ClausF said..
Ask any of the surfers on this video from Denmark about how their day was on the water, - they will all reply "It was bloody fantastic!!"...

As cold as it looks in that video, those guys should be crashing their brains out, they can't feel their hands or feet

ClausF said..
Ask any of the surfers on this video from Denmark about how their day was on the water, - they will all reply "It was bloody fantastic!!"...

As cold as it looks in that video, those guys should be crashing their brains out, they can't feel their hands or feet

Te Hau said..

ClausF said..
Ask any of the surfers on this video from Denmark about how their day was on the water, - they will all reply "It was bloody fantastic!!"...

As cold as it looks in that video, those guys should be crashing their brains out, they can't feel their hands or feet

Yes, actually in some parts of the video the temps were below 0?C.. Vikings..

Not quite to sort of crashes. Most of those are everyday standard get offs.

I was thinking more like these:

Does anyone know of an online apparent wind calculator that can give me an answer for these numbers? All the ones I have tried to use give me rubbish numbers or nothing at all. Or someone who can do the math?

What I am looking for of course, is the apparent wind angle for the boat/board in this scenario:

True wind speed 38 knots.
True wind direction 270 degrees
Sailed course heading 135 degrees (=135 degrees off the true wind angle)
Boat/Board speed 45 knots.

Thanks Roo, but that still does not make sense to me without an explanation.

For instance. the true wind angle should not change??

Anyone like to elaborate?

Thanks John. That makes sense to me.

So assuming a sheeting angle of about 30 degrees from the centerline of the board in the above example, (just a guess - could be more or less), that would give the sail approximately a 25 deg AOA. If the sheeting angle is less, say 20 deg. AOA is more, in this case 35 deg.

I wonder if we can somehow get a ball park figure for the optimum sheeting angle to the apparent wind on different courses from the true wind for a windsurfer using these calculations and assumptions?

For instance, I observed I could do around 26-27 knots speed on pretty close to a square reach, in, I estimate, around 16-17 knots of wind. (Flat water, IS86, Tribal speeded 27cm Koncept 6.2m sail) I am sheeted so the foot of the sail is right in against of my leeward rear footstrap. That suggests around 10 degrees sheeting angle to me, and I can feel the apparent wind is very forward.
But in 26-28 knots of wind I can only pull about 30-32 knots on a beam reach. Sheeting angle is very similar but maybe a little further out. (Flat water, IS50, 20cm speed fin, 5.0m Koncept.) Note: Quoting these speeds is tricky, because just a small change of course angle broader will quickly add knots to the speed, especially in the latter case.

On a speed board at top speed in 38+ knots of wind the sheeting angle is definitely out further.
What angle does this look like?


And another random example:

Quick question, what are you taking as sheeting angle? Center line of boom seems obvious to me, buy obvious to me isn't always correct.
In which case maybe 20deg, would be much easier to judge with an overhead drone shot

closer to 20deg than 30deg

So applying the above logic to a recent session I had at Lake George.
Wind speed = 30kts
Board Speed = 40 kts
Direction of board angle to wind = 135 deg
Apparent wind = 28 kts
Apparent angle of wind to board = 48 deg
I don't think I was holding my boom at 48 degrees to the direction of the board. It was probably more like 30, so I was chocking the sail, i.e. not sailing efficiently. I suppose this is why others were sailing faster

John340 said..
>>>>>
I don't think I was holding my boom at 48 degrees to the direction of the board. It was probably more like 30, so I was chocking the sail, i.e. not sailing efficiently. I suppose this is why others were sailing faster

don't forget the angle of the leading edge is very different to the boom angle. if the boom angle is the same as the wind, you're not getting a lot of lift.
I think max lift is when the leading edge is aproaching the apparent wind angle?

Nothing to forget for me Ian.

VRBones said..You will need to add some angle of attack on your sail to generate the power. Typical wing AoA is around 15-20 degrees, so you holding at 18 is in the ballpark. Each sail would have its own optimum AoA though.

Yes, I think the AOA will be different for different sails, but I think even different sails would be very close, maybe within a few degrees.

barney831 said..

The attached video is of a quad going 25 knots in 5 knot winds with a 3.4 m^2 sail. There is zero leeway and the rear blades are running exactly in the track of the front blades.

Can't be zero leeway, the video shows the blades throwing ice particles to the lee side, Can't do that without an angle of attack. Maybe the rear blades are running the same track due to a chassis mis-alignment?

barney831 said..


The blades are asymmetric (chisel profile), rockered and aligned. They are generating lateral resistance without any yaw angle of the board. The centerline is in the direction of travel and it tracks like it was on rails.

Ah. So there's a fair bit of tuning to get the rear blades to slot in behind the front ones? You don't just bolt them on square and hope for the best?