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.
We do that already to some degree by raking sails over our heads. That's what got me thinking about fins. I was initially thinking that the gain sailors were claiming by raking the sail to lift the board would be swings and round abouts. ie raking the sail would reduce the forward lift -and it would all balance out. But it doesn't! It's the hypotenuse, opposite adjacent thing. If you swing a lift vector by 15 degrees you get the sine of 15 degrees of original lift in the new direction but only lose (1 - cos 15 degrees ) in the old direction. You'd more than gain the 4% loss of forward lift by easing the weight on the hull by 25 % of it.
It's all about consolidating your foils to act along a common direction.
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. An Aussie mathematician has written an analyisis of and prototyped such a craft. He actually uses struts in tension rather than wire and has to incorporate a crew pod/ floatation device in between , but the principal of consolidating the foils to work along a common line is the same. Don't know if it will be controllable so as to ever reach its potential.
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 :)
Hadn't thought much about nose lift, reality check on bright ideas, thanks Slowboat. So the nose has an area of just under square metre, it's running in an apparent wind about 20 or 30% stronger than your 5.8 sail, and more on the nose - due to the wind profile. It's not really ground effect, ( water depth > 50cm
) it's a light fluid over a dense fluid. The water surface will react under the higher air pressure before the hull makes contact and then continue developing downward momentum as it does. More efficient than using the hull to produce lift in free air but how does it compare efficiency wise with the lift gained from just water? I'd see it as creating a longer easy entry, virtual rocker line, sort of cushioned, sort of self adjusting to the ideal lean entry rocker curve - great for control.
But whatever the exact value of the lift, it is in the vertical direction, at right angles to the fin lift. Good for control, not good for efficiency.
Haven't tested the aerodynamics of my CA at anywhere near its top speed yet. But in the meantime I've found it to be the best jumping board I've been on. Self balancing underfoot. Is that another feature of the unique aerodynamic nose, or a placebo effect brought on by its incredibly good looks? ( Ah.. for warranty purposes that's lowish, moderately longish, old blokey sort of jumps in the moderate Broulee swell - I try to keep Barney off it )
- 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!
errr yeah you probably shouldnt jump it 
Yeah they are very stable midflight... I prefer the 47- its a tad more stable in 25kts+ at *some* height off the water. Just clearing off the seaweed of course 
And keep Barney off it... he might enjoy high speed floaters too much...
It's not the jumps that cause the problems, it's the landings!
As long as you float down on the sail, no worries.
As for the balance thing, well the vertical lift is an obvious reguirement. Something has to support your body and the rig weight. You can use the board hydrodynamic and aerodynamic lift, and the rig canted to windward. I like a lot of controlled aerodynamic lift. The nose acts like a solid wing flying close to a relatively solid boundary ("ground effect") which gives a boost in lift. It also smoothly balances the downward pitching moment from the rig.The apparent wind is a much more stable thing to be generating lift from compared to the water surface using hydrodynamic lift of the tail. Basically the tail is exposed to a lot of relatively high frequency force excitation which is coupled to the rig. This makes it more prone to becoming unstable, and hence it is more critical to achieve some form of damping other than active (pilot). The more lift you try to get from it, the bigger the shocks and disturbances. Usually this is done with rocker, concaves, and other hydrodynamic tricks which increase the drag. This can be a good solution in very rough water where moderate to high speeds are not realistic. The CA's are designed with maximising the use of aerodynamic lift. Thats why they have that "magic carpet ride" feel, where they float over the chop rather than bang through it. It works well up to a surprising chop size.
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.
I think one of the problems here is too much focus on a number/s eg. "how to get to 50kts" "Who's gonna do the first official 50kt 500m"
Humans are different and unique and have their own individual preferences. The guy who wants to push his yatch to 50, doesn't really want to do it on a windsurfer or kitesurfer!! They might be annoyed that windsurfers and kitesurfers can generally go faster, and some in power may even try to change the rules, so they can claim it for their own favoured sport and disqualify other sports.
If all I wanted to do, was to do 50 kts on water, I'd cut all the crap and heart ache and buy a small water craft like a jetski, and go and do 50 kts whenever I felt like it. I'd then feel the rush, it would be an absolute buzz whether you like jetskis or not!! For me it is eeking out every tenth of a knot I can on a windsurfer, because that is what I have invested 25 years of my life into and it is the sport that makes me feel the things that have kept me coming back for 25 years, and if God or the Universe is willing I want another 25 years of those feelings on a windsurfer.
For me all I'm interested in is how to make me and my windsurfer go faster. I tend to sail mostly with 45 degree weed fins because of the water conditions where I live. When I sail in flat water, that's deep enough and weed free, I find pointer fins are faster than 45 degree weed fins, and my 40 degree weed fins are faster than my 45 degree weed fins. I think you'll find the fastest speed fins have rake of about 12 to 30 degrees. Maybe slowey can describe why different degrees of rake produce different effects.
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?
Have also thought previously about a back foot pressure weighted fin angle - use the footstrap connected to a mechanism to raise angle of attack
your pod needs to be suspended by the kite like http://www.amphi-kiting.com/html/body_the_future.html
these ideas have been around (in public domain) for at least a 1/4 of a century.
see www.ayrs.org/Index08.html
And here is a video of kite & foils with boat in the air in between.
foils.wordpress.com/2007/08/11/kiteboatspeed/
PS how do you link to just the youtube vid?
There are some others, just search youtube for kiteboatspeed.com 2 ( or ... 3 )
Select to expand quote
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.
Thanks for the links yoyo - suspected it had been tried - hadn't seen those links before so interesting. Hardie, forgive the theoretical banter, not sure about others but I grew up sailing boats and dreaming about any way to go fast powered by the wind on the water. That eventually led me to windsurfing which I'm extremely glad of. Now that yoyo has set my mind at ease I can drop all talk of kites and concentrate on getting decent results for GPSTC. Keep up the discussion Ian - better something to read and talk about than nothing
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.
