Simplest Proa Rudders
That’s an interesting thought about moving the lower staple to below the water surface. It involves a little more drag but could also limit cavitation. But I’d be guessing about that. Of course, the further away from the top staple you move the bottom one, the more the rudder head (and tiller) has to move f&a.
The rudder does not move very far - just enough to locate an edge in the restraining slot. I think the best solution would be to put the restraining slot in the bottom staple; this will minimise the extent to which the tiller and the top of the rudder have to move f&a. The further apart the staples are the smaller the angular travel of the rudder and the less the rudder head moves.
A possible reason for having the restraining slot at the top staple is that it would be more convenient for rigging a sheet to pull the ruddder out of the restraining slot so that it could be used as a rudder (in emergency or for low-speed tight manouevring).
I’m not sure that I can claim credit for moving the lower staple underwater - Dave Culp talks about the lower staple being immersed in #18.
“Have nothing in your house that you do not know to be useful or believe to be beautiful.” William Morris
Have you ever tried to get rid of a ‘plus-one’ guest on that basis ?
:o)
modeled up “simple rudders” for Herbie. This is a take on Chris’ rudders, being non-linked, steer only with aft rudder. Herbie’s hull extensions naturally function as “rudder holders”, and I love the idea of using negative space as the cam, talk about less is more! There is a high density plastic top and bottom cam, the top cam has a “keeper slot” to hold the rudder in place as the “forward” rudder. The white disc on the rudder keeps the thing from falling through the slot. I would use bungees and small control lines to make sure the rudder stays firmly in/out of the keeper slot.
Come to think about it, Michael, the unlinked simplest rudders are a really great fit for Herbie in two ways. One, you have the real estate in your Sampan overhangs. Two, the schooner rig puts the rudders and boards in the right place for the sail.
I was thinking of them in the context of a fixed mast, boom less Gibbons set up. But the CE is aft of the midpoint length of the Vaka, and the fixed board is ahead of it. Not so good. But your fixed board could cancel the forward rig on upwind courses, and you’d essentially be steering with the aft rudder and rig—the same way you balance a sloop with the mainsail.
Clever!
I love to see these kinds of builds here.
BTW, I do not believe the fixed rig is *better* than Dave’s flexy rod counter-rotater. Just another idea.
I only have one worry about these rudders- the strength of the rudder material, versus the leverage that may try to push on the rudder itself. In other words, will the slots/frames dent the wood of the rudder if you make them too large? It isn’t something that’ll stop you from making the rudder, it is just something you have to take into account when making it, I would expect.
I really like the one piece bendy tiller idea. It might be a good thing to use with ‘properly’ hinged (centre hinged) side hung rudders, because the return spring effect would help to counter the overbalancing tendency, so there may be no need to move the rudder fore and aft on each shunt.
This is a very good point, Mal, standing alone. It occurs to me that, as someone else suggested, where this bendy tiller bends—including where it is pushed by the skipper—will effect the relative angles of attack of both rudders. Perhaps there is a geometry here which will completely eliminate the over-balancing issue?
Imagine a rudder with a tail fin, like say the sail from Saildrone scaled down, turned upside down and hung under the boat. There would be two of them, one forward and one aft. Each rudder is steered by its tail fin and the tail fin linkages are connected so that they act in opposite directions. These rudders will only be able to produce a turning moment, they will not (as a pair) be able to resist leeway, so their sole function is to steer the boat. Leeway would have to be prevented by some other means.
What’s wanted is some form of “collective” in order to get the rudders to both steer and to provide leeway. The big monohull canting keelers in AUS use this in order to get more lift off their front “rudders.” By collective, I’m talking about something like a V-tailed airplane uses.
It also reminds me of an old scheme to overcome the very strong weather helm in monohulls such as old-fashioned catboats (not catamarans) Both the placement of the mast and the very wide hulls cause these to have murderous weather helm, and huge “barn door” rudders to overcome it. Fundamentally poor design, but…
What was done was to mount a tail fin on the rudder, just as you describe, including a 90 degree control horn. The horn was connected by a rod right to the transom itself, so the tail fin becomes self-actuating. The fin size and control horn length can be altered to get the amount of rudder force wanted.
Dave
I’m not so sure about the third ‘staple’ - I’d be concerned about misalignment and binding. Why not use the locking slot in only one of the staples - the other would be simply a butterfly slot for steering which would locate the rudder fore-and-aft. When the boat is shunted the was-rear rudder would pivot aft in the non-locking staple and lock in the other one. No alignment needed…
You’re right, Peter. Aligning three of them calls for accuracy. But there are ways to jig them together and bond them to the hull as unit. More fiddly, for sure, but quite doable.
I think you guys are over-thinking things—a little. A middle staple, if wanted, can be aligned by first mounting the top and bottom ones, then installing the rudder and using it to align the middle one.
I understand the desire to have the locking slot in the bottom staple, but I think I’d rather have it at the top, so I could affirmatively see whether it’s locked or unlocked. TheEnemy makes a good point here as well—this little slot is going to be highly loaded; will the rudder fail in this small area? (Imagine grabbing the rudder’s edge with a big pair of pliers and rotating forcefully. You might just tear out a “jaw-full” of rudder leading edge)
Having gone this far with the discussion of locking slots etc—some very clever stuff! I have to say, I don’t favor locking the front rudder at all. 99% of proas need to get their CLRs to migrate aft on the shunt. Locking a forward rudder to align with the hull does just the opposite.
Now, if you locked that rudder to align with the hull’s course rather than its heading (ie: heading plus leeway), you’d have a better setup, but then again, leeway changes all the time; it’s a moving target. Leaving both rudders unlocked automatically takes care of this—you push the tiller sufficient to point the boat where you want and then, only if you want to, fiddle with the tiller to get the loads and centers where you want them. Great accuracy will make the boat faster, but isn’t required so long as things more or less balance.
Dave
TheEnemy makes a good point here as well—this little slot is going to be highly loaded; will the rudder fail in this small area? (Imagine grabbing the rudder’s edge with a big pair of pliers and rotating forcefully. You might just tear out a “jaw-full” of rudder leading edge)
Dave
The notch stops the rudder from rotating but not by gripping the front edge and twisting. The rudder is located laterally by the narrow waist of the slot. The rudder is prevented from turning about its long axis by applying a force at the front edge. This is provided by the edge of the notch bearing against the leading edge of the rudder - over a relatively large area. The forces being applied at the waist are also applied over a reasonably large area also.
The notch stops the rudder from rotating but not by gripping the front edge and twisting. The rudder is located laterally by the narrow waist of the slot. The rudder is prevented from turning about its long axis by applying a force at the front edge. This is provided by the edge of the notch bearing against the leading edge of the rudder - over a relatively large area. The forces being applied at the waist are also applied over a reasonably large area also.
It may be over a relatively larger area, yes- what I was worried about, is that the rudder has quite a bit of length, vs the ‘pivot point’ of the staples. I was worried about side force in general, especially when forced to do odd things by a gust, rather than ripping out the front when the rudder is locked in place. You don’t have that big ol shaft in the middle like a spade rudder, here, just the rudder material itself to resist bending and denting. (I"d probably just put a metal plate over the rudderblade where it sits in the staples, and be done with it, and never worry about it again.)
Imagine a rudder with a tail fin, like say the sail from Saildrone scaled down, turned upside down and hung under the boat. There would be two of them, one forward and one aft. Each rudder is steered by its tail fin and the tail fin linkages are connected so that they act in opposite directions. These rudders will only be able to produce a turning moment, they will not (as a pair) be able to resist leeway, so their sole function is to steer the boat. Leeway would have to be prevented by some other means.
What’s wanted is some form of “collective” in order to get the rudders to both steer and to provide leeway. The big monohull canting keelers in AUS use this in order to get more lift off their front “rudders.” By collective, I’m talking about something like a V-tailed airplane uses.
Sorry if this is a bit off topic….
A “collective” would be easy enough to do. The problem is, with a conventional fixed fin for leeway resistance the lift coefficient varies proportionally with the leeway angle, so a fixed fin is ‘self adjusting’ with regard to the amount lift it produces. OTOH trim tab type rudders will always produce the same lift coefficient regardless of leeway angle, because they adjust their AoA relative to the local flow. So while it is entirely possible to use them for leeway resistance, it becomes a bit of a manual process to adjust them depending on speed and heading. I thought it might be simplest to separate out the steering and leeway resistance functions to separate devices.
There is a way to automate the use of trim tab rudders for leeway resistance, you could use a ‘leeway sensor’ (basically a water vane) and feed the output of that into the trim tab control system, more complication, but again entirely possible.
Interesting info about the cat rudders by the way. I’ve seen a similar system designed for large ships.
Mal.
Having gone this far with the discussion of locking slots etc—some very clever stuff! I have to say, I don’t favor locking the front rudder at all. 99% of proas need to get their CLRs to migrate aft on the shunt. Locking a forward rudder to align with the hull does just the opposite.
Dave
In which case the whole device becomes really simple - a few sturdy components at (towards) each end of the hull and a flexible pole between them. The pole would have a bridle at each end - just a slot that’s a good fit on the rudder laterally and longer than the rudder is broad. Two identical staples. Two rudders. Control lines - downhauls, uphauls, bungees, whatever. The pole would sit above (directly on top of?) the top staple. The bottom staple might need a break-away or crushable capacity to cope with grounding.
In which case the whole device becomes really simple - a few sturdy components at (towards) each end of the hull and a flexible pole between them. The pole would have a bridle at each end - just a slot that’s a good fit on the rudder laterally and longer than the rudder is broad. Two identical staples. Two rudders. Control lines - downhauls, uphauls, bungees, whatever. The pole would sit above (directly on top of?) the top staple. The bottom staple might need a break-away or crushable capacity to cope with grounding.
Yep. Maybe I shoulda titled the thread “Really Simple Rudders.” 😉 But aren’t you missing that the forward rudder is still going to be unstable—unbalanced. Yes, cross-connecting them will help, but I still think you’re going to need some rotation on both rudders to get that forward one to balance.
Was also thinking that, on a small boat (likely candidate for simple rudders) that the one-piede long bendy tiller is going to be all over where the crew are trying to sit. Is there any reason the tiller attachments can’t leave the rudder canted outward (or inward) perhaps 5-10 degrees? This puts the tiller, when rudders are amidships, out over the gunwale instead of between your feet.
Dave
Yep. Maybe I shoulda titled the thread “Really Simple Rudders.” 😉
Got me !
But aren’t you missing that the forward rudder is still going to be unstable—unbalanced. Yes, cross-connecting them will help, but I still think you’re going to need some rotation on both rudders to get that forward one to balance.
Was also thinking that, on a small boat (likely candidate for simple rudders) that the one-piede long bendy tiller is going to be all over where the crew are trying to sit. Is there any reason the tiller attachments can’t leave the rudder canted outward (or inward) perhaps 5-10 degrees? This puts the tiller, when rudders are amidships, out over the gunwale instead of between your feet.
Dave
Sure - the front rudder is always going to be unbalanced. Can’t be avoided as far as I can see. The pole doesn’t have to come straight out of the bridle as drawn; it doesn’t even need to be straight. If the boards are to leeward the pole would be out of the way, too. On a really small boat (Bionic Broomstick size) the crew will possibly be out on a tramp or sitting on the ama. The SPR is not a universal solution but a real contender for small proas. A really neat thing about the pole is that it has a higher bending stiffness against both rudders going the same way than it has against them counter-rotating. In the first condition the pole has to take up a shallow S-shape and in the second it’s just a smooth arc.
Whilst i love the simplicity of this concept, my concern is that will it fail on 2 accounts:
- at speed it will shale & vibrate like crazy?
- there will be too much friction?
Mark
...the front rudder is always going to be unbalanced. Can’t be avoided as far as I can see.
Sure it can. That’s why I want to trail the boards back a little bit (tillers forward) after the shunt (maybe go back and re-read the very first post in the thread?). If the cheeks force the rudder to rotate vertically and they’re trailed aft, they *can* be balanced, so long as the trail angle is correct (it’s fixed, by the length of the trunk—remember that the original “simplest” version of this setup is a plain garden variety rectangular sectioned trunk, 160% as wide as the board and long enough to allow/force about 10-15 degrees of trail in the boards
The pole doesn’t have to come straight out of the bridle as drawn; it doesn’t even need to be straight. If the boards are to leeward the pole would be out of the way, too. On a really small boat (Bionic Broomstick size) the crew will possibly be out on a tramp or sitting on the ama. The SPR is not a universal solution but a real contender for small proas. A really neat thing about the pole is that it has a higher bending stiffness against both rudders going the same way than it has against them counter-rotating. In the first condition the pole has to take up a shallow S-shape and in the second it’s just a smooth arc.
Yup, you’re dead on. I agree with all this. The bridle you show can replace cheek pieces, though things fet more and more flexible.
Dave
Dave
That’s why I want to trail the boards back a little bit (tillers forward) after the shunt (maybe go back and re-read the very first post in the thread?).
Dave
Oops. That’s the trouble with these long threads - just ask Ariadne (or Arachne)...
:o)