WISHBONE – JUNK SCHOONER RIGGED PROA
One problem with a single mast is that the mast is where you wish to have the hatch, especially if you sail in cool climates and that is the place to be. A schooner rig gives excellent control, though normally is not as efficient. The reason for this is that the overlap, as with a jib and mainsail, cannot be achieved as the boom / battens will clash.
Or at least it cannot on a monohull. As the proa sails do not swing through the wind, it is possible.
The sketch shows how this looks in plan on a Madness hull. The sails are junk-wishbones. As shown without any articulation.
The problem is the danger of being caught aback. In which case as the sails lie along the hull centreline, the part of the rear sail in front of the mast hits the rear part of the front sail. Eventually (& hopefully) one sail should push the other out of the way, allowing both to swing free.
Jzerro & Pacific Bee, both have mains that snagg on the rear stay, if caught aback. They have not found it a particular problem, so with care the schooner should be ok. For a snoozing singlehander, it would be safe if reduced to one sail.
A schooner rig gives excellent control, though normally is not as efficient. The reason for this is that the overlap, as with a jib and mainsail, cannot be achieved as the boom / battens will clash.
I have to disagree with the statement about more overlap being more efficient; it’s better to have more separation.
Here’s a quote from an article on proafile about schooner rigs.
“It appears that the amount of separation between main sails is important, though I haven’t been able to locate any data or testing on the subject. Sidewinder, the Kelsall designed Atlantic proa was schooner rigged and notorious for the foresail back-winding the aft sail. The more separation between the two sails, the better.”
It might be better to avoid that overlap altogether for other reasons too. In a sudden back winding, rare as it might be, the battens of one sail might easily be smashed and broken on the other mast; think of how violent a jibe can be. Otherwise I like the design of a Junk schooner rig.
I’m cartainly no expert on schooners or overlap, but Nigel Irens’ recent schooners have loose footed sails on the fore mast. I assumed that that was to create overlap. It sure isn’t to make them self tacking!
http://www.nigelirens.com/FRAMEcruising.
Chris
I have to disagree with the statement about more overlap being more efficient; it’s better to have more separation.
I’m cartainly no expert on schooners or overlap, but Nigel Irens’ recent schooners have loose footed sails on the fore mast. I assumed that that was to create overlap.
You should ask an aerodynamicist. I only have a vague memory from reading Marchaj that there is no such thing as a slot effect, that the jib does not increase flow over the lee side of the main, but on the contrary, that the main increases the lift of the jib. That doesn’t yet say whether overlap matters. However, I notice that in one-way boats, the main is usually flatter than the jib, at least when the jib is as large or larger than the main. So my working hypothesis would be that the schooner rig on a proa suffers from too close spacing when the two sails have the same profile. If you could change the profile quickly enough that the aft sail can be flattened very quickly when shunting, you could have them close together.
Now you have to get someone like Tom Speer interested to find out whether I am just blowing hot air.
Regards
Robert Biegler
I have read Tom Speer say that to get lower center of effort it is generally more efficient to change the planform of the sail rather than shorten the mast as in what a schooner essentially does. I haven’t seen him address closeness or interference of the sails of a schooner; I would like to hear that from him as well.
I’m cartainly no expert on schooners or overlap, but Nigel Irens’ recent schooners have loose footed sails on the fore mast. I assumed that that was to create overlap. It sure isn’t to make them self tacking!
http://www.nigelirens.com/FRAMEcruising.
Chris
http://www.nigelirens.com/FRAMEcruising.htm
Gunnar - 16 July 2012 01:22 AM
I found an interesting thread about the (lack off) efficiensy in shooner riggs. Here a directlink to a post abut Wharrams wing-gaff in schooner configuration http://forum.woodenboat.com/showthread.php?11292-Schooner-rigs&p=643645#post643645
I really like the wingaff-sail on my Tiki21. So simple. No turbulenc from the mast.
So far, I would prefere the easy handling http://www.balancedrig.com on my next boat. But on an atlantic or passific proa, an scooner rig would leave more room for the main hatch..
Where you have two or more sails operating close enough to each other so that the flow around one sail affects the flow around the other, I think it’s best to consider the rig in it’s entirety as a single slotted aerofoil. So if we consider a schooner rig to be a single aerofoil, it becomes more obvious where the the notion that schooner rigs are less efficient comes from, that is, if you look at the overall planform of a typical schooner rig, it will have a lower aspect ratio than a typical una or sloop rig and hence a lower overall lift to drag ratio. However, multi slotted aerofoils can potentially develop hight lift coefficients, thus schooner rigs are known for being powerful rigs for off wind sailing.
Whether or not an overlap is desirable is more of a design detail than a general rule and the lack of overlap is certainly not the reason for the upwind inefficiency of the schooner rig. The purpose of the slot is to re-energise the airflow on the low pressure side of the aerofoil, delaying separation. There are many ways of achieving this with both overlapping and non overlapping elements. The choice usually comes down to practicality.
As Robert Biegler mentions, one way to increase the upwind efficiency of a schooner rig is to separate the masts far enough so that the sails don’t interfere with each other. Then you have two high aspect ratio single element aerofoils rather that one low aspect ratio slotted aerofoil. This may be difficult to achieve though within the confines of the length of the hull. Another way to increase the efficiency of a schooner rig is simply to use very high aspect ratio sails on each mast so that the overall aspect ratio is still pretty good and you get the benefit of the slot.
Cheers,
Mal.