The purpose of this project is to design and mass-produce kits for a floating tiny house that can sail. It combines high-tech modeling and fabrication and low-tech assembly that can be carried out DIY-style on a riverbank or a beach. This boat is a four-bedroom with a kitchen, a bathroom/sauna, a dining room and a living room. The deck is big enough to throw dance parties. It can be used as a river boat, a canal boat or even a beach house. It's rugged and stable enough to take out on the ocean.

Tuesday, January 13, 2015

The Centerboards

On most point of sail QUIDNON will sail just fine without using a centerboard. The chine runners will trap enough water along the almost vertical sides to prevent leeway. This was my experience with HOGFISH, which did a fine beam reach with the boards up, with barely 10º of heel. However, when sailing close-hauled, the chine runners become overwhelmed, and hard on the wind with the centerboard up HOGFISH could only manage a course about 60º to windward. This was still useful in tacking out of shallows, where the centerboard could not be deployed. But it wasn't possible to make good progress to windward out on the ocean without lowering the centerboard.

In addition, HOGFISH was impossible to dock or to maneuver in and out of a slip with the centerboard up. This was a mistake I made on multiple occasions, and at one point I even wanted to make a little brass plaque, to screw to the dodger right above the companionway; something along the lines of “Remember to lower the centerboard, you idiot!” With the centerboard up, the rudder would quite effectively alter the orientation of the boat, but not its direction of drift. Luckily, dropping the centerboard only required ripping a line out of a jam cleat and could be done in about 3 seconds, so the embarrassment was generally short-lived. With the board down HOGFISH happily pirouetted within its overall length.

And so QUIDNON has to have a centerboard—the same kind I had on HOGFISH, since that design worked so well. It was a kick-up centerboard: when it hit things underwater, it bounced up. It was ballasted with a slug of lead embedded between its layers of plywood, and the amount of lead was calibrated so that the centerboard was almost neutrally buoyant when immersed in salt water. It descended through gravity, was reasonably easy to haul back up using a 3-part purchase, and didn't get damaged when it hit something underwater even when moving at hull speed.

In addition to providing lateral resistance and good tracking, the centerboard also served as the depth sounder of last resort: by keeping the centerboard purchase under a bit of tension, I could get a warning of an impending grounding. When it started to droop and jingle, it was time for a quick 180º course correction. Working together with the kick-up rudder blade, the centerboard also made for very responsive steering when being dragged through mud or sand. Thus, even when I found myself in barely 4 feet of water and heading for 0, I did not lose hope, because I could still execute a 180º course correction and head for deeper water.

The design of the centerboard on HOGFISH had the following major shortcomings:

1. The centerboard trunk took up the entire middle of the cabin—the most prized piece of real estate in the entire boat. Going from the v-berth to the heads required a circumnavigation of the entire boat—all the way to the galley and back again on the other side. The centerboard trunk made the already narrow cabin feel even more cramped.

2. Having been made of 3 layers of ¾-inch plywood screwed and epoxied together and sheathed in fiberglass (3 layers of cloth) and suspended on a pivot made of 3-inch bronze pipe, the centerboard was relatively indestructible—except for its forward edge. Coral heads were especially hard on it, taking big chunks out of it, which I had to fill in with thickened epoxy during subsequent haul-outs. The obvious solution is to screw a stainless steel rub-rail to the leading edge of the board. The screws have to be long (3 inches) and bedded in epoxy, so that they don't pull out even if the rub-rail takes a big enough hit to crimp and drive it into the plywood.

3. The centerboard was quite difficult to service even with the boat hauled out. Fully extended, it protruded by about 6 feet out of the bottom if its trunk, and blocking the boat at that height is not possible using standard jacks. Dropping the centerboard and putting it back in would have been quite an adventure. And so the top of the centerboard, or the inside of the centerboard trunk, was never serviced in any way or even looked at. This didn't seem to matter much, but in general it would be better to make the centerboard easy to service. This I intend to do by making the top of the centerboard trunk into an access hatch, so that the centerboard can be hoisted out for service with the boat in the water. The lid of the access hatch can be made sacrificial, so that if the boat hits something underwater while sailing fast and the centerboard goes flying into its trunk, the resulting damage will be confined to heads sheared off a few bolts that hold the hatch in place, which will be easily replaced.

On QUIDNON the design of the centerboard poses an additional challenge: given its wide hull, a single centerboard hanging down from its center-line will be rather ineffective when the boat is heeled over, because only part of it will be submerged. My solution is to have two centerboards instead of one—one on each side of the hull. Note that this solves Problem 1 above: the centerboard trunk will no longer take up the prime real estate in the middle of the cabin. Instead, each centerboard trunk will sit off to the side, making a comfortable 2½-foot-high foundation for a pilot berth and forming one side of the fresh water tank (because everything must serve more than one purpose).

Problem 3 will be solved by providing an access hatch at the top of the centerboard trunk, with hoist attachment points screwed into the cabin-top above for lifting the centerboard out for service with the boat in the water. Since the bottom of the boat will be surfaced with copper sheet, it will never need to be hauled out, and so the ability to service the centerboards in this way will be essential.

An additional problem remains: with the boat heeled over, the centerboards will not be at an optimal angle. The optimal angle is at 90º to the surface of the water, but if the boat is heeled at, say, 30º, then its effective surface area is reduced by some 15%. The solution is to camber the centerboards out by a small amount—10-12º.

Both centerboards will be lowered for maneuvering in tight quarters, but when sailing to windward on a given tack for any great length of time, only the leeward centerboard will need to be deployed. This will cut down on drag somewhat. Of course, this is not something one would want to do when short-tacking through a channel or out of a crowded harbor.

What remains is a very, very minor problem: the problem of “clunking.” On HOGFISH, when tacking, or when temporarily lying ahull with the centerboard down, it would clunk back and forth in its trunk. This wasn't dangerous, but some people found it unnerving. It also interfered with sound sleep while underway. I would like to solve this problem by providing packing along the pipe that serves as the centerboard pivot, restricting the motion of the top of the board, and by surfacing the board with a strip of HDPE plastic (cutting board material) on each side at the point where it contacts the aperture at the bottom of the centerboard trunk. This should reduce clunking to a sub-audible level.


  1. I understood Leeboards to hang from the side of the boat? Perhaps a more accurate description, if one is needed, would be Bilgeboards?

    1. Yes, calling them "centerboards" does seem like a bit of a misnomer. "Off-centerboards"? "Bilgeboards" works, although some people might think they are something else.

    2. 'Leeboards' do hang on the side of the boat, but the important point is that they only work on the lee side (water pressure from leeway presses them against guards). On the windward side, they 'wing out' away from the hull... if left that way (sometimes for short-tacking), they turn and drag... the maneuver is called 'sailing broken-wing'.

      We sail with a preventer cable on what would otherwise be leeboards, converting them to Off-CenterBoards (OCBs, we calls 'em). This follow's Phil Bolger's lead, who used them often.

      Problem is, ya gotta explain that to just about everybody! 8\

      So... Boards? LR Boards? Twin Boards? Lifting or Kick-Up Boards (singular form used to be common)?

      Dave Z

  2. Dmitry,
    My centerboard used to ring like a bell from side to side on my MIrage 5.5. I found a nifty solution online similar to yours. On the part of the board that was up inside the boat (not hanging down) I glued a layer of carpet on both sides with 5200. It never came off, and I shaved it to the right thickness so the board went smoothly up and down, but was snug from side to side. Never budged after that. Bushings and centerboard bolt should also be serviceable, I'm sure you know. I used self-lubricated bronze bushings from McMaster Carr, and stainless bolt in my salt water boat, and never had any corrosion. 1/4" Brass plate on each side of CB trunk.

  3. Dmitry-

    Nice solution; the space between the centerboards and the hull is the freshwater tanks - taking care of that odd space!

    When the centerboards are down, what if there was leaf spring in the housing? Would that prevent clunking, or are the forces too high? Gary's solution sounds good too.

    1. I thought about tightening them down in some way, but decided it's easier to just provide for tighter clearances and some damping.

  4. Wow! So cool man!

    I have been reading you since you first started writing now you have opened a subject which I am embarrisingly obsessed with. I have sailed all my life, lived aboard, designed and built all kinds of boats. I have always leaned toward non-convential designs from the likes of Bolger, Wharram, Colvin, etc.

    Currently I am somewhat land locked in the middle of the Willamette valley Oregon so I built a 30' electric sternwheel river boat that can regenerate when anchored. We have lots of fun spending days at a time exploring the river and all of its sidings but we get constant ridicule for going so slow, 3 to 5 knots.

    I really miss the ocean and dream of new designs for live aboard. I love your concept and have a ton of info as to how and why it is sound and practical, (not that you need me to tell you).

    Just a couple of quick thoughts; The Gray Marine gas engine went out on my 40' kettenberg I lived on and so I used my 10' dinghy with 10hp outboard as a yawl boat tied off to the stern quarter for propulsion for years. With someone at the helm and one in the yawl boat you have an amazing amount of maneuverability. Make it fast and easy to drop the lines and the yawl boat can zip around and push or pull the mother craft in any direction. On open ocean the motor can be locked in position and I could climb back on the mother boat to do other things. The Yawl boat stays with the swell so little or no cavitation. In narrow ways, which are usually flat water I either pushed from behind or pulled. The added benefit is you have a strong shore boat and only need to maintain one motor, (I actually carried a 5hp backup sometimes).

    As for center boards you might look at dagger boards. That is what all singlehanded ocean racers use as they can be designed to be bullet proof and simple for one person to use. They also wedge in so as not to wobble.

    I will bookmark you and follow your progress.


  5. Hi Jef,

    Chris Morejohn used a daggerboard on his HOGFISH MAXIMUS, with good results. It lived in a slot that went from the deck all the way down to the bottom. He made it buoyant, so that it took a purchase to horse it down. It had a notch at the top of the leading edge that hooked onto a pipe. If it hit something underwater, the notch would slide off the pipe and the centerboard would pop up. I played with this same idea for QUIDNON, but decided to go with the older design he used on HOGFISH, since there is so little wrong with it, and since it continues to work even when dragging along the shallows.

  6. Another thought about off-centreboards/bilgeboards is that they can be profiled to produce lift in the required direction. I assume that a centreboard is equally shaped/profiled on both sides, as the one board is used on both tacks, but with twin boards, each board can be shaped to produce lift to windward. (Port board shaped to produce lift to starboard, and vice versa)?

    1. That's right! Thanks for reminding me. The hulls on a Hobie and the hull on proas are sculpted to generate lift. Could do the same on QUIDNON by layering some more veneers on the windward side of each board, while keeping the leeward side perfectly flat. There will still have to be a broad flat spot in the center in order for the board to slide in and out without damage and to provide a big contact patch where it will rest against the edge of its trunk when on the opposite tack. But some amount of asymmetry at the leading and trailing edges, is possible.

    2. Hi Dmitry,

      I got lost at 'windward and leeward sides of each board' which I would imagine would change with the tack. Did you perhaps mean inboard and outboard sides?

      We shaped ours about as you describe, to generate inboard lift... when heeled, the deep immersed, near-vertical leeward board's lift would strongly counter leeway. The shallow, windward board (it's lift vector oriented downward and weakly to leeward) would counter heeling moment. Unfortunately, it would also partially counter the leeward board's lift, but a net gain is better than nothing.

      To tell you the truth, I can't tell you if the theory panned out or not.

      But it SOUNDS cool! 8)

      Dave Z