Proa steering

I found the drag from the little hull could be amazingly high. also if youwant to tilt or rake the rig, the abilty to move things around is nice. the ability to experiment it fun too. sorry about typing i'm doing this on my pda while walking the dog.

paul

 

I think the sailrocket video of the crash goes a bit towards proving my point. if the pilot had been able to move his ce forward while he trimmed in his main, the round up and unintended tack might not have happened. proas like to round up. imho, rudders are not enough.

paul

 

Lots of proas are functional without moving the mast.
I just looked at some sailrocket pictures. Isn't the pilot able to swing the aka forward?

If we disregard the hull(s) (SR hardly has any lw hull), I don't see any difference between moving the sailforce forward and moving the leeboard back.

 

Sigurd, that's true to some extent, but SR's pod to windward is still in the water, and the main hull seems to lift and pivot around the pod. I'm guessing the ama had more resistance through the water than the main hull? I suppose he could have moved the pod backwards, but it seems he sheeted in in only and the pirouette began. if he could have let the rig move forward, the aero sail force would have helped him do it, possibly attaining some balance as natural outcome of the forces at work. As it is, he would have had to pull the pod backwards, which is working against the way the forces were wanting to go. Easier to let a line out under control than pull it in against the forces present. I'm echoing what Steve Clark says about control of what you've got. On a windsurfer, you can drive the board forward with your legs when accelerating, but as anyone can tell you, it doesn't always work. As your face will tell you as it kisses mylar. I don't know where it is anymore, but there are some pics of proa model lagoon racers in the south seas that race each other without r/c, but under control with a balanced configuration. And these model proas REALLY have their rigs forward. And if you draw the lift/force lines throught he sails, you'll find that they are on a straight line witht the ama. SR wasn't as far as I can tell from the video. At least we know that the rudder was way overloaded, and that says something about the balance of things. Why deny yourself another control? And the speed that comes with balance?

Paul

 

My 2 cents worth in the proa steering grail is below. I understand I have almost no real understanding of a proa. I have talked to Dick Newick, have been on Russ Brown's Jzero and read Rob's stuff.

I like cassettes for rudder boxes - easy to make. If the cassette was trapezoidal then it could flop back and forth at the top. This would rake the rudder for a shunt. When the forward rudder rakes back its aft upper tip goes into the case which locks it in the centre. The aft rudder flops back pulling the centre of pressure forward and making it feather.

The rudders would be symmetrical sections and have a centreline shaft. They would not spin but need a system to pull them forward and aft in the cassettes. If the cassette design was clever they may be able to kick up if they hit something.

I have attached a very quick sketch - the proa is going to the right.

I haven't got time to do it myself, my own little project keeps me more than busy. Maybe when I get older.

cheers

Phil Thompson

 

Correction

Should have said if the cassettes are rectangular in a trapezoidal case then the rudders can flip back and forth at the top.

cheers

Phil

 

G'day,

Paul, the drag on all harryproa ww hulls (from 4m long on the 7.5m El up to 10m long on the 15m is not sufficient to need both rudders. They all steer with only one, if required. They also steer with the main only and one rudder. The only time there can be a problem is if the big roach mains are sheeted on too quickly during a shunt. It is also interesting to note that when they fly a hull, there is no sudden decrease in weather helm.

You need a deep, narrow foil for optimum windward performance. If you make it turn, then you have a rudder for no extra weight and not much complication.

Phil,
We have tried your system, and it didn't work very well. The rudders need to be moved by hand (Side pressure is far higher than drag) which is a nuisance. They are also hard work to build, prone to damage in a collision and on the bigger boats, get growth in the case. If they have to turn, it is far easier to make them turn 360 degrees than to muck about with sections which work (or don't) in both directions.

I strongly advise you not to start playing with proas until you have done all you want to dowith cats, tris and monos. Once you start seeing all the challenges and advantages, you will never go back.

regards,

Rob

 

Hi Rob, you are right that a well designed proa needs only one rudder and ideally should have a consistent leading edge and trailing edge. However where you put it is the issue for me. I like rudders directly under the leeward hull. It is much more efficient there than somewhere in the space between the two hulls. Also what is the most efficient method of changing the single rudder's position after a shunt? Have you tried a solution with only one rudder fitted to a proa? I didn't like the solution on your Elementarry when we sailed together and you had 2 rudders which could turn at least 180 degrees and the forward one really needed lifting the blade right out of the water to cut down on a lot of drag. Good to see that you are going to try the KSS infusion method and also enter the Transatlantic race.
Regards, Sam

 

rob, is there any decrease of weather helm, slow or otherwise when you fly a hull? There was, I think, some on my windsurfer proa, but sometimes it was hard to tell. The whole thing remained a mystery to me. The only thing that really cured it was a windward hull the same length as the main hull, but that was heavy. So I wound up with a short planing hull. Any way it didn't make it worthwhile to try to fly the windward ama. Actually, I would keep it on the the water, just to keep the drag constant. I was using only one assymetric ogival foil, in the middle, to lessen the WS of two foils. Thought about trying the foil toward one end or the other, but never got around to it. So I had to put the rig really forward to keep things balanced, which made it hard to find a good place to stand, without torqueing my body most horribly, and burying the bow some. It was why I moved on to other things. Made me realize why the south seas proa makers made their hulls so long.

 

Single rudder idea

Hi, I thought about how one rudder might efficient for a shunting proa with minimal effort by the crew to change its position after a shunt.
It came to me that the rudder needed to be on a shaft thru the leeward hull. The blade should be shaped liked a proper aerofoil, as on a high performance monohull yacht, and it should align very closely with the bottom surface of the hull, to gain end plate effect.
As the shaft is only single and not movable it must be in the very centre of the leeward hull, fore and aft and also laterally. The tandem sail concept used by Rob Denney is an excellent solution for a proa, and I would favour that arrangement. (As in Elementarry) The proa could almost be steered while on a leg of a course without rudder input, but with the relative sheeting pressure of the fore to the aft sail. Therefore the loading on the rudder is very little in a well designed proa except when initiating and exiting from a shunt.
To allow the CLR to always be astern of the midpoint the blade would have to be constructed at the end of a streamlined shaped horizontal appendage.
The shaft would enter into this appendage at opposite end to where the appendage sprouts forth the rudder blade. I will try to make a drawing of this idea, either on this post or a following one.

 

Wouldn't it be very compromising to speed, to have to use the sheets for balance?

 

G'day,

How's the tri going? I have payed with one rudder on tracks to move it fore and aft. Worked well. However, it was too heavy for the 50 footer so I reverted to 2 rudders on the little boat to try out a few different ideas. What I now have is better than before, less drag with 2 rudders down, easier to lift and lower the front one. Both turn 360 degrees and are used in light air maneuvering. Once you are going faster than a couple of knots, only one is required. I am not convinced that under hull rudders are more efficient in practise. I should be sailing on Monday if you want a look.

Paul, no noticable change of helm at all. The boats steer remarkably well and over a large apparent range of coe/clr discrepancy.

Sigurd. Certainly is.

regards,

Rob

 

This is my diagram of the one rudder proa. All comments very welcome. Does anyone know if it has been tried before?

 

interesting

Interesting, perfect brake when shunting.

 

quick thought

Just a quick thought based on frosh's idea, profile view same as his.
Sort of servo rudder to reduce tiller loads.

(edit) Hate to think of all the extra drag involved but on the narrow dutch waterways a quick shunt will be valuable and speed was not my ultimate goal anyway.