quarter hung rudders

More basic questions about proa design - Most of the proa community seem sold on quarter hung rudders - the alternative being stern hung, I suppose...
I've been mulling over design issues - optimal placement for a reversible (shunting) hull, Crossbeams are convenient, but if cross beams are at, say 1/3 and 2/3 of vaka length, wouldn't further fwd and aft work better?
Would a single mid-line rudder foil work?
What about lateral position - stern hung is in line with center line of vaka, but the micronesians, and the vikings, didn;t seem to think that was important. What if rudders were in the ama, and the vaka turned around the ama as it were?

 

A rudder in the center of the hull would not work very well, it would cause the hull to skid rather than change heading.

The further aft the rudder is located the small it needs to be so it is has effective control authority. so the smaller the rudder the less weight and less drag, that is why on many small racing classes you will see rudders way out past the transom on frames or beams, it can be smaller and have the same control authority (less drag).

You also would not want a front rudder, they tend to be unstable causing an over reaction with control inputs (very twitchy), and they overreact to sudden perturbations like waves or odd stray currents.

The only practical solution for a proa would be using two retractable rudders, one at each end. having two linked rudders, one at each end, that defect the same amount would work in generating a yaw of the hull, but it will be very twitchy at higher speeds. You must have more lateral area in the water aft of center or it will not be directionally stable. hence the reason all boats have their rudders in the back. On a shunting proa the only solution is to have the "forward" rudder retract or lift out of the water and you only use the aft one.

 

thanks for your reply!
"On a shunting proa the only solution is to have the "forward" rudder retract or lift out of the water and you only use the aft one"

Or lock it as a way fwd foil? If they were actuated separately, couldn't you ie set the fwd one a few degrees to windward to aid windward performance, like a trim tab?

 

any surface, even a fixed one, in the forward part of the underwater profile, will be destabilizing for directional stability. You would have to have a larger aft rudder to compensate, both would add drag. so the most efficient way to have an effective rudder is to have the "forward" one retract or lift out of the water.

The only benefit to a trimable forward surface would be to aid in maneuvering in tight places while docking at low speeds. At higher speeds it only will add drag and reduce directional stability.

 

thankyou so much again for your clear and succinct reply