Bow Thrusters

I need help in looking for diagrams (or text) which illustrate how bow thrusters aid in turning a ship. Angle to desired turning arc and ratio of power to overall interia may be relevant in these studies and or examples.

I assume that most information out there is for single thru-hull ship bow thusters and not dual systems. Center of buoyancy, center of mass and center of gravity are not all the same at speed, right?

If you have a pen and paper and think you can just describe it better yourself, then please post your sketch. I've been doing some sketches myself, But I'm not sure that I'm on the right track.

My thinking about this is in this direction: the bow thruster is not used so much to "point the nose" as it is to counter " interia" from taking the craft off the intended path. However if this assumption is true, then the best location for a thruster is the center of the craft, not at the bow of the craft. This is where I'm having my understanding challenged.

 

How do you think you are going to open a tunnel in the middle of the boat where the beam is generally maximum? The reason behind the installation to bow side is the slenderness of the hulls towards fore.

 

Maybe you are missing the point of my query, you just have to look at "POD DRIVE" layouts to see that you could put the drive or thruster unit anywhere you want. You could also put a water-jet just about anywhere you want. Don't limit your thinking to a single application, although specfic examples are useful in illustrating a point.

http://www.windwardtechnologyhawaii.com/cruise_ship_capt_000008.htm


Tug boats are another example of multiple vector drive units being essential to getting the job done.

Again, I'm not really interested in bow thrusters, water jets or pod type drive units in their mechanical sense. I'm interested in how the path or arc of the vessel is changed for the better as compared to "without".

I would imagine that most examples are for non-planning ships, I am more interested in high speed planning boats as far an actual application is concerned.

Thinking way out of the box, I recall seeing a talented students work in the Gallery section about a year ago. He designed a tripod hydrofoil sailboat in which each submerged foil could be steered. It was kind of like letting the sail stay put, and moving/rotating the craft underneath it.

If you find a nice graphic, please post it, thanks.

 

Try drawing the free-body diagram (top view) for the ship with the bow thruster active.... the way I see it, the thruster does provide a lateral force, but far greater is the moment it exerts around the centre of resistance. Thus the ability if activated on its own to turn the boat without much fore/aft motion, or if used in concert with main engines and rudders to counteract the moment exerted by the rudders leaving only two lateral forces in the same direction. I've rarely, if ever, seen them being used while underway except when in very tight channels; a strong side thrust to push the bow in the desired direction and slight opposite rudder can be used to keep the boat tracking through a turn without kicking the stern out of the channel.

 

Did you make up the term centre of resistance? I like if you did, if it's a term I should of known already, then I guess you now know how little I really do know and how much I have to learn on the topic.

If the boat is planning then the centre of resistance is near zero, right? This is when a side wind or momentum can cause the boat to skip sideways over the waves (sideslip) and overturn. This brings up the idea that a side-thruster could counter this dangerous situation and keep the craft on a smooth turning arc.

I think we are all familiar with drifting as a spectacle of automotive showmanship. Real performance racing typically avoids this type of phenomena as it decreases speed in the desired direction. I can imagine performance racing boats facing the same predicament, the boat which overcomes sideslip having the advantage.

 

re "centre of resistance"- not an official NA term, not as far as I know, but rather just physics-speak for the effective point around which the object will pivot. For an object in a vacuum it's the CG, but for a boat, you'd also have to account for the tendency of things like keels, chines and skegs to resist lateral motion. So the physicist ends up with a gross simplification (hey, we've been known to call a 'horse' a 'sphere' to make the math work out nicer), but the idea is that the thruster is as far from the centre of the boat as it can be, thus it creates a pretty big moment from a relatively small force.
As for boats on plane, that's a whole other story- take a look at the Unlimited hydroplanes, they counter the side-slip thing with a 'turning fin', essentially a big skeg hung off one of the forward pontoons; also competition ski boats, which often have a few mini-keels of perhaps 4-6" deep to do the same thing.

 

Horse?

Now you have me thinking of animals.

1. Thruster at the rear.

2. Stablizing fin in the middle to exaggerate the center of resistance, which magnifies the turning of each end.

3. Port and Starboard thrusters.

I know I've seen this before.