At the slow speeds most expect seasteads to go at propeller efficiency issues will require large slow propellers.
Water treatment mixers or agitators could be used as thrusters. A google search for mixers.
Mixers used in mobile aquaculture experiment
Cliff Goudey used some slow speed mixers to make a self propelled aquaculture cage which was tested near Puerto Rico. Some nice video of the propellers on the cage. These mixers can produce thrust of 3.4 kN/760 lbs from power input of 4.6 kW/6.2 hp. With two of these mounted in fixed positions he was able to control the 62 foot diameter cage like a normal twin screwed boat at 0.5 to 0.6 knots. He said a propeller/motor unit costs around $20,000. He used Toshiba Model VFS11-2037PME variable frequency motor controllers which cost about $1,600 each.
These 8 foot diameter propellers are off the shelf equipment used for very efficient mixing in the waste-water treatment industry. One model gets 530 lbs of thrust off of 3.5 Hp.
- main Flygt mixer page.
- slow speed mixers - larger diameters and most efficeint
- compact mixers - smaller diameters and cheaper
These are noisy, not efficient at slow speeds, use a lot of gas, and probably not the best way to go for 24/7 thrusting. But for use just when near a port they could do fine. Also, as a backup to an efficient electric thruster they could be reasonable.
For up to about 300 lbs of thrust you can get electric trolling motors. Seems to be about 1 amp from a 12 volt battery for each 1 pound of thrust (so about 12 watts/lb-thrust).
- Minn Kota Riptide 101 lbs thrust for $649 - salt water trolling motor people seem to like
- Torqeedo is a high-quality electric outboard from Germany. Some versions include a Lithium Ion battery pack. Range from 40 to 121 pounds of thrust. Expensive, but compact and energy dense due to the Lithium Ion. Not really meant to be trolling motors but as replacements for small outboards, so maybe doesn't belong in this section. West Marine sells.
Big ships have electric or hydraulic thrusters to ease up to a pier. For example, bow thrusters. These might be reasonable for a seastead thruster.
Small ships can also use electric thrusters. Yachts for example may use ones like these from Lewmar. The 12 to 24 volt models that West Marine sells which range in price from $1,500 to $6,600 with horsepower from 3 to 14.5 and thrust from 92 to 550 pounds.
Some ships use large azimuth thrusters. Not clear these come in small enough sizes for seastead use.
Starter motor on outboard motor
The electric starter motor on a gasoline outboard motor has enough power to turn the prop with significant push (Vince has done this when there was no gas getting to the engine). So it might be reasonable to convert an old engine to electric only use.
Outboard motor converted to electric
Some electric boats have used outboard motors converted to electric by removing the internal combustion engine and bolting an electric motor in its place. The motor mounts vertically via an adapter plate. This leverages the cost advantages of mass-produced outboard motor parts. Electric motors are like those used for small electric cars.