junkrigexperimenter
Marine/Ocean
- Nov 27, 2002
- 1
Hi,
First please let me set out the scenario in which my questions arise:
Like several hundred other like-minded enthusiasts around the world, I have a sailboat that I have converted to a Chinese Junk Sail rig. Since the traditional Chinese experts never wrote down their secrets, over the last 50 or so years, there has been a great deal of research going on to adapt the Junk rig to "western" vessels. (Reinventing the wheel!!) (On reflection in this case it may be a case of Mr Gurney having reinvented the wheel!!)
The latest discovery (?) is that on some traditional Chinese Junks the sail, (at low speeds an effectively flat, fully battened wing on its side), has a Gurney flap on its trailing edge, or "leach" in sailor jargon. This flap is created by making the rope which is sewn into the length of the sail's leach/trailing edge somewhat shorter than the fabric of the sail. So, when the sailboat is under way, and travelling across the wind or to windward, the baggy section of sail created along the trailing edge/leach by the short leach rope, is pulled out of alignment (to windward) compared to the rest of the sail's surface. Unlike conventional western sails the junk sail is controlled by a rope/s attached at several points along the trailing edge/leach, pulling at roughly right angles along its length. This tension enables the Gurney flap effect to be created.
(It should be noted that when wind speeds are above about 15 knots that fabric stretch in the flat cut junk sails, along with some bending of the battens, results in quite a good airfoil shape to the fabric between each batten. It is in the lower wind speeds that westerners have trouble with stalling, i.e, their junk rigged boats tend to not point well to windward. Hence, the current interest in the traditional Chinese use of Gurney flaps.)
People are now considering using either the traditional Chinese method of creating the Gurney flap effect, or more modern methods, such as hinges on the battens near the leach/trailing edge.
What are the best chord length:flap length ratios for flat planes and near-flat planes, at very low combined wind and vessel speeds, say from 1 knot to 20 knots? Also, what would be the ideal angle for the flaps? Once the combined wind and vessel speeds go over the roughly 20 knot mark where the sail already has a good airfoil shape, will the presence of the Gurney flap be detrimental to performance? If so would reducing its angle of incidence to the sail improve matters? In other words is there a "happy medium"? If you can't assist me, where might I find this information?
Thank you for bearing with me during this long enquiry. Paul Stewart-Day
First please let me set out the scenario in which my questions arise:
Like several hundred other like-minded enthusiasts around the world, I have a sailboat that I have converted to a Chinese Junk Sail rig. Since the traditional Chinese experts never wrote down their secrets, over the last 50 or so years, there has been a great deal of research going on to adapt the Junk rig to "western" vessels. (Reinventing the wheel!!) (On reflection in this case it may be a case of Mr Gurney having reinvented the wheel!!)
The latest discovery (?) is that on some traditional Chinese Junks the sail, (at low speeds an effectively flat, fully battened wing on its side), has a Gurney flap on its trailing edge, or "leach" in sailor jargon. This flap is created by making the rope which is sewn into the length of the sail's leach/trailing edge somewhat shorter than the fabric of the sail. So, when the sailboat is under way, and travelling across the wind or to windward, the baggy section of sail created along the trailing edge/leach by the short leach rope, is pulled out of alignment (to windward) compared to the rest of the sail's surface. Unlike conventional western sails the junk sail is controlled by a rope/s attached at several points along the trailing edge/leach, pulling at roughly right angles along its length. This tension enables the Gurney flap effect to be created.
(It should be noted that when wind speeds are above about 15 knots that fabric stretch in the flat cut junk sails, along with some bending of the battens, results in quite a good airfoil shape to the fabric between each batten. It is in the lower wind speeds that westerners have trouble with stalling, i.e, their junk rigged boats tend to not point well to windward. Hence, the current interest in the traditional Chinese use of Gurney flaps.)
People are now considering using either the traditional Chinese method of creating the Gurney flap effect, or more modern methods, such as hinges on the battens near the leach/trailing edge.
What are the best chord length:flap length ratios for flat planes and near-flat planes, at very low combined wind and vessel speeds, say from 1 knot to 20 knots? Also, what would be the ideal angle for the flaps? Once the combined wind and vessel speeds go over the roughly 20 knot mark where the sail already has a good airfoil shape, will the presence of the Gurney flap be detrimental to performance? If so would reducing its angle of incidence to the sail improve matters? In other words is there a "happy medium"? If you can't assist me, where might I find this information?
Thank you for bearing with me during this long enquiry. Paul Stewart-Day
