The design of soft wing sails for cruising

Will, I recognize several of your insights from theory and practice. As sailors go I am more theory than experience myself but I have not managed to put together a complete monohull model. Fudge factors are acceptable in my level of work as long as long as the variable I need to tweek isn't in the fudge. My interest is not so much about accurately modeling current designs as it is about finding value in what should be -like closing off to the deck or is there a better control variable than just angle of attack like a flap and or a slot. I also want to quantify how difficult it is to sail a boat -how much control does it take per variation in wind.

Keep up the good work! I am looking forward to the spreadsheet.

 

Interesting old Naca video on flow, separation and slots etc.

Hope you get something out of it, the section on slots interested me.

 

OOPS
https://www.youtube.com/watch?v=TqTSyFz6DJc

 

Now that Bamfield Music by the Sea (a superb series of classical and jazz concerts) has ended, I'm cruising again, and can give a first sailing report:

1. Remembering my terms of reference - an easily handled rig for ocean cruising, and cruising to wild places, with all the benefits of junk rig but with windward performance as good as I can get it - I think I'm getting there.
2. I'm sailing rather closer to the wind than with my "Fantail" junk sail, with good boat speed. Don't ask me for numbers that would apply to other boats - because Tystie is quite unlike any other boat (except her one sistership), they would be meaningless.
3. Two days ago, I beat through some narrow channels in the Broken Group, Barkley Sound, with only enough room to gather boat speed before it was time to tack again. I was making 90 degree tacks in 5 - 10 knots of wind, on the GPS track (ie, with leeway taken into account). Pretty good, I thought. Bermudan rigged boats couldn't have/wouldn't have done it.
4. Yesterday, with 15 - 20 knots of wind, I beat into the entrance to Ucluelet Inlet in the fog, making about 120 degrees between tacks on the GPS in the choppy sea. This is about as good as I'd expect to get, in the conditions.
5. Helm balance is neutral in about 10 knots of wind, and with a little weather helm in one-to-two reef conditions (that's all the wind I've had so far).
6. The sail is taking a good shape from the centreline of the luff and around the lee side.
7. The windward side of the sail, from the luff back to the half way, or widest point, of the nose is tending to collapse inwards, as it did with my previous wing sails. I addressed this in those sails by adding three "riblets" between noses, and it seems as though, if I want a sail that's good to look at, I'll need to do the same again. They probably ought to extend far enough aft to bear against the mast. They can be made from, say, 10mm alloy tube. Will it have any effect on performance? I don't know, though it ought to have, if the first few % of the windward side are in their designed shape.
8. And finally - most of the bermudan-rigged boats here are motoring a lot of the time. I can't find one with whom I can surreptitiously try to sail alongside. If they're so good at sailing to weather, why don't they do it?

 

Hi David, excellent to hear about the sailing qualities of the new rig.

The collapsing windward leading edge need not be a problem, it might actually be beneficial. My reasoning for say that is: it effectively adds more leading edge camber to what would otherwise have been a short symmetrical foil (the forward double-sided part) with a large thin flap at the back. Picture a line extending forward from the luff and tangentially to the camber - forward camber "bends" this line closer to the wind and should extend the maximum angle of attack of the foil as a whole before it stalls, giving you a little extra margin for changing apparent wind in pitching and heaving conditions.

One thing that this benefit depends on the the impact of the collapsing on the leading edge radius. If there is a pronounced ridge that forms as the concave part to windward transitions to the convex leading edge, it will cause a local low pressure spike which will have exactly the opposite effect, i.e. reduce stall angle.
If this ridge is far enough aft, the leeward side will be unaffected. I would love to see a picture if you ever get a chance to take one...

 

Thanks for the update David. Short tacking is a wonderful utilitarian measure of a rig.

About the windward nose shape, I don't think it represents much loss as long as the crinkle is not moving. I was thinking about putting an inflatable tube in my luff temporarily or otherwise to test what's worse the problem or the fix. Chafe would be my biggest concern.

 

Hi Will,
You're right, the collapsing of the weather side flattens out the sail section here, and this is beneficial, or ought to be - except for the fact that the cloth develops vertical corrugations here. I don't think this is too deleterious, as the air is moving very slowly at this point. More important is that the stagnation point is just a little bit aft on the weather side, and the luff ought to be round as far back as this point.

 

I see what you mean David. Is there no way to get rid of or reduce the corrugations with some more luff tension perhaps?

It is nevertheless very encouraging to know that despite the imperfections it still performs as good as it does. There are obviously improvements possible, but with no guarantee that the benefits will be worth the extra effort.

As a possible compromise you might want to to focus your improvements on the top section only. Any additional weatherliness in reefed conditions would be worth the effort if it means getting off a lee shore quicker!

 

Actually, Will, the top section sets quite well. That's because weight has more of an effect than tension, in a rig like this. When deeply reefed, of course, this weight effect is lost, as the lower battens are supported by the lazyjacks. Yes, I can apply tension, but it doesn't pull out all the wrinkles, so I prefer not to.

 

Perhaps the nose section could be fabricated from a fancier carbon reinforced cloth with reinforcing orientated to respond to a bit of tension?
I realise David that you will use what you have built and probably wouldn't go to the trouble. Just thinking aloud.

 

For what it's worth where my head is going with this is;
Double sided wing mast section, standard single sail rear section but with a slot between the two all mounted on a balestron boom !

 

Sorry, can you please explain why bermudan rigged boats couldn't have done it? I can tack my 28 footer bermudan rig through a passage probably 40-50 feet wide with no problems. She can actually sail into the typical marina, tacking up the normal gap between the boats attached to the finger piers without issue in 5-10 knots.

As background - I sail and own cat rigs (both conventional "pinhead" and square tip), sloop rigs (both "pinhead" and big-roach full-batten), cat rigs with double-sided luffs, wing masted sloop and (in the past) wing masted cat rig, so I have years of experience with various types, but I can't see any reason why a decent bermudan rig could not go upwind where a cat rig like this cannot.

As to the question, why do bermuda-rig boats motor around? Perhaps it's because it's such a useful and efficient rig that there are lots of them around.

 

I have spent a lot of time and concept sketches on exactly that: a two-segment soft wing with a slot. One of the important geometric arrangements to keep in mind is to position the rear section leading edge to windward of the forward section trailing edge, such that the fwd section leeward flow gets dumped directly in the region of lowest pressure of aft section LE.
If the aft battens have their hinge axes right on the fwd section TE while the aft section LE is positioned a few inches further aft, this could be quite feasible.

The main benefit would most likely be in very light air or when the sail area is limited because of some other constraint and one still has spare righting moment to play with.

 

Yes, the Omer wing sail uses some high-tech stuff, and tensions it vertically, and suffers less from distortions. The Beneteau wing sail uses normal Dacron cloth, and I see the same kind of vertical corrugations as I get, in some of the magazine article photos.

Actually, I think the most effective way of maintaining the luff's shape is to add "riblets" between the main battens. I did this on my previous sails, and it worked. I'd hoped to get away without them this time, but having gone for the Mustang cloth, with its greater stretch, I think I'll have to add some to this sail. I made them of 8mm alloy rod before, and added three to each panel. Maybe two would be enough. Certainly the law of diminishing returns applies.

 

With respect, that didn't answer my question. There are lots of pointy-tops around, yes - and lots of them motor both upwind and downwind, when there's a good sailing breeze.

You're a sailor, and know how to use your rig in confined spaces, and that's good. Sadly, those skills are getting rarer, as so many sailors turn the starter key when things get tricky. But how do you manage when the wind's astern as you enter a tight spot? The halyard of my sail is the throttle - I can lower a panel or two to slow down, if I can't spill wind. I did that the other day, as I ran through Sulphur Passage in Clayoquot Sound. A gust hit me just as I was about to gybe around a corner, and I lowered two panels and gybed in one fluid operation, as the pilotage was getting a bit tricky and I didn't want to go around the corner lickety-split. Even with two reefs, I sailed away from a bermudan rigged boat that was also running through the passage.