Darren Bos wrote:  Fair enough, although I think much about how sails and wings produce lift gets mixed up when we talk about low pressure pulling a boat along.  I find the concept that you are trying to redirect a mass of air and that doing so pushes you along more intuitive.  

It would be interesting to see the results of the vortex generators.  I like your idea to try them on a few panels which might allow you to directly compare their effect (I've wondered what battens do to spanwise flow).  Perhaps the easiest way to figure out where to place them would be to first outfit the sail with many tufts of yarn to see where the flow is separating?

Darren, I agree with you in your thinking about lift as a result of deflecting incoming airstream (air mass). However, I don't think the  Newton model and Bernoulli model need to crash here.

As for the test of vortex generators. That is  a low-risk test. My idea is to fit them on the port side and only to panel 2 and 3 from bottom. My hope is to see a different reaction of the leech telltales behind the vortex generators when sailing on sb. tack. Normally all the leech telltales fall behind the sail almost simultaneously as I fall off without easing the sheet. I would easily be able to spot if one or two telltales kept flying for longer than the others. If I see no difference, then I just drop the idea, no big deal.

Arne

Anyway, if we believe the separation bubble theory you should be turbulating the mast rather than sail. I'd epoxy a pair of wires running full length, each just forward of the maximum width of the mast viewed from the angle of the apparent wind when close hauled... I remain sceptical but would be delighted to be proved wrong. Turbulators should be small; you're trying to entrain air into the boundary layer to add energy, and even in the worst of our flow regimes that layer won't be particularly deep.

Bryan,

My armchair guess is that the vacuum cleaner doesn’t suck the dust the way tube mail systems work. What I think happens is that the foot of a vacuum cleener is designed to leave a thin gap between it and the carpet/floor for the air to pass through. The combination of this thin cross section for air and the cleaner motor’s high capacity leads to a local “triple hurricane” in that air gap. Thanks to the viscosity of the air, it will drag with it dust , sand, and wotnot, just as tornados do.

Enough about that.

I wouldn’t be so worried about that separation bubble (which mainly occurs on foils with sharp leading edge, I think). Practical sailing has shown that the wind reattaches and then does good work. I worried about that when I made my first cambered panel junk sail, but the telltales at the leech soon showed me that the wind stayed attached  to the lee side until I over-sheeted the sail. Just have a look at “Broremann” on Youtube. Broremann is sailing at the “bad” tack  -  and doing quite well!

Arne

PS: Separation bubbles is not a JR phenomenon – they can be found (with smart tufting) on any jibs.

Scott,

I start with writing the tekst, like the one below, but without posting it yet. Then I open the JRA site in a second window. There I find my way to my album via Directories - members  -  my name (do not go via View Profile). From there I find the album I look for at the bottom of my profile. Then I open the album and right-click on the photo I want to show (don't open it by left-clicking on it). Now, the pop-up menu lets me choose Copy, which I do.

Then, I go back to the drafted posting and Paste the photo into the position I want. Finally I post that draft. I don't know if this is the easiest way, but at least it works.

Arne

Darren,
We all know that there is no such thing as negative pressure. Zero pressure is vacuum.

However, in normal life we can still use the term suction as long as we know it is just a pressure drop from the atmospheric pressure. If the atmospheric pressure is p, then the suction would be Δp with a negative value, never bigger than p..

A good vacuum cleaner can produce a suction of -300hPa (=Millibar); if connected to a glass bottle, it will reduce the pressure in it with almost one third of the atmospheric pressure.

The suction we have on the leeside of a sail in a normal breeze is no more than one hPa, except, maybe in small areas of the sail.

This good-bad tack discussion and separation bubble has become of new interest to me. I have recently made a number of vortex generators which I want to try on a couple of panels on Ingeborg’s sail. The idea is to increase the stall angle. However, I am unsure of where to put them  -  very close to the leading edge, or a couple of feet aft, that is, away from the separation bubble. Sooo, I guess that is another major thread drift. If I go ahead with it, I will start another topic.

Arne