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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.












7 comments:

  1. Nice job. I sail cat ketch dinghy and i love this rig. Just curious, why you called it "volksvagen"?

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  2. Hello. Thank you for your question.
    The idea 35 years ago, was to introduce a simple offshore boat, not unlike what was going on in France with the craze for amateurs to build chined vessels destined to sail the oceans.
    My twist was to pursue the goal further by not only the use of 3 chines, but also a flush deck for ease of construction and more importantly, rig the boat as a Cat-ketch on a platform capable of handling ocean crossings in comfort and simplicity. A do yourself project with appeal to the folks. Voila!
    A little side story:
    Being introduced to Alan Payne, the famous Australian Designer (12M. Gretel)and having given my name, he immediatly reacted by saying " Ah! you are Tanton of the Ocean Going Volkswagen". I guess the name works.

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    Replies
    1. Thank you. Yes the name works. How long is the hull?

      Is there a suggestion to use a schooner configuration in heavy conditions, by reducing the main? This summer I took my cat-ketch 17 ft dinghy to Finland again to join the small open boats cruise. There were all kinds of weather including a front from the west passing over us with a black squall which catched us underway. It took about 1/2 hour since we noticed small dark line on the horizont to be hit by strong blow (40 knots) and rain from big dark low cloud with a rugged edge.

      That 1/2 hour I decided to use full sails heading to the coming front to get away as far as possible from a lee rocky shore of the big island we were passing along. Wearing rain clothes , fixing gear etc.

      Few minutes before the big blow we fixed mizzen in the axe of the boat to keep the bow facing the wind, pulled down the main to reef it. Next, it was too late to try to reef the mizzen. We started to continue sailing having a schooner rig, just like on your second drawing. It was really nice and efficient sailing, rewarding enough to ignore heavy rain. We have had good drive to be really safe from the lee shore. After another 1/2 hour the squall was over.


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  3. Hello. OGV is 37' long.
    I found out that by keeping the mizzen and lowering the Mainsail the boat heaves-to very well. Further more, the sails being of equal size the reduction in sail area permits to keep on sailing at a good clip. Its all in the balance. I think the reasons for the tame behaviour under this sail configuration is that the boat has a long fin keel and that the windage on the forward mast reduces the weather helm that you might expect otherwise.

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  4. Thanks for directing me to this page which has given me something to think about.
    Limiting drag behind the mast is more important for windward performance in a cat ketch than in a sloop and some sort of solution is necessary for satisfactory performance. The idea of separating the foil from the sail avoids some of the problems created by other solutions to this problem:
    two ply sails : a lot of friction, the bulk and weight of two-ply sails
    the “Wharram wing sail” looks difficult to reef or remove as the foil would bulk up at the base of the mast. I had thought of doing this but had my doubts.
    rotating masts etc. are a complex and expensive solution for a simple 28-footer
    I can see separating the foil from the sail facilitates conventional jiffy reefing of the sail and removal of the sail which I see as the drawback of the Wharram wing sail.
    Shearwater differs from the Freedom rig in having a conventional boom supported by a rigid boomvang with jiffy reefing which works well with this idea along with its freestanding carbon fiber masts.
    The practical implementation of the idea will require some discussions with my sailmaker.

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  5. Where are you in this development? I have a Freedom 39 Pilothouse Schooner and will be going to wishbones and would love to try a sock sail like this too especially since the masts are not only round but much larger in diameter than stayed rigs the benefits of a better airfoil should be more pronounced.

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  6. Hello. Besides what has been written here,nothing more. I think a conversation with your favorite sailmaker would be the next step. So, see what he thinks and if the concept is valid.

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