Arto
This model is quarter scale of the proposed beach proa. The full size sail is intended to have a foot of 4m and a length of 8m. That means the mast height is around 6m depending on the minimum cant angle.
The head of the sail should be constrained on a sliding guide so it is held to the mast when adjusted lowered. This means when the sail is reefed the cant angle increases so the sail gives higher proportion of total force to lift as the sail area is reduced in stronger wind.
The model has a lever arm off a servo connected to the rigid vang. Control was too coarse in that first test so we have altered the lever length for the next test.
I doubt if battens would give any benefit. It would be like placing battens in the luff of a typical jib - no point. Both edges of the sail can be tensioned to the limit of the rig so can be held taught. The remodelled sail will have wire edges so we can get them tight and keep the sail flat. It is best to keep the sail as light as possible so it will lift under air pressure and form the correct airfoil shape in light winds Under pressure the unbattened triangular plan form gets close to an ideal airfoil with the maximum camber controllable from about mid height to the bottom, less camber at the top and the bottom camber controllable by the foot tension.
A semi-circular track for the foot of the sail might be the simplest way of setting up the highly canted sail on a larger boat. There would be two track trolleys with a block that sail sheets would pass through. The two sheets would control the track trolley position and the foot tension. Releasing the sheet on the trailing edge would spill air in the same we we can release sail pressure by releasing the boom vang on the revised model.
This rig is intended for a high speed boat that has potential to plane - where displacement is say less than 20 times the drive force. Canting the sail provides vertical lift component at the expense of drive but that lift component reduces displacement and hence reduces water drag. The L/D for a slender hull under planing conditions will be well under 10. It depends on how trim can be controlled but 4 or less is typical. The rig we are using has a lift to drag around 9 so it is more than twice as efficient for producing lift than the hull.
At high speed the boat gets closer to an airplane connected to the water so it can generate apparent wind. The two slender hulls provide a stable platform and good light wind performance.
In displacement mode a slender hull has a displacement to drag ratio above 20 meaning the canting rig is not likely to offer the same benefit there. It does offer potential to better balance drive and drag by adjusting the lateral position of the CoE.
Rick
On 28/03/2013, at 8:45 AM, Arto Hakkarainen wrote:
I have given this quite a lot of thinking time after my initial comments. This seems now more promising than what I first thought.How do you control the rig? Lines to the ends of the boom? Is the top of the sail going to be free or will it be controlled by something when raising and lowering the sail?Full battened version that can be controlled while lowering, raising, reefing etc. could be promising. How big are you thinking of when full size?ArtoFrom: Rick Willoughby <rickwill@bigpond.net.au>
To: harryproa@yahoogroups.com.au
Sent: Friday, March 15, 2013 8:56 AM
Subject: [harryproa] Canted Bi-directional Sail
A friend has been playing around with an idea I had for a canted sail and has built a quarter scale model of a 7m beach proa he is thinking about building. There is a video clip of testing here:The controls are still a bit coarse and rig too flexible but it sails quite well and responds quickly.The aim with the canted bi-directional rig is to get better alignment of drive and drag forces in 3D. The flexibility of the rig tended to reduce the vertical component of the sail drive. Also the sail was a bit large for some of the gusts. A smaller sail could have increased canting angle to improve the vertical lift component. In addition the mast could be canted to the windward side rather than being vertical to give more vertical component to counter healing.
Rick Willoughby
Rick Willoughby
__._,_.___
.