On the 18m proa with its two relatively small, under hull rudders and single central board on the inside of the lee hull, I find steering in light conditions to about 8kts easiest using the leading rudder for directional control as it gives faster feedback.  The trailing rudder is cranked on enough and locked to get neutral helm with the leading rudder in line. 

The board is always lifted off the wind as it is cambered so generates negative leeway once sailing lower than 60 degrees to true and is just added drag.  This board has very low angle of attack unless pinching high on the wind - above 45 degrees.  That means the edges for bi-directional operation are only 6mm radius so very little drag from the trailing edge; unlike a Speer bi-directional rudder.

Above 8kts, the leading rudder gets too sensitive unless waves are above 1m or so.  The leading rudder is left free.  The trailing rudder is used to both balance and steer..

At various times both rudders are used.  A good helmsman can work the boat over waves using both rudders at the same time.  Its worthwhile noting that both helms are steered intuitively so you turn either wheel to the direction you want to head irrespective of whether on port or starboard tack.  In tight spots both rudders can be used to crab sideways, turn fast or just act as brakes.  

I was pleasantly surprised at how easy it was to set up hydraulic orbital motors on rudders and helm to give continuous rotation.  They work quite well in closed loop even without an accumulator but get a bit notchy when cold and blew a nylon ferule when lines heated by the sun.  The nylon ferule should have been copper.  A small accumulator that elevates the system pressure accommodates both oil expansion and contraction and the system is smooth at all times.  It was easy to fit a electric-hydraulic pump for autopilot into the system although we are yet to work out the best way to reverse the autopilot action during a shunt.

The orbital motor/pump combination has no problem with continuous rotation.  The biggest challenge is setting up the rudder position indicator.  We have a magnet coupled arrow above the deck but need torches at night to check rudder position during a shunt.  I have worked out a way to use a remote position indicator but not installed that system.  If a rudder is set the wrong way around at the start of the shunt it spins the wheel very fast once the speed overcomes the system friction.  

We already had gearboxes on the rudders so both pump and motors have the same displacement.  It also kept the operating pressure low.  The motor has the torque capacity to drive the rudder directly and selecting a smaller displacement on the pumps, that the wheels are mounted on, enables setting the required ratio.  Around 5:1 ratio is good for 800mm helms on the 18m proa. 

BMR 315 orbital motors/pumps (rated at 360Nm) came from China with the 4 required landed for under AUD1000.  Exchange rate is not so good now for Aussie buyers.

https://www.renold.com.au/media/2057344/orbital-motors_disc-valve-heavy-duty_-bmm-bmp-bmr-bmh.pdf

Rick

On 21 Mar 2019, at 6:58 pm, 'Peter Southwood' peter..southwood@telkomsa.net [harryproa] <harryproa@yahoogroups.com.au> wrote:

You sit at the back for three major reasons. 

You can see the rig

You are better protected from the weather and bumpy ride

Because that is where the steering gear is usually mounted

On a power boat you tend to sit further forward because you can see where you are going better and do not need to see the rig

Getting used to it takes a few minutes

On a proa it would make sense to sit in the middle to windward of the rig with a single wheel between seats facing opposite direction. You put the top of the wheel to leeward, the linked rudders turn the boat to leeward. Opposite to turn upwind. Rudders turn in opposite rotation. You sit on the seat facing the direction you are sailing. That’s the theory. In practice it may not be as simple, and will probably take some tweaking to get acceptable balance. The rudder balance could be problematic as centre of lift will be in front of axis of rotation both shunts. Continuously rotatable rudders could solve that, but would need a chain or geared drive. Lots of room for experiment.

Cheers, Peter