| 1
|
Photo 1 shows the sailrig set for sailing downwind - note that the C of E is positioned to act along the fore and aft centreline of the hull. During trials only minor applications of a steering-oar were required to maintain heading downwind. |
2
|
3
|
The other end of the cross-arm has a dead-eye to guide a rope having each end attached to the opposite sides of the sailboard-boom. The whole arrangement is free to rotate about the stubmast axis. The lengths of the radius-arm and cross-arm are adjusted to achieve practically zero torque on the stubmast for all points of sailing.
Photo 3 shows the sailrig set for sailing into the wind. Note a disc attached to the stubmast which was used to select and fix the angle of attack of the sail. Also note the absence of a mainsheet. A windward heading was maintained solely by shifting the crew-weight (me) fore or aft.
A
| Fig A shows operation of the sailrig when shunting as a pacific proa. |
4
|
Photo 4 shows the sailrig feathering the wind during shunting. This corresponds to position 2 in Fig A. Note the cross-arm rope - pulling it draws the boom into contact with the cross-arm.
|
A number of mast-support bearings (such as the holes in the deck-beam in photo 2) were provided to allow the rig to be shifted when exploring hull/sail balance.
In practice this rig performed very well and was used for two years in trials sailing without rudders and centreboard. In July 94 twin spade-rudders were added as shown in photo 5. In operation, on shunting the rudders were rotated 180 degrees, the bow rudder preset to a positive angle of attack such that a positive angle of attack by the stern rudder was necessary to maintain a windward heading. The logic for this that both rudders provide a hydrodynamic lateral force acting to oppose leeway, leaving the hull to move with zero angle of attack. This certainly improved windward performance.
For these craft to maintain a heading, this torque must be countered by a hydrodynamic-torque produced by the hull adopting a positive angle of attack relative to the heading. The resultant drag increases as these craft lean to leeward. The balanced- sailrig does not produce sail-outboard torque, does not require the associated hydrodynamic torque and thus allows me to sail without rudders and centreboard. (Sailboarders eliminate this torque by sailing with their sailrig hauled to windward).
The sail-outboard torque appears to be omitted from mathematical treatise on the theory of sailing. I find the treatise are vague in areas of angles of leeway and attack of the hull. Bethwaite, Bruce and Morss reported lack of correlation between these theories and practice.
I welcome discussion on these points. E-mail to: smithvanaalst@bigpond.com