A Photo & Discussion Forum for Wharram Design Enthusiasts
We are speaking here of un-stayed masts buried in the hulls, carrying junk sails of moderate size. 4 sails of 15 m2 make 60 m2, while it needs 30 m2 sails on two masts, or the same on one mast. High aspect ratio, that means a square sail of 6 x 2.75m with 8 battens and 4 sheets, built very light. Reefing for heavy weather by binding 2 battens together, so stiffening the sail (reefing from the top).
Well, I have done and tested it. Broke 4 test masts in the process (glass/epoxy).
Has anybody thought about this?
I think 2 masts on each hull only if the boat is very big.
According my own searches I think more you have sails less is the ability of the boat to close the wind. The ideal is to have only one sail but it's difficult to have a light and strong mast able to do the job so it's why I've chosen one mast on each hull.
But why not to experiment your idea of 2 masts on each hull. I'm intrerested with your experience and I'm curious to know the results.
For information 3 years ago I made a little article about the sails ( with a lot of mistakes) . I'll correct and improve it after my next experiences when sailing my new boat :
J'ai visité api.ning et lu avec grande attention.
As most others, you got it, unfortunately, completely wrong. Never mind. It's one of the things that are hard to beat.
The scientific truth is as follows: Air flows (wind). You can deflect that wind, but it needs power. Take a sheet of plywood and stand in the wind. If you put the edge facing the wind, the sheet is feathered, nothing happens. Now turn the sheet so that it deflects the wind. You immediately feel the power.
When you sheet in your sails so that they deflect the wind, this power propulses your boat.
What about Bernoulli and all the formulas? Of course there is a low pressure behind the sail, but this is the consequence, not the reason, for the event.
The sail shape is important to 'take' the optimal power, in the optimal vector, out of the wind deflection.
You then have two situations: laminar flow, like on a Genoa, and turbulent flow, like on a spinnaker.
So, as you can see, I play with deflecting the wind. Promise I shall report on the results. Watch this space.
Merci pour ta contribution, Bertrand.
(I write mostly in English because it's easier for me than French.)
I had read before that it was a mistake to make a very close comparison between airplane wings and sailing rigs, being so very different situations.
So, when I read your article, (although it was too complicated for me), I was glad to see this concept again.
I also enjoyed to see that the lug rig, (as in Iain Oughtred's designs, wich I so much admire), is considered very efficient.
Even then, it still amazes me to see the "sails" used on the America's Cup, wich again, more than ever, imitate as much as possible, an airplane wing. They even aren't called "sails" any more, but "wings"!
What other solution do you find for installing a junk rig on a catamaran without sacrificing valuable living space on the hulls? Isn't it possible to install the mast of a junk rig on a mastbeam, with a stay and shrouds?
Just for the record. Aircraft wings and sails really act in the same way. The profile on the wing, also on wing masts, is to smooth out the air flow and avoid turbulence. The wing/sail deflect the airflow, and it needs power to deflect this flow. In the glider, this power is used for both propulsion and lift. In the sailboat, it's used for propulsion, while the keel takes over the second function of the wing.
There is no law according to which the masts have to stand in the center line of the hull. I put them off center, outboards, on a bulkhead in a box, which has a shelve at each stringer. The mast goes through all shelves and the structure continues to the keel, along the side of the hull. This column of boxes is glass/epoxy coated all around and extremely strong.
I'm happy to know Tom and Susi are in Argentina. Give them the good day from me and Marie-Helene and have a good sailing aboard AORAI.
Sure it's always possible to fit a junk sail on a mast held with shrouds but you can't reef it easily at all the points of wind and you can't open the sail at 90° at down wind. I suppose your Surf Song will be rig with a such rig.....
For information, Marie-Helene made a little web site about our boats, and her searches around the life of Eric de Bisschop : http://maheyo.free.fr/
At my eyes there is a great difference between a wing and a sail :
- a wing needs and engine to make enough wind speed to be abble to carry an aircraft. The engine of a glider is the force of gravity (its own weight) and if it don't find vertical winds it can't stay in the air and has to land.
- a sail absorbes a part of the power of the wind (cinetic energy) to push a sail boat in diverting the direction of the wind.
A sail is able with no angle of attack (and so with no deviation of the apparent wind and in this case only the real wind will be deviated) to produce a sail force only in curving its shape.
Sure, even a flat door is able to produce a sail force with an angle of attack but the deviation is very rough and produces a lot of drag comparing with a deep curved sail which will deviate smoothly but strongly only the real wind (with no angle of attack) with a minimum of drag.
I wish to continue to experiment this type of deep curved sails as soon as possible and for that I've to continue to work on my boat building ......Next month I'll begin the building of the wood masts .
Thank you for admitting that the true wind is deflected. That's all we need to know. Now please don't confuse this with the apparent wind. the apparent wind is the resultant of true wind and forward speed of the yacht (over ground). This leads, by the way, to the interesting phenomenon that you can sail at 5 knots dead against a tidal stream of 5 knots, not gaining and losing any ground, so that true and apparent wind are the same.
The resultant means that there is a (momentary) balance between the forces exerted on the sail (or the wing, no difference). A curved sail will generate more power, a flat one less. So in low wind you need a curved sail, while in high wind you need a flat sail. Normally the junk sail cannot cope with this demand. So you reef. Another option is to reef from the top, binding two battens together (including the panel sail cloth), to make the sail stiffer also.
Just remember. You need sail area to drive the boat in moderate winds. In storm conditions, you will not need any sails. Bare poles and the boat structure are enough to send you down wind at a speed that you might want to control.
I have changed the original concept by balancing all sails fully, so that the mast is roughly in the center of the sail. Like this, there is very little stress on the sheets. For an illustration of the idea, check out Maltese Falcon