Proa steering

[sigurd]

frosh, and rob, by no means did i intend the geometry on the paint drawing to be any sort of guideline. Just showing the principle of a very low friction and simple way to achieve both auto kickup of the front rudder, and, with a sacrificial "fuse" (short length of weak rope) in the forward stay, the rear would kick up if grounded. No reason you couldn't have the rudders close to the vaka. You might skip the rudder cases too, just pivot the board back or fore if in shallow waters.

Frosh, as it is only a single rudder and the hull resisting leeway, (as per elementarry) it would not make sense to have the rudder too far aft I think, since it would be beneficial for it to take a large part of the lateral load off of the hull.
Looks like rob is using about 1/3 on the vaka length on his little schooner.

[sigurd]

got a little idea by looking at rob's one-rudder-proa in his yahoo group's photo section. Perhaps the rudder kick-up mechanism could be linked to the sheet somehow, so that when the boom passes 90 deg one rudder is lowered and the other rised?

[rob denney]

Worth a try, but allow for accidental sheet release, say you were broad reaching and the sheet was released, either accidently or deliberately (say to avoid a nose dive), then the rudders would swap and you would be in trouble.

regards,

Rob

[sigurd]

Hi, here is another sketch, with double proa section rudders like the Speer P30xxx series. This time too the exact geometry is a bit off, it is ment just to show the principle of changing the pivot axis instead of raking the rudders to get them balanced.

[Paul Scott]

Hey all, here's an idea I have kept coming back to for over 15 years, but have not done. I wish I could draw this like Sigurd. Really Cool Sigurd. But, here goes-

On the main hull;

Base of the mast moves on a, say, jibtrack, for and aft.

A single ogival foil moves for and aft on it's own track, also for and aft.

On a small proa, my favorite gedanken experiment controls are;

a tiller for the foil,

and a line from the hull mid point for the mast.

Pros: The idea has some simple elegance. The mast shunts automatically, steering with mast/sail position straightforward and fairly intuitive. Steering proa with foil straightforward, uses tiller, intuitive, shunting of foil fast. One less foil in the water= less drag. Very little machinery involved.

Cons: details, details, details. for example, where to put foil? to lee gives quicker handling, but putting the tiller under the mast track is a bit awkward. I suppose you could use lines, but that is not very elegant. Foil to windward makes larger inputs necessary. It's an ogival foil, surface piercing. This is windsurfing thinking. So learning fast course changing and sail control with control might involve a steep learning curve.

There are a number of ways to control the mast/sail, all entertaining to think about, from bullet proof, to the rig falling down if you screw up. From rigidly upright to as dynamic as a windsurfer rig.

Paul

[sigurd]

Scott, Here are some pics of rob's rudder track.
http://au.ph.groups.yahoo.com/group/...os/browse/eb5d
there is also some recent discussion about rudders tracks in that discussion group.

[Paul Scott]

Looked at Harryproa site. Even though I have a Yahoo account, I couldn't get to the appropriate forum. I think now that I didn't express myself clearly. If you you look at the windsurfer Serenity, notice the location of the one dagger board, no skeg. What I am proposing is a track running on the edge of the proa's main hull, either on the lee or windward edge, running fore and aft, to the approximate relative position for and aft that the single Serenity daggerboard lives in, if the Serenity moved both directions fore and aft, instead of the one that actually goes. The foil moves on this track. The foil does not move like a rudder. Except for it's fore and aft movement, it does not move in any other way. The only thing that the moving leeboard/windward board moves is the center of lateral resistance fore and aft. My idea is that it is moved by what I call a tiller, but the tiller moves the foil for and aft only. So the foil moves the CLR, and the mast/sail moves the CE. Whether this would give enough manoverability for a canal is something I do not know. But it is simple, in some ways. Given a main hull that has the right amount of course stability, you might have something that works.

Paul

[sigurd]

I see now what you mean.
That sounds nice and simple, except maybe for the track construction details? the hull now has to be strong enough to hold the full force of the board all the way between the track ends.
I previously dabbled a sketch for a track with a "normal" symmetrical rudder trunk.
I am however not sure if UHMWPE can have a low enough friction against a (insert material here) tube that is large enough for the loads. It would be annoying to have it jamb all the time, best would be if it were dragged back by the water on each shunt.

[SeaSpark]

My proa project ground to a halt in the last few months for a number of reasons but i hope to pick it up again some (soon i hope) day. Will keep you all updated.

[Paul Scott]

Sigurd- the best idea I have, I think, is to have the foil attached to a spar set at 90 degrees to the centerline of the main hull like the seat of an IC that moves for and aft. The tracks are likewise fixed to the edges of the top deck. The foil curves smoothly from horizontal to vertical in a curve. You might need a couple of fences. The foil projects out to leeward a couple of feet to leeward, maybe a metre. The spar moves underneath and above the tracks for the mast, which are located directly above and below each other. The mast/sail rides on the top track. The spar is attached on it's top and bottom in the middle to both the upper and lower middle for and aft tracks. The spar is stabilized by 2 spars at each of the ends of the ends of the spar at 45 degrees or so and they attach to the track that the spar moves on, or even better, the bottom mast track. Each of these spars is stabilized back to the the IC type spar, in line with the mast tracks, forming two opposing triangles. I think this would give the location of the mast/sail a great degree of freedom for and aft. But a lot of tracks. I intimated that this might be a cool ICD, as it would be within the rules. SC pointed out that it might be legal but it would be wrong. And so it goes. Thanks for paying attention.

Your idea seems to cry out for unobtanium? Maybe my tracks too. At least as far as weight.

Paul

[sigurd]

why unobtanium?
uhmwpe stands for ultra high molecular weight polyethylene, i think, and is obtainable and very slippery.
I am thinking about it because those sailtracks are so expensive, a sliding bearing seems simple enough if it is good enough. I'd like to eliminate through-hull bolts and other bolts (stress concentration) too.
In your idea, i am not sure i get it, a sketch would be great!
One thing though, with the cambered leeboard that you could use, there could be very little leeway. Because of that the hull will make very little sideforce, which makes its location lengthwise relative to the rig unimportant?
so, what i am thinking is that as long as you can move the leeboard, why move the mast?

[Paul Scott]

Sigurd, as unobtanium goes, a leeeetal joke here among my Boeing Engineer friends. I'd like to have the capability to send a sketch, but until then, maybe this will help-

three tracks on the main hull, all running for and aft.

one track on each side, one track down the middle.

the cross spar that the foil and outrigger hull is attached to is connected to the three longitudinal tracks on sliders.

a fourth track is fixed across and on top of the spar, on top of and along the middle track, attached to the middle track by sliders.

the mast slides along this.

I'm not sure if the spar and the top (4th track) will need stabilization. That's why I included possible support via triangles, whose ends slide for and aft.

A lot of track, initially. I figure (hope) it will stiffen the hull. I experimented with windsurfer proas for a time back 79-82, and found that EVERYTHING needed to move. Especially if the main hull planes, the ama planes and if the sailor wants to move back to minimize wetted surface, as windsurfers do. It is useful to remember that Ken Winner's first funboard had a track almost the whole length of the board. Look how long tracks are now.

I'm still trying to wrap my head around cambered bi directional ogival foils. It seems you need some control over incidence with them, and that is going to be dependant on hull type.

Hope this helps.

Paul

[sigurd]

on windsurfers, the mast is a bit forward of center. if the mast were stationary, the proa board would be a bit longer, which would mean the bow is extended. I don't see any big problems with that.
I guess the tracks were long because they wanted to experiment? I hardly use my 20cm track, I'm no expert though.
The fin problem: flipping up the tail fin on each shunt is what I imagine to be the easiest solution. linked flippable centerboards are normal in bigger boards - no big problem I think.

Why did you use an outrigger? stability? could you still tilt to steer then?
Thinking about it, a shunting windsurfer might be utterly pointless if one did not have a good reason for the outrigger?

[Paul Scott]

sigurd, I was looking to reduce wetted surface , get the sail out of the water with stability, and eliminate stabilityproblems while tacking and gybing. I could steer by moving the CE and my weight, etc. It was faster than early div 2
boards and stopped falling in the water so I could gunkhole and rest more easily. paddling was easier too. had to use planing ama though, otherwise the ama drag lead to rig having to be way forward. tried to keep ama out of the water in the light stuff.

Paul

[sigurd]

By the way I like in your idea that you can move the ama back with the leeboard, you need not so long ama then.

The leeboard will assume an incidence angle as required by the sail load.
Take for instance Tom Speer's P3 02 12 (series, camber, thickness).
Zero lift occurs at about -2deg.
If you size the board relative to the sail, and choose a moderate target CL in order to have good margin - say CL=0,3. If my spreadsheet is good, that lift coeff will occur at about 2,5 degrees of leeway, depending on the planform (AR=6, oswald efficiency = 0,8). I think this leeway is so low that the hull will not appreciably add to the side resistance. If that is so, it will not affect helm balance either, appreciably, so that is why I am asking, why do you want to move the mast? the helm balance is already almost entirely decided by the leeboard location relative to the rig?