same foil/board and wind speed, how does different size sails affect speed?

Has anyone measured how much faster they go with the same foil/board and wind speed using different size sails? I am trying smaller sails in conditions that I would use a larger sail, so a 5.8 instead of a 7.2/8.0. I find with correct technique I can foil with the 5.8 but feel like I am going a little slower than with a larger sail. Am I imagining the speed difference or is it real?
Thanks

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LeeD said..
I find it's not sail size, but also sail power, and how it's tuned. Pumping once already up has a bearing, as does what angles you go to get up to speed.

Thanks LeeD

I haven't measured but smaller sails tend to go faster, but the other big dimension is rig tension. I had a 5.2 Sailworks Revo wave sail that I could foil with in 15-20knots on a soft mast and 20-25 knots on a stiff mast. It was quite fast on top end with the stiff mast as I assume that the stable airflow over a sail that does not distort is more aerodynamic.

I find the same with Ezzy Hydras. As I increase sail tension with all other equipment the same, top end speed increases.

So how do you know what sail size is as big as needed for the wind with a given foil/wing, other than logging gps speed? I know I used an online sail size calculator for windsurfing, any for foiling?

I find (as above) smaller sails are faster. In the cross over of wind strengths they'll function in. But that's only one piece of the puzzle. My 8m (talking race foil sails) starts to function properly at around 12 knots. My 9.5m also functions at 12 knots. my 8m will be faster upwind than the 9.5 will, but I won't be able to point as far upwind (or downwind) as with the 9.5, and when the wind lulls to 8 knots it'll stall. The 9.5 won't.

Wing sizes are simple (for me at least.) my small wing won't be powered properly below 15knots of wind, so that's when I make the change. By that stage I'm on the 8m regardless. Of course, that's for a course racing scenario. Going purely for speed, where you're not going upwind or chasing vmg, I'd just go for the smallest sail that would stay powered.

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Subsonic said..



For me it's just something that comes from sailing them in different wind strengths. Trial and error (lots of error). Then reviewing what happened on the wind graph afterwards.

Yes

A lot of time on 9.0 and racefoil made it easier to just deal with overpowered situations when using freeride foil kit (slingshot). I worry more at the high end of getting into survival mode but it's not as common here. I know that if it gets a bit lit up on my 6.6 and i76 that I can safely make it back, just point deeper downwind and upwind to deal with overpower with apparent wind or with using the foil lift to push upwind harder. Same with being massively overpowered on a fin but the angles are less (and my experience is now less as well). But the i76 doesn't point as high as the race foil, and it's very noticeable in comparison.

After more TOW I know that if it's not happening with a 6.6 and vigorous pumping on the i76, it likely isn't happening with a larger sail. I don't have a board appropriate for the very aft lift of my i99 so I don't bother now (saving it for a wing maybe). But the 5.0, 5.8, and 6.6 cover me for 90% of the time I want to use freeride gear on a foil.

When I first started foiling I couldn't figure out how to deal with overpower and would limp home by flagging the sail and shlogging back. Getting over that fear and getting deeper angles made a lot of difference, but it wasn't "comfortable".