Slieve McGalliard wrote (in another thread):I have seen a number of very experienced junk rig sailors sail my rig and not get good performance. They always seem to stall it by oversheeting, and some could not be persuaded to ease sheets.

@Slieve,

I was paraphrasing you poorly, from memory. I should have said "...sheeted [rather than sailing] more free...". Or better, just quoted you in the first go! Sorry about the confusion.

Meanwhile, thanks for your answer, which is quite clear, and your good advice!  8)

@Dave and Ueli

Thanks for your answers! I appreciate the thought that went into your comments. So much  to learn!

Dave Z

Hello, Dave Z.

Since starting sailing about 3 years ago, I’ve wanted to understand how the sails generate power to drive the boat. 

My longwinded understanding of the usefulness of twisting off the upper sail in comparison is as follows;

What any sailor is trying to achieve is to generate maximum thrust and lift from their sails, regardless of the type of sail. Understanding how the airflow works is useful. And a few underlying principles apply to all sails. Every sail will work using part or all of the 2 following principles; Even a flat piece of plywood can generate some thrust using them.

A cambered sail generates "thrust", "lift" or "drive" in a couple of different ways which combine;

1. According to Newton; "Every action has an equal and opposite reaction". A directing of energy in one direction tends to move the body upon which the energy acts in the other direction. Rocket engines work this way. Aircraft wings do too. So, I believe, does airflow diverted rearward over the windward side of the sail. Rearward movement of air drives the sail and boat forward.

2. Bournelli’s theory. Airflow which encounters a curved surface (camber) will tend to flow faster over that curved surface, reducing the pressure acting on that surface. This creates a suction and tends to suck the surface forward. I.e., "Lift" is generated, tending to pull the boat and sail forward. An additional item which I feel comes into play is that the suction generated also tends to keep some airflow flowing back along the leeward surface of the sail until it exits the leech and recombines with the flow coming off the windward side, helping to support somewhat the airflow coming off the windward side and keeping it moving in a rearward direction.

To generate "lift", the degree of camber needs to be such that its change of direction is not so great that the momentum of the airflow as it passes the luff of the sail overcomes the suction generated by the curved surface (camber), causing the air to detach at the luff and not follow the curve of the camber. Nor so little camber that it generates no suction.

So, there are 2 factors at play in generating thrust and lift to move the boat forward.

In order to create thrust on the windward side of the sail, the air must move smoothly over it and exit out the back. Take a sail that is feathered to the wind. Airflow is not being diverted and its energy is not being exploited. It moves the boat nowhere. As the sheet is hauled in, the airflow is diverted from its preferred "straight downwind" path, and some of its energy acts on the sail and moves it forward. The more the sail is pulled in the more energy is harvested to create thrust. But only as long as the airflow has a relatively easy path to the rear.

When we oversheet, (generally less than 45degrees to the apparent wind, which changes as the boat speeds up) the airflow no longer flows easily and freely out past the leech and becomes stalled. This can be seen by using leech telltales. When useful thrust is being generated, the telltales will stream to the rear, indicating airflow out the back of the sail. When the sail is oversheeted, the telltales will fall to leeward, indicating a lack of thrust to the rear. Also, the boat will heel more, due to reduced forward velocity.

As windspeed increases with altitude, if a sail has no twist at all, then the upper part of the sail is experiencing greater airflow momentum than the lower. If the lower part of the sail is sheeted correctly to give good airflow, as judged by the rear-flowing leech telltales, then the upper part will be oversheeted, telltales stalling, falling to leeward and not producing optimum thrust. Thus the need for some twist in the upper part of the sail. Or if the upper is sheeted to to provide a flying leech telltale, the lower part of the sail will not be sheeted in to its optimum. Though at least it shouldn’t be stalling and will be supplying some reduced thrust.

So some twist-off of the upper part of the sail at the leech would be beneficial to overall sail thrust. So that’s airflow dealt with (in a basic manner) over the leech and windward side of the sail.

At the front half and leeward part of the sail, things are somewhat different. In order to generate "lift" and add to the thrust generated at the leech of the sail, camber is normally added. If the change of direction of the camber is too great in too short a distance, (eg, a completely flat sail) the airflow momentum will overcome the suction generated by the camber, detach from the sail and cause a stalled luff or leading edge, generating no lift. So the key seems to be to have a gradual curve up to maximum camber rearward to approx. 1/3 of the length of the sail chord.

(Before the flatties get up on their high horses, obviously, flat sails do harvest enough wind energy to sail,  they have been doing it for many years, just perhapsnot as efficiently as a well-designed cambered sail)

If we have arranged our sheeting to allow for some upper sail twist, then the amount of camber at the top of the sail will be need to be less than the bottom for several reasons;

1. Higher wind speeds at the upper part of the mast will create greater airflow momentum, thus increasing the likelihood of the airflow detaching from the luff if the camber on the upper sail is the same as the bottom and the sail is sheeted correctly for the bottom telltales.

2. Due to allowing the leech of the sail to twist off, if camber was the same top and bottom, the upper luff would tend to be luffed by the wind at the top end.

3. Due to the higher windspeed (i.e., available energy ) more lift can be generated with less camber than the slower windspeeds available lower down the sail.

Perhaps the ideal junk sail might have some upper sail twist built into its sheeting system and varying (lessening) camber at the front of the sail from bottom to top in order to generate maximum thrust and drive

I think that as long as we have some understanding of the airflow over our sails and how it’s generating power, we are likely to be able to extract more useful power from the wind regardless of the type of sail we are using.

Telltales allow us to see a little of what’s happening with the airflow. Stick plenty of them all over your sail

Dave Z, Hope this is of some use and sorry about its extreme longwindedness.

Dave D.

Hi All,

This from Vincent Reddish (http://www.thecheappages.com/junk/tutorial.html#Reddish -- 1998 Update from Bill Samson):

The fan arrangement of the battens means that sail twist is converted into horizontal aerofoil sections and the amount of lift is controlled by how much twist you allow.

As I understand this (plus related comments) - especially in fanned sails - wind blowing roughly horizontally across canted battens encounters an aerofoil section, whose camber (draught) is proportional to the amount of twist (confirmed by observations of model and actual sails I've encountered).

Is this correlation generally accepted in the JRA, or has it been challenged?

As JR sheets potentially afford control over leech shape (therefore degree of twist), then I assume that one does have control over camber... more 'real-time' control, even, than that  apparently available to most other rigs.

PJR emphasizes flat leech sheet mechanics and sail position. My own experience on flat-cut, HM standard is that twist helps significantly, if not over-done. Others have pointed out the curved leech (twist) of their flat-cut sails.

Has twist control been tried and proven helpful on cambered JR sails? If so, is its effect on a par with Bermudan camber controls?

Slieve, you've noted that SJR should be sailed more free than most of us are used to... do you think twist control is a helpful option for SJR? Or is the attack angle of the jiblets paramount? Intuitively, it seems that eased sails, assuming they're well-shaped and drawing, would swing the sail's force vector forward.

Has this been discussed elsewhere, that anyone can point me toward? Or any fresh thoughts?

Thanks for walking me through this!

Dave Z

"... your boat is a home and a companion ... if you let in racing you are letting in the serpent ..."  Hilaire  Belloch

If I were to begin racing, I would just fit a big JR to a common boat type, with sail area only limited by safety (mast weight and windage) and practical handling. I would accept any handicap as my point with racing would not be to collect silverware. My point would rather be to see how we fared compared to other boats and in particular compared to the Bm-rigged sister boats.

If I were to race a Splinter, I would give her 27sqm and just have fun. It is, in my view better to win on sailed time and lose on corrected time than vice versa, in particular if one wishes to show off the JR.

Cheers, Arne