ROV underwater propellors

There seems to be a few knowledgable people here so I thought I would throw this one in and see what you guys think

the topic is ROV propulsion on electric Robots that weigh about 1/4 ton (mass only, they are neutrally bouyant)

small Remotely operated vehicles use electric brushless motors which can spin up to 5000rpm or higher if nessesary. The size of the props have to be 200mm or less approx (diameter) just for logistical reasons. They tend to be made of plastic or aluminium for mass and cost reasons.

These robots tend to work between 500m and 3000m underwater so cavitation is less of a problem than for you surface guys. This is just as well as they only tend to move at about 3 to 4 knots maximum. This is mostly as they have the dynamic properties of a brick and are towing a big long umbilical behind them.

Generally they are fixed thrusters arranged with 4 horizontal and 3 or 4 verticals. These tend to be arranged in a vectored layout so if you are moving forwards, the thrust from each of the 4 horizontals is generally at 45 degrees to the direction of travel ie 2 to the left and 2 to the right.

The thrusters sit in a kort nozzle with a direct drive from the motor. One of the trickiest bits is that they have to go from full speed one direction to full the other in less than a second (another reason for going plastic). They also have to be able to operate effectively throughout the speed range, preferably close to a linear speed to thrust output though again this is not a major issue, we can modify the controller to take account of a certain amount of non linearity.

So here is the question, for maximum thrust from the lowest HP and to try and get the speed up to a more reasonable 10Knots, what would be the optimum outline design for the prop, high speed, low pitch, low speed high pitch blade numbers etc. revs/min etc

Thanks
Alkemist