waveslave said...

Mark _australia said...

Wow Mr Slave

a few posts that are very scientific and well informed. Your reply????

The geometry of the rider has the greatest effect on the amount of drag with poley speed-sailing.
A high-tech, lightweight fairing-speedsuit with built in GPS meter together with a streamlined aero-helmet would sex-up the sport.

Like lycra in cycling?

Maybe you kitey's should sex up your image and start wearing your speedo's on the outside of your wetties[}:)]
Less drag too

Ian K said...

Less drag than skimming over the top. Control is the problem. Kayakers cannot make a flat bottomed kayak plane, but they can make them get up on foils - a foiling kayak is faster than a racing 8 over a short distance.

Exactly. A foil windsurfer may be slower at the moment but that is due to the practical flaws in the current designs. In theory a hydrofoil windsurfer will be much faster. The few yachts that have got foils working properly invariably outperform similar non-foil yachts.

Nah I don't know what kind of foils you're thinking of but they're no bigger than a fin. I used to have foils on my 14ft yacht and they were about 6in each. The faster you go, the more lift is generated, so the smaller the foil needed. So in your theory if you were doing 100kmh you would only need a tiny foil in the water. Hence why some foil craft have a kind of tapered foil system, so the more the craft rises out of the water, the less foil remains below the surface.

The other alternative which has been speculated on from time to time is having a downward fin with two sets of foils - a large upper set to generate lift when the board is moving relatively slowly. These then rise out of the water at speed leaving only a smaller, lower set below the water.

Ps: - I will be very surprised if kites don't eventually end up being faster. When kites have been around as long as sailboards have now, they will be pretty advanced and a hell of a lot more stable and efficient. A big advantage that they will have is that they won't need to use large fins, hence they will be able to run in very shallow and flat watter and also won't have the same issues of cavitation that conventional foil shaped fins have. They also can operate on very small wetted surface areas.

Then the big question will be, "Are kiters really sailing... ? And what defines a sail?"

Greenroom said...

You can certainly see whos serious and whos not in this discussion...
Hey does everyone wanna see me suck fart?

LOL Yeah why not

Greenroom said...

You can certainly see whos serious and whos not in this discussion...
Hey does everyone wanna see me suck fart?

then burp and light it !!!

I think the biggest challenge with optimising gear (boards, fins, sails etc) for really high speeds is retaining the ability to get from 0 to 60 knots.

As mentioned with regard to hydrofoils, the amount of lift generated with 50 knots of water speed flowing over a very small foil is enough, but a foil small enough to be useful at 50 knots will do nothing very useful at 15. One of my mates has a foiling moth, and it has active trim stabilising which uses a trim tab on the back of the foil to adjust the amount of lift it sees.. even so, it maxes out in around 25-30 knots of wind and creates so much lift it becomes unsailable.

If we scale the board size waay back because we don't need lift from the board if a hydrofoil is giving us the lift, and we're already using a smallish sail then getting the thing moving to start with could prove challenging.. Maybe there's a whole new discipline - barefoot/boardless windsurfing!

waveslave said...

Brien said...
Then the big question will be, "Are kiters really sailing... ? And what defines a sail?"

Kiters aren't sailing......
Kiters are surfing on steroids.
lol.

I have real trouble with the term "kitesurfing", I prefer kiteboarding as that more accurately describes the dynamics as performed by 99% of participants of the sport.

As for kiterboarding being likened to surfing on steroids
At best; kiter are like waterskiers on redbull.

Kiters aren't sailing......
Kiters are surfing on steroids.
lol.

Only a kiter could lol at their own statement.

Hey CAN you barefoot behind a kite, or waterski? I can't see why not. Waterskiing would be very lol worthy. How about a sea biscuit?

Water is 800 times as dense as air, so anything that gets hauled through the water will be the dominant drag factor.

For example, land and ice sailing is easily capable of 150kph, so its quite clear that the drag due from the human will be insignificant for water-sailing at the speeds that we currently achieve.

Since the fin is being pushed through water with some non-zero angle of attack, it will be the highest drag component; the next highest would the be part of the board that is in contact with the water.

mathew said...

Water is 800 times as dense as air, so anything that gets hauled through the water will be the dominant drag factor.

For example, land and ice sailing is easily capable of 150kph, so its quite clear that the drag due from the human will be insignificant for water-sailing at the speeds that we currently achieve.

Since the fin is being pushed through water with some non-zero angle of attack, it will be the highest drag component; the next highest would the be part of the board that is in contact with the water.

What Mathew is referring to is skin friction.
Skin friction is all part of the drag equation.
But the geometry of the object in motion (rider & rig) has the greatest effect on the amount of drag.
For example,
if you bolted on a sheet of plyboard square to the front of your mast,
the induced drag would probably be greater than the lift generated by the sail.
In other words, the forward motion would be zero.

....and as someone has pointed out I think that the friction of air or water increases with the square of velocity.

So the faster you go then the more difficult it is to go faster again....

A theory which some of us smaller and thus slower guys have speculated on - as to why we have trouble keeping up with the bigger guys...

Due to our lower body weight, we dont have the leverage to maintain a low board-trim-angle which is less than the fin-rake (eg: board angle could get to 20deg, while the fin rake could be 15 deg) -> so we are seeing that raked fins are more slippery compared to upright fins.

Note that often the smaller guys are using similar sized sails to the bigger guys - so this board-trim thing assumes that both guy-sizes are in 100% control of the rig - which isn't the case as the bigger guys will have better leverage, so more control/faster.