The design of soft wing sails for cruising

Only if you measure the returns in knots or pointing angle alone.

For a tinkerer the returns can go beyond that, if a little more subjective: knowing you are getting the most out of the design's potential and, perhaps more importantly, having a beautifully shaped leading edge to stare at for hour after hour on a long voyage!

 

David, just a question on this feature of your sail: I assume it works the same as on conventional junk rigs and relies on lazy-jacks and proper batten length?

In the pursuit of a cleaner rig, would it be possible to substitute the lazy-jacks on either side of the sail with a single lazy-jack rigged inside the double-sided section of a soft wingsail? For simplicity's sake, picture a simple symmetrical foil with no hinged aft section or other camber complications. The wishbone-battens will have a narrow framed slot along its chord line, stretching from just aft of the mast to fairly close to the trailing edge. This slot will allow the wishbone to slide up and down an inclined lazy-jack unhindered, but will arrest the sail head from twisting when halyard tension is released.

Could such an internal stay - regardless of its usefulness as a lazy-jack - be used for trimming twist of square or fat-topped sail by adjusting its tension?

 

My topping lifts are of 3/16" Amsteel, and I can't imagine that they generate much parasitic drag. They are essential to capture the sail as it reefs and furls, and an internal lift wouldn't do this satisfactorily. It might capture the battens, but it wouldn't capture the loose cloth. And a split batten would weigh more than a single tube, which is always the best for strength/weight ratio. I do actually have an internal lift, of 3/16" Amsteel, to support the mid point, at the hinge. It droops a little too much if this is not present, but I wouldn't think it to be essential, on a smaller, lighter rig. I don't think such a stay would control twist,as there is not enough tension on it.

 

I really meant that one riblet is certainly worthwhile, two are better but not twice as good, three are marginally better than two, four are ... you get the idea.

 

What IS the perfect or beautiful leading edge shape for a sail?
So many different airfoils, is ONE more beautiful or more perfect?
I'm not trolling or picking a fight.
Just wondering if there is a majority endorsement for one particular airfoil for sail leading edges.
Is there an ideal shape, independent of mechanical or engineering considerations and compromises?

 

If you look at all the high-lift foil shapes, I think you'll find a common factor - the leading edge radius is quite close to 2% of the chord. So for my sail, I'd like to see the luff with a radius of about 110mm.

 

thanks.
I'll try it on a teetertotter lateen yard/luff spar.
In the fall.

 

I doubt that you would find one particular foil to be perfect. There are just too many variables at play. These could include sail material (rigid vs soft), mast diameter, sail planform, camber, sheeting angle, sea conditions (changing apparent wind angles due to pitching and rolling), presence of a headsail, sail size vs wind strenth (Reynolds number) and point of sail.

Just like there is no perfect rig, and yet the bermudan sloop has become the default "best compromise", you could very well find a foil that provides the best all-round performance for the majority of sailors.

I have played with 2D foil solvers like DesignFoil that allows independent manipulation of variables such as camber, camber location, thickness and thickness location for 4-digit NACA foils. I was surprised at some of the results I got for thin foils with maximum thickness and camber very far forward. For instance, a NACA 6203 with a slightly enlarged leading edge radius and maximum thickness at 10% gave a max l/d of 95 and Clmax of 1.7 at Re=1000 000 (equivalent to 10ft chord at 10kts). It is a really odd looking foil, it looks like an over-twisted wingmast. Makes you think...

By comparison, here are results for some more conventional foils:
_____________l/d___Clmax
NACA 6310.......90 1.6
NACA 4412.......85 1.5
Eppler 422 .......78 1.55

I need to mention that for all the above foils I forced the boundary layer to go turbulent over the entire foil. This is because one cannot realistically expect any laminar flow on a soft wingsail.

For a rigid wing with smooth, clean surfaces where laminar flow is possible I got the following results:

______________l/d__Clmax
NACA 6203........125 1.7
NACA 6310........128 1.6
NACA 4412........106 1.5
Eppler 422.........124 1.55

A special high l/d foils such as the e422 needs some laminar flow to reach its true potential. Once again the thin NACA 6203 was able to keep up with the best of the rest.
The Clmax predictions from these programs are not always that accurate, but can be useful for identifying foils with potential for further investigation.

 

I have followed the OPs thread with interest.
I'm great admirer of the Chinese lug sail and Chinese junk design in general.
One of the problems, and a problem of all fore/aft balanced rigs, where part of the yard and sail is forward of the mast, is the "bad tack".
Where the mast is to leeward of the sail and the wind causes the sail to foul the mast.
The lateen is another ancient sail form. Simpler than the lug sail since it only has a single yard. No boom or battens. it also has a "bad tack" and fouls the mast.
Unless, it tacks like the wings in Herreshoffs design below.
Teetertottering to tack, allows the sail to always be to leeward of mast, no fouling, no bad tacks. And the windward and leeward sides of the sail are ALWAYS the same side.

What is of especial interest to me, it also allows an asymetrical rigid airfoil shape in the spar as the leading edge, with no need of mechanical reversal or a "universal" shape.
I can't imagine how it would be applicable to the Chinese lug sail. BUT, it's another way of having a soft wing sail. A simpler way, perhaps. I intend to try it.

 

`Makes you think" What I've thought all along... that is , a rotating wing mast and sail can keep up with all but the most advanced 2 element wings ... want proof? Hydroptere uses a soft sail and there isn't many that can match her...

IMHO , wings aren't worth it unless chasing a racing trophy ...

 

Some junk rig sailors claim, to their surprise, that in actual experience the 'bad tack' actually was the 'good tack'. Their ad hoc explanation was that the sail draped around the mast apparently helped smooth the airflow on the windward side, while not creating that much extra turbulence on the leeward side.

 

Was that just an impression?
Or did they measure more speed or pointing higher?

seems counter intuitive.

With the full battened junk sail perhaps the sail is flattened laying against the mast.
Unbattened sails look like a dromedary camels humps.

 

Over the top is one way to avoid the bad tack, you can also 'outside gybe' some designs or wishbone mast. Personally I think Herreshoff was more amazing for the designs that didn't get produced than what did.

 

Fat old masts have lots of drag and less if the sail is up against them. If sail shape was flat or poor the other way it is possible to be better against the mast.

 

Measured.

see https://groups.yahoo.com/neo/groups/junkrig/conversations/messages/16064