One of the best motor/drivetrain combo is right there in front of your sub,a cordless drill.
With a sub of that size and round bow there is going to be needed some seriouse power without overheating the powerplant, and still give you scale operating speed.
I would sugest the following from my experiance since I go for running endurance and reliability.
Dewalt makes a drill that the motor and planetary gear drive are crewed together not held together by the outer housing.Very ifecient and high quality with a built in clutch adjustable,thrust bearings and 2 or three gear ratios ,I use the high setting to get plenty of power on even a 5 inch 7 blade prop on my Robbe U-31.It turns at 1400 rpm at full thraughtle but not ever used ,she will also turn very very slowly on standard 25 anp ESC.
She runs for about 2.5 hours and is water cooled,now with live wireless tetemtry.
For large subs these drives are ideal and very inexpensive with parts readily available.
Another top knotch unit is made by Makita and its in my 5 ft Alfa it has all ballbearing construction and a very powerfull 12 volt motor.I can send details if needed.
Robot stores sell these units but directly from power tool parts houses is cheaper or you might find one at the flea market ,I got mine for 4 bucks and it is still going strong!
Bruchless motors are good to but much more money and hard to find the proper size and ESC combo and much less robust and durable as a drill and its hard to find the gear train,with the drill you can chose the best gear ratio in secounds and if not good sellect the next ratio.Evry part is available without having to replace the hole motor.
No mater which one you chose I would recomend water cooling it,it will last and wont draw as much power.
Also for a ballast system I would sugest a compressor based one,works just like the real thing,you have plenty of space and available power and most of all it will blow the large ballast tank this sub requires to bring her to the proper waterline with ease ,just pump her up!
Fast dives and serface and go as deep as you like.No gas to buy or concerns about a rubber bag.
Again if you need info or complete kit just pm.
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