Simplest Proa Rudders
The concept is based on the rudder(s) fitted, without gudgeon or pintle, into a so-loose-its-sloppy daggerboard trunk. If the trunk is rectangular in cross section and is about 125% as thick as the board, said board may freely “wobble” up to 5° either side of centered. Make the box 180% as wide as the rudder is thick and it can wobble +/- 10° either side of centered. See attached for a solution with tighter fitting board.
Do this at both ends of the boat and you have counter-rotating, double rudders able to offset an aggregate 20° from each other. This may not sound like much, but anyone who’s played around with counter-rotating rudders is likely to concur that it’s plenty, for anything save close quarters docking and the like, for which you already carry a steering oar, right?
Interconnecting is simple; get yourself a lightweight, flexible pole—imagine a pole-vaulting pole—or a long chunk of wooden closet rod, and attach it to the top of both rudders—perhaps you’ve glassed a short socket onto the rudderhead into which you insert the pole. This holds the rudders (flexibly) in alignment. You can now push this pole to windward or leeward; sit back and use your feet if you like. You’ll discover you have a pair of inter-connected tillers nearly half the length of the boat, and the rudders self-center to boot.
If your rudders are, say, 16’ apart, you’ll need to push or pull this flex-tiller about 15” to get 10° on the rudder. It’s OK to partially retract the forward rudder to reduce wetted surface and increase leeway/AoA on that rudder. You can eliminate the flex-tiller if you like and use rigid tillers—connected to each other amidships with a bolt. The tillers will need more length to slip into and out of their sockets at the rudderheads if you do.
And let’s do one more sloppy injustice to those daggerboard trunks, shall we? Let’s cant the forward and after ends about 10° from vertical, making the trunks longer at the tops than the bottoms. What we want is for the daggerboard to be able to fit in the trunk—and wobble in pitch as well as yaw—so set either vertically, or swept back that 10°. This is how we’ll get rudder balance: After the shunt, grab the flex-tiller and give it a push forward—wracking both rudders so they sweep back (water flow will hold them there). Pick a sweep angle that properly balances the rudders and Bob’s your uncle.
You’ll need to allow the rudderhead sockets to pivot slightly in pitch, while remaining rigid in yaw—or they could be replaced by “proper” rudder cheeks. The point is to allow the rudders to sweep back but also to force the tillers to pivot the rudders vertically—if they don’t the rudders won’t necessarily balance.
[Added a second sketch and changed “wrack” angle from 15 to 10 degrees in the text. 2-22-14]
Dave
This is a very cool idea Dave! I love the balance restoration through tilting and the flexure linkage.
But, assuming the flexure rod is glassed to the tops of the boards, when the rudders shift fore and aft, won’t that effectively shorten the rod by bending it into an S? So let’s attach the rod to the rudders with a horizontal bolt—now it’s a parallelogram linkage that can accommodate the tilt of the rudders as long as the tilt is equal.
But if you raise the forward board a bit to move the clr, now you have that length change problem again. Telescoping rod with a friction collet? Threaded at one end like an adjustable tie rod on a car?
In any case I love the creative elimination of hardware through geometry. Very nice thinking, Dave.
Chris
Would the concept be applicable for a non-symmetrical daggerboard? The foil would work better as a rudder on one tack than on the other.
Would the concept be applicable for a non-symmetrical daggerboard?
You bet. The only difference between a sym foil and an asym, of the same family, is that the asym has a curved mid-chord line. So we just curve the mid-chord line on the trunk as well, and it still fits perfectly.
The foil would work better as a rudder on one tack than on the other.
Not sure what you mean here. The rudders (let’s call them rudders, OK? They’re technically “rudder-boards” but that seems to confuse most.) The rudders will work equally well in either direction.
Dave
This is a very cool idea Dave! I love the balance restoration through tilting and the flexure linkage.
Thank you.
But, assuming the flexure rod is glassed to the tops of the boards, when the rudders shift fore and aft, won’t that effectively shorten the rod by bending it into an S? So let’s attach the rod to the rudders with a horizontal bolt—now it’s a parallelogram linkage that can accommodate the tilt of the rudders as long as the tilt is equal.
But if you raise the forward board a bit to move the clr, now you have that length change problem again. Telescoping rod with a friction collet? Threaded at one end like an adjustable tie rod on a car?
Chris
Um, yes… But, erm, no. I think we’re saying the same thing. Read what you said then read this. Aren’t we addressing the same thing?
“You’ll need to allow the rudderhead sockets to pivot slightly in pitch, while remaining rigid in yaw—or they could be replaced by “proper” rudder cheeks. The point is to allow the rudders to sweep back but also to force the tillers to pivot the rudders vertically—if they don’t the rudders won’t necessarily balance.”
Any length changes are addressed with the tillers sliding within their sockets (or cheeks), any alignment issues via pivoting the sockets in pitch. Probably need some keepers/limiters, but that’s just an exercise for the reader. 😉
Dave
The foil would work better as a rudder on one tack than on the other.
Not sure what you mean here. The rudders (let’s call them rudders, OK? They’re technically “rudder-boards” but that seems to confuse most.) The rudders will work equally well in either direction.
Dave
I was thinking that the lift at a positive angle of attack would be different from the lift in the other direction at the equivalent negative A-o-A. It’s probably not enough to worry about and I could be simply wrong…. :o)
FWIW A-o-A would make a cool name for a proa since it’s completely symmetrical. No idea how you’d pronounce it, though.
I was thinking that the lift at a positive angle of attack would be different from the lift in the other direction at the equivalent negative A-o-A. It’s probably not enough to worry about and I could be simply wrong…. :o)
FWIW A-o-A would make a cool name for a proa since it’s completely symmetrical. No idea how you’d pronounce it, though.
Ah, gotcha! Actually, I think it’d be biased in the “right” direction. Lesser lift from forward board, greater from aft. So, more efficient than sym boards. Good point!
You remember the Grateful Dead album AOXOMOXOA? These guys had the “proa mindset” a long time ago. The term was actually coined by a surfer, to describe surfing a perfect wave. It has been re-birthed by kite flyers as that transcendent feeling you get when sailing fast and everything balances perfectly http://tug.com/blog/1995/08/01/01.html
Friend of mine tags all his email with the signature “OXO.” Easy to pronounce
Dave
I showed this concept above as daggerboard-type rudders in interior trunks, but it can be as easily applied to leeboards or beam-hung rudder-boards. The “trunk” becomes a skeletonized “cage,” perhaps of 1/8” x 1.5” aluminum, with the fore and aft pieces turned 90 degrees so as not to impede the water flow. The wavy trunk sides will make the rudder-boards quieter and offer a bit less drag at the trunk opening, but they aren’t necessary; simple rectangular cross sections are fine, too. If leeboards or beam hung, the bottom of the cage can be above the waterline, and so offer no drag at all—or run well below the waterline, with all the pieces aligned with water flow—offering better support for the boards.
Dave
Great idea. In my cartoon-line imagination, I can imagine running aground with these linked rudder boards - the front one rotates forward as it’s bottom is forced back, and the force on the flexible pole yanks the aft rudder out of its trunk. Then the flexible pole oscillates, stabbing the rear rudder into the deck repeatedly. It would make a fascinating video. I still think it’s a cool idea and worth fleshing out.
Hey, this would be fabulous for side-hung rudders in Dierking rudder cases. You could raise/lower the rudders by lifting the pole. The pole wouldn’t be going across the deck where it’s in the way. There are many ways to apply force to it to steer - feet over the side, lines on blocks (both leeward and windward if the rudders are on the windward side). This might even be a good way to steer two rudders with one autopilot mounted on the centerline of the boat.
Great idea Dave! You could also place the boards in the extended side deck of Herbie, to create side mounted rudders, perhaps.
Hi folks,
the rudder construction shown before could even a little bit simplified by fixing the rudders at the lee side of the aka with two silent logs for each then asymmetrical foilers, which gives a lift to the aka and simultaneously is used to steer the boat. The tillers are fixed together on a midship traveller, which is moved by an additional tiller. This construction works efficiently and reliably on my 6m proa. Groundcontact does not damage the foils because of the elastic attachment by the silent logs. Together with the fast movable crab claw sail shunting works within seconds and is manageable by a single person. I plan to enlarge the sail from 13 to 16 sqm and construct an additional foiler for the ama to lift the whole boat out of the water, but I am in doubt, cause it´s quite heavy
( estimated 230 kg, cylinder-mold-construction, after Kurt Hughes ).
Best regards
Dieter
Hi folks,
the rudder construction shown before could even a little bit simplified by fixing the rudders at the lee side of the aka with two silent logs for each then asymmetrical foilers, which gives a lift to the aka and simultaneously is used to steer the boat.
This sounds really interesting, but what is a silent log? If you happen to be Dutch (name sounds Dutch) I can possibly translate it for you.
Have you got any pictures or videos of your boat online?
-Thomas
hi Thomas,
My English is a little bit rusty, so what I call a silent log in german is named Silentblock and commonly used to reduce vibrations from a motor. It´s a cylindrical peace of sturdy rubber, in the tops of which a metal plate with each a female thread is welded. When you use the rubber mounting as a connection between boat and rudder, this connection will be flexible, but by the velocity of the boat and the relationship of levers the rudder will run out of course; so you need 2 rudders, which connected with each other will neutralize the levers.
I will post some photos later
Regards
Dieter
Here’s another hypothesis. Instead of counter rotating rudders on a flexure linkage, what if the front rudder locks itself?
In this scenario (see attached diagram) the board locks on a port tack (going to the left in the diagram) and is free to rotate on starboard. I didn’t think of this until I started writing, but you could move the locking notches (top and bottom) a few degrees counterclockwise and now your fixed board has a little AoA!
Each board would have a simple tiller screwed to it, and that’s it. Two moving parts—the boards. It eliminates (by my estimate) two (pitch) pivots with large thrust bearings (to prevent movement in Yaw relative to the flex rod), the flexy rod, and the linear bearing for the rod. Maybe we don’t need the linear bearing to account for the change in length of the rod when it flexes, but I think we might.
Because the top bearing only has to cope with one tack, we might be able to make it a little less sloppy. But then again, because our rudder needs to turn further we might open up the slop again. But I think we can get the forward one, that’s acting like a board, pretty well located.
I like that the “now board” can be raked more aggressively than the rudder to move the CLR aft. Notches could provide some pre-programmed “gears” for bearing away. Or maybe there’s a little bungee you put on like a laser to keep it up or down.
Now picture this. Dory canoe body (Tamanu-like). Simplest rudder with locking board. Fixed unstayed mast with boom-less gibbons rig. That’s a pretty simple proa. Two sheets. A barber hauler for downwind. Two “downhauls” for pivoting the yard. One halyard. Two one piece boards that tack themselves. That’s a pretty short list compared to a lot of proa concepts I’ve seen.
If the simplest rudder is going to be locked in one direction and steerable in the other then it may be possible to take advantage of this in a couple of ways -
Shape the foil to act as a leeboard when locked.
Arrange the slot so that the effective pivot point of the foil is closer to its center of lift, reducing the steering forces.
Of course, the slot in which it works gets a bit wacky….
I’m not so certain that the “weird shaped slot rudder” concepts actually are simpler. Fewer parts? Yes. Auto-locking on flow direction change? Yes. Simpler on an Internet forum? Sure.
In the real world, attached flow must be established for any of the weird slots to work. I’m sure most here will publicly admit to getting caught on the precipice of a shunt/tack that runs out of juice at the wrong moment, resulting in frantic tiller wagging, expletive practise and a sotto voce plea “I hope no one saw that”. I’ve sure been there.
I’ve been locked in mental combat with proa rudders for a while now. I strive for discovering that perfect, elegant solution that my predecessors have missed and so far every time I “solve” one problem I conclude it makes worse many of the other competing criteria for rudder success.
We ask many things of proa rudders:
* Rotate one or more vertical (more or less) blade(s) on a yaw axis to enable directional change for the boat
* Lift on that same vertical axis to allow for adjustment of lateral area for helm balance, as well as anticipation of hitting things.
* Able to deal with unanticipated hitting of things without breaking anything. This usually means some way of rotating out of the way on a pitch axis - but only when the impact threatens damage.
* Able to functionally handle working in two opposite flow directions without increasing drag
* Hopefully not be too ugly and make our Industrial Design gene segments look like the double helix became a monkey’s fist knot
* Be very effective at turning a boat that we really want to be directionally stable 98% of the time. When we are inside that 2% window of needing them to work, failure to turn the boat means trouble.
Sometimes complexity IS necessary, and that complexity is the result of evolution not the failure of evolution.
Just a different perspective.
—
Bill in Ottawa