2.5/1 is fine,
950 D SERIES (385 MOTOR)=4.5 TO 15 VOLT, @4.5VOLT=1890RPM,6V=2520,9V=3375,12V=5040,15V=6300RPM.
919 D SERIES (540 MOTOR)=4.5 TO 15 VOLT, @4.5VOLT=2250RPM,6V=3000,9V=4500,12V=6300,15V=7900RPM.
BOTH THESE UNITS ARE AVAILABLE IN RATIO'S FROM 2.5/1 TO 3000/1
HIGH SPEED MOTORS WITHOUT GEARBOXES ARE ABOUT AS MUCH GOOD AS A CHOCOLATE TEAPOT ON A SUBMARINE,THEY BOIL THE WATER ON THE TIPS OF THE PROP (CAVITATE)AND KILL THE BATTERY IN NO TIME,THE SHAPE OF MOST SUBMARINES RESTRICTS THE FLOW OF WATER TO THE PROP,KEEP THE REV'S DOWN AND ADJUST THE PITCH OF THE PROP TO GIVE YOU THE SPEED YOU REQUIRE.
I generally use around 6/1 on a single prop boat, although I have a feeling that I fitted a 11/1 on the Holland,which Andy now has, on the X craft I fitted 6/1, on my type 205 its a 6/1,on the turtle its 50/1 but thats a very low speed boat, I allways run 7.2 to 8.4 volt, the run time is very high on the 205 it is upwards of 6 hrs,and I have never blown up a speed controler in weeds,as you certainly would be likely to do if you have no gearbox.