Funny you should ask that Paul, as I was looking for confirmation of that number just yesterday, and the answer is not clear. Horses for courses comes to mind as different points of sail seem to make different demands.

In Sailing Theory and Practice by C.A.Marchaj there is one diagram for a single sail with max camber at 33%. 50% and 66% chord and from the resulting graphs he suggests the the 50% should be better for close haulled and the 66% for broad reaching. He suggests that the 50% case would be the best compromise. However, looking at the force plots for each one it would be easy to form different opinions. One thing which may effect those results is that there was a mast at the luff of the sails which should not be ignored. My question is that if 50% is possibly the best solution then why does virtually every diagram in the book draw the max camber point at just under 40%?

I’ve always aimed for 35 to 40% for a headsail and more, up to 50% for a mainsail but certainly not more than 50%.

As the junk rig is a swing rig and the close hauled setting simply rotates as the vessel bears away, then high L/D is important then my choice of max camber position has been 37% as being between 30 and 40%, and with the leech area a straight line.

I spent a lot of time and effort when R&D secretary trying to protect readers from the many theories of Bernard Slootboom. Please don’t open that hornets nest again.

Cheers, Slieve.

Paul,
this is what I learned from fiddling with junkrigs on my Malena in 1990-1994:

• ·         With the first flat sail (1990), the helm balance was mainly the same as with Bermuda rig. However, the CP clearly moved forwards and backwards as one headed closer to, or further away from the wind. The performance when close-hauled was really bad, and sometimes Malena refused to tack.
• ·         With the very same sail with hinged battens that created a 10% camber at the middle (1991), the performance was restored to that of the of the BR. Weather helm went up a good deal, so the CP must have moved a good deal aft. Easy tacking.
• ·         With the blue cambered panel sail with max camber around 35-40% aft of the luff (1994), the performance was about the same as with the hinged batten sail. However, now there was a noticeable lee helm when close-hauled in light winds. I cured this by shifting the sail aft on the mast, and if I remember correctly, I even adjusted the mast’s  rake a little aft (less than 1°).  The result was a fast and easily-handled boat (32sqm on 1400kg...). Easy tacking.

I have come to the conclusion that the actual shape of the camber is less important than having camber at all. When shaping the round along each panel’s edges, I give it a moderately gentle curve at the forward end. The after 40% part is kept dead straight. Right or wrong, this has proven to work on both tacks.

The only improvement I can think of is the soft wing-like sail which David T designs (mainly to cut drag).

Bernard Slotboom aired many theories, not least on the Yahoo JR website. However, he never presented a real sailing rig on a real boat. My practical experience with JR has taught me that the rig sometimes does what it likes, so I have grown sceptical to elevated but unproven theories about this rig.

Cheers,
Arne

Members of the JRA now know that cambered sails, rather than flat sails, are the future of our Association. But where do you place the position of maximum camber?

My wife Wilma and I built our first sail with symetrical wishbones in 1988 so the camber was at the 50% point from the luff. Vincent Reddish in 1989 designed his sail also with the effective PMC at 50%. This was also the norm among Bermudan sloop owners for mainsails who recommend the PMC in the middle of the sail. 

Arne Kverson designs and builds his sails with the PMC at 35-40%. Asking the internet for an opinion gives the PMC between 40-50%.

There was an interesting letter in magazine issue 42 from January 2004 to Slieve McGalliard, then the Research and Development editor, from Bernard Slotboom of the Netherlands in which he said - “…are based on the generally accepted profile with the PMC roughly in the middle of the chord.” He goes on the recommend putting the PMC at the far end of the sail near the leech citing an increase in lift and lift/drag ratio compared with a flat sail. 

So we all have an idea of the perfect position. I now recon on 45% from the luff. 

But what do members think?