Soft wing sails, an evolutionary leap towards sailing bliss – Part II

To continue the last article from a few weeks back, soft wing sails are a natural step forward from the cambered junk sail. The highlights of the junk rig are easy low-tech construction and ease of handling. There have been forays into the world of wingsails but most of them do not share those two mentioned advantages of the junk rig. David Tyler’s designs however does. I’m going to go through a brief(?) background on his designs, so I can move forward with my current one.

Design background

In a thread on the Boatdesign.net forums he introduces himself, goes through his recent history of sailing 40.000 miles with his soft wing sail with the junk rig properties of easy reefing and handling, and acknowledges that his last design had a few flaws that needs to be corrected. The thread is very informative and goes through a lot of aerodynamic theory, as well as include the usual internet forum naysayers, but I highly recommend reading it from start to finish if you have interest in this topic. I’ve read it myself three times now I think and I’m soon going to read it a fourth time.

Here is an illustration of the design he sailed 40.000 miles with:

The basic recipe is as follows:

1. Find an airfoil with decent properties for low speed sailing (as opposed to a high speed aircraft)
2. Use a hinged aft batten to create an assymetrical foil shape
3. Create a forward batten that encapsulates the mast, most of all to increase aerodynamic efficiency

The think thank starts off with a design using a V-shaped aft batten design:

Lots of hemming and hawing later, he starts to doubt whether his V-shaped aft batten is the best idea.

He decides to yet again use the single batten aft and makes a sail using these specifications. This time however with a hinge that articulates both horizontally and vertically, which will later prove to be a mistake.

This works well for a while, he reports the following:

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?

Source

But the joy is somewhat short lived. Soon after this, he finds himself in the position where he is running before the wind and tries to take in a reef. The top battens “capsize” (leans to one side) and refuses to come down by themselves. This is obviously an inconvenient scenario. Not long after, the same thing happens again and some of the battens break. Back to the drawing board.

I’m now absolutely certain that there cannot be a second, horizontal, axis of articulation in the battens. There can be a malfunction, with the noses tending to rotate about their axis so that they are almost “on edge”, as much as the mast diameter inside them will permit.

Source

After a lot of thinking and going through options, an old idea he had already forgotten about resurfaces. Instead of building the front wishbone using two pieces encompassing the mast, with their needed crossbeams which weaken the whole structure, a single sturdy batten with a U-shaped jaw could be used:

It’s this particular design that peaks my curiosity and I start considering scrapping my idea of a junk rig and instead construct a soft wing sail! The design goes on to evolve into this:

Shortly after, yours truly joins the thread and the rest is a work in progress.

My own drawing looks like this currently:

Wing sail construction details

The design is still unfinished. Here are the details I’ve come up with so far.

Aft battens

Aluminium, square, 50x50x2 mm rectangular, 60x40x2.5 mm, approx 2.3 meters long. Reinforced somehow (GRP? CFRP? Unless I find a good sized square profile that has an appropriate inner diameter).

Hinges

A single double wedge made out of hard- or laminated plywood, reinforced using GRP. Pivot shafts maybe a bit loose to allow for some vertical articulation. The wedge will be bonded to the aluminium tube using Sikaflex, epoxy or both.

Edit (19 January, 2015): I made some calculations and realized that a single joint will be too short as it has to articulate 20 degrees to each side. The only viable option is to use a double wedge. This also requires me to use square or rectangular both fore and aft, which isn’t a problem like it is on the junk rig where the battens can rotat rather freely, as the U-shaped jaw keeps the battens positioned correctly. This also slightly simplifies construction of the jaws.

Front battens

Aluminium, round, 50×2 mm rectangular, 60x40x2.5 mm (maybe! need to do some strength calcs), approx 1.3 meters long. Rear part reinforced with 54×2 mm tubing.

Jaw/ring

Plywood, laminated. Not sure whether to build the jaw and nose in the same piece, build them separately and attach them using screws/bolts or build them separately and glue them together permanently.

Sail

The sail will be made out of polytarp, 2.7 m wide, three pieces. The part at the aft batten will be a single sheet. The part at the forward batten will be in two parts, each around the mast. They will most likely be joined using a zipper (velcro was on the list, but David’s experiences speak against it).

The sail will be attached to the aft battens using batten pockets like the ones illustrated on the right in the above image. The front (doubled) part or the sail will most likely be fastened to the jaw/nose using screws through the sail into the nose/jaw using grommets.

Where to go from here?

That’s about it for now. I will keep updating the site with further developments as we go along, so stay tuned!