Centerboard or Leeboard(s)

Discussion in 'Sailboats' started by brett2634, Mar 20, 2010.

  1. brett2634
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    brett2634 New Member

    First off, I want to thank anyone in advance for reading this and responding to this post. I'm not a boat builder or designer, I'm basically just trying to piece together a kayak into a sailing vessel/trimaran. The bad news is that I don't really know what I'm doing. I had to sell my O'day Daysailer, but have an extra kayak and really want to sail this spring/summer.

    So...what I have done is purchase canoe/kayak stabilizers to attach to the deck of the 13'8" kayak. These are only 42", but hopefully they will provide enough stability/buoyancy. Also, I purchased a 14' windsurfing mast with a 5.1m sail.

    My original thought was to build a centerboard box and centerboard, however, this might be more than I can undertake. I have considered leeboards, but don't know much about them. So, the reason for the post is this: Can I get by with one leeboard that would act like a centerboard, or do I need leeboards? Any thoughts?

    And please, excuse my ignorance. I know little about sailing and boats in general.

    Lastly, this is what I am going for:
    [​IMG]

    Starting with this:
    [​IMG]


    Thanks for the help!
     
  2. gonzo
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    gonzo Senior Member

    Asymetrical outriggers will act as keels or centerboards. That is all you need.
     
  3. sharpii2
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    sharpii2 Senior Member

    More information is needed.

    When you say '42"', what do you mean?

    Is that how long the floats are? Or is that how far they are apart?

    Perhaps a picture of this assembly would be helpful.

    It seems you are starting out with too much sail for a kayak conversion.

    You have two possible problems.

    1.) the float being driven under, and
    2.) the bow being driven under.

    A tall sail rig like the one you have, both are likely to happen.

    A smaller, shorter rig is probably called for. One of maybe 3.0 sm. Or maybe 4.0 sm at the outside.

    The Hobie(r) you show in your post was designed to be a sailboat to begin with. Your kayak was not. It can be made to work as one, but it will not be as good as something originally designed as one.

    A single leeboard will work as long as it can be held down on both tacks. In that case, it is really a side mounted center board.

    A true leeboard is only used on the lee side of the boat, so sailing presses it against the hull. It is only held in place with a hinge or a lanyard at the top, so, on the windward side of the boat, it simply floats up and outward. For this reason there are usually two. One on each side.

    As a side mounted centerboard, it has to take sailing pressures from both sides, leeward AND windward, so it has to be attached to the boat differently to stand the loads.

    On straight sided scows I like to design, the 'lee board' is bolted to the side of the hull with a single bolt, halfway up from the bottom.

    The bolt acts as an axle allowing the board to pivot fore and aft only.
    On one tack, the board pushes against the part of the side below the bolt.
    On the other, it pushes against the part of the side above the bolt.

    A rack can be designed to straddle the cockpit of your kayak and have a side mounted centerboard on one side. It will have to be firmly attached to the kayak somehow. The simplest, cheapest way to do this is to have a line go from one side, under the kayak, to the other side.

    Another thing you have to think about is how you're going to hold the mast up. You will need either a very sturdy mast step or a system of stays leading outward to the floats. The sailing loads at the base of the mast, if it is just held up by the step, can easily amount to several hundred pounds, even on a boat as small and simple as this.

    The last thing you have to consider is the Center of Area (CA) of the sail.

    It has to be directly over the CA of the 'leeboard' or very close to being over it. It can be behind it a little, but never in front of it. The area of the 'leeboard' should be about 3% of the sail area.
     
  4. brett2634
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    brett2634 New Member

    floats/outriggers are 42" in length

    I was afraid of my sail area being too large. Guess I'll have to go back to ebay.

    My challenge will be trying to mount the single board to act as a side mounted centerboard.

    Back to the drawing board...
     
  5. messabout
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    messabout Senior Member

    Reinforcing Sharpies advice..5 square meters is waaaay too much sail. About half that size is a good starting place. If you use a windsurfer type sail, it is best to use it as a free standing rig. That is to say; no shrouds. The reason is that the sail is cut to complement the bend of the mast. Windsurfer masts are deliberately bendy. If forced to use shrouds the bend characteristics may be compromised. More than that, there will be no place to put mast tangs unless you cut holes in the luff of the sail. I think that you would do better with something low and simple like an Optimist pram rig.

    Be advised that ther is a finite speed that you will be able to obtain with the kayak. Putting more powerful sails on it will not deliver the performance that you probably want. What too powerful sails will deliver is misery,danger, and broken gear. The amas (floats) themselves will place a lower limit speed on the boat because they are so short. It is conventional to have the amas close to the same length as the main hull. What you have will work, but there will be limitations in both performance potential and righting moment.
     
  6. Petros
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    Petros Senior Member

    Do not over complicate what you are doing. I think your sail set-up will work just fine, a larger sail means you will make better headway in light winds. You have have to be wary of high wind conditions.

    A lee-board is will also work just fine, though not as efficient, it is easier to build, it will not leak into the hull and you will not permanently alter the kayak hull. Two to 3 square foot of total area should work just fine for a lee board, make sure it is strong enough to take the side loads from the sail. Figure about 60 pounds later load on the lee board will be plenty strong. Place the lee board on the hull so the center of area matches about the center of area on sail rig to balance the forces on the sail and lee board.

    You also should consider making a larger rudder (or modifying your current rudder) since most kayak rudders are way too small for a sail boat.

    Keep everything simple and you will enjoy it more, but also consider that the kayak was not really designed to be used as a sail boat. My experience with sailing rigs on kayaks (and I have extensive experience with both sailing and kayaking) is they do not make very good sailboats. there is limited room to move around, it is difficult to make adjustments to the rig while underway, and they do not tack very well. One local kayak builder says "do not go sailing in a kayak, and do not go kayaking in a sailboat, they are not compatible activities" This pretty much squares with my experience too.

    Keep it simple, make it strong, and just go have fun with it.
     
  7. ancient kayaker
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    ancient kayaker aka Terry Haines

    The combination of wind and water forces on the sail and board creates a heeling torque or moment that will try to overturn the kayak. That in turn is counteracted by the stabilizers, floats or amas, whatever you want to call them, which generate a righting moment. These 2 moments duke it out continuously while you are sailing.

    Now for the important part. There is no reason why those forces should pass through the kayak. In your place, I would attach a mounting structure to one side of the kayak that would support the mast and the board. I would make the board about 5-10% of the sail area.

    The mounting structure should be attached firmly to the beams. The kayak can be attached quite lightly to the beams, in a way conventient for removal. It only has to withstand you wiggling about in the cockpit and the rudder forces.

    I have sailed a kayak, it is a blast. The sensation of speed is out of all proportion to the actual speed but who cares? it is fun.

    The sail area sounds a bit high but is probably in the same range as commercial systems. Sail area is not needed to overcome the force of pushing the kayak through the water, which is very little. It becames important when trying to sail upwind, because in a kayak you are forced to sit bolt upright and the wind force on you is more than the drag on the boat at the speeds you are likely to achieve upwind. With a small sail it will force you to sail off the wind, and create leeway, both of which can make it difficult to make any progress upwind at all. Been there, done that. If you want a good upwind performance you will need that sail area. However, there is nothing to stop you making a smaller sail for learning.
     
  8. sharpii2
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    sharpii2 Senior Member

    Very good points.

    I am working on such a design myself.

    It is basically a short stubby catamaran with the kayak lashed under its cross beams. It will have 38 sf of sail, a 7 ft Beam, and 6 ft floats. The balance lug sail powering it can be reefed down to a little more than half its size. The floats will each have long shallow keels on them and will be held with their bottoms just clear of the water when paddling.

    The idea here is to create the most basic motor sailor. Paddling will be done when the wind is light or non existent, and sailing will be done when there is enough wind to make it really worth while. Switching from sailing to paddling will not require the sail to be struck, making paddling through brief calms a real option. At least, so the theory goes...
     
  9. Doug Lord
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    Doug Lord Flight Ready

    ===========================
    You might consider duplicating the Hobie as closely as possible right down to the Mirage Drive. If the cost is not outlandish then it would take care of your lateral resistance requirements w/o an additional leeboard or centerboard...
    Good luck-you'll have a fun boat!
    Correction: the boat I saw did not have a daggerboard-just the Mirage drive, mast,sail and amas- but I just doublechecked and the Hobie Adventure Island is offered with the Mirage Drive AND a daggerboard-sorry.....
     

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  10. sharpii2
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    sharpii2 Senior Member

    Her's my idea of a kayak to sailboat conversion. It's pretty much like the one I suggested in an earlier post on this thread, except I made the floats a little longer. Except, for the rudder the rig is a complete unit which is strapped onto your unsuspecting kayak. The floats are kept clear of the water to facilitate easier paddling.
     

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  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    Bob, lot of stuff to like in that design. The rig is almost exactly the one that I have decided to create for my small (10' 3m) sailboat, except it will not have a boom. I already have the sticks.

    I like the way the floats are above the waterline. My first venture into a sailing kayak/canoe used one float sized to support my full weight half-immersed when seated on the gunnel to keep it down full-time. The performance was disappointing until one day it took off, smooth silent and fast. Then I saw the float was out of the water so I shifted position to put it back down and the speed fell off. Seems there were things I had to learn about the shape of the float. IMHO they are just for insurance, not actual use, just as you have them.

    The beam looks like a 2 x 4 (5 x 10 cm) which is simple and should be adequate. I found shrouds a nuisance; I was using regular hardware store doweling but eventually changed to for an unsupported spruce mast. The mast looks about 1-1/2' (4 cm) dia, which may be enough to stand alone. A 2" (5 cm) dia mast would certainly do the job without shrouds, if you can get 8/4 unfinished lumber for the blank.

    Your floats are about the same size I used, 7' x 8" x 8" (.2 x .2 x 2 m), which I think is larger than you need. They will allow you and a partner to perch on the gunnel or withstand a monster gust: with an unstayed rig you can just release the sheet.

    An offset pivoted daggerboard may be more efficient than the two low aspect ratio keels shown, unless you have to work in shallow water a lot. Oh, and make sure you have space to swing the paddle including sweep strokes.
     
  12. normwei
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    normwei New Member

    sail for kayak

    There is a much simpler way to attach a sail to your kayak. I bought a WindPaddle and it works like a charm. Go to YouTube and type in normwei and you will it in action. You don't need a mast. I have since added a rudder to my 10-foot kayak and built a centerboard on the starboard side. Very simple design and it works!
     
  13. sharpii2
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    sharpii2 Senior Member

    Thanks for your kind words. AK.

    You have a very good eye for dimensions.

    The floats are really just 7" wide and only 6" deep.

    The design is not meant to be all that efficient. Efficiency was sacrificed for simplicity.

    It is designed to sail with one float in the water and the sailor just sitting there.

    I figure it can go up wind about as well as a boardless Hobie Cat(r).

    In some conditions I would rather sail upwind than paddle upwind.

    The stays are necessary not so much to hold the mast up but to stabilize the whole structure.

    The floats and the connecting beams are bound to compromise paddling. I have thought about that a lot.
     

  14. booster
    Joined: Apr 2009
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    booster Senior Member

    Guara Boards

    Hi!
    Under the post "Old quarter-tonners Magic Bus" I have discussed the early development of guara boards. The design "Kon-Tiki" by Thor Heyerdahl is shown below.
    Regards,
    Booster
    [/ATTACH]
     

    Attached Files:

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