I've been researching windsurfing rigs trying to figure out which setup would be optimal for an iceflyer. After doing a considerable amount of research, here are my thoughts.

At high apparent wind speeds and low angle of apparent wind, drag becomes very critical. What works great at the relative low speed of a windsurfer would not be ideal for an ice sailor.

The standard tips for reducing drag on a foil are:
* Minimize twist - Some twist would be good for control, but a modern windsurfing sail has a lot of twist built in. Older windsurfing sails might be better as they did not twist as much.

* Flat is fast- The draft of the sail would have to be minimized, other wise the sail back winds. Even with maximum out haul I'm not sure if you could get the sail flat enough to be optimal.

* High aspect sails reduce tip drag- Short booms and long masts reduce tip drag, only negative is the high center of effort which makes you heel more. I suspect the best aspect ratio for an ice sailor would be much higher than a windsurfer!

I also sail a DN and a skeeter. The skeeter will do over 100mph and I suspect it's due to its high aspect ratio and very flat sail profile. Check out this photo and you can see what I'm talking about. .

With these factors in mind I've been trying to identify a windsurfing sail that has a higher than normal aspect ratio and a very flat profile. I'm looking for something in the 7.5 to 8.0 range.

Any comments or suggestions?

David Woodward
http://www.davidwoodward.com/iceflyer

Very cool picture!

Here's my opinion on the "optimum" sail for a rear sail boat this size.

On aspect ratio, a higher aspect ratio sail will have less induced drag but as you mentioned, it will also have a higher CE which will tend to tip the boat more. The fix for this is to go to a wider rear beam so as to maximize the balance between down force on the lee runner and the tendancy to hike. Then, with the longer plank, you would likely need to extend the front spring board to overall balance the boat. When taken extreme, the boat ends up looking more like skeeter - definatly much faster but its no longer a small relitively inexpensive easilly transported boat.

So I think you have to at some point set some constraints on the design and this will have a fairly big influence on the sail design. For example, setting a sail size limitiation for racing (typically done in lighter winds) will result in a raked back sail and if no limitations are placed on the mast, it will end up being a wing mast. If you restrict the mast to round, you will end up with a sail that looks (and has rake) like what is on the Fed 5 land sailer. If there is no restriction on sail area (like the ice flyer), the sail will likely end up more vertical (vertical has less induced drag for higher top speeds but tends to also produce less power for a given sail area at the typical wind speeds where racing is done) and will keep on growing until the boat becomes too small to "utilize" the area.

Since Id like to be able to race other ice flyer's and also like to mess with things, I "suggested" some contraints for this design which are basically that the boat size and configuration is set but sail area is not at all. I also suggested that the mast be kept round but Im not sure if this is what people will want to be contrained to. So what comes from this, who knows but based on what Ive seen so far, it will positive and its a new "equation" that I dont beleive has been explored before. Here's my wish list for an sail:

1. It would probably still use windsurfing masts but designed around one stiffened by putting the top peice of a second mast inside the bottom peice of the mast the sail is rigged on. One important reason for this is that they are cheap and easy to get and when stiffened, they dont break no matter the torture. Overall mast lenght (including the extension would be around 525 cm or so. These boats do get tipped over (partially because of in generall sailing in some wild conditions) so the mast needs to tolerate this.
2. No monofilm.. This material is nice if you need light and dont care if the sail falls apart in 3 years but Id prefer a sail that lasted longer.
3. The overall windsurfing sail planform is a good one so keep this. The sail would have some of the area at the boom end moved lower so that it did a better job of closing the gap. The aspect ratio of the sail would be modified slightly but not that much.
4. Fullness of the sail - I wouldnt mess much with what windsurfing sail currently do. With an adjustable outhaul, I can make my sails too flat. The windsurfing sail is very tunable with outhaul and I find that very flat is rarely the fastests unless you have too big of a sail in high winds. Having a sail that will go full also gives it a huge range on the low wind side.
5. Windsuring sails in general get too much curvature right after the mast (in my opinion) when you try and make them very full which you need to in very light winds. I still end up modififying my windsuring sails to stiffen the battens just behing the cambers and I beleive this helps in the light winds.
6. This optimum sail would likely be somewhat different than a windsuring sail but Id still want the staying system to work with windsurfing sails. In high winds, windsurfing sails really work well and Id still want to be able to "throw on" a windsurfing sail "just in case".
7. I think your optimum size is about right. Im currently using a 9.0 a lot (because its fast in 3 to 12 mph winds which Ive seen a lot this year) so a really efficeint 8 to 8.5 m**2 might be a good overall sail size.
8. Twist - There is a wind height gradient and when the boat moves, it has an effect of changing the angle of the wind as height increases. Even at 5 times wind speed, there is still an angle shift in the wind with height. Based on some simple calcs I did at one time, it looked like you needed about 3 to 6 degree twist (or close to that, I didnt go back and look up the calcs) in the sail to maintain a constant angle of attack to apparent wind depending on the boat speed to actual wind speed ratio. Also, windsurfing sails are supposed to also use twist at the top to minize induced drag at the top of the sail but I havnt really seen good evidence to suport this (on the fastest day Ive gone, I had a square top 6.7 do 72 mph and buddy on a 6.4 m**2 triangle top sail did 71.9 mph - at these speeds, induced drag should have been very important but we were about the same speed). Anyhow, I think some twist is good even for an ice boat. Another example of controlled twist in the Windjet. They have a three section mast that they can dial in the twist on.

Overall, Im happy with windsurfing sails (except the durability). With a 9.0 m**2 sail that can be made very full, these boats really do well in very light winds and the 9.0 will still work and be fast in some fairly high winds. And in high winds with this geometry, windsufing sails are awful darn sweet. And, they have a really huge range.

Regarding induced drag, there are some things that "aparantly" you can do to improve the induced drag in this applicaiton. Ive been experimenting lately with "end plate skirts" and other things like that to improve the end plating at the sail bottom. In general with end plating, its either "do it right or dont bother" as most of the benfit only comes when you get close to perfect but the limited and uncontrolled experiments Ive been doing so far look at least interesting.

Regarding wing masts sails - these are cleary faster higher performance sails than round mast sails. Ive never gone to one partlially because I dont know how to build one and the big wing masts require a lot more attention to sail - which could be a significant problem of not being able to readilly see the mast and flow meters attached to it. However, Ive been wondering about some sort of structure at the very leading edge of a windsurfing type sail. It would seem that the first few inches on the leading edge of a sail can have a fairly significant effect on the sails performance - but shaping must be done very accuratly or dont bother. By the time the flow is back to the mast sleeve, its much less critical (for example, I cant tell much difference between a sai with a wide mast sleeve vs. a narrow mast sleeve - they both get a big seperation bubble on the windward size and feel about the same speed wise). Some of the ideas have been possibly a couple maybe 3 foot long sections of a plastic "snap on" leading edge that give the sail a "perfect" leading edge and since they are attached to the luft sleeve, rotate nicely with the sail. Or, maybe an inflatable bladder ONLY on the leading edge of the mast sleeve that created a fairly rigid perfectly shaped leading edge on the sail but still allowed the sail to be rolled up?

Anyhow, just some ideas. I will be interesting to see what you come up with..

wh..

Thanks for the insight. I plan on using a production windsurfing sail but have been trying to sort through everything on the market. Seems like there is tremendous variation in profile, aspect ratio etc.. Freerace, freeride, race, cambers, camberless, etc. Is there a production sail that you feel is the best fit for the iceflyer right out of the bag?

I'm considering a used Neil Pryde V8. Specifically what others should I look at?

David.

I think the NP V8 is likely a very good choice. In the big sail, I like 2 or 3 cambers and the only reason for this is that in the very light wind they will hold the foil shape and so seem to get going a little sooner. I think heavier is better as its generally also more durable. I personally have not tried a lot of brands but I think most are good. The one thing that I dont know how to look for is a sail which is very tunable with outhaul and also has a "good shape" when full (they all have good shape when flat). I dont think age of the sail matters that much and ice season is generally a good time to buy in the US as dealers are pretty motivated to get rid of stuff they didnt sell during the last season.