Originally Posted by telrunya
I'm interested in using twin waterjets and I'm curious on how it provides a lateral thrust to the boat without yawing.
You don't want want to produce no yaw, but rather the "right" amount of yaw. The WJs only produce forces, essentially at the transom. Only when combined with other forces are moments produced. Thus if they only produced a lateral thrust, this would only move the stern. It needs to be combined with a yaw moment to move the bow too. (The "right" yaw moment moves the bow at the same rate as the stern.)
Can anyone explain what's the combination of machineries (i.e. nozzle, bucket, throttle) that produces a lateral thrust to the boat?
Without independent steering, it cannot be done (you can walk it in by repeatedly alternating steering and forward/reverse, similar to twin screws but more effecient due to better thrust vectoring, but you cannot translate laterally).
With independent steering, to move to port:
Have the stbd WJ in fwd and turned "some amount" to stbd (thrust to port to turn bow to stbd) but forward of the combined center of lateral resistance, which produces a forward thrust component, a port thrust component and a small yaw moment to move the bow to port. Now set the port WJ to equal thrust (probably slightly more throttle due to reduced efficiency of reversing bucket) and turn to port (thrust to stbd to turn bow to port if it were in fwd) the same amount and bucket set in reverse. This produces a reverse thrust component, a port thrust component and small yaw moment to move the bow to port. The net is zero forward thrust, (2x) port thrust at the transom and a (2x) port yaw moment.
The "some amount" of equal/opposite steering will result in a balance of the yaw moment with the stern translation so the boat moves smoothly sideways - more steerage angle both increases the port thrust and decreases the yaw moment. Increased throttles (note plural) will result in increased lateral thrust and probably a slight change to the yaw moment balance which would require a sight correct to the equal/opposite steerage angle.
The above applies to zero wind/current and pure beam wind current
. If wind/current are from some other angle, then this can (probably) be compensated for with other combinations of steering and throttles. For example, assume you have a stern wind/current and you want to move sideways into a dock
without drifting into the boat in front. In gross terms, starting from the settings above, increase the thrust on the port WJ, say 20%, and decrease the thrust on the stbd WJ the same amount. This results in net reverse thrust (40%), the same port thrust (1.2x + 0.8x = 2x) and the same port moment to overcome the forward thrust of the wind/current.
Finally, if you have a joystick, I would think you have independent steerage at low speeds and synchronized steerage at high speeds. (Remember the Suburbans and pickup trucks that had four-wheel steering - opposite dierction at low speed and same direction (but different value) at high speed? Are those still available?)