I think it is safe to interpolate this value which is half of the half blade distance, thus in between 1 and 0.789 which amounts to about 0.89 of thrust meaning that 89% of the full thrust is available at that moment.
The efficiency, generally indicated with the nabla sign η, is the quotient of thrust and torque.
A different somewhat crude approach. I measured on the photo the width of the ship. In my case it was 17.5 cm but that might be different on larger screens. Then I measured the top of the rudder to the waterline which was 1.2 cm. A bit difficult because of the wake. So the rudder and the prop as seen are 1.2/17.5 * 51 m ≈ 3.5 m above the waterline.
Normally the propeller’s tip is in line with the top of the rudder so then the part of the blade that is out of the water is also 3.5 m. For a ship of this size I estimated the propeller’s diameter at 9.6 m, could not find this value on the internet. Interpolating this as done earlier the efficiency is 84%, still not too bad considering.
The 84 % assumes that efficient is max at h/R = 0. But the tip of the blade at the surface is not considered efficient. At least not in my experience.
We measured propeller immersion by the radius of the prop, not the diameter. The minimum required immersion was 0.7, that number being the distance the tip was submerged. For us it worked out to an aft draft of about 7.2 meters or thereabouts.
Using the h/R method minimum allowable submersion would be h/R = 1.35
I posted this before but here at the min drafts for the Panama Canal.
Here’s the min drafts for transit through the Panama Canal for a 200 meter ship.
Over 625′ (190.50 m) – 24′ (7.32 m) forward, 26′ (7.93 m) aft, TSW
We had a min draft fwd as well for bow thruster effectiveness.
What matters is the Y-axis velocity (breaking down the chartlet into a Cartesian system). A toll in advance distance was going to be exacted past a certain point with the velocity she had approaching the dock. If the chartlet is scale, you could calculate the speed and accel vectors of the port quarter. And of course the propeller efficiency table doesn’t really tell anything about the astern efficiency.
But of course there is the matter of tugs and environmental conditions… while the math might amuse, the recommendation seems obvious without calculation.
My first thought was the transverse thrust of the RH fixed pitch propeller would be increased due the the incomplete immersion in an astern movement and I haven’t seen much to change my mind. The rudder operating in highly aerated water would have little effect.
Class rules will vary but match up to IACS requirement guiding compliance :
M25.1 In order to maintain sufficient manoeuvrability and secure control of the ship in all normal circumstances, the main propulsion machinery is to be capable of reversing the direction of thrust so as to bring the ship to rest from the maximum service speed. The main propulsion machinery is to be capable of maintaining in free route astern at least 70% of the ahead revolutions.