Lateral Motion by Twin Waterjets

[telrunya]

Hi all

I'm interested in using twin waterjets and I'm curious on how it provides a lateral thrust to the boat without yawing.

Can anyone explain what's the combination of machineries (i.e. nozzle, bucket, throttle) that produces a lateral thrust to the boat?

Thank you!

[James S]

Welcome to the forum.
I would think that if you placed your jets at equal distances fore and aft from the center of underwater lateral resistance you could achieve lateral movement with out yawl, assuming the same trust at each jet.

To consider the other questions: If you could be more specific as to the objective i.e. movement in port and starboard directions, speed of movement, distance, and of course type of boat.

[telrunya]

Hi James S

Thanks for your reply.

I don't quite understand the definition of the center of underwater lateral resistance. The position of jets have been decided by the boat builder, which i assume is typical. I've noted your point on having the same thrust on both jets.

If I'd like to achieve lateral movement of the boat to the PORT side, without yawing, what would be the placement of nozzles and buckets on each jet?

[James S]

Not being very familiar with jet drive systems, are the buckets the things that drop down and divert the water in the opposite direction i.e. reverse?
Are you able to control the port and starboard steering and thrust separately?

[telrunya]

Yes you're right about the buckets.

I'm able to control the port and starboard thrusts separately, but not too sure about the steering. Alot of these are undertaken by the waterjet system, hence I'm trying to find out more, whereby sending a "move port laterally" command using the waterjet joystick, what will the physical position of the machineries be at.

Thanks anyway.

[kolbywhite28]

I don't think so if it requires steering, just ask the experts regarding twin waterjets. Maybe it can help you for what's bothering on you.

[James S]

I have to go shopping now (I'm actually only the bag-man) but if you can tell me what boat you've got I'll do a search and try to learn something about it.
I'm thinking its high end and if so can see some elaborate individual controls of the jets that could produce those results.
PM me if you like.
One other thing...I should have asked from the beginning...do you have bow and/or stern thrusters?

[GordMay]

Greetings and welcome aboard the CF, telrunya.

Operating a twin jet drive boat is a lot like handling a twin inboard boat.

Likely, you do NOT have independent Directional control of the drives, but DO have independent Throttle control. Unlike an inboard, a jet boat does not have a rudder. In this respect, steering is more like twin sterndrives, that also do not have a rudder. The drives are articulated, and steering is accomplished by the lateral movement of the water jet exhaust nozzle.

Putting one engine in forward (bucket up), and the other in reverse (bucket down), will turn the boat on station, spinning 360 degrees within the length of the boat,

By turning your wheel hard to Port, and having the Port engine in forward (bucket up) at idle, your Starboard engine in reverse (bucket down) at idle, you can move the boat sideways to Port, by increasing and decreasing the throttle power to the Starboard engine. You'll still get "some" yaw.

[Lee Jerry]

Originally Posted by telrunya

Hi all

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?

Thank you!

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?)