How to build a Kick-Up Centerboard and Rudder

[Free Pirate]

How could I make a kick-up centerboard and kick-up rudder a for 20 foot plywood stitch-and-glue catamaran hull? The catamaran's still in the design phase, so if I need anything special that's fine. I want it to be able to be beached safely and without damage even if the crew forgets to pull up the centerboard and rudder. Any pictures would help a lot, thanks!

Reccommend anything? Books, "maybe-try-to-look-at-a-(insert boat here)", anything? I can't find anything on this subject, and since anything is more than nothing, anything will help! :)

[Raggi_Thor]

A quick search on google will give you plent of ideas, for example,
http://www.dougpile.com/mac19/cbcable.jpg
found in the search
http://images.google.com/images?&q=centerboard

[Free Pirate]

That image you linked to is what I had in mind. It's funny that it and similar ones are right on the front page of google, I put in "kick up centerboard" and got three images that didn't have anything to do with centerboards at all. I still don't get how I can have the rudder to be able to get pushed back (aftward and up) into its slot if it hits the bottom, but still stay put when it's in the water.

The simplest thing I can think of is to have the cable continue past the centerboard and go back up, allowing crew to be able to pull the centerboard into and out of the slot. There's probably a simpler way that works just as well or better.

I attached two animations for a conventional centerboard and mine.
A conventional centerboard has one line, which is red in the animation. It's used to pull up the centerboard, and when given slack, allows the centerboard to lower. Water going past it helps to keep it in place. This design does not allow the centerboard to be pushed into the hull if it hits the bottom, and if you hit the bottom hard enough (like when you're coming ashore, riding the surf and having only paddles to slow you down) the hull or centerboard will be damaged.

To allow the centerboard to go into its slot if it hits the bottom, everything would have to be flipped around. (the bow is on the left in the concept centerboard animation). Because water will now be pushing it towards its slot, in addition to it being wooden and trying to float into the slot instead of staying down. I added the green line and a hole for it through the hull so that tension on that line will pull the centerboard down. The red line still pulls the centerboard up. If the green line is pulled tight and secured to something, it will not be able to recede into the hull if it hits the bottom.
I need some way or device to hold the green line taught, but release the line if there is enough force (when it hits the bottom). Could anyone help me out, or suggest something better?

You have to click the thumbnails to see them move.

Thanks!

[Raggi_Thor]

I have never seen a centerboard oriented "the wrong way". They allways "flip back" like your animation with red and green line. Some ballast in the board will make it sink, this is common in traditional dinghys. The board can be kept down by friction also.

[Free Pirate]

It makes more sense that it wouldn't be backwards, I was confused.
So I guess I ought to do it with only the red line and ballast.
And ideas about ballast materials? I suppose lead is the best, where could I get lead the right size to put in a plywood centerboard? Something like this: http://www.amateurboatbuilding.com/ProjectJustRight/chap08/03--Lead-In-CB.jpg

[Raggi_Thor]

If you are on a (tight) budget, you can try to collect lead from car tyres shops, don't know what you call it there, it's used to balance the wheels.
Regular "ballast lead" costs a few dollars per pound in small quantities.
You can melt lead over a gas burner, 300 something celcius.
If you are thinking of extreme lightweight boats (like catamarans) you don't want any extra weight. Then you need some kind of downhaul and friction lock or a spinte or something.

[versavice]

Free Pirate: I have also been stumbling around with CB designs and somewhere in my design pile have a drawing like yours. What was the outcome? What did you do?
I wish to have an unweighted CB so will also need two controls. As well will need a kickup design. My best design consideration so far would have the control ie the lever or pulley that lifts and lowers the CB in the cockpit mounted on a plate that can slide forward (removing all tension from the control cables and thus allowing the Cb to swing freely). The degree of slide would be controlled either from a spring (bungee cord) or friction plate. Hope that is somewhat clear. What do you think?

[versavice]

Just another question/thought in this matter. I have been struggling in the design to keep the volume of water that will fill the CB box to a minimum. Perhaps this is not necessary. What effect does a greater volume of water in the box have? Will it function as a ballast? (obviously equally decreasing the boats bouyancy.)thanks

[Karsten]

One of the uphaul or downhaul ropes can be just an elastic. I recommend making the downhaul (green rope) elastic so that you hit an object the elastic lets the centreboard up automatically. It works very well.

To "seal" the centreboard case you have to glue or screw some "flaps" to the hull bottom on each side of the centreboard case. The flaps can be made from doubled sailcloth and overlap each other about 5mm. They will form themselves around the centreboard and seal off the open bit at the back. There will still be water in the centreboard case but no turbulance that increases drag.

[yokebutt]

Pirate,
You might want to consider buying a hobie 20 rudder assembly or similar off the shelf.

Yokebutt.

[Maciek188]

Why not to buy Hobie 17 or Hobie 21? They are design exactly that way. I drove my Hobie 21 at full speed into flouting telephone poll in Jamaica Bay, NY without any damage to the cat.
Maciek