sailquik said..


The situation I theorised before regarding doing 28 knots in 15 knots of wind, works out to a Beta of 32.4. Hmmmmmm..........

Perhaps it is possible for a windsurfer to get to Beta 33 degrees

The method is probably best at places like Sandy Point where you can run down the slowly curving course in 15 knots of wind. At some point you'll hit a max speed which will be very accurately recorded by your gps. Then you can use that right angle assumption to determine Beta. At places where you're running down a canal or behind a straight breakwater the angle may not be giving the best speed for the wind conditions so Beta will be overestimated.

And how good is that 15 knots of wind going to be at the position and instant of your max speed?

Barney's suggestion that we put tell tales on front of our boards has a lot of merit. It's a measure of efficiency not dependant on us accurately knowing wind speed or heading. Speed sailors could then determine Beta at all points of sail, in the gusts and in the lulls . Might give you a few tuning clues to be transferred to the heading of most interest?

So the 'rocket science' of speed sailing is reduced to a telltale on the board with protractor marks, speed up to max speed crosswind, check the telltale for your beta angle, bear off keeping the same beta until telltale is perpendicular to true wind direction?

Reducing the beta angle would then be the highest priority. At the the absolute minimum it would be only the angle of attack of the sail (close hauled), but evidence from the pics and sailquik suggests that as outright speed increases, more and more additional beta is required:

sailquik said..
On the occasions when I was not feeling totally out of control (as I felt on 27-09-2009 ), I am fairly sure my sheeting angle was close to what is shown the photograph I posted from Luderitz earlier in this thread. Probably about 20-25 degrees from course, maybe slightly more, but certainly not 'closing the gap' in the way we do on a slalom board.

Compensation for parasitic drag from the sailor & other non-lift surfaces?

VRBones said..

Ian K said..
But to reverse that calculation you hit the top left reverse button on your mac calculator. You'll get a sin -1 button pop up. Divide 38 by 46 and hit that button. should get Beta = 55.7 degrees.

Something is off. 55.7 degrees would mean bearing away at 145.7 degrees, where sailquik has been stating 130-135. A 10 degree difference would be significant? Maybe they are going a heading of 135 and the apparent wind direction isn't perpendicular for some reason?

Another wierd thing would be that the 55 degree beta would mean a drop of apparent wind by 57 percent from a crosswind tack, which is also supposed to bring about a 57 percent increase in speed (~4 times the speed from the same relative apparent wind). That only seems to make sense if there is simply too much power to hold at a crosswind tack and dropping the 40Kn to a more manageable 32Kn apparent wind on such a steep downwind tack?

Yes thinking about it that way it does seem a bit strange. I get the apparent wind in the case of bearing away at 145.7 degrees, while doing 46 in 38 true, as 25.9 knots. i.e. 46 sin 55.7 = 25.9. This assumes the right angle between apparent and true wind of course.

If Sailquick was doing those speeds at say 132.5 degrees then we have dropped the right angle assumption but we have enough information to calculate Beta.

Use the cosine rule to find apparent wind

Va = sqrt( 46^2 + 38^2 - 2*46*38*Cos(47.5) ........ 47.5 = 180 - 132.5

ie. apparent wind at this tighter angle is Va = 34.61 knots.

And use the sin rule to find Beta which is 54 degrees.

So Sailquick at that tighter angle is sailing at a slightly better Beta to do 46 knots.
But the course theorem requires that the same lift to drag ratios of rig and hull remain as he bears away further. He's probably picked a sail to give best lift to drag in an apparent wind of 34.6 knots.

If he bears away he will only go faster if he maintains the same lift to drag efficiency of both hull and sail. Trouble is that at that optimum angle that we worked out initially his apparent wind drops to only 25.9 knots and his sail is too small to work at max efficiency. Also at that angle his fin will have to do less work, (and at at a faster speed) so ideally the fin should be smaller! He was probably on a 4.5 and a 20cm fin..Can he get started on the course with a 15 cm fin and a 5.5 sail? Probably not, but that's what he'd have to do to take advantage of this theory. And even then if he could get that bigger sail on the course with that slightly better Beta, he'd do 46.9 knots. 0.9 knots! Not a lot of extra speed carrying an extra metre!

I suppose we have to look at the assumptions made in the course theorem before applying it too strictly.

OK. I remembered this article and found it again. I think there are a LOT of answers in here.

Of particular interest to me is the included graph of speed v's windspeed which agrees very much with my own experiences, except that the course angle to true wind is 140 degrees. That may well be the angle I am sailing.

Note. Low 40's speed in 30 knots and 47 speed in about 38 knots: (Note: the caption under it is wrong. It obviously should not have the bit on the end that says "Wind speed 40 knots")




Also note that the article places an emphasis on the influence of the weight of the sailor. I have obsevrved that the bigger sailors (90K +) are able to do better in the 20 to 40 knots range than this red line suggests. Perhaps the line for them is curved a little higher between 15 and 40 knots of wind?

The full article by Rick Hanke and from Boards Magazine is here: boards.co.uk/how-to/how-fast-can-we-go-the-science-of-speed.html#6li5y5WChVpfkGuS.97

sailquik said..
OK. I remembered this article and found it again. I think there are a LOT of answers in here.

Of particular interest to me is the included graph of speed v's windspeed which agrees very much with my own experiences, except that the course angle to true wind is 140 degrees. That may well be the angle I am sailing.

Note. Low 40's speed in 30 knots and 47 speed in about 38 knots: (Note: the caption under it is wrong. It obviously should not have the bit on the end that says "Wind speed 40 knots")




Also note that the article places an emphasis on the influence of the weight of the sailor. I have obsevrved that the bigger sailors (90K +) are able to do better in the 20 to 40 knots range than this red line suggests. Perhaps the line for them is curved a little higher between 15 and 40 knots of wind?

The full article by Rick Hanke and from Boards Magazine is here: boards.co.uk/how-to/how-fast-can-we-go-the-science-of-speed.html#6li5y5WChVpfkGuS.97

Andrew, I told you some time ago the Boards article is wrong. I thought that you could figure out why on your own but here is one example.
In Fig 7 above, the curve labelled Vs/Vt = 2 must pass through coordinates (0,0) (20,10) (40,20) (60,30) etc. The other curves are also wrong. Furthermore, as it stands, Fig 7 shows the board speed is about 7 knot with 0 wind speed. Sounds like a violation of the first law of thermodynamics to me. FWIW there are other errors.

The influence of the weight of the sailor has been known for at least two decades. It is in my 2001 paper that I sent to you a long time ago.

You might also benefit from reading my latest paper. It was not written specifically for windsurfers but there is a couple of things in it the may be useful.

Hyperwind sailing means sailing faster than the wind (Kenney, 2001). This paper is available free from the AYRS website but the reproduction of the figures is very poor. Because it is only 3 pages long I have attached a better copy including the original colour photo below.

It is easy to tell if you are hyperwind sailing because the faster you go the more the apparent wind comes from the front. For example, when sailing at the 3 times faster than the wind apparent wind is only 18 degrees off the nose. If you try to gybe an iceboard while hyperwind sailing without slowing down first you WILL end up on your butt.

You have three choices for completing a success gybe:
1) slow down to near the wind speed before you gybe,
2) do a precision duck gybe with the sail aligned perfectly into the apparent wind,
3) go around the front of the mast like a tack (I think it is called a 'monkey gybe').

Free article here is interesting:
www.ayrs.org/catalyst/Catalyst_N34_Apr_2009.pdf

Sailboat Speed v's Wind Speed - P21