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| Physics of Sailing |
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Years ago, I wanted to better understand how a sailboard worked. Rather than guessing where to put the mast base, fin, boom height, etc., I wanted to know why these changes did what they did. So, I read some articles, and I kept reading about how the sail’s center of effort (C.E.) had to be right over top of the fin to balance the loads when sailing in a straight line (which is true). But, there was one part of that idea that did not make sense to me. How could sailors put so much lateral force through their legs in front of the fin and not drive the board down wind? The articles went on to explain that the board itself created a lateral resistance relative to how much of the board was in contact with the water. They suggested that the relatively vertical shape of the rails and sometimes bottom shapes were "biting in" (making a high pressure) on the leeward side, much like the fin itself. This still did not seem right to me. I thought back (and have since looked many times), and you can clearly see that water is in fact flowing out from under the board on both sides. Also, the leeway angle would have to be very high to get the rails at the stern to "grab hold". In light of this, it is my opinion that virtually all lateral resistance is provided by the fin or fin and centerboard when planing. (When I say planing, I mean riding on nothing but the bottom of the board). What about the lateral force from the sailor’s legs? I still didn’t know why sailors could push that hard in front of the fin. So, I started looking at the whole sailboard, putting the known forces down on paper and here’s what I came up with. Since sailing in a straight line is a matter of all the forces being held in check by equal and opposite forces, we could start with any of these forces, but I prefer to start with the sail. Let’s start by looking at the sail’s C.E. (the point where all the forces are centralized) – See Fig. 1. |
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In theory, you could hold the sail by this point only and it would not tilt in any direction. (Note: In reality, the C.E., as many of you know, does not remain fixed in one spot on a sail. Because of designed-in shape and aerodynamic variation, the C.E. tends to move a little. But under normal sailing conditions with newer rigs, this movement is too small to have much effect on this discussion). However, we do not hold the rig at the C.E., we in fact hold the rig below the C.E. This causes the sail to try to tilt (lever) above the boom to leeward, below the boom to windward. So when the rig is fixed to the board by the "u" joint and sailed, there is a windward force at the "u" joint resulting from the fact that the C.E. is above the boom (See Fig. 1). It is this "u" joint force that allows sailors to push in front of the fin. This "u" joint force must be offset by an equal and opposite force. This is where you (the sailor) come in. We now have two windward forces acting on the board – the fin and the "u" joint. You must ride the board somewhere between these forces to counteract them (See Fig. 2). |
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It would be nice and simple if it ended right there but there are variables that affect these forces. Below is a list of the five variables. From this I’ve tried to work out a method to set these variables. Note: I said "try", because each of these settings has an effect on the others. It will probably be necessary to come back to some of the settings you did first, make small changes, and try sailing the rig again. Sail size: Changes in sail size affect the windward "u" joint force by moving the sail’s C.E. While the boom height remains fixed, a larger sail increases the windward "u" joint force; a smaller sail decreases "u" joint force. See Fig. 3. |
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Boom height: Changes in boom height affect the windward "u" joint force by moving the fulcrum between the sail’s C.E. and the "u" joint. Raising the boom reduces the "u" joint force; lowering increases the "u" joint force.
Fin fore-n-aft: This affects where the sailor will have to stand (footstrap placement). The farther back the fin’s C.E. is, the farther back the sailor will ride the board, and vice versa.
Mast foot fore-n-aft: This setting also affects footstrap placement. The farther back the mast foot, the farther back the footstraps and vice versa. This setting also affects mast rake since where the sailor holds the boom will always have to be right in front of him or her, (mast foot forward – more rake; mast foot back – less rake).
Footstraps fore-n-aft: This setting affects how evenly the sailor’s legs will carry the lateral load. Too far forward and more of this load will be carried by the back leg, and vice versa. OK, now let’s make all our settings: Sail size: Piece of cake. Pick your size and rig up. Boom height: Since the boom is a handle, it becomes a matter of comfort and making good use of your weight. Set the boom so your arms are approximately perpendicular to your torso when you are powered up. This works out to be about shoulder height for slalom sailing, a little lower for wave sailing, with most sails. Fin fore-n-aft: What matters here is the C.E. of the fin. Approximate where the C.E. of the fin is and for now set it under the center of the fin box. Mast foot fore-n-aft: For now, put it in the center of its adjustment. Footstraps fore-n-aft: For now, if you’re a skilled sailor, set them in the aft settings; a beginner to footstraps, try the front settings. Now go sailing and check for two things. First, when powered up (in the footstraps), is the foot of the sail running parallel to the deck of the board? It should be on slalom and race sails. If the sail’s foot is up too high, move the mast foot forward. If too low, move it back. Next, when cruising along, are both legs handling about the same amount of lateral load? If your back leg is taking all or most of the load, first, move your straps back more, if you can, then try it. If your straps are already back, next try moving your fin forward. If you still have too much load on the back leg, re-check the mast foot settings. Next check your boom height again. The leg loads should be feeling pretty good by now, but here are a few more things that can be factors. In very high winds, say over 35 mph, your apparent wind will be off to the side of your board requiring you to lean back off the board instead of out to the side. This leaning back may make you have to sail with your back leg bent, causing it to get tired fast even though your settings are ok. It is a less common problem to have the front leg overloaded, but if you do, make all settings in the other direction. A note about steering: The preceding was about going straight, but when it’s time to turn or change course, here’s what I think is going on. When powered up in the footstraps, the sailor can turn downwind by putting more of the lateral force on his/her front foot and less on the back foot. To turn up wind, just the opposite. The sailor does this by leaning fore or aft. In essence, the sailor moves the arrow in Figure 2 marked "leeward force from sailor’s legs" fore or aft to turn. (This type of turning is not foot steering). I, as well as others, have been testing out this theory of sailing for a few years. It seems to work. See if it improves your board sailing experience. Brad Miley |
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