@RossJ Sorry but a small thinking mistake here. The nozzles actually are more efficient on slow moving vessels such as tugboats when engaged in towage.
At higher speeds it is better to have free running props so the water runs unobstructed to the propellors.
That is also the reason that on todays modern azipod propulsors the props are actually pulling the ship instead of pushing.

The nozzle will restrict the flow of water from the prop, that will increase the velocity hence faster vessel. It might not be as efficient going astern tho.

@Chum444 same on a 1:1 supply boat

Thank you for the link to the Kort article. My experience with steerable Korts on models is that when backing, the nozzle acts like an outboard does on a 1:1. I emphasize model since those of you with experience on 1:1 tugs may feel differently.

Nozzles have been used to improve on bollard pull however nowadays they are usually fixed around the propellor (Kort nozzle) . I have sailed on a supply boat which was equipped with steering nozzles. Makes for interesting maneuvering when reversing so keep that in mind.
It goes different then with fixed nozzles and conventional rudders since the thrust direction changes.
Have fun at: https://www.kortpropulsion.com/kort-nozzles/

Ronald This is an image from the net and from what I can recall reading about tugs it's the way it works Col

Ron, I’m looking forward to hearing about the results. That is really an interesting arrangement & would be a rather easy retrofit. I’m a strong advocate for steerable Kort nozzles which most all of my tugs have. I think I could predict the results but will await your report of the results.
Good luck!
Bill