Q: Sail tunig by carbon tube battens - any good?

Gun is offering a set of carbon tube battens for their 'M7' race sail - not cheap..

Would those have any real advantage for recreational sailing / race sailing besides a very small weight loss?

Theory and experience are both welcome!

Thanks,

No experience with this change on this sail, so read on only for theory. Changing battens might or might not change how the sail feels and the shape it takes in certain wind conditions. Unless Gun says these battens are seen as an improvement in some way other than weight and we can take them at their word, they would be moronic to substitute one batten for another without consideration for performance change. You need to speak directly with them to see how it will affect you at your weight.

They claim a 15% improovement (speed?)

What company making such claims and trying to sell an extra product for 98 Euro (about 95 $) can be taken at their word?

Thats why I ask.

Thanks,

In theory stiffer battens (presumably Gun's accessory battens are stiffer) will make a generally flatter rear area of your sail which will tend to make the rig easier in the upper end. Also there is less tendancy for the shape to move rearward making the upper end easier. The low end is possibly compromised if stiffer battens are used. At ART some years ago the TNT race designs actually included xtra stiffer battens with the sails. There was a difference in performance, optimizing possible for the racing conditions... Gun does their R&D here in Tarifa, we have good relations, they have been experimenting with tubes of greater wll thickness...

Saludos, Monty

Monty,

Great to see you posting here. I hope your can contribute your expertise here a lot. We have several sail designers chiming in here from time to time and we feel very lucky to have some access to your expertise.

Hans

On 19 Jun 2002, Monty Spindler wrote:

(...) So I gather for early plainig conditions and as I´m not a racer the normal battens should do...

Thank you very much for your kind help!

Regarding batten stiffness: I vaguely remember someone posting here about making battens stiffer for use on his ice flyer and Windwings. Interestingly, he was making them stiffer near the mast edge of the sail. Then again, he was going 50mph or more. A search on 'ice flyer batten' gave the article.

tom. (Boston) PS: Monty, the 8.4 02 is still kicking! I put a hole in it with my harness last year, but it was where it was reinforced...

The mod you are refereing to is at http://iceflyer.com/ptb2.html

Interestingly, this 'funky' mod has had the largest gain on my 8.8 race sail used for windsurfing when the winds are just enough to plane and the sail is run on the full side. Ive wanted to race it in the local racing scene but job and family have prevented that. Im using the sail with a 8:1 adjustable outhaul (which I adjust a lot) and a Star board 155 but I dont own anything over 30% carbon masts and this combo -at least from a recreational sailing point of view - ie, BAF'ing- has so far been 'remarkably' faster than everything else Ive sailed with. If I were a sail manurafucture, I would at least try this....

What I think is happening here is that this change to some extent addresses 'seperated flow' behind the mast and the flat spot behind the mast to some extent minimizes this. Localized seperated flow actually accelerates air faster than the sail is going and drag is simply how much air mass gets accelerated by the sail moving by. Seperated flow has high drag and also degrades the pressure difference's which produce lift. Dealing with seperated flow is I beleive one of the big advantages of wing masts and I believe the current trend towards larger flatter sails is because it minimizes seperated flow.

I am fairly convinced based on my testing and what I see in other types of sails that dealing with seperated flow can have big gains in sail performance and Ive also just started to play with another idea based on this. Given my apparent lack of common sense in not embarassing myself with publicly talking about crazy ideas, here it is..

Vortex generators are devices that sit on the surface of an airfoil and 'energize' the boundry layer and delay seperated air flow. One application for Vortex Generators is to decrease the stall speed of airplanes and they apparantly work well in that applicaiton. Some web sites on Vortex Generators are:
http://airtab.com http://streamlining.com http://home1.gte.net/pjbemail/VortexGen.html http://www.cubcrafters.com/cc/products/ vortexgenerators.asp http://www.avweb.com/articles/vortexge.html

The reason Vortex Generators might be interesting for windsurfing sails is that the single surface sail with the round mast typically operates with some level of seperated air flow in the regions just behind the mast even when they are not 'stalled'. It just might be that the extra drag of the vortex generator is hugely offset by a reduction in the drag of the seperation bubble behind the mast?

Here's my first attemt at an experiment (which I hope to try today) http://iceflyer.com/analog/vortex_gen.jpg

Wally Hall Denver CO

Walt, vortex generation is a compromise that might have some merit if we were sailing at stupidly tight angles and at 100kts. The question on a windsurfer sail is not separation of flow versus laminar flow, but separation versus turbulent flow. With all the bumps, bends and nonsense going on all over the sail (like battens non-aligned with the airflow, a boom head that makes a wake two feet wide, and the big fat blob holding onto the booms to windward) laminar flow is hardly a viable concept. Managing separated flow correctly is the key. VTGs induce a rotating cone of air to redirect or, as you say, re-charge a space. But that space being re-charged usually is one that would otherwise be turbulent in the absence of the VTGs. F1 cars use VTGs to direct 'clean' air into a spear-like cone onto surfaces that otherwise would be covered in turbulent flow, like the rear wing, thereby increasing down force or reducing drag from excessive turbulence.

Turbulence at 200 mph imparts much more drag than at apparent wind speeds of 50 kts. Obviously we all can benefit from any net reduction in drag, but we run the run the risk of decreasing lift as well. The L/D ratio ultimately rules supreme, not simply forcing cones of swirling air into turbulent flow. If we could reduce tip vortices with VTGs then we really might see something faster. Cool links.

I aggree there is probably very little laminar flow on a windsurfing sail and likely even if VG's could work, it would probably require a wind tunnel and some luck to find optimum shapes or even a shape that produced a net positive effect (or maybe something like that truck Gaastra used to have where they would mount a sail on and drive around on a dry lake bed).

You might find my observations on seperated flow interesting. Ive been sailing for over a year now with a couple of tell tails within several feet of the mast and using the sails on both the ice boat and windsurfer. The ice boat has the sail more upright, sheets harder, in higher wind uses larger sails and goes faster (to some extent, operates at higher reynolds numbers). Seperated flow is when the pressure gradients get too high for whatever flow type exists to stay attached to the surface and the flow at the surface actually changes directions. During seperated flow, with the sail more upright, the tell tails will point more directly at the mast however as the sail is raked back, the tell tails will tend to point upwards parrellel with the mast. On a windsurfer with the racked mast, leaning the sail to windward probably creates an 'upwind gradient' which probably has some effect on cutting down on the upwards flow.

On the ice boat, usually when I start off, the flow will be attached but at some speed, it will become seperated. As the sail is made flatter, the speed where the seperation occurs will increase but I also lose power and would need a larger sail. Very often, both the boat and the windsufer are fastest even when the sail is operated with some seperated flow going on. Even with the flat spot behind the mast with the 'funky' batten mod, I still get seperated flow but it does still seem to allow the sail to be faster with a little more camber.

Here is an article that you may also find interesting (Im sticking with round mast's by the way - Im getting all the performance I need and they are WAY cheaper): http://www.tspeer.com/Wingmasts/mast.htm

Wally Hall