What Happens when you put the Kite up?

Towards the end of 1996 there was a long thread in the rec.boats.racing newsgroup about the effects of asymmetric spinnakers on the trim of a boat. Basically the discussion was between those who sail such boats, and those of a more mathematical disposition. The former reckoned that the moment you put the kite up the bow lifts, the whole boat steadies and pitchpoling becomes less of a problem. Therefore the kite must be lifting the bows of the boat. The second group worked from the logical viewpoint that the result of a large sail pushing the boat along from a centre of effort well above the water can only be to press the bows down. The strong views expressed inspired Mikko Brummer, of WB-Sails, Finland, to model an asymmetric rig (actually based on the pictures of my Cherub from the then class website) into his company's sophisticated mathematical model of a sailboat rig. This article is prepared from his original paper, published on his site, and the copyright for the data is retained by WB-Sails. The conclusions however are my own, and Mikko is not be held responsible for inaccuracies or errors in them.

To summarise Mikko's findings, the rig creates a great deal of upwards force which must have a big effect on the performance of the boat - and certainly explains the ever increasing tendency of Cherubs to "get some air" from wave tops in extreme conditions. In spite of this there is no doubt at all that the drive from the spinnaker presses down the bows.

This table summarises some of the main figures.

Sail Drive Force Lifting Force Heeling Force Heeling Moment Pitching Moment
kg lbs. kg lbs. kg lbs. kg/m lb./ft kg/m lb./ft
Spinnaker 68 150 60 132 52 114 129 87 172 115
Main 36 79 7 15 28 62 80 54 117 78

Thus in this model the rig is producing an upward force of 67kg - around a quarter of the total weight of boat and crew. You can add to that the considerable upward force created by the shape of the hull. The dramatic effect the spinnaker has on the performance of the boat in this situation is also well illustrated. Even the mainsail is displaying a small amount of lift due to the rake of the mast and bow-up trim.

I must apologise to the purists for displaying the forces in kilograms and pounds, but I felt this would be a more accessible format for the casual reader than the technically more appropriate Newton.

Graphic - Computer wire frame image of Cherub Rig.

This side elevation from the computer model demonstrates the way the combination of bow up trim and mast rake enables both mainsail and spinnaker to create an upwards force.

In my opinion, bearing in mind this data, the reason why the 3 sail reach feels so much steadier and seems to require less hanging off the back of the transom must be to do with secondary effects, and not directly caused by the lift from the sail, which is most definitely pitching the bow down. One can then speculate on what these effects might be.

The first must surely be the " reduction" in displacement caused by the lift from the sail. Its well established within the Cherub fleet that heavy crews have much more trouble with nose-diving and pitchpoling than light weight ones. Whatever causes this effect, which one presumes must be partly to do with the shortness of the boat, a reduction of effective displacement of the order demonstrated above must bring it into play.

Other forces that seem likely to have an effect on this phenomenon are to do with the differences between the mainsail and the spinnaker. In his (highly recommended) book, High Performance Sailing, Frank Bethwaite notes that a sail that has a long leading edge angled to the apparent wind (like a delta wing of an aircraft) can develop "roll-over" vortexes which scrub stagnant air off their suction (lee side) surfaces. This mechanism can maintain attached flow up to abnormally high angles and create a great deal of lift (and drag). Thus the low aspect ratio spinnaker should be much less sensitive to changes in its angle of attack caused by gusts, the boat rocking, accelerating or decelerating in waves etc., so the power produced by the rig should, I think, be much more even. In addition on a two sail reach the main - which produces its energy further above the water than the more triangular spinnaker - is the main source of power, whereas on the 3 sail reach it tends to be used more as a "trim tab" to control the pointing angle and is less often fully powered up. Therefore on the two sail reach the ratio between drive force and pitching moment is less, and the delivery of power from the rig is much less even. My memory is that the two-sail reach unsteadiness was much less of an issue a few years ago before we all adopted the very rigid stable and low twist mylar sails of today.

I also wonder about a steadying effect in pitch. This is a model of a boat sailing on flat water, which is all well and good. But supposing there are lumps about? The craft is pitching relative to a point somewhere around the the centre of buoyancy and centre of mass, probably both pretty near the stern on a short boat. So the kite will be going up and down significantly... As the boat pitches up I would think that the lift from the kite reduces, and as it pitches down I guess it must increase. So I wonder if this reduces pitching, which would also make the boat steadier. I think I can see a sort of "hang" effect on the kite on the many videos of twelve foot skiffs you can find on the net - try YouTube of course .

I've just (2013) been alerted to another potential factor in the increased stability experienced when sailing under spinnaker. In an inteview of C Class catamaran designers, conducted by Sailing Anarchy, and hopefully available here, one of the participants (I fear I haven't worked out who) noted that spinnaker equipped catamarans gain extra stability in pitch and yaw because of what he called a tow effect. As I understand what he said this means that when a boat is towed, with the centre of drag well aft of the centre of effort, then there is an automatic stabilising effect. Its obvious enough when pointed out, but it hadn't occurred to me in eighteen years!

I'm not sure how reliable these figures are as an emulation of actual sailing performance. Any kind of mathematical model is only as good as the data you can put into it, and ideally one would like to feed in some real data from measured performance on the water. Mikko has assumed an "across the deck" wind speed of around 20kts, apparent wind at 70 degrees to the centreline, and a forward velocity of around 15 knots. The photo was pre GPS days, and 15knots might have been conservative, but who knows.

The following tables are extracted from Mikko's paper. Elsewhere on the WB-Sails site are more details on the mathematical modelling techniques he uses, as well as some information on their practical application. Its well worth spending time on their site: there's a lot to learn.

Asymmetrical - section forces [N] & moments [Nm]

Section DriveF SinkF HeelF HeelMom Yaw Mom PitchMom
1 24 -13 12 3 1743 -19
2 143 -97 141 151 5116 77
3 182 -122 153 343 2646 332
4 171 -140 124 408 -1247 533
5 123 -161 73 311 -3031 585
6 26 -53 10 48 -558 174
Total 669 -586 513 1265 76 1682

F stands for force (in [Newtons]. 1 N = 0,1 kg = 0,2 lbs.)

Mom stands for moment

Section 1 is at the foot of the sail, 6 at the top (every section between the red lines).

The third column, SinkF, is the sail force component perpendicular to the sea surface. Negative sink is LIFT, so the kite is lifting the boat at a force of 586 N, or about 59 kilos.

MainSail - section forces [N] & moments [Nm], including mast

Section DriveF SinkF HeelF HeelMom YawMom PitchMom
1 46 -5 38 31 -2691 42
2 78 -9 61 102 -5615 142
3 73 -11 56 145 -6003 207
4 65 -12 49 174 -5760 250
5 60 -15 46 205 -5461 293
6 35 -12 23 125 -2052 211
Total 356 -65 273 781 -450 1146

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