RE: 5' pt boat
Hello Rye,
In my opinion, if i were to run a 5' PT boat, i would use 3 Astroflight 25's or 40's direct drive. They are very reliable motors and will easily run your boat. The motors have a variable voltage range so that you can decide how fast the boat will go full throttle by X amount of volts added. (see astroflight motor specs)
I am overhauling a Dauntless (4' 1.5"), in which the hull is based on a PT boat design. I will be running it on (2 )Astroflight 25's with (2) Astroflight Harbor Master speed controls. (That speed control is supposed to be very good and also has reverse) That will give Dauntless plenty of power to plane out and look very scale when underway.
The original Dauntless ran on (2) Packard PT boat engines and was 66' long topping out at 40+ knots.
PT boats ran on (3) Packard engines at 78-80 something feet long and topped out around 40 knots.
Hope this helps.
Best of luck.
-Rich.