Friday, April 2, 2010

Part of my windsurf calculator is in a free iPhone app

For a while there has been an iPhone application that uses the phone's receiver to calculate the wind strength. Michael Alex over at "The Peconic Puffin" windsurfing blog says it works pretty well.

Now, apparently, there is another iPhone application that uses the formula I published in my windsurf calculator to relate wind speed and body weight to the ideal sail size for planing. Unlike the anemometer application, this one is free. You can check it out here.

It would be interesting if they combined the two applications so you could just enter your weight then hold the phone up in the wind to see what size sail to rig.

PS- If you're looking for my KITE size calculator, it's here.


PeconicPuffin said...

Are they actually using your work? Did they ask you in advance? "Inspired by" ...was it inspired or copied?

James Douglass said...

Puffy- Yep, they're actually copying my exact sail size formula, but they give me credit for it, so that's cool.

In truth, I myself copied the sail size formula from an ancient thread on the Starboard Windsurfing forum, and the only changes I made were creating the mph and kmph versions of it.

The board size and fin size formulas in my calculator I came up with myself, based on personal experience and regressions I did on published fin size recommendation from the True Ames fins website.

Brian said...

Thought I already posted this comment, but couldn't find it, but i guess you might have just answered part of my question. The sail size calculator works pretty well for me at 208 lbs, but way under-sizes the sails for my 108 lb girlfriend. I was curious how you calibrated for weight?

James Douglass said...

Hi Brian- You left your comment on the kite size calculator page, so I answered it there. The formula that the sail size calculator uses is 1.34 times body weight in kg divided by wind speed in knots. So it's directly proportional to body weight. So since your body weight is twice your girlfriend's body weight, it recommends about half the sail size for her at a given wind speed. It might not be as far off for her as you think. I.e. it says she could plane in 12 knots with a 5.5 and you would want 23 knots with the same sail. You should get her to test it! :)

Brian said...

She definitely cannot plane in 12 knots with a 5.5 m2 sail, despite being a pretty proficient sailor. I'd like to know who developed the original formulas.
I think your comment about the combined sailor + rig weight might be important for the lighter sailors.

James Douglass said...

Brian- 12 knots is 14 mph. Does that make a difference? I have no idea who developed the original sail formula, but I did the board and fin formulas myself. If I can get some good wind-meter-checked data from very lightweight people that shows the calc is consistently recommending too-small sails for them, then I might revise the formula.

Brian S said...

Yeah, the regression on a good data set would be easy, that's why I was curious who did the original sail formulas. Most people don't go out onto the water with a wind meter. As I said, it's pretty much right on for me. Thanks again.