I am just in the middle of installing a bow thruster in my 40' Palmer. Based on opinions everywhere, hydraulic is a better way to go in the long run, and once I have the hydraulic system, I can add things like a hydraulic powered windlass and capstain system, prawn trap pullers, davits, 3" bilge pump, etc. As far as choosing a bow thruster, the MaxPower CT HYD 125 had some of the best specs for power at lower pressure and rpm in a 185 mm tube (7.28"). That was the model 317588 with a 11 cc or .671 cu in motor that produces 260 lbs of thrust with 9.25 gpm at 2320 psi. The math shows that to be 12.5 hp at motor 3184 rpm although the prop rpm is closer to 1/2 that I believe. Is it high prop rpm on some of these thrusters that makes them so noisy? I have sized my hydraulic pump to give me max thruster speed at 1100 engine rpm which allows me to throttle back the main to idle and allow the thruster to work at 2/3 speed when I need less than full power. I am running an open center valve with a remote cable to the helm which also allows me to feather the oil to the thruster. Cost wise I am just going to a hydraulics shop to buy a Permco P124 clutch pump - 8.8 gpm @ 1000 rpm. Max speed is 3000 rpm and 3500 psi so specs well over operating conditions. This pump is $500.
I did find a Kobelt thruster, KP 8 which is more of a commercial unit with brass prop and good specs but I could not get info out of the supplier on all the specs to design the pump myself. I still think it is a better unit and bigger 8" tunnel but also $1000 + more cost. Prop speed was 1500 rpm. I like the simplicity and reliability of hydraulic systems, as long as you don't get into fancy electronically controlled valves and controls. Nothing wrong with a good 24 V thruster, I just chose hydraulic.