Anonymous wrote:

I’ve been pondering that conundrum and testing it with little stick and fabric models… As far as I can tell. The angle of incidence is the same (or at least on the scale of my models the difference imperceptible). What did come to light though is that the effective area is noticeably diminished for the same angle with the twisted rig. For the fanned model the effective area looked to be about 2/3 of the true area. In order to then receive the same power from the sail the angle of attack needs to be 1.5 times greater, thus incidence necessarily increases? 

Many boats have downwind and upwind quoted sail area and I wonder whether a fanned and twisted rig has this feature automatically built in. Then the question is whether the sail needs to be sized to the area requirement for upwind work using the effective area.

A problem I foresee with my musings is that if my models are accurate (and let’s be honest with sails twenty centimeters (eight inches) high I highly doubt they are) it would suggest that the average Bermudan to junk conversion would require the twisted junk sail  end up being 1.5 times bigger… this doesn’t seem to fly with what is practiced and has been shown to function well as far as I can tell.

It would be fantastic to hear the opinions of other more experienced members of the group! What do you think guys?

All the best,

Zachary.

Yes, I observed too that the area of a twisted fan flat-cut sail develops camber over max 2/3 of its area. The more contemporary sewn-camber sails don't have camber over 100% of the area though, the top panels are nearly always flat and the lower panels often resemble flying duvet covers with the baggy camber only developing over some of the area of each panel. I think its very difficult to understand what is really happening aerodynamically though. Both types of sail are not simply objects with the wind passing by in the horizontal plane along the chord-line, there's upward and downward vortex inducing phenomena on both sides of the sail and it all gets a bit complicated and I reckon one just needs to get out there and see what happens on the water. I too would like to hear some discussion from more experienced members than us on the subject of angle of attack on flat-cut vs cambered sails... 

Anonymous wrote:

 I’m still flabbergasted by the cambered shape when the fanned sails twist. 

It's a thing of beauty.

What I'm not sure about, and maybe some people with experience could chime in here, is if the twisted fan sail doesn't have as good an angle of incidence into the wind as a sail with camber sewn in, and whether this matters anyway. The Westerly22 (with Mark I bilge keels) is so poor to windward that I can't really judge it. 

Thank you for the extra information! That looks like a proper interesting layout. Especially in the precise amount of twist it lets you experiment with. I’m still flabbergasted by the cambered shape when the fanned sails twist. 

Anonymous wrote:

Hi Jan,

That looks like an absolute cracker! Glad it's working and that it's given you some cool projects for over the winter! Gives me some hope for my build...

One quick question, do you have a sketch of your sheeting arrangement? Still undecided as to how I'll be sheeting Peggy and your method sounds to work well. I'm just struggling to picture the details haha.

All the best,

Zachary.

Hi Zachary, this works better than expected but it is a small sail and easily manageable, 12 sq m. I'm not sure if it's transferable to a bigger sail but I don't think it has ended up being more faff or more spaghetti in the cockpit than any of the boats I had the pleasure of sailing on in Roscoff. 

See attached pictures. The sheet to batten 1 and batten 2 is a loop that passes through the cockpit coaming, so it can't run out and be lost. Likewise B3 and B4 form a loop. You can pull the loop and sheet two sheets at once. It's all very easy. 

The main sheet is a 2:1 purchase. In full sail the rig is operated much like any gaff/Bermudian sail, the boom and main sheet taking the loads. The batten sheets can then be adjusted to share the load on the sail or finely tune the fanned shape. I'm not getting enough twist yet as I have to move the mast sheave right up to the top of the mast. When reefed the batten sheets play their role in holding the reefed panel down and taking the load at the bottom of the sail. If you haul the boom right in amidships (by pulling on the sheets well over to windward) you can really feel the drive increasing. It seems to work a lot better on the port tack to windward.

The arrangement at the stern will be replaced by a horse that will arch over the outboard well and will mean the sheets all go through from the centreline and won't need adjusting when we tack/gybe. The main sheet won't foul the tiller either, not that that is a problem now really. 

One thing that I didn't foresee (but an expert would have guessed) is that the centre of effort of the whole sail-plan is lower than the original and so the boat hardly heels over... and then the bilge keels don't work (they're rubbish anyway) and we drift to leeward close-hauled , when I can get the whole sail higher it should help. If I have crew they can lean out to leeward in the complete opposite to most other boats.

We've had a few more really nice trips and learn more each time. Just about finished now before winter sets in. 

Nice work, well done.