For a propultion motor from what I can see your sub is quite streamlined with a small cross section.
With a 3.5 inch prop I would sugest the standard setup I use for subs of this size.
A 12 Volt RS -550 HS type motor are very effecient and very powerfull and run cool.
Just instal this gear box a double ballbearing nylon gear and brass pinion sealed gear box unit by MasterAirscrew and the ratio is 3.5.Have never had to replace one yet all these years.
You will find with this combination that plenty of power and very slow prop speed is obtained without high amprage use.My subs nurmaly run for 2 to 3 hours continuesely with this motor/grearbox and similar prop,even turning a waterpump though not realy necesary.I like to watercool the drive motors.
remove the setscrew on the prop adapter and a 3/16 shaft is there.I then install a MILL spec U joint of stainless steel,Ill sell you one if you like.Or just use a different one.
Tower hobbies has them.http://www3.towerhobbies.com/cgi-bin/wt ... LXZ900&P=7
You can perchase every part of the gear box fom the manufafturer and great prices.
Get the motor on Ebay or surplus for around 5 bucks
Ill get that list of parts for the ballast system for you soon .