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by Bernd Kohler - The Netherlands ikarus342000.com

Part One - Part Two

The Case for the Wishbone Gaff Rig

I developed the wishbone gaff rig in 1996 for our "PELICAN" catamaran. We owned a full battened main sail before, but it was a lot of work getting the sail up because we had three reefing points with single line reefing for each reef. This meant a lot of ropes to get up. Reefing down was of course easy and quick. This is a necessity on the Mediterranean Sea where strong winds built up in minutes in the afternoon. I was researching vertical roll reef systems. Our main sail had a big roach. To get the same sail area with a triangular mainsail the mast had to be a lot longer. But this was out of the question.

So I sat down to develop the wishbone gaff rig. The rig was from the beginning a big success. Out of the harbor under engine for the first time with this rig we had no wind. We got out the wishbone mainsail for the first time in a matter of seconds. There was a slight zephyr, nothing more. The boat picked up speed immediately and was "overrunning" the engine. Setting the sails on our Pelican with the wishbone gaff took about 20 seconds! We also had a roll jib. That was it, we used 5 liters of petrol in the whole year because handling the sails was so easy. Also, room permitting, anchor maneuvers we did now under main sail alone, sometimes with a partly rolled in sail. We spent the next five years sailing and living on the boat until we sold her and never a problem with this rig.

In the meantime sails where developed with ever increasing roach. To hold the sails in shape more and more battens where necessary. This was making the sails more heavy and difficult to handle. The next step, as you can see today on many new boats, the top of the sail is almost a square. The idea is to catch more wind in the upper part of the sail. The idea is sound. The problem with these sails is that there is no way to control the twist in the upper part of the sail.

Here comes my wishbone rig. The upper part of the sail area can be controlled by the wishbone gaff. The twist is always under control. For this sail no (costly) battens necessary, so less weight and a easier to handle sail. Next step was to use a vertical furling system.

Besides the good behavior when using the rig, it has other advantages. With some small modifications it can also be used for other designs.

1. Normal round Aluminum tubes can be used instead of a mast section with sail tracks.

In the case of the rig shown below you need the following aluminium tubes 150/140 mm diameter, 6m long costs $366.00 140/130mm diameter 4.4m long $314.00 total = $680.00.

The prices are from a German store as an example.

2. Weight, the whole mast 10.13 m long has a weight of only 41 kg!

Compare this to a suitable mast section.

3. The wind speed increases with the height from the water surface. A normal roll reef main sail is a triangle. So there is not much sail up in the wind. On a windward course a great part of the sail is in the wind shadow of the mast. Mast turbulence's diminishes the effectiveness of the sail.

The wishbone gaff rig has a big sail area up. Many racing sail boats have today main sails which are almost square to catch the higher wind speed on the top. There drawback is, that the can not control the twist in the upper part of the sail effectively. Much force is lost. With the wishbone gaff the twist can be controlled for the most effective twist, thus there is more force up in the sail. These new main sails need a lot of battens to stabilize the sail, which are expensive. Reefing battened sails can be a hassle. The vertical roll system to furl and unfurl take only seconds.

Often main sails which are rolled inside the mast jam. In this case, when it is time to shorten sail because of a wind force increase, it can be dangerous. An open construction can not jam.

The gab between mast and the leading edge of the sail will have no negative effect.

The argument that a mast should have a oval respectively airfoil cross section is only as long as the mast can rotate. Otherwise these masts produce more turbulence and resistance on given courses to the wind.

See drawing below. Ideally in low speed flow the airflow follows the shape of the mast, but when the speed increases, in normal sailing conditions, the air particles can no longer follow the mast, because of the inertia of the wind "particles". This in turn creates some recirculation zones along the mast and a separation bubble is formed.

Now a rotating mast with the sail on a jack stay, the gap is smaller and the air will follow the sail surface more directly. Thus a smaller separation bubble will be the result and the sail will be more effective and the overall resistance lower. See also bibliography (1) and (2).

Depending on the height from water surface the wind increases. A generally recognized rule of thumb is that wind speed increases as the 1/7th power of the height above ground. The following curve below illustrates this theoretical increase in wind speed.

As result the power of the wind increases as shown in the next diagram.

Here the formula how much power the sail can generate:

F = ½ x ? x A² x C x V²

F = The expression of the aerodynamic force in Newtons.

? = density of air. Depends on the air temperature. We assume 1.22 (corresponds to 20 ° C) A = sail surface in m²

C = aerodynamic coefficient. These can be greater as 1, we assume 1.2. This means the recovered energy on the lee and windward side. The coefficient depends also on the wind direction*. V = Speed of the apparent (not true) wind how the sail "feels" the wind.

* This is the general formula for wind power in the sail. Because a sail is in general a wing section. These generate the force depending on the angle of attack to the apparent wind (CL). And also drag (CD). These depends on the apparent wind. The result is then the maximum force obtainable for a given angle to the wind. The Reynolds numbers play here also an important role, but to explain these is out of the scope of this article.

Now lets look at an example A = 20² = 400 C = 1.2 V = 5 kn (2.572 m/sec) V² = 6.615 1.22 x 400 x 6.615 x ½ = 1614.06 N or for person who are more familiar with 1190.469 ft/lbs.

Now lets presume that this 5 knots is the wind speed as you feel in the cockpit. On the mast top this will be 2,8** x 1614 = 4519.37 N or 3333.31 ft/lbs.

**see table below

These explanation is summary but sufficient to understand the merits of the wishbone gaff rig.

If you would like to understand the aerodynamic of sailing better see at the end FUTURE READING for additional material.

This explains why a pure triangular main sail is inferior in respect to a sail with a roach or more so with a (almost) rectangular mainsail.

By the way, this explains also why a good gaff, sprit or crab claw sail sometimes perform better as Bermuda sails.

This is why I am consider my wishbone gaff rig as the best rig for the money. The realization of the rig can be very different. Compare the KD860 rig and the DUO1000, both with rotating masts. We used on the Pelican a wishbone gaff with a hinge. The gaff respectively the twist in the sail can be controlled by ropes from the cockpit.

The DUO1000 is by the way a catamaran with a biplane rig.

FUTURE READING
1. "Aero-hydrodynamic of sailing" by C.A.Marchaj
2. en.wikipedia.org/wiki/Forces_on_sails
3. www.ikarus342000.com/RuDag.htm

Copyright © 2013. All Rights Reserved, by B. Kohler.

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