I know that bow thrusters don’t do much with any speed on, but like a lot of things people know I am not sure I know exactly why.
I was helping try to get a high-freeboard vessel hooked up to a mooring with no painter. I took a tender out to the mooring, tied a line to it, and stood by to hand it up. The boat came right at me and then hit the bow thruster. I could see that they had enough way on for the bow thruster water exhaust to be streaming aft, the boat didn’t change course, and I had to fend off from being rammed. We did get the line up though and all my limbs are still attached 
So…the actual part of the assembly that pushes the bow I think would be the prop thrust in the tunnel. If the prop was loose it would tear itself off the shaft and depart, wouldn’t it? Why does it care where the water goes next?
It clearly does though, every vessel has a speed beyond which the bow thruster is just wasting electricity. I used to install the things and warned the owners about this, but I can’t quite get exactly why offhand?
the pivot point for a ship with way one is about 1/3 the loa back from the stem. with way on bow thrusters have very little lever arm to change the ship heading. Conversly - when going astern the pivot point moves to near the stern - giving the bow thruster a big lever arm to change the ship heading.
Note to self - ask if boat has a stern thruster next time!
Most thruster tunnels I’ve seen have a deflecting bump on the outer edge at the front to guide waterflow past the tunnel when underway to reduce the effect of the water colliding against the trailing surface of the tunnel. I worked on one yacht where the owner would demonstrate his marginal helming abilities by traveling at excessive speed in a harbor and using the thruster. The result was a lot of increased noise and cavitation from the thruster with little effect, other than everyone in the harbor swiveling their heads towards the racket.
I believe the physics is that the propeller “screws” itself in one direction or the other, with the actual reactive force being on the propeller and all it’s supported by. If it’s cavitating, it’s acting more on a gas than a liquid.
May be this?
If a water stream crosses a perpendicular opening, (even water in a pipe if it passes a lateral and pressure less bifurcation) it will create suction in the bifurcation; the higher the speed of the water flow, the higher the suction force in the bifurcation.
The thruster’s tunnel is just such a bifurcation… on both sides of the ship. Beyond a certain speed of the ship, the thruster may not be able to overcome the suction forces.
Thread creep a bit, but one of the more interesting thrusters I have seen was installed on the rudder. The rudder stock was bored out to carry hydraulic fluid to it.
The active rudder was fitted to naval vessels used as minehunters as apposed to the minesweepers based on the Ton class vessels used by the British Royal Navy.
A bow thruster is most efficient when a vessel is stopped because basically it is a clean flow of water and clean flow out of the thruster. When moving ahead or astern , the flow of water across the bow thruster tunnel causes the flow to reduce and become less efficient.
Moving above say 5 or 6 knots ahead or astern ( less with a weak thruster) the thruster becomes completely inefficient and not effective at all.
A bow thruster is most effective when moving astern. You can imagine for practical purposes it is the long lever from the bow thruster to the pivot point moving aft but is actually the hydrodynamic effect of water on the moving vessel.
or look for the painted decals
rough terms a prop creates thrust from 50% push and 50% suction
If you force water onto a prop, lose thrust
If you restrict to cavitation the suction side you lose thrust
yep the bump is there for 2 reasons, to mitigate resistance while the vessel is going forward under standard operation and to allow for clean water without bubble/resistance while going at slower speeds when thruster is in operation. Propellers are designed to operate fully submerged so the bubble create cavitation and cavitation is not good for efficiency
On a bow thruster they are evenly balance to have equal amounts of thrust depending on which way they are turning, screw analogy works
Generally speaking you will not see bow or stern thruster tunnel much longer than 6x the diameter of the thrusters propeller as that is generally the size that efficiency completely falls off
Research vessels often have water jets in lieu of tunnel thrusters to allow for thrust underway
It’s hydrodynamics, I’d imagine it gets complicated.
One interesting detail I found in search results is wrt increasing speed the effectiveness bow and stern thrustsers differ. Depth may be another factor.
In deep water, the hydrodynamic forces produced by the stern thruster do not decrease as much as those from the bow thruster. In shallow water, the lateral force due to the stern thruster increases with ship speed, and may amount to several times the lateral force produced at zero forward speed.
From here: EXPERIMENTAL STUDIES ON THE EFFECTIVENESS OF THE SIDE THRUSTER
Plenty of data on DP vessel showing calculated thruster performance versus speed
That data is in your dp system to create the holding capability screen
Isn’t that because the pivot point is forward, increasing the leverage?
On ships I’ve seen with bow and stern thrusters, the bow thruster is twice as strong as the stern thruster. I don’t know why, but that ship also has a 70 degree rudder so you don’t really need much of a stern thruster.
You can certainly make this more complicated - but, on a practical level - I still think this explains 90% of it - at least
The effectiveness of thruster depends on whether the ship is moving ahead or astern.
As a practical matter, mostly what we’re actually dealing with in the wheelhouse wrt a bow thruster is the shifting pivot point. Controlling the heading while moving astern takes a much lighter touch on the thruster than does while moving ahead.
I’ve no experience with a stern thruster.
I was thinking it was all about water flow, the pivot point issue makes sense too.
Hope nobody mind a little aside to the discussion about the functions and effect of bow trusters.
The history of bow thrusters goes back to the 1950s:
Source: https://www.standbyengine.it/en/bow-thruster-vs-tugboat/
The last statement is not entirely correct. Side thrusters appeared on purse seiners in the 1960s.
The idea came about when the fishing boat owners and brothers Laurits and Lodve Gjendemsjø came to the motor manufacturer Brunvoll in Molde with the idea of having a propeller made that could make a boat manoeuvre sideways to avoid getting the net in the propulsion propeller when fishing with a power block. (which was also introduced in the 1960s)
PS> The Gjendemsjø brother’s first boat, “Klaring” was completed a few months after “Siljo”:
I remember the excitement when “Young America” arrived in Singapore as the first OSV with a bow thruster:
Built at Burton Shipyard for Offshore Logistics in 1970. Now Parkit, under Indonesian flag and still in operation.
Excellent research Dr. Bugge.THX