Try these

1/ Prop or shaft not balanced or shaft bent

2/ New motor not correctly lined up with the shaft

3/ Motor – shaft coupling out of alignment

4/ New motor not secure in its mounts

5/ Is the rudder loose or have any sidewards play in its shaft

And questions would be:

Does the shuddering occur across the full speed range or just at certain revs?

Is the new motor the same type and size as the old one?

Did the boat suffered any underwater damage or strike anything during the new motor test?

Paul

Is the prop shaft moving slightly backwards and the prop catching against the rudder ?when in reverse.

Len

Hi Gordon. 

I assumed you were using a bobs bored because you mentioned these before.They did cause a stutter if they were not attached to the servo properly.

The 12-ok then 6-not-ok is a common problem when you try to use bec with a speed control on a 6 volt supply which is not enough.

Some older 6-12 speed controllers used a  6 volt reversing relay whose coil was powered by the drive battery via a resistor. The extra resistance meant the pulling force too weak.

More info would speed the process along

Tom

Ah….sea hunter!

It is indeed a low voltage problem.

The unfortunate side-effect of the overcurrent protected transistors in some escs is that they have a minimum threshold voltage.

Tom

The speed control drive stage does not work at a low voltage so you would have to go with a 7.2 minimum  battery.A motor with a lower stall current would not trigger the voltage dropout but a 6 volt lead acid battery would not have enough headroom to supply the receiver anyway (through  the speed control battery eliminator circuit).

I wonder what motor and prop you use because the scale prop for that corvette is rather large for direct drive for 385 and even 380 motors.Perhaps a 555 motor from RS components and a small 12 volt or  7.2 or 8.4NiMH battery would be in order anyway.

You could kill two birds with one stone.

Tom
  

All..  I have 385`s on my HMS Devastation battleship with 40mm brass props on 6v and they run all day at just being warm, same for me Titanic at 35mm props. Dont see a problem,  385`s are nice torqy little motors, and quite pokey when run at 12v.   Ashley

Tom. It may be that for  optimum efficiency the prop should be smaller than I use, but it seems to me that they WILL spin a larger prop. The current draw is very modest to start with so perhaps even overloaded a bit they still only draw a small ampage.  I have never bothered to put a meter on one, The pair on the battleship last for  well over  1 1/2 hrs on a 6v 4.5Ahr battery which is usually enough for me !   Ashley