I did some calculations on the shortest stopping distance of the Milano Bridge when decelerated by one tug that should be pulling with full power and in line with the ship’s center line. From the link shown I used for the calculations the data of the 3200Hp Z-peller Harbour Tug as used in Busan with a maximum bollard pull of 42.5 tons.
Beware, these are rather rough calculations by lack of all the data. For example the backing power is unknown, I assumed a ship with a stopped engine. Also the friction resistance was left out. At these low speeds and the fact that the speed factor (V^2) will go down quickly this is not very important.
First was calculated which power is required to stop the ship in one ship length of 365 meters.
S = 365 = 1/2at^2 = Vav*-t Vav is the average between the speed of 6 knots and the end speed of 0 in meters per second.
t = 365/1.5 = 243.3 sec or about 4 minutes and a = 0.012329
F = m*a m = weight of the ship. ∇ = 365 x 51 x 7 x 0.77 = 100334850 kg
Dan is F = 123.7 ton = pulling power needed.
It can be calculated that for a modern tug with a maximum bollard pull of 80 ton that stopping distance is 564 meters and the time required 376 sec or about 6.27 minutes With a good sized quay slot this would be about right.
For a tug as used in Busan the stopping distance is 1057 meters and the time 704 sec or about 11.7 minutes.
Looking at the figures I have the idea that for this type of mega ships the present tugboats are unsuitable, larger power is required to control these ships in the right way especially under adverse circumstances.