Proa steering

Hinges

When the rudder is placed between hull (near the hull) and ama the drag inducing hinges and slider can be placed above water level. The rudder will have to rotate towards the helmsman then, a bit unnatural. Will try to produce a more clear drawing but am a bit cad tired at the moment.

 

Sigurd, I have experience on two very different craft both using a tandem rig and the relative sheet pressure has much less practical effect on boat speed than you might imagine. The first example is a self designed tandem windsurfer where there is no steerable rudder fitted at all. Yet by getting the longitudinal balance correct by having the appropriate sizing and positioning of daggerboard and rudderfin, and being able to lower and lift daggerboard when changing from a beat to a reach to a run, on the fly, allows the hull to have almost as much maneouvarability and directional control as a regular large dinghy including fairly quick tacks and slowish gybes. However fine direction adjustment while on a course is easy by using relative sheet pressure alone. The two sails are equal and it is the front one that does more trimming to stay on course. Doesn't seem to slow things at all. The rear sail is the one that initiates a tack by sheeting on hard while the front sail eases. After passing thru the eye of the wind the front sail is sheeted on quickly and hard for a few seconds on the new side, to fully turn the hull onto the new tack. Then both sails are rapidly optimally trimmed and the boat accelarates quickly up to max speed. It is the most fun I have ever had windsurfing, even better then high speed blasting on a short board in the ocean.
Sigurd my second experience with tandem rig was sailing with Rob on his Elementarry. I found it had some very good design points but also a few not so good. (With all due respect to you Rob, for your vision and fantastic ability to make a modern proa concept work.) However small sail trim differences from optimum I believed had almost nil performance influences once up to speed. It likely could be a steering method also in part, but having two rudders fitted, of which the front blade was mostly raised clear of the water did not allow us to properly test this theory, as naturally having a tiller in the hand you naturally relied on this method of steering.
Seaspark, please elaborate on your method of steering with a clearer diagram and better description. It might have promise.
Rob I will send you a PM about the progress of the tri as it as a fairly long story and well off topic for this thread. I still await fairly analytical comments on my diagram. Regards to all proa enthusiasts.
Sam

 

hi Frosh, the rudder looks like some rudders on Yankee catboats. They've been around for a hundred years or so, so there must be something to it. It is interesting to feel the difference between flow deflection and lift with the setup. There is a delay in reaction, so you have to think ahead some. Subtle, but fascinating. I made one like it to steer a short board, hooked the tiller to a bungie chord, tried to steer with my foot. Egads. How long do you make the trailing arm? Hmmm? Hmmmm!

Paul

 

Powered shund

Inspired by Frosh's central pivot i present the powered shunt. She "servo" rudder was dropped in this drawing, may still have potential.

A) Going to windward ruddercase (not drawn) is pressed against the hull by lateral forces. The rudder is a fin type, the boom connecting ruddercase to the central pivot point does not touch the water.

B) Starting shunt. When boat has decelerated a bit you start pulling (1) the "shunt stick", this breaks (like in handbrake) the proa, the "paddle" action of the rudder helps the pointing of the new bow in the new direction.

C) You can start pulling (2) the telescopic tiller to complete the shunt and help accelerate the boat in the new direction the "shunt stick" is not needed anymore.

D) self explanatory i hope

E) Here we go.

This system can only be used on small proas, have no visions of hydraulic shunt sticks yet. Mast and other rigging cannot be mounted at the centerline of the proa.

(edit: sorry for the typo in the title)

 

Picture

3d picture of the basic elements of the powershunt rudder.
Not all objects are to correct scale and i know the end of the boom will collide with the hull when shunting, it will have to be bend,tilted or the whole construction will have to move up. Notches (English?) on the hull will fix the rudder in place when the lateral forces push on it. Among the things that worry me is the length of the tiller.

(edit: also the tiller is missing, tiller extension is drawn directly on balance part of rudder won't work)

 

SeaSpark- the misspelling could have been worse! Your idea of the rudder pinned against the hull raises a question Ive been trying to answer- If a rudder or leeboard, say, of ogival section with a flattish hull side fit flush, or is pinned to the hull, does it experience less drag than a symmetrical rudder fitting flush (-ish!) to the hull? Is this drag the same as a bump on the hull? How much does a bump on the hull aft of the point of max beam slow things down?

Like the lateral thinking.

Paul

 

Drag, steering

Can't answer your question about the extra drag but am curious about the answer.

Another aspect bothering me about my proposal is the directional behavior of the proa when pushing it through the shunt with the rudder(now paddle). As the rudder cannot be used to control direction. Guess this will be hard to answer until build in practice. Anyone making a guess is welcome.

 

Leeboard style rudder drag

It is pretty much beyond dispute that leeboard type daggerboards, or in this case a rudder creates a number of inefficiencies compared to a blade through the hull with a close fit to the hull skin. Turbulence is huge, and the blade will definitely flop around as the side pressure varies as it naturally will, under normal sailing conditions. This will be an additional source of huge turbulence and probable changes in angle of attack to the water. A reasonable idea for a practical solution, just as my central shaft mounted system could be a practical nightmare. However the efficiency produced (read performance) should be streets ahead in my design, I think, but it needs testing to make sure. Also the engineering and construction of my design needs to be of the highest order, to withstand forces that will try to break it away from the shaft.

 

Frosh, your throughhull thing, is it a rudder or locked board? If a rudder, it is *heavily* underbalanced! How can you hold that? And, pointing anywhere but fore/aft it will loose the endplate of the hull.
If it's locked, how will you shunt it? lots of blocks and rope? lock it before you sheet on?
If it's locked anyway, I think I'd rather have two db's flippin up like on longboards - safer when grounding and maybe easier to make strong?

Btw, look at all the free development classes without throughhull rudders.
I don't remember too well the things I read about foils close to the surface (some in Tspeers paper on foil configs).
I guess that usually rudders don't suffer as much from the extra induced drag without the endplate, as would a dagger-rudder.

 

Leeboards

Any other options?

 

On our "Mi6" proa http://members.optusnet.com.au/mal.smith1/mi6.html, we used two side hung rudders which were linked together so that they turned in opposite directions. The rudders used a bi-directional section shape so that they would work in either direction. We hung each rudder blade on a transverse axle which in turn was connected to the rudder shaft. The idea was that the rudders would be able to swing on the axles to trail aft a few degrees so as to keep them balanced correctly with slight 'lee helm' type loading on each tack. We were relying on rudder drag to swing the blades aft on each shunt but we found that even though we had pretty good bearings on the axles (rulon bearings on polished stainless shafts), the rudders still had to be shunted manually. If left, the rudders would swing aft by themselves eventually when the speed was high enough, but that wasn't soon enough for smooth uneventful shunting, as steering was very difficult until they did swing to he new aft position.

However, we eventually found that the rudders worked perfectly well if we just fixed them in the vertical position and didn't bother to move them on each shunt. The rudder loads were low and manageable because having them linked together in opposition meant that the turning moments tended to cancel each other out. The only thing to get used to is that using this system, weather helm feels like lee helm and vice versa.

For this linked, centre pivoted rudder system to work well, the loads on each rudder need to be fairly similar i.e. the boat needs to be balanced about the mid point between the two rudders on each shunt (close to neutral balance). The Mi6 is essentially an "Atlantic" proa with the rig on the windward side and is inherently well balanced, so the system worked OK. However on a pacific or Harry type configuration, with the rig well to leeward, getting neutral balance will be more difficult, requiring either the rig or an additional leeboord to be moved on each shunt. This might still be easier than moving the rudders though, especially as during the shunt is probably the time you need the rudders the most!

Mal.

 

drawings

Hoi Mal,

Thanks for your considerations and the detailed information on your site. I found that the link to "rudder linkage conceptual drawings" http://members.optusnet.com.au/mal.smith1/mi6/Rudders.html did not work.

Jeroen

 

Single rudder idea

Sigurd, you are probably right about the heavily underbalanced foil. It is a rudder and not designed to be locked at all. It won't probably work on anything large, however my interests are in lightweight off the beach proas and tris.
I spoke at length to Rob Denney who had just read my postings and saw the diagrams. His main criticism was how do you get it up for launching and retrieving the proa? He made no comment regarding huge loadings or needing additional purchase to exert sufficient force. My many experiences in strong and medium winds, with my tandem windsurfer, (20ft. long and shaped like a hard chine very skinny racing dinghy), is that most of the time you can sail with neutral helm and make fine adjustments via the two sails. It must be neutral on my windsurfer as there is no movable rudder at all, and knowing that, any unbalance would mean that I could not maintain a straight course, which is NOT borne out in practice.
As far as losing the endplate, this would only happen infrequently, as I would think that tiller movements should be minimal 90% of the time. During shunting you would need to wash off a lot of speed before initiating big tiller movements as the loads could be huge, if you tried to crudely push or pull the tiller at high speeds. As I said before, nothing less than vacuum molded carbon, and great engineering on the shaft system.
Rob Denney also gave credence to the idea of endplate effects being good, but said he personally didn't bother because of practical problems to raise the thing, which for him is quite important. He and I also agree that most of the time a forward foil in the water serves no purpose and creates large amounts of turbulence=drag. It is again a practical solution for a shunting proa, and he likes to lift the forward foil clear of the water straight after a shunt. This means that during each shunt, one foil needs to be lifted, and the other foil lowered, quite an effort in practice. I noticed, and Rob admits that even the rear foil, working off the crossbeam, and not under the leeward hull, creates lots of visible turbulence.
All in all I think my idea still has a lot of merit for a small, light proa, but I stand to be corrected after a practical trial. Unfortunately I am not at this time able to do it, as I am heavily involved in finishing and then learning to sail efficiently, my experimental tri. For info on this see the thread :
http://www.boatdesign.net/forums/showthread.php?p=139762#post139762

 

Thanks Jeroen. The link has now been fixed.

Mal.