Kite-surf, explained by a sailor
- An addendum to Guara-steered raft sailing -
Don't forget that we are talking apparent wind - not the thrue (geographic) wind. The wind window move and change together with the board speed.
The expresions 'LIFT' and 'DRAW' we have from the airplane industry. The 'lift' is the lifting force across the wing - and 'draw' is the resistance of air sweeping over the same.
There is no direct connection between the "sail" and the "vessel". The force from the kite goes to the pilot, where normaly tied to his waist by the power line. He is "handling" the board with his feets - and he is steering the kite with his hands.
An analyse of the kites draw is simple, because the force is by a lonely main line. As long as there is an angle between this line and the centerline of the board, we can divide the drawing kite-force in two components: a lateral force across and a forward directed along the board, and those two wind-components will drive the board.
A kite board is a simple shaped board with a minimum of weight and no bouyancy. As any wind powered vessel it hold a LOW forward hydraulic resistance and can yield a HIGH lateral dito - specially when tilted against the movement.
And as with any wind powered vessel: the lateral kite-force is up against the HIGH lateral hydraulic resistance and give a leeway, whereas the forward kite-force will accelerate the board to planeing - speeding it up until the gained hydraulic resistance have reached the kite-force level.
That is kiting !
Board across too can be used to kite directly downwind. Drifting a sailor would say.
That is not possible to sail directly downwind with a kite-board. A kite-board is so swift moving, that the kite because of lost tension in towing-line will fall out of the sky.
Some notes around power kite special facilities:
Therefore, going upwind, kite boards doesn't tack - they shunt as a ferry = change aft to for and so they zig-zag up against the wind, keeping heel-side or keeping toe-side.
They can weer!
They weer when sailing slalom (downwind of course), or they could chose to zig-zag downwind, but they can't go directly downwind, as told.
Toeside style versus heelside style.
In the same way as any other wind-powered water-craft the forces of wind attack in the CE=Center of Effort, and this has to be in balance with CLR=the Center of Lateral water-Resistance - but that may be outside concern from a kitesurfer. As athlet he is concentrated on own body balance. CE and CLR balance is for sailors.
But - because of no boyancy from board the draw from the kite have to be compensed against capsizing by the weight of pilot: excentric.
The professional interest in kites and sails is bunker savings
- but too environmental aspects -
These huge kites as tested can't be handled by men. They need an automated servo system (probably full automated) with a launch and recovery arm + a powered winch - and that is a technology still under development.We look forward to more experience.
As difference from sails on masts, a powerkite give less heeling, due to its lower CE = the point where the kite is made fast.
The other option for shipowners is the Flettner sail = the Rotor sail: A vertical rotating cylinder as with wind abeam will be able to yield as a sail. The "Magnus effect" it is called. The Flettner sail is considered as a sail, even if the rotation of cylinder - a few hundred RPM - is powered by electrical motor.
The Magnus-effect will only work with wind abeam - and never running downwind with wind from abaft.
A key point by Flettner sails seems to be the surface speed of the spinning cylinder in relation to the apperent wind speed.
That means, if sail/kite is mounted in fore peak, then the ship is secured against excessive leeway.
We look foreward to more openness about professional results. For both Kites and Flettner sails I expect, that after the tests, their place will end up in the bow, simply because of the higher lateral hydraulic resistance with the wind abeam - due to the stem rammed into the sea and thus hindered in side-sliding.
And here come the relation to Kon-tiki rafts
- becourse the sailing speed will be low - as with any raft there is little dynamic influence.
Tie the main line of the kite to the root of the mast, and fasten your steer-lines when trimmed - and then you should be able to steer the direction of the raft by lowering /lifting the Guaras.
But because we moved back the CE, when we tied to the mast, then the raft probably will need more Guaras down aft - to resist leeway of aft end.
To what use to sail a balsa log raft with a powerkite ?
I have no argument. I only say, that with a Guara steer-system, a powerkite is directly possible !