The role of the rigid straight boom in the stability of the main sail but also in the loss of propulsive power are known since modern aerodynamics looked at sailing. Airfoil theory principles highlighted these two conflicting factors. Consequently, industry has conceded to the need for a rigid straight boom and developed sophisticated concave surfaces and management technics to minimize the loss of propulsive power that nevertheless remains significant.
In best practice today, maximum driving force is produced in the middle and upper part of the sail. In the lower part, the flow is increasingly deflected and ultimately interrupted by the turbulence produced by the straight boom. Neither the angle of attack in the luff zone, nor the outflow angle at the leech are directed to the angle of attack of the boom. The barrier of a straight boom, even for main sales loosely attached to it, remains insurmountable.
In summary, significant wake turbulence is formed along the foot of the sail, caused by the exchange of air from the luff and the lee sides, combined with important losses due to the straight boom effect.
A proposal
We are proposing a different approach in tackling this apparently insurmountable conflict with a simpler, more reliable and more efficient main sail rig. The Flexible Boom and its Sail System (FBSS)*.
It entails an innovative concept with the boom resembling a bow with an eccentrically positioned arrow consisting of:
• semi-rigid boom with a capacity to bend under considerable load,
• expansion rods applying the load introduced by hydraulic or mechanical means,
• strings that enables the bending process.
Simplified drawings and images of a demonstration hydraulic rods expansion system are shown in the following attachments 1 and 2. A model of the proposed concept is shown in attachment 3.
In best practice today, maximum driving force is produced in the middle and upper part of the sail. In the lower part, the flow is increasingly deflected and ultimately interrupted by the turbulence produced by the straight boom. Neither the angle of attack in the luff zone, nor the outflow angle at the leech are directed to the angle of attack of the boom. The barrier of a straight boom, even for main sales loosely attached to it, remains insurmountable.
In summary, significant wake turbulence is formed along the foot of the sail, caused by the exchange of air from the luff and the lee sides, combined with important losses due to the straight boom effect.
A proposal
We are proposing a different approach in tackling this apparently insurmountable conflict with a simpler, more reliable and more efficient main sail rig. The Flexible Boom and its Sail System (FBSS)*.
It entails an innovative concept with the boom resembling a bow with an eccentrically positioned arrow consisting of:
• semi-rigid boom with a capacity to bend under considerable load,
• expansion rods applying the load introduced by hydraulic or mechanical means,
• strings that enables the bending process.
Simplified drawings and images of a demonstration hydraulic rods expansion system are shown in the following attachments 1 and 2. A model of the proposed concept is shown in attachment 3.With this arrangement the boom can acquire any cumber and remain stable with adequate residual rigidity to absorb instability forces on the sail. The kicking strap in combination with the main sheet ensure a tensioned leech line. A flat sail attached assumes its optimum airfoil shape on all its height including the boom itself when the latter bends.
Two additions enable taking full advantage of a bending boom, a modular under the boom skirt with inversed open T profile, as shown in attachment 4 and a computerized control system that sets the boom to optimum camber according to apparent wind parameters.
A video demonstrating the principles of operating such an arrangement in detail can be found in www.youtube.com/channel/UCPcNQ5rqb3w6oPvDzxN_oMA
FBSS requires negligible electrical power and stored energy. In cases of hydraulic or electromechanical power source the control can be easily done from the cockpit. As a replacement package, it can easily be installed on existing vessels.
Ensuing relative benefits of FBSS
- Flat surface sail and the potential to use modern materials.
- Radical simplification of the rigging and operating around the main sail.
- Reduced relative boat righting couple due to lower sail center of effort.
- Acquisition of sharper tacking behavior.
- Smaller ballast.
- Lesser heeling.
- Lighter boat.
- Faster vessel
- Safer boat.
For more details contact: optir@proximus.be
*EPO patent 2873606
Attachment 1
Expansion mechanism folded-at rest
Expansion mechanism activated starboard
Attachment 2
Expansion mechanism folded at rest
Expansion mechanism activated at starboard
Attachment 3
