The Physical Art of Sailing

Thanks for posting Leo. Certainly all research on body kinetics to propel the boat is welcome, and like they say there's not much academic papers available on the subject. We have been doing this since 2010.

I don't quite see what they mean with "exotic heave", though. The reason for the torso movements upwind in waves is two fold:

1) With the bounce upwards, you instantaneously lighten the boat, to ease passing over the wave, in an analog manner to a horse back rider hopping over a fence. So you allow the boat to heave easier, insted of pushing it through the wave. By leaning forward, you press the bow down, by leaning backwards you lift it up.

2) By "flicking" they mean the flicking of the leech of the sail, through a jerk with the thighs & legs against the deck/gunwhale. The jerk flicks open the leech of the sail, to avoid stalling, when apparent wind shifts to the side, caused by the motion of the boat climbing over the wave. Likewise, the leech flicks back, avoiding luffing of the sail, when the bow drops down and apparent wind goes forward. You can observe this in the video they provide, even if the wind is light here and the laser rig is not the best for this technique. Observe at the wind indicator at the top of the mast and the leech of the sail.

 

Attached a little article about the mast/sail dynamics in waves.

 

Propelling the boat downwind by body kinetics is another story, plenty to research there for a better understanding. Nowadays, most classes allow "free pumping" in winds above 10 kn (RS-X allow it it in all winds). This has made the sport much more physical and makes a huge difference downwind. The pumping of the sail (repeatedly sheeting in & out) is combined to +-30 degrees rolling of the boat, achieved by moving from a hiking out position standing in on the lee side and back. See the stills from a video from a Finn.

The period of the rolling in this light wind/flat water is 4 sec, the optimum probably varies with wind strength but we don't know exactly how. Rolling doesn't only work for a cat-boat like the Finn - if you observe a Star boat running dead down wind, you will notice that the wind indicator rarely points downwind - it's pointing to one side or the other (+- 90 degrees), depending on which way the roll is going.

 

In the process of kicking this concept around, perhaps we can pay some attention to the influence of the board. If we roll the boat the board will surely play into the game. It is presumed that the board is running with some AOA. A vector diagram suggests that some thrust (or drag) might be generated when the board moves in the arc that rolling motion will produce.

 

True... If we think about the rolling tack, the roll will induce a very large angle of attack at the tip of board, which will not help to propel the boat forward, on the contrary the board will stall. If you could make the board to flex from the trailing edge to windward when pressed, that would give a boost - difficult to achieve as most of the lift is concentrated in the front part of the foil. On the other hand, the rapid rolling could create a strong spanvise vortex on the leading edge of the foil, momentarily sucking the boat forward. I guess this is one of the things the Cornell team is looking into.

Downwind the board is lifted, so it hardly has much of a role in the propulsion.

 

Nice stuff Miko. Regarding the C/board of course a deliberaltely flexible one (sideways) may also be helping to ride the waves as it too will react to body movement such as jerking. The Finn is slightly unusual in a relatively thin flat sided beast for a board but other classes which are far more open about shape and size may benefit.

I'm not sure how far boards are lifted either especially downwind. My experience both in una and two sail (with and without spinnaker as well) is that often significant board is required at speed to maintain control and heading. Such that a Merlin Rocket may require almost 2/3rds board down when 'running' regardless of wave conditions. Equally on really windy races I have left the board in one position all race long, just over half for all points of sailing on a una rig. 30-37 knot conditions....inside Hayling Island harbour mouth.....

 

Petros' sister is a sage. She called like it is.

 

Guys, check this video out, I think you'll find it very interesting.

See 2:30-4:00 for downwind pumping.
https://www.youtube.com/watch?v=N_rYAyl81qY

 

/\

That's pretty much the same technique used in Lasers, but we have to stick to one pump per wave.

Windsurfing shows you how important it is to move the rig forward into clean air, rather then just continuing to beat up the air that has already been churned into turbulence by earlier pumps. Inexperienced pumpers on longboards in light winds can end up furiously flapping the rig but creating no forward motion.

Regarding the effect of the centreboard; rolling a round-hull boat with reasonably efficient foils can propel even a small yacht in a dead calm, even with the sails down. On Division II boards (the round bottomed former Olympic class boards) you can move in light winds by just rocking the board from side to side, even when the rig has been left ashore.

 

I knew Oli when he was 7 years old and still living in the UK, he was big for his age. Not many people can pump a Finn sail like that.

of course a flexible foil still mustn't break when you stand on it to right the boat.

In the RS400 we leave the board down all the time. Same on a fast trimaran. However on a catamaran it's worth lifting

Richard Woods

 

I thought I'd read of his name in Y&Y years ago: I didn't realise he was an expat.

I was unclear in my earlier post; I was just wondering how much of the lateral rolling motion was about steering down the wave and how much of its effect was projecting the rig sideways into clear air in order to make pumping more effective. Obviously the steering effect is real, but windsurfer racing experience seems to emphasise how important clear air* is when pumping.


* that's clear air as in air not previously disturbed by one's own pumping, not air clear of other boats' bad air.

 

Some of the dynamics are addressed here (on and around page 167)

https://books.google.com/books?id=-...n drawings delft series hull sections&f=false

 

Hmm... In my experience the RS-X are just flapping the upper leech when ever they can. See below from about 2 min. on. Interestingly, it is done with the pelvis/torso, via the trapeze, not with the arms. I guess not nearly enough power in the arms. 470 crews shake the upper leech (and the board?) in exactly the same manner, through the trapeze as well, 1-2 minutes after the start, when free pumping is permitted in above 12 kn.

 

In Finns, rolling (heel and counter-heel) is used very much for steering the boat , simply through putting the sail center of effort on either side of the centerline - bear away on counter-heel and head up via heel.

 

Boats with long bowsprits develop plenty of leehelm with the sprit extended and asymmetrical flying. Hence, they need to keep the board down to avoid all too much push in the tiller. This is the case of the 49er, who sail with board all down, and I guess the same with RS400.

Another thing is that skiffs are actually never "running" downwind - they go so fast that apparent is always from the front, they are reaching downwind. In a 49er (or a Tornado), you always sail as high as you can downwind for max VMG - max VMG is limited totally by righting moment only - you sail as high as you can without capsizing, because sailing higher you go so much faster your VMG is better. In contrast with a traditional boat, where you always try to sail "deep" for a better VMG. In this respect it's a shame they need to keep the board down - that trips the boat over more easily. A very lof aspect board more forward would probably be more efficient for downwind.