Please click through to the source link above to the read the entire lengthy discussion. Below is a snippet.
I have personally found that more boats use bowthrusters rather than sternthrusters, for the simple reason that the keel and rudder are located towards the back of the boat, thus making the sternthruster “unnecessary”.
However, by observation, the back portion of the boat is usually heavier, due to physical build and equipment stored, such as trim tabs (exterior), generator, batteries, inverters and engines (in engine room). It would be logical to require extra ‘help’ in the form of thrusters.
At which point is it necessary to use bow/stern thrusters? What are the criteria involved in making the decision? Have there been instances where only sternthrusters are used without the bowthrusters being present?
There are many reasons why some ships and boats use bow thrusters.
. A twin engine boat or ship will need only a bow thruster and move sideways with no major difficult. .
. A one screw boat will not necessarily need an stern thruster. With some experience you can make the vessel to move sideways without a stern thruster. Of course if you have both, it will be easier.
. The bow thruster is relatively easy to install in the bow because the width of the hull -in the region where you need to install the tunnel for the bow thruster- is not a big one and you do not have other machinery, shafts or equipment in that area.
. In the stern, the installation is mainly more complex because you have shafts, machinery, engines pipes, etc and usually the stern is wider with some exceptions so you need longer tunnels, etc. Mainly the stern is thin only in the places where you have the main shaft in one screw vessels.
. No matter what is said before, there are ships with both stern and bowthrusters.
. Contrary to popular belief, a lot of big ships use bow thrusters with pretty big HP
As Fleet Manager for a previous employer, one of our vessels had a rotating nozzle in the stern (no rudder) and a bowthruster. The rotating nozzle acted as a sternthruster. I don’t believe I’ve ever heard of any ship with sternthruster only installed, although this particular vessel operated fine and maneuvered well without utilizing the bow thruster at times.