On rudders.....
It seems to me that using only side thrusters to steer
would work of course, but as Vance described, there are
limitations to the effectiveness under some scenarios. For
sure it would work OK on a tiny sub but maybe less so on
a larger one like a K boat.
To steer a straight course using only side thrusters
there would need to be constant adjustment of the motor speeds
which would possibly use more electrical power.
With a rudder, small adjustments to the rudder position
could be made while using a steady speed on the thrusters. Any
forward movement would make the rudders act as "control
surfaces".
On Git Kraken I opted for a combination of these. I have
the two rotating side thrusters mounted midship for use as
depth control and fine maneuvering "on location". Up, down,
left, right. forward, back.
The stern thrusters ( 2 ) are mounted into prop rings
with "rudder vanes" and my hope is that a "vectored thrust"
effect will be achieved. With 120 degrees of angle and both
props turning in unison the vectoring effect should push the
ass end around for a relatively tight turning radius.
The "rudder vanes" are large enough that even without the
rear props spinning they should act as rudder
control surfaces as long as some forward movement is
made. The main benefit is that the rudder angle is
changed with just a small "bump" of the rudder actuator which
uses very little electric power.
As a quick "burst" all 4 motors can be activated. In
theory that should give 404 pounds of thrust. With the two
rear thrusters at full speed it's 202 pounds pushing the stern
side ways. Each thruster can be activated alone so any amount
of thrust can be applied up to the maximum.
Each motor circuit is completely separate from the others
including dedicated individual battery banks, speed/direction
controllers, thru-hulls and motor.
One problem I'm expecting is that both props in the rear
turn in the same direction. If this turns out to be a problem
as I suspect it will, I'll need to get a left handed prop to
fit one of the motors. My hope is that the vanes in the prop
rings will reduce the problem to a minimum but only testing
will confirm what's needed here.
On oil compensating......I think having each
motor compensated separately would be safer. No chance a leak
in one would foul the others. A common "bellows" could still
be used if the plumbing connecting it all together had some
sort of "trap" in each line so water couldn't get to an
adjacent motor.
The "air cushion" from a truck looks promising as a
bellows. I know they can take some abuse without failing.
Frank D.