Rotomolded kayak sail conversion: stiffness

Discussion in 'Boat Design' started by peterchech, Jun 26, 2013.

  1. peterchech
    Joined: Aug 2010
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    Location: new jersey

    peterchech Senior Member

    So just for fun I was thinking to convert something like this:
    [​IMG]

    to take a small sail, say 15 sq ft or so, just for some fun off the beach. Maybe not even put in a daggerboard, just a foot-pedal rudder.

    It's for playing around in, serious sailing is done on my keelboat.

    Obviously rotomolded hulls, esp. lightweight 8-10' kayaks with no bulkheads, aren't known for stiffness.

    Any ideas how to stiffen it up to take a small sail? (not those little chopstick downwind kayak sails, a proper mast and boom type rig) Does anyone have any links to someone who has successfully done this? (google is a clutter when I tried to look around)

    I have two ideas.

    1. Could insert a 1/2" ply ring frame at the location where the mast should be (say 2' forward of the cockpit, right over your legs), through bolted to the hull at several locations. Step the mast on top of the deck at that point, and set up lightweight stays/shrouds.

    Downside is it would clutter things up right where my (long) legs should be. I could keep the ring frame thin, but it couldn't be less than 1" wide I am thinking (a mast step collapse could trap someone inside this kayak, very dangerous obviously). Even a partial ring frame, under the deck only from gunwhale to gunwhale, would make things tight.

    Other downside is that if I start leaning to balance the boat as the wind loads up, it could bend the crap out of the thin rotomolded hull, maybe even break the deck-hull joint. Stiffness is the limitation obviously. This is less of a problem if I use a stayed rig because they would be swept back to just forward of the cockpit at the rail, and I could lean right there, preventing twist from being a problem.

    2. Make up two small v-shaped amas s&g style, mount the crossbeam to the gunwhales using screws, and stay the mast to the amas. The crossbeam takes most of the load, and the amas could even act as daggerboards and allow a little upwind ability. Obviously this would allow more canvas to be flown to a certain extent, and would be more comfortable, but is also much more work to build and transport from cartop to beach. And I'm not 100% sure that I still wouldn't need a ring frame or bulkhead of some sort.


    Any thoughts? Suggestions? Other internal components which could add more rigidity to such a hull besides bulkheads?
     
  2. 805gregg
    Joined: Apr 2010
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    Location: Ojai, Ca

    805gregg Junior Member

    Just get a Hobie Outback and the sail kit, they sail well and are fun
     
  3. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    you will not be able to handle much sail at all with such a narrow hull. I have tried it, a kayak is not suitable for sailing. I would go with the outriggers and detachable beam structure that holds the sail, this will give you something that you can have fun with at all points of sail, and likely keep you up-right. Personally I do not care for foot pedal rudders, I would rig a conventional tiller with an extension, this way you can move around on boat while sailing rather than be stuck in the seat.
     
  4. kvsgkvng
    Joined: Jan 2012
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    Location: *

    kvsgkvng Senior Member

    Hello, Peterchech. Here what I would do if I have a need for such thing. Best of luck.
     

    Attached Files:


  5. ancient kayaker
    Joined: Aug 2006
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    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    Been there, done that.

    The issue isn’t the physical stiffness of the hull it’s the lack of stability; the close confines of the cockpit makes it impossible to hike out in a hurry to balance a gust of wind. Fastening the mast rigidly to the hull doesn’t solve a thing, it just makes it easier for the sail to flip the boat. You’re better off with a hinge so you hold it up, but that gets busy with the sail to and rudder to control as well.

    A 15 sq ft sail will give you wet elbows from the bow wave on a breezy day downwind and on a reach across the wind. It won’t make progress up wind because your body is creating drag; the best you can expect is to sail backwards and forwards along the same line. However a15 sq ft sail can still flip you over in a sudden gust.

    There are three options that I know of, I’ve tried 1 and 3:

    1. This is your option #2 i.e, sail and mast attached to a cross-beam with amas. The amas react the sideways effort of the sail to flip the boat, the boat’s longitudinal stability reacts the sail’s forward thrust. This information will guide your design of the rigging, but from experience I recommend a minimum of rigging to reduce setup and dismantling time and for safety. This is the safest option IMHO. The amas can cause a surprising amount of drag and the best results are obtained by using body weight to get them out of the water, which is why I found it worked better on a canoe than kayak.

    2. A kite. These are readily available, not nearly as easy to make as you might think. They can be difficult to raise in a low or really high wind, but they don’t exert a lot of force that might flip the boat. Also they get up high where the wind is strongest. This is the performance option but I could never make it work for me.

    3. I may be the only guy to try this. The concept is an inclined sail and a Bruce Foil. You can research BF’s on Google, it is a slanted keel that partially converts the sideways force of the water into a vertical force. Mounted on an outrigger, it counters the upsetting force (or torque) of the sail. In my version I had the sail inclined the opposite direction so the BF wasn’t doing all the work: the SailRocket is the ultimate example but it is strictly one-way.

    Safety note: most important thing of all: either be able to drop the rig instantly if you get into trouble or ensure you can detach it from the hull so you can do stuff like recovery (if you wear a skirt) or bail, right the boat and re-enter. Get really good at this kind of stuff before you start sailing. A lot of rigging lines becomes a serious threat to life after a knock-down.
     
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