Good evening gents, thanks for all your replies. Sorry for not updating sooner, but ive had a busy week.
I have followed your advice and checked that all the shafts are free to turn by hand and that non of the locking nuts are binding.
I have checked using a multi meter the amps being drawn by the port motor. Connected to the shaft and running on the workbench im getting between 3.5-4 amps. When running at full speed in the water this increases to 7 amps. Obviously these figures are doubled because of two motors, so 8amps on the bench and 14amps in the water.
With the batteries wired in parallel their total amperage is 7400mah (7.4ah) so i can now see why the batteries are getting a thumping. What I don't understand is why the motors are rated as 2.75ah no load by the manufacturer, but im getting a reading of 3.5-4ah.
Also I am wondering what the discharge rate of the NIMH packs could be, what I mean is, how good are they at delivering the power? Please understand that although I have been building model boats for many years, my understanding of electronics is basic.
Would changing the propellers make a big difference if much at all?
Im willing to give it a try, but one snag is that the Drumbeat appears to have M5 shafts and the range of 3 blade props in the 30-40mm range is non existent, apart from prop shop, but im a bit reluctant to spend £30-£40 on a new set if the problem still exists after.
I have tried to attach some photos for clarification, but unfortunately they are too large to upload and I don't know to resize them for the forum, but i will try.
Anyway guys, i enjoy reading all your comments so please give me your thoughts.