Proa steering

Steering

Bob,

I must be missing the point. How does the outrigger provide steerage? Getting back to SeaSparks design, I think he is interested in simple operation, as opposed to simple design, which would be nice too.

Tom,

I guess I should have included smileys after my negative leaway statement. Thanks for the input though. The sailor's holy grail: sailing dead to windward.

 

Collective

As Tom mentioned there are other threads that go into the way a twin foil collective system or gybing board works. Collective works very well just not exactly as most people think it does. The hull does not generate leeway resistance and moves virtually straight thru the water when collective is used and the sails can be eased very slightly.The whole "normal" boat moves at a leeway angle thru the water. Comparing two equal boats both equipped with twin foils and collective "steering" and one not using it the non collective boat will appear to "point" higher but the collective boat will have better vmg. Use of collective on CBTF leadbelies is a proven race winning technology and in close tactical situations the boat can actually move sideways for short periods if desired.
This illustration may help: draw a line coming toward you(wind) and a line going from close to you out from the wind line 45°(course line). Now draw a little boat viewed from top with it's centerline at the bow 5° to windward of the of the course line and crossing the course line at the middle of the boat. Now just ahead of the first little boat draw another boat but put it's center line on top of the course line. Draw two small rudders at the front and back of the second boat angled 5° to the course line.
That is an approximate illustration of the difference between a "normal" (first boat) and a boat using twin foil w/collective. The practical benefit on a canting keel boat is less drag and better upwind performance; it would seem to me that there should be some degree of similar benefit on a proa.

 

LP, I'm sure that Bob is referring to a tacking proa, usually called an outrigger. Therefore it has a permanent bow and a single rudder at the permanent stern. Sort of a catamaran with unequal hulls and the rig mounted directly onto one hull.
While on the topic of leeway and twin rudders I have grave doubts that twin rudders even with collective steering will actually improve VMG to windward!
My thoughts are maybe a 2 degree improvement in pointing angle and a 5% plus speed loss due to various additional drag forces. :?:

 

Twinfoils/collective

Frosh, as I tried to illustrate above "pointing angle" is NOT improved using collective-but speed is. The use of this technology on leadbellies is well proven in racing...

 

I am working on the design of a proa for the Dutch inland waterways.

Focus is on portability (the reason why a choose a proa type) and ease of use. I am looking for a proa steering system that needs as less as possible user interaction when changing bows. Potential buyers may be scared of the proa concept so i want the steering system to be very easy to use.

Any suggestion is welcome.

Seaspark,

Good idea. Proa rudders are a lot of fun. I have tried about 30 combinations over the last 10 years, including all those mentioned in this thread. 2 x 180 degree foils mounted in the hull works, but there are safety and ease of use issues. A better system is what we have on Blind Date, a 15m harryproa on the Ijsselmeer. see http://www.harryproa.com/blinddate.htm These have no holes below the waterline, work in both directions, can be lifted and steered and kick up in a collision. On my 7.5m test boat http://www.harryproa.com/Elementarry/SailingPhotos_4.htm I currently have a single rudder which I move from end to end. Works well, although there are some low speed issues, and the end for ending mechanism is not yet perfect. Unaided, it takes me 20 minutes from trailer to sailing on this boat.

Harryproas track very well, almost regardless of the rudder set up.

(Sam) Frosh pretty much summed up my experiences with two rudders. They are more drag and work than they are worth on the little boat, although they have some benefits for maneuverability, particulalry after a shunt when speed is low and luffing tendency high. On the big boats, they are necessary due to the hassle of end for ending them.

Doug's collective steering may work on keel boats where speed is limited by hull length and they have excess power. Did not seem to work on any harryproas which are long and skinny so can use all the power available. Having the foils visible shows up the added drag and turbulence from steering with them very quickly. It is not fast sailing with one rudder having weather helm, it would be even slower with 2.

I do not agree that they are well proven. No dinghies, RC, America's Cup, Volvo, Open 60's, minis, Orma 60's or any other multihulls use them. A few boats with canting keels do use them, but there are so many related issues that it is a big call to attribute their success to the steerable front rudder.

regards,

rob

 

Twin Foils/proas

Thats an interesting take on twin foils ,Rob. Thanks to you and Sam for relating your experience.
There are many sources of biased info on the use of twin foils on CBTF boats and the results attained by boats using them including the CBTFco website, the Reichel-Pugh website, the CBTF thread here and some relatively unbiased info from the likes of Russel Coutts on his website and some others ,I imagine. Their use on multies is very interesting to me- particularly on cats- and I've been wanting to build an rc cat to test the idea. I have built numerous rc leadbellies where the system has worked well.
I'm surprised at your results but I appreciate you sharing your thoughts on the subject.....

 

Oh, the leeway angle can indeed be negative. It's just that the boat's heading won't be the same. Basically, in the long term the boat rotates about the foils, not the other way around.

 

First hand information.

I want to thank Rob Denney (and all the others that replied of course) for sharing his information with us. To me this is prove of how valuable boatdesign.net is.

Jan Schippers is offering his Blind Date for charter so i will be able to get some first hand proa experience! Have never seen a proa on Dutch waters so this a great opportunity.

 

Sharpii2, Your suggestion of a proa including large ones with a central keel and a spade rudder at each end deserves further comment.
I have never experimented personally with such an arrangement but surely others, maybe Rob Denney has done so.
My thoughts on this are that the front rudder which is free to move would oscillate, ventilate, and cause a relatively huge amount of turbulence adversely affecting speed as well as steerage.
Retraction of the front rudder fully into the hull would be a desirable aspect of any proa design, but the method is cumbersome on small proa where a person would have to move to the rudder and manually pull it up. On a large proa the manual method is not even practical, so some type of mechanical (perhaps electric motor driven) method of lowering and raising the rudder blade is required. This is a difficult design exercise but must be possible.
I don't think that a practical solution is to just leave it to "free wheel".

 

I have thought of that.

In one of my design sketches, I drew a rudder blade that pivots fore and aft about 15 to 20 deg on its rudder shaft. The boat itself was to have small transoms at each end. Each transom was was to have a fork like projection to grab the blade on the forward transom and lock it straight. Due to drag, the stern rudder blade would tilt well aft this fork like structure on the stern transom.

Now back to my original concept (I LOVE arguing )

A well designed spade rudder should not oscillate at all with a free helm. Whether or not that is actually the case is another matter. I would imagine that with a careful foil section design and a proper amount of ballance, this can be made so.

But now another thought has occured to me. :0

Who says it has to have two rudders?

Why not just have one rudder. One that spins around, as I previously suggested, when the boat shunts to then become, alternately, a bow then a stern rudder. By avoiding the temptation of putting the rudder at the very end of the boat, the worst problems with bow rudders, such as turbulance, aireation, and then shimmy, may well be avoided. Then you only need one helm.

For starters, with this scheme, I would put the rudder shaft at about 25% of the WL from the end of that WL. I would then eschew the temptaion to have a retracting board, but have a shallow fixed keel instead. The rudder would project down about 75% of its depth. Then I would go nuts on the shaft strength specification, so it would take more than a minor mishap to KO the rudder.

Just some more thoughts for ya.

Bob

 

G'day,
I have sailed a variety of proas with two rudders, some with and some without daggerboards. Leaving the front rudder free to rotate is added drag for no lift, so is not recommended. However, of more interest is that when left alone, the front rudder will adopt an angle of attack slightly more than the leeway angle, presumably because the sideways load is applied at the middle of the rudder (aft of the shaft), and the forward speed is rotating the rudder round the shaft. Anyone with better theories, please say so. Regardless, the upshot is that the boat tends to bear away, occasionally quite dramatically. I would not recommend it.

Lifting the front rudder is a better bet, but is hard work. A single rudder is the ultimate solution and works very well on my 7.5m/25' proa. The sailing proa in my last post had a single rudder, no daggerboard. Worked well, albeit with some peculiar slow speed attributes.

I do not like foils fixed fore and aft (dagger boards or fixed keels) as they are draggy and tend to stall during a shunt. Two large rudders work much better, but need to be pretty solidly built.

Regards,

Rob

 

Rob, if you only need a rear rudder, couldn't you do something like this? The red dot is a universal joint like a windsurf mast foot or something. I didn't bother drawing the other rudder, it is mounted in the same way and will be, with some clever geometry and counter-acting bungies, pulled up automatically. You'd have to give the new rear tiller a kick though, until the tip of the rudder enters the water, from where the drag, and a bungy (getting more leverage the farther down the rudder is) pull it down the rest of the way.

This is based on the assumption that moving the rudder and putting it into the new trunk can be an undesireable task though. Perhaps that is no bother.

 

Hi Sigurd, strikes me that a rudder about half way between main hull and float is an extremely inefficient method of trying to steer, and might not even work at all. My thought is that the best position is under the main hull very near the stern, followed by being hung from the stern itself. (second best). Rob Denney's proas usually have the rudders mounted immediately alongside the main hull towards each end, (but not very close to the end) and this is clearly less efficient that than better positions that I mentioned already.
Perhaps Rob would like to comment on your proposed idea.

 

G'day,

My rudders are 500mm/20" inboard of the hull. They work well. No reason why they could not be further inboard, except that when you want them most is just after a shunt, when speed is low. They would add more to the luffing force the further to windward they are. Your geometry looks ok , but there are a whole lot of details which need sorting out. Automating the kick up using bungy is a good idea, but also needs close attention to details.

Mounting the rudders at the ends of the boat make for better steerage, but make balancing the boat mre difficult, especially if you have a fixed water foil as well. Lifting or locking the bow rudder is also not as easy to do as it is to write. We have one or two oversized rudders, no other foils. Cheaper, lighter and more efficient than any other set up.

Most proa rudder systems work to a greater or lesser degree in their primary role as boat steerers. The holy grail is still a single rudder which can be raised under sailing loads and will kick up in an impact. This has yet to be achieved, but we are getting closer.

regards,

rob