Rudders to a Sharpie-proa?
I have been thinking back and forth about rudders to my proa.
Currently i use a simple aft-hung rudder attached with cable-ties, but its only to test my ideas. I have been meaning to try casset-rudders that i would attache to a protruding part of the leeward hull-side. I dont like side-hung rudders, as i consider them unsafe and very inefficient.
Now i want to test daggerboard-rudders like Russell Brown and Dick Newick use. I think they would be very easy to build and use in a sharpie-hull. I can place them next to the side of the boat, and that way build a vy strong box onto the inside of the hull. They would be a part of the hull and not use much space inside.
Should i place them towards the windward side or the leeside? The windward side could lift them partly out of the water if the boat is knocked down in a blow, and im afraid they would loose steering when its needed the most.
If i place them on the lee-side of the hull, im afraid the could be snapped right of if a giant wave would crash on to the hull from the windward side. They would be part of that very large lateral area to lee that would resist leeward drift or slip, and a such they would have to withstand enormous forces. Im i thinking right in this matter? Its hard to imagine every aspect of the dynamics around the rudders.
Johannes.
My thoughts are for a cassette rudder/board, placed on the lee side. Lee side will be most efficient, therefore the smallest area required. It has to be strong enough, but..
The cassette is not built solidly into the hull, it sits in a recess and the outer face of the hull and the cassette matches the outer face of the hull (perhaps cut out after it is built?) It can then be held with strong elastic and pop-out if you hit anything. It would also work well with a non-lifting 360 deg pivotting rudder. The whole cassette can be lifted in shallow water. Or separate dagger boards and pivot rudders (linked?), allowing the pivot rudder to be much closer to the stern, so more effective and smaller.
Mark
The cassette is not built solidly into the hull, it sits in a recess and the outer face of the hull and the cassette matches the outer face of the hull (perhaps cut out after it is built?) It can then be held with strong elastic and pop-out if you hit anything
This is an interesting take on rudders for a proa.
Thanks! I will have sketch some different ideas around this.
I am leaning very much towards dagerboard-rudders like Dick Newick and Russell Brown uses in their proas. I think a daggerboard-rudder is very easy and simple on a sharpie, as you place the dagerboard-box next to the hull-side inside the boat. It will not use much area at all, and i think it will be plenty strong. The side of the hull will probably bee built out of 3/4 inch plywood and several layers of glasfiber and epoxy.
I will probably reinforce high-stress areas like the daggerboard-boxes with additional plywood and glas+epoxy.
I have been trying to shunt my little model-proa with external cassette-rudders in my head, and i think its a bit complicated. I dont like the idea of having to walk all the way out to the bow of the boat. I think daggerboard-rudders is much easier.
Johannes.
I’ve also become a fan of the Brown/Newick dagger rudders, mostly due to Sven’s explanations here on the forum and also that they are well proven on ocean passages. Placing them next to the hull side will open up space in the ends for a berth or whatnot.
Should i place them towards the windward side or the leeside?
From what I’ve read, the Pacific islanders would place their steering paddles on the leeward side of the vaka, apparently they are more effective, perhaps due to higher pressure, cleaner flow or that they are buried deeper when heeled, most likely a combination.
If i place them on the lee-side of the hull, im afraid the could be snapped right of if a giant wave would crash on to the hull from the windward side. They would be part of that very large lateral area to lee that would resist leeward drift or slip, and a such they would have to withstand enormous forces. Im i thinking right in this matter? Its hard to imagine every aspect of the dynamics around the rudders.
I would think that if a giant wave moves the boat laterally, it won’t matter on which side of the hull the rudders are mounted, the water will grab equally well? You may get away with smaller rudders than say, Pacific Bee, since you are relying more on the chines for lateral resistance. Smaller rudders = less force, both good and bad.
Rudders are probably the biggest headache in proa design 😉. For what it’s worth, here’s MY thought process.
I would mount them on the windward side @ 25% back. get the rig far enough to windward to leave them both down most of the time, super simple shunting. easy access from the nearby cockpit with short tillers/linkages.
hardest part in my head is cleanly supporting the mast without stays to bump the yard on my junk sail;-)
I’m sure to leeward is more efficient for the reasons stated, but then I’m running linkage all over the deck, and if I do want to raise/lower I have to leave the cockpit…...every shunt?
I love The brown/newick cassettes, but they can’t kick backwards and take up precious volume on a smaller proa.
Tom
I wonder if there is any mileage in combining the ‘cassette’ design of kick-up dagger board used by Gary Dierking with the approach of Harmen Hielkema in which a dagger board is raised and lowered to steer the craft. A sketch of the idea is shown below.
This shows a yellow dagger board which runs vertically in a green U-shaped cassette. The slot of the cassette is closed by a dark-blue protective cover plate held in place by a bungee cord or similar. This cover plate can be pushed out of the way if the dagger board strikes an obstruction in either direction but will return to place when the obstruction has passed. The shape of the cover plate can be tweaked to make it blunter or sharper as desired. A dagger board which moves freely will tend to float upwards within the slot and this effect can be reinforced by a bungee cord. The vertical position of the dagger board is set by the red downhaul line which runs over a sheave and off to the cockpit. The dagger board, its bungee cord and cross-pin can simply be lifted out of the slot after the downhaul is detached. The whole unit is self-contained and one would be attached to each end of a ‘sharpie’ hull. This allows investigation of shapes, sizes and aspect ratios of the dagger boards as well as simple repair in the event of a crash. Although shown here with a vertical stem/stern it could equally well have a raked profile.
If both dagger boards are extended by the same amount they resist leeward motion (to a degree which is fully adjustable). If one is extended more than the other then the end of the boat with the more-extended dagger board will tend to turn to windward. The steering inputs will have maximum effect because the dagger boards are as far as possible from the middle of the hull. I suspect such a system will not have any ‘feel’ – with nothing to show that the dagger boards are working correctly except the heading and the lack, or otherwise, of leeway.
This is potentially brilliant.
It would work very well for a fast proa that would rarely sail with the wind aft of the beam or dead aft. This kind of steering doesn’t work very well on a run since the side forces required for steering are absent.
A pic or two showing the daggerboard in different positions, or with the cover plate pivoted opened in the event of hitting an obstruction might be helpful? You can upload more than one attachment by clicking Preview Post, BTW.
It would work very well for a fast proa that would rarely sail with the wind aft of the beam or dead aft. This kind of steering doesn’t work very well on a run since the side forces required for steering are absent.
I hadn’t considered that…
Rather than bolting the cassette hard to the ends of the vaka, maybe mount it on a cylindrical (or conical) end surface as Skip Johnson is using on the Bionic Broomstick?
I’m probably not understanding the quote but are the steering forces not produced by the fluid flow over the curved surface of the dagger board? Why would these be absent in a run? I hereby claim my right as a newbie to ask dumb questions. :o)
I think there are two different (but mostly agreeable) concepts under discussion: pr1066 is varying the lift to windward created by asymmetrically shaped daggerboards at either end of the vessel to cause a turning moment, while the Ed is shifting the CLR relative to the sailplan’s CE to cause the vessel to turn. Buth work, just in different ways and at different times: lift is only created when the boat has forward motion while changes in CLR can be used to turn a boat at rest, which is important in a shunting craft for example.
I think that any board creating enough force to steer the boat (one way or the other) will be a real pain in the behind to adjust whilst loaded underway and doubt that a bit of shock cord will do the trick, but I like the concept.
I’m probably not understanding the quote but are the steering forces not produced by the fluid flow over the curved surface of the dagger board? Why would these be absent in a run?
Steering a sailboat by adjustment of wind force resultant (Center of effort, CE) vs. water force (center of lateral resistance, CLR) is a fascinating subject. The attached diagram shows a boat with a fixed mast and a centerboard that is able to slide fore and aft along a central slot.
The boat on the left is beating to windward, the CLR is forward to properly balance the CE. CLR shifting works very well on a beat.
The center boat is on a broad reach, the CLR is shifted aft to balance the aft shift of the CE resultant.
The right boat is on a run, and we can see that the CLR would need to move infinitely aft in order to balance the CE. CLR steering ceases to be effective.
...changes in CLR can be used to turn a boat at rest, which is important in a shunting craft…
Will the change in CLR from raising one board and lowering the other be insufficient for this purpose?
It is probably worth mentioning that my thinking on all this has been colored by my target which is to build and sail a schooner-rigged beach proa of modest length (16’ plus the two cassettes) which, thanks to Luomanen, may well finish up very Scampi-like and maybe even a solo-tourer. I’m attracted to the two-mast solution because it keeps major forces from the masts and akas close together and resolved in nice strong bulkheads at each end of the cockpit. It also allows steering when the proa is stationary. The forces providing lateral resistance are, admittedly, at the farthest ends of the vaka but nothing is perfect…
Will the change in CLR from raising one board and lowering the other be insufficient for this purpose?
I believe it will work beautifully, in fact… especially when used in conjunction with a schooner rig. My only intention was to point out that there were two different theories being discussed, and they are not [necessarily] exclusive of each other. Varying degrees and directions of lift along with altering the balance of effort and resistance can all work together very, very well in my opinion. Sailboards are probably the best example of this…
Happy holidays
Will the change in CLR from raising one board and lowering the other be insufficient for this purpose?
I believe it will work beautifully, in fact… especially when used in conjunction with a schooner rig. My only intention was to point out that there were two different theories being discussed, and they are not [necessarily] exclusive of each other. Varying degrees and directions of lift along with altering the balance of effort and resistance can all work together very, very well in my opinion. Sailboards are probably the best example of this…
Sailboards are a very bad example for this, the only surfboards that are steered by shifting of the CE of the sails are the really old ones of the 1970. Since they grew up and became faster, steering is done by canting the board with the feet.
Iam sure this will not work properly. Perhaps you can steer a proa on the open sea using the forces of CLR and CE of sails, but as soon as you have to avoid a crash with another boat or entering a tight habour you will see, that this system responds by far not fast enough to change direction.
The next thing is that you have to realize that the centers of efford on a boat are not fixed, they are moving a little. If a puff hits the boat, the boat speeds up and the sail is deforming under the pressure of the wind, both effects change the centers. Imagine a hard puff lifts the ama and the boat wants to go into the wind, but you want to steer off the wind to bring the ama down to the surface to avoid a capsize.
If you want to steer straight ahead, you will constantly trim both centers ... I cannot imagine that this is comfortable and/or effective to steer. Or think of steering it in big short waves. And last you have to have a buttersoft sliding of the “dagger” in its case up and down against the side pressure of the water ... no easy engeneering.
But the idea itself is ingenious. In this open slot there can be a “traditional” Newick/Brown Daggerrudder. The biggest shortcoming of the daggerrudder is eliminated. It can kick up now if hitting the ground.
.
But the idea itself is ingenious. In this open slot there can be a “traditional” Newick/Brown Daggerrudder. The biggest shortcoming of the daggerrudder is eliminated. It can kick up now if hitting the ground.
This was my thought to when i saw pr1066 rendering.
I really love this very simple and efficient way of making the daggerboard-rudders able to kick up if they hit something. If the rudders would be slightly canted forwards they would not be sensitive to sucking down air when the aft part of the vaka is lifted by waves.
I have been thinking a lot about this kind of daggerboard-rudder for my sharpie-proa.
Thank you pr1066 for a great idea!!! I think this is a brilliant solution for rudders on a shunting craft!
Cheers
Johannes
Been away for the holidays for a few days, this is almost like finding an extra present under the tree.
I had been planning on having the whole end pop up but pr1066’s idea is much more elegant. The Bionic Broomstick’s a little too far along to change cassettes in the middle of the stream but once I whack one hard on something there will probably be a new end piece or two.
Regarding control running downwind, having a good controllable rudder is ‘just’ barely enough in some circumstances, an extra sweep (oar) can be awfully handy. A more static system of changing the draft of daggerboards might work but it looks like a system that’s begging for a broach and Murphy is sure to oblige.
cheers,
Skip
The right boat is on a run, and we can see that the CLR would need to move infinitely aft in order to balance the CE. CLR steering ceases to be effective.
I disagree. If you were to sheet the sail in a bit so that the thrust vector from the sail passes ahead of the CLR, the boat will bear away. You would steer just by using the sheet. The direction of the sail’s thrust vector is independent of the wind direction.