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.