The shunting behavior of proas demands a new kind of sail, different from conventional westerns sails because it must be bidirectional. A bidirectional sail is consistent with the bidirectional nature of proas. The new sail is a development of the AYRS Bolger sail – a radical design which afaik has proven dangerous and uncontrollable. Afaik, it never been made to work. I believe the Penny variant solves both the “roach collapse” problem and the “loaded shunt” problem. It is also simple, should perform well to windward, will permit quick and easy shunting and will be reefable. (If it does all this is will be truly a major advance in design).
Sail
The “Penny Bidirectional ” sail is a symmetrical triangle, about 2:1 aspect ratio, made with draft but without battens or boom. Halyard at peak. It can be thought of as a 200% genoa, flying to leeward of the mast, the difference being that luff/leech are interchangeable by reversing tack and clew points.
Deck and cockpit tackle.
The “tack/clew” points of the sail will go to blocks or fairleads on deck on centerline of boat, equidistant from mast foot.
Some adjustment of the position of these points may be necessary in practice. Tack may be brought fwd for CE balance and adjustment of helm, and possibly steering by sail. Tack may also be brought to windward for closewinded sailing (?)
Sheeting points will need to be far aft of clews to permit good sail curves when closehauled. Lines to aka or ama may be desirable for downwind sailing (no pole necessary).
There will be 2 sheets per sail, the fwd one usually cinched down at tack. The aft will run via block to winch and cleat in cockpit.
A third line, the ‘shunt cinch’ will go from the center of the sailfoot to block and cleat at leeward side of mastfoot.
Mast
Mast will be unstayed and stepped on deck (allowing unstepping for transport) using my “sliding mast-plug” technique. Mast height on the full-scale boat will be 20’ (that’s the length the 4” tube comes in J). Halyard runs through masthead sheave to mastfoot sheave to cleat on deck (or mast). The mast is to windward of the sail.
In normal operation the fwd sail corner will be the tack and will be cinched hard to deck, tensioning the luff to masthead. (Sail will be made with non-stretch Kevlar tape stitched into luff pockets). The aft sail corner will function as sheeting point, with sheet running to deck block or fairlead and to cleat in cockpit.
Shunting
- Approaching a shuntpoint, boat will fall-off and sheets will be eased to broad reach.
- Shunt-cinch will be pulled taught, forming a vertical axis along which the sail will fold.
- Current sheet will be eased fully, unloading aft side of sail.
- Current tack will be eased fully, unloading fwd side of sail.
- Boat will slow.
- Old sheet will be pulled tight to become new tack.
- old tack will be sheeted in to become new sheet
- Boat will leave shuntpoint in the opposite direction from which it arrived.
Model motor control
A major project for the radio controlled model is not simply to prove the sails work, but to permit testing of shunting procedure. Control will be by pulling on ropes, this will require DC gearhead motors driving custom winch/windlass drums.
Each sail will require 3 motors, all reversible, with PWM control and quasi infinite rotational sensing. With the rudders then, there will be 8 motors to control on the boat, requiring control of 4 motor boards.
Rotational sensing will be via custom hall effect sensors, with 4 (?) magnets in each winch drum.
We will also want to implement current sensing to indicate when tack and shunt-cinch ropes are tight (ie: motor is close to stall).
Radio controller
This new control scenario will require new design of the controller. Sheet/tack motors and shunt cinch motor may each be controlled by a double throw- center off toggle switch (providing sheet in, sheet out and stop functions), with an led indicating stall condition. Layout of switches and leds on the controller surface will mimic the positions of tack/sheet points and most foot on the boat, ie six switches in a row across the top surface – possibly set into a line drawing of the boat hull.