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.

Wednesday, May 22, 2019

The Rudder

Rudder assembly
Quidnon’s steering has evolved quite a lot since the original concept. Now all that’s left of the original concept is the idea that the rudder should have a kick-up blade: when sailing across shallows it should gently float up instead of getting torn off or getting stuck, and when the boat settles on its bottom at low tide the rudder blade should automatically get itself out of the way. Only now has a good solution to this problem finally been found.

Early on it was thought that twin rudders and wheel steering made sense, but this made the design complicated and expensive. Twin rudders require a complicated steering linkage that uses something called Ackermann geometry, which is also used in cars: when turning to the right, the right wheel has to turn more than the left wheel because, being closer to the point around which the car turns, it has to follow a tighter circle.

Later on, after single-handing a 36-foot sailboat from Boston to South Carolina, I discovered that wheel steering is a bad idea and that I prefer a simple tiller. There are very few steering positions that are comfortable with a wheel: sitting behind it and standing behind it are more or less the only choices, and they both get tiresome rather quickly. On the other hand, with a tiller, it is possible to steer the boat while standing, sitting or lying down, using hands, feet and hips, or, with a tiller extension clipped on, with the inside of the knee or the armpit. It is possible to operate the tiller remotely, by tying a bungee cord to one side of it and pulling it to the other side using a lanyard.

In turn, a tiller on a boat of Quidnon's size is only workable if the rudder is a balanced rudder, with about a third of its area ahead of its rotational axis, so that the boat can be steered with a fingertip instead of your heel on the tiller pushing with all your might, as is the case with an unbalanced “barn door” rudder that rotates around its forward edge.

Further on, I discovered that Quidnon doesn’t heel enough to make twin rudders necessary: just a single rudder would work fine, and so the design was changed to a single rudder hung off the center of the transom. But this arrangement was still somewhat problematic. First, the rudder assembly cluttered up the transom and made the boat a bit longer (which is a problem because marinas charge slip fees by overall length). Second, the pivot point of the rudder was too far from the cockpit to give the tiller a useful swing range.

Rudder assembly installed in engine well
And so the rudder was moved from the transom to the back of the engine well, where there is just enough room for it. This made it possible to solve a few more problems.

Quidnon doesn’t always need to have a rudder. It is a houseboat, and houseboats mostly just sit at the dock, where having a rudder is not just unnecessary but also rather inconvenient. The tiller tends to whip around and hit things whenever the tidal current shifts or a boat wake hits. Since it sits in the water, it tends to accumulate marine growth which makes it not work very well when the time comes to move the boat. A better solution is a rudder assembly that is easy to install and just as easy to take out again when the boat is at rest.

Gudgeons in engine well: top view; aft view

With the rudder assembly removed, all that remains on the hull are two gudgeons bolted to the back of the engine well along the centerline. To install the rudder assembly, it is turned 90º, so that the tiller faces directly sideways and lowered into the engine well using a hoist. The rudder shaft has two pintles that engage with the gudgeons. The lower pintle has a longer pin than the upper pintle, so that it can be engaged first rather than having to try to line up two pintles with two gudgeons at the same time.

Once the rudder assembly is dropped into place it can be turned to the 0º amidships position, with the bottom part of the assembly sliding under a recess in the bottom of the transom. This recess serves several purposes: it provides an exit path for the stream from the propeller; it also provides an exit path for the outboard motor’s exhaust when it is in idle (when it is in gear the exhaust goes through the propeller and into the water); lastly, it provides a space for the rudder assembly.

The bottom part of the rudder assembly consists of the rudder blade case and the rudder blade. The case is a box, welded out of mild steel and galvanized, with its bottom and rear open and forming a slot from which the rudder blade protrudes. It is welded to the bottom of the rudder shaft (a steel tube) and reinforced using a triangular gusset. The gusset has a hole in it for attaching a lanyard by which the rudder assembly is hoisted out of the engine well. The sides of the box have specifically shaped cut-outs in them.

The rudder blade is made of a 3/4-inch (20 mm) piece of plywood sheathed in fiberglass and painted. Close to the bottom of the blade is a circular cut-out that is filled with a lead disk, to ballast the blade to counteract the buoyancy of the plywood and to exert a certain downward force when submerged. The top of the rudder blade is surfaced with epoxy that’s loaded with graphite powder, to create a hard bearing surface.

Rudder blade detent mechanism: roller guide and rollers

The rudder blade is joined to the rudder blade case using 4 rollers, 2 on each side, that are through-bolted to the blade and ride inside the cut-outs in the case. The arrangement of the rollers and the cut-outs acts as a detent: in order to get the blade to kick up the force acting on the front of the blade generated by an obstacle has to be more than 4 times the downward force on the blade due to gravity.

The lead disk is sized so that this force is significantly more than 1/4 of the force generated by drag with the boat moving through the water at its maximum speed. Once this initial resistance is overcome, the rudder blade kicks up rather easily. Once the external force acting on it is removed (because the boat is again in sufficiently deep water) it floats down into vertical position and the roller mechanism clunks into place. When the blade kicks up, it fits in the recess under the transom.

Rudder blade in kicked-up position

The only necessary precaution is to avoid running aground while moving astern: the rudder blade will not kick up backwards. Most of the time the centerboard will strike bottom first, because it hangs down lower, and will stop the boat, shattering if it has to, in which case it will be time to pull the remainder of the centerboard out of its slot on deck and to drop in a new centerboard. But if the centerboard somehow misses the underwater obstacle and the rudder blade doesn’t, then the rudder may suffer a bit of damage.

If the bottom is soft and the boat is moving slowly, it will simply stop with the rudder blade stuck in the sand or the mud. If the obstacle is hard or the boat is moving fast, the bolts holding the rollers to the rudder blade, which are designed to be the weakest element, will shear off and the rudder blade will drop off. Then it will be time to pull out the remainder of the rudder assembly and to jump down into the water (all 4 feet of it) to recover the rudder blade and the rollers. Then the rudder assembly can be put back together with new bolts. There is also the chance that the rudder blade will strike and damage the prop, in which case it will also be time to pull up the motor and replace the prop. In short, don’t run aground when backing up!

To summarize: the rudder assembly easily installed and easily removed when not in use and for maintenance. With the rudder assembly removed, all that remains in place are two gudgeons mounted to the back of the engine well. It doesn’t kick up unless it encounters a hard obstacle, with no amount of moving water able to displace it from vertical. Its action is fully automatic, never requiring any operator intervention. It provides for fingertip steering using a tiller because the rudder blade is balanced, with 1/3 of the area ahead of its axis. The use of the tiller makes it possible to use the simplest and most affordable kind of autopilot: a tiller pilot that clips onto the tiller. The tiller itself is of a telescoping type, with a handle that slides into its body, so that it isn’t left swinging about the cockpit when the boat is on autopilot.

After all of the various evolutions, I dare say that this rudder design is very close to final.

7 comments:

  1. Dmitry,,,
    I really like the steering changes, though personally I'd be more comfy with a more robust--durable setup, and a lanyard to manually raise the kick-up rudder. KISS all the way.
    Quidnon just keeps getting better and better.

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    1. What do you see in this set-up that isn't robust or durable? A lanyard to pull up the rudder blade is easy to arrange, but isn't really necessary. Having sailed for 5 years with a kick-up rudder, the only occasion where I needed to manually pull it up was when the boat was getting hauled out and blocked, but Quidnon will never need a haul-out.

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    2. Thanks again, Dmitri--this time for your patient explanation to me. My initial concern was the strength of the 2'x4'x 3/4" thick piece of marine plywood forming the rudder blade was not strong enough, but encapsulating it in fiberglass will solve that problem nicely, and engineering easily obtained sacrificial components in the event of a major screw-up, pretty-much removes my rudder concerns altogether. Carry on......

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  2. I too can see the advantage of a lanyard to raise the rudder. I'm thinking of a situation like backing out of a shallow Mangrove swamp after a hurrican (for example). Overall, I think this is a great design.., thank you.

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    1. Probably the simplest solution is to make a 3/8" hole in the back of the rudder blade near the bottom. Then, for backing out of the mangroves, you could loop a length of 1/4" line through it and pull up on it to get the rudder blade all the way up. Then, once in deep water, you release one end of this line and pull it out of the hole. I haven't thought of this use case. Thanks. I'll add the hole.

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  3. Check out a better kick-up rudder used on race boats and Michael Storers scows which kicks up also if going astern and hitting something. No heavy handed tiller as it kicks up either. Article on it here and may work better still in a well: https://www.storerboatplans.com/foils/the-standard-storer-boat-plans-kick-back-dagger-rudder/
    Maybe use bicycle inner tubes for the shock bands?

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    1. That's a cute idea and great for a dinghy but not for a 36-footer. It's very far from balanced when it kicks up, but steering a little boat doesn't require balance. Not sure how it kicks up backwards. With Quidnon, kicking up backwards would result in the blade hitting and damaging the prop, and props are expensive. Bicycle inner tubes are expensive too, considering you'd need a dozen of them and they would only last a few weeks (even trucker's straps don't last a season in salt water). But, like I said, a cute idea for a dink.

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