The new 50 cm rule got me thinking about one advantage of kite boards. Instead of dividing the required water lift into a vertical and a horizontal component as windsurfers do, kiteboarders use just one lift producing surface angled at the right direction. A bit of geometry shows that combining the vertical and horizontal foils into one inclined foil has a big drag advantage.

Suppose hypothetically you need 100 units of vertical lift and 100 units of horizontal lift . Assume for the exercise that the lifting foils/hulls all have a lift to drag ratio of 10:1. Using a foil, and a hull, each would have 10 units of drag for a total of 20 drag units. (diag 1)

One foil inclined at 45 degrees developing the sqrt ( 2 ) * 100 = 141 units of lift can do exactly the same job with only 14.1 units of drag. (diag 2)

The vector diagram with the fin canted to only 15 degrees still shows a significant drag reduction. (diag 3)

The double foil disadvantage is greatest when the two components are roughly equal and acting at the widest 90 degree angle. If you can't dispense with one of the foils altogether there are still advantages to be made by shifting most of the load to one or the other. In reality the board, which lifts against gravity, is working harder than the fin, so shifting a bit of lateral work to the board is one way - as kiters do. But on the other hand the fully submerged fin has a better lift to drag ratio than a planing hull so canting the fin windward and relieving the hull of some work may win out for windsurfers.

In the hypothetical example the fin at 45 degrees can lift the board just clear of the water, but that may be a bit much for a few reasons. You need a bit of board in the water for control, it's good to endplate the fin, it wouldn't work at all on the other tack,. A fin canted 15 degrees to windward would be worth a try. The vector geometry shows that for the hypothetical requirement of 141 units of lift at 45 degrees the total drag is 17.4 units vs 20 units for an upright fin. In the hypothetical example even a moderate 15 degree cant relieves the lift needing to be supplied by the board from 100 units to 68 units.

Formula boards have already achieved a degree of this effect by having very flexy fins. Sean O'Brien on the carbonsugar website describes formula fins as being flexy, and have the lift centre ahead of the twist axis. ie as the fins load up they don't flex off but load up even more. It appears that the lightening or unsticking of the hull as very noticeable with bendy fins, and they win races. A bendy speed fin preset to 15 degrees windward might incline to an even more favourable angle at speed and come back to 90 on the wrong tack for an easy ride back up the course.

Instead of loading up the poor fin to produce vertical lift, has anyone else except I considered using the sail to do this?
After all, the sail is the primary source of power, the fin is an interface.
Closing the slot may have sent us in the wrong direction.
Offsetting the mast base towards the downwind side of the board allows a planing configuration providing the center of effort of the sail remains over the centerline of the board.
If someone tries to patent this idea I weel keel them.

Sort of semi-foiling, and its just a fin modification. Sounds like a great idea.

Interesting idea Ian,

Dont forget the most significant lift contribution- from the nose of the board. This is a very efficient lifting surface since it is working in ground effect off the water. The fin and board tail do not need to produce a lot of vertical lift at speed. Canting the fin would require much more weight to be applied to the rail to compensate- reducing the power available from the rig, and also reducing the capability of balancing the aerodynamic nose lift. Besides that its very difficult to control the foil geometry under load, to get a reasonable balance. That pic of Sam shows what happens when you go a bit far...

The noses of the CA boards are relatively wide for a reason :)

- I try to keep Barney off it )

I,d watch that he doesn,t put a wave fin in it and see how many flips it can do!

It's not the jumps that cause the problems, it's the landings!
As long as you float down on the sail, no worries.

Hi Guys - Ian you're always thinking aren't you - saw your thoughts on kite / foil combo.

"The most efficient sailing device would have a foil submerged in the water (underwater kite) connected by a fine wire to another kite up in the air. All lifting forces would act along the wire - no need for ballast."

I too have been thinking along these lines - below is a post I put on gps-speedsuring about 18 months ago. Have seen that mathematician from Adelaide's idea too but it loses out on the kite benefits from memory. In any case - have a read of this and I'd be interested to hear what yourself and slowboat think of these concepts. Look forward to seeing what people think - maybe we could test the theory at Sandy Pt later in the year. Hope I get time to make my new weight jacket idea and show you at Sandy Point later in the year too. The thread this was posted on is www.gps-speedsurfing.com/default.aspx?mnu=forum&forum=1&val=3447

John Rohrsheim 1-25-2007 06:37
I have spent many years trying to dream up fast hydrofoils etc. but in the last year or so one idea has been dawning on me. Realistically I am not going to ever get to build it myself to test without help of others. So, I figure why not raise it in this topic and see if the concept is fundamentally flawed or not.

So here goes...
Firstly, I think kites are the answer to the source of power - simply by being in the stronger airflow and by having the ability to dive the kite to help get initial power up when building speed. (this hurts being a windsurfer who's yet to try kiting)

Secondly, it has been raised in this discussion again - the question of leverage. Most of the speed craft being built by the yachties are based on leverage being maximised by wings, trapeses and geometries like MI. Kiters and windsurfers haven't really pushed this except with weight jackets. As far as I can tell David and Tilman & co will be limited by this at some point. This is where my 'genius' idea comes in. Please forgive me for being long winded here.

The beauty of a kite is that the force of the kite is always directed along the strings (lines). That's the beauty of rope/line/string is that the force has to be in one direction only. Now imagine that instead of having a bar like kitesurfers use you have a tube to direct the lines into. The tube could have a smooth entry (or pullies etc) to guide the strings without unneccessary friction. At the outlet of the tube would be a set of pulleys to direct the lines to the rider / pilot of the water craft. So, effectively at the outlet of tube if you were to fit a universal joint of some kind you would be assured that the direction of force would be along the direction of the tube. Now imagine that sheathing the tube is a rotating symmetrical foil shape designed for minimum drag. The tube is most likely carbon of 2 - 3 metres in length. This would then allow the outlet of the tube to be underneath the water surface some 1m+ depending on waves etc. So effectively the force from the kite has been directed under the water surface and not to a mast or rider sitting 0.8m or more above the surface. As such it would be possible to locate the fin (be it ventilating or not) at the exact location of the force and consequently eliminate the physics and limitations of traditional yachting leverage!! A riderless craft could perhaps use some fancy electronic control and simply attach a fin / foil of big enough size directly perpendicular to the tube to counteract the force. However unless we were using scuba gear a rider would want to sit in some type of craft positioned above the surface from where he/she could control the kite lines / fin angle / steering etc.

The biggest problem I can see with this setup is working out how to tack / gybe the craft. but it may be possible to overcome this with a clever off-centre rider and rotating connection between the fin and the tube outlet. By adjusting the angle between the fin and the tube you would be adjusting the amount of lift the kite transferred to the craft and as such your fin could in turn act as a foil to lift the rider's seat off the water surface.

Now obviously some well thought out pitch control etc would be needed to avoid danger and if the fin /foil were sized to be capable of delivering enough lift / resistance even if ventilated then you could feel pretty safe so long as you're able to keep the foil in the water.

So, to sum up an efficient yet controllable kite is exposed to better wind / angle at height and is able to have it's force resisted directly from beneath the water surface so the only limitation is the drag of fin and kite and the materials / weight of all the equipment.

On a tangent - so far land sailors and ice sailors have been able to reach far higher speeds than water sailors. However there are 3 elements that we deal with - solid/liquid/gas. They are dealing with a solid/gas solution to converting wind to movement. They are using solid land and gravity to resist the pull of sail/kite. They are constrained by the amount of weight they carry, the friction they can develop with the solid surface and because they cannot pierce the surface they are also constrained by leverage issues. The difference with this solution is that it is a gas/liquid solution. The only liquid of abundance in the world is water (the only variable is salt content of course). The beauty of using a liquid to resist the gas born force (wind) is that you can direct the force to a point below the surface of the liquid (the beauty of keels / fins etc). The great thing is that you can direct the force directly to the centre of effort of the fin / keel and thereby do away with the necessity for weight / leverage in the equation.

This design concept would avoid the need for specific water states for high speed but would demand a reliable water depth to ensure the resisting foil remained below the surface. I would be interested in seeing what people think and then maybe drawing on the different expertise in this forum to put into action. A prototype arrangement could probably be done quite cheaply if someone would design the fins for it. Could just sit on a sailboard as the craft and have a go.

Anyway, thought I would put it out there for comment amongst the kite/windsurf speed seekers on this forum.

Someone said that the only way to overcome the effect of cavitation was to use ventilated foils - these are high drag so you need lots more power. The amount of power available is limited by leverage. Do away with leverage and the amount of power is only limited by how scary you want to get!

On another note I looked at some vector diagrams of apparent wind in 20 knots of true wind. Once your craft reaches a speed of 60 knots in 20 knots of wind then the angle of attack is basically the same whether you are sailing at 135 degrees to the wind or 90 degrees to the wind - in fact if you sail closer to the wind the apparent wind speed increases so it may actually end up being faster to sail closer to the wind using such a craft - throws everything we do now out the window!! If the foils/fins were efficient / powerful enough this may be achievable seeing as MI is going for 50 knots in similar windstrength.

Anyway, thought I would put it out there for comment amongst the kite/windsurf speed seekers on this forum. I am not an expert so please don't cut me down but I am interested in the opinions of those more expert than me on this forum. Please let me know what you think.

Well he's in Greece at the moment and might be there for a while so its safe. I liked the description of concaves as "out of the bag of hydrodynamic tricks" to solve instabilty at the expense of drag. My 115 litre freeride board has em in spades, can see what you mean.

Enough of reality, back to the thought experiment. I've put up a picture of a deep V power boat to best demonstrate the lift/drag disadvantage. Resultant hydrodynamic Lift adds as a vector sum but the drag adds as a scalar sum.

Thinking outside the set square, fins are traditionally set at 90 degrees because we wanted the boards to sail symmetrically on each tack. Once you get into an asymmetryic mindset there is nothing special about 90 degrees.

Need a diagram for your control system Seahorse, it looks like a variation on the Adelaide mathematicians concept?

Here's a diagram for you Ian - not the best - no scanner to do it by hand.

seahorse said...

Here's a diagram for you Ian - not the best - no scanner to do it by hand.

Ian I know you're posting to get some input on your theoretical ideas, which is what these forums are for. There are some great minds here that you can bounce ideas off.

I'm geussing that what I'd and some others, would really like to read about on a windsurfing speed forum is how to make our current windsurfers go faster?? Do you have any ideas in this area.

I'm geussing that what I'd and some others, would really like to read about on a windsurfing speed forum is how to make our current windsurfers go faster?? Do you have any ideas in this area.

The theories we've been tossing around are the big picture approach to fluid dynamics. Maybe good for indicating a revolution or a new evolutionary direction that might be tried, but the detail only comes from a lot of test and development. Any new direction, although possibly having potential, faces stiff competition. The traditional designs have had 20-30 yrs of trial and error and are pretty finely tuned - fluid dynamics 101 is not in the game when it comes to fine tuning.

I sometimes think the current windsurfer, is far more efficient at doing its intended job than something like the space shuttle - a one hit design using the best fluid dynamics program on the super computer of the day. If you gave that same computer the task of optimising the fastest single person sailing craft using the general parameters of a windsurfer, it would be lucky to come up with a Wally.

As far as I'm concerned an understanding of exactly what's going on underneath me, is an aid to evolving my technique and the design of my next board.
I find the technical discussions here invaluable, in my opinion, it's what this forum should be about.
All the dumbed down, fashion, twisty twirly, cool macho stuff, can go somewhere else.

I see a potential problem structually canting the fin. The width of a tuttle box may not allow you to get the amount of cant you are after. You may have to build a fin with a predefined kink in it, or cant the whole tuttle box in the board.