Thursday, March 22, 2012

Ocean Going Volkswagen Cat-Ketch with "SockSail".

                           Aerodynamic shape around a free standing spar and a partial solution.
Over the years, I have adopted many options. The track on the mast and a sail without battens is the simplest but worst scenario. Next, the track on the mast with battens is an improvement, however, track on the mast with full battens is the best normal set-up. I have also tried a "Thick" sail with foam inserts, as well as wing mast and full battens. This is certainly a valid system but is complicated and expensive. On the original 40' Freedom Cat-Ketch, Gary Hoyt adopted a double ply sail going around the mast. The result was producing too much friction, causing difficulties hoisting, lowering and reefing the sail. I propose another solution; the "SockSail" and I am using the Ocean Going Volkswagen 37 to illustrate.

The mizzen mast (aft mast) is fitted with a regular mainsail and battens. On the image, I kept it as such so that you can visualize the difference between the main and the mizzen. The Sock sail on the mainmast consists of a wrap around canvas over the free standing spar. The sock is zippered on one side to permit easy installation and removal. The sock has section cut out to give access to the fittings located on the mast
which are necessary to control the wishbone. The sock has a permanently attached headboard to the masthead. The tack has a Cunningham to control the tension at its base. The fore and aft dimension of the SockSail is sized to provide rotation around the mast without much friction. The chord is calculated to achieve that effect, respecting a reasonable span thickness ratio. A grooved luff, similar to the headfoil for a jib, is sewn on the backside of the socksail and runs the full height. The groove receives the luff of the sail with its own halyard and tack line. The sail is fitted with normal length battens. Full-length battens cannot be fitted because of the obstruction given by the rigid foil.
The behavior of wind on a circular leading edge is well known. Viscosity, pressure and friction prevent the air particles from staying attached to the mast. Therefore, the objective of this set up is the limit drag in the wake of the mast, to streamline the air flow, and to maximize sail power. The classic jiffy reefing is the preferred method for shortening the sail. After reefing, the forward part of the soft sail leaves the 'Sock" full height. The mainsail is received between the arms of the wishbone into lazyjacks which, when cruising are left in place.

A cruising boat such as this Cat-ketch can benefit in performance with a SockSail. To what extend, it is hard to say and experimentation is in order. Maybe this half measure solution ought to be interesting to follow in practice.