Hi Roger. If I may I'd like to recommend a power meter. These are not expensive and install temporarily in between your battery and motor.
I paid about £12 and fitted XT60 plugs to suit my models and batteries.
You will of course have to test it in water with an assistant to hold it still or tether it.
I did this with my Stevn's Arctic tug, you might have seen the video? it proved the maximum current at full throttle to be only just over 20 amps each despite 85mm propellers
If you want to try it you can borrow mine but I won't be visiting Balne for a while.
I've added a link to an example but I would advise to find a seller with UK stock as charges can be heavy from China since Brexit. I believe the £15 threshold for duties has been scrapped.
EDIT II There is a UK seller at £13.28 that's best I could find, delivery 14th