guess more than an answer, but you get the idea. Best of Luck, Rob
I believe Rodney had it right. From my flying days, I recall that greater lift can be achieved on a wing by introducing a leading-edge slat. Your asym spinnaker was acting as a slat, accelerating the relative wind over the top (arched side) of the mainsail. In effect, your mainsail was probably where the majority of your lift vector was being generated, as it had a greater pressure difference between its top side and bottom (concave) side. Kind of counter-intuitive, considering the size of that big chute... Kinda make sense? Well, its a guess more than an answer, but you get the idea. Best of Luck, Rob
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Dave
Weather Helm
I invite Mr. Cherubini whom I acknowledge as the resident expert on this subject to join in. My shot at is is similar to many. As the boat heels the center of lateral resistance from the keel is rotating from a vertical plane to some other angle and the lateral resistance is now made by the shape of the hull and the vector component added by the keel shape in the angled position. As the rudder is applied it has a lot of verticle force component that gets worse with the greater heel angle, thus you are trying to force the bow into the water and lift the stern out of the water versus rotating the boat in the horizontal plane. The center of effort of the sail when the boat is vertical is close to the centerline of the boat in comparison to where it is as the boat heels so the moment applied to the boat by the center of effort (combined for both sails) increases as the boat heels futher and further. In several discussions we all agree that sailboats should remain upright for efficient sailing. I use a rule of thumb of 20 degrees max heel angle before reefing. I didn't make this up I read it somewhere. Hope this helps. A good engineering analysis of this would require a major effort with supercomputers to 3D model the boat and the applied forces. Dave
I invite Mr. Cherubini whom I acknowledge as the resident expert on this subject to join in. My shot at is is similar to many. As the boat heels the center of lateral resistance from the keel is rotating from a vertical plane to some other angle and the lateral resistance is now made by the shape of the hull and the vector component added by the keel shape in the angled position. As the rudder is applied it has a lot of verticle force component that gets worse with the greater heel angle, thus you are trying to force the bow into the water and lift the stern out of the water versus rotating the boat in the horizontal plane. The center of effort of the sail when the boat is vertical is close to the centerline of the boat in comparison to where it is as the boat heels so the moment applied to the boat by the center of effort (combined for both sails) increases as the boat heels futher and further. In several discussions we all agree that sailboats should remain upright for efficient sailing. I use a rule of thumb of 20 degrees max heel angle before reefing. I didn't make this up I read it somewhere. Hope this helps. A good engineering analysis of this would require a major effort with supercomputers to 3D model the boat and the applied forces. Dave
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Miles
Also...
As you start to round up from a broad reach your apparent wind speed will increase and make the situation worse. Then when you start heeling a lot the rudder becomes less efficient at steering since it's now closer to being parallel to the water and that doesn't help either. Your angle to the waves has changed now too and that might also feedback into the situation. Best to let out a loud "Yahooo!" and try to convince the crew you had it planned all along ;.)
As you start to round up from a broad reach your apparent wind speed will increase and make the situation worse. Then when you start heeling a lot the rudder becomes less efficient at steering since it's now closer to being parallel to the water and that doesn't help either. Your angle to the waves has changed now too and that might also feedback into the situation. Best to let out a loud "Yahooo!" and try to convince the crew you had it planned all along ;.)
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