Sorry but when you said 6v I assumed it was a SLA so your NiMh, must be a 5 cell as they come in multiples of 1.2v.

So I agree the simplest solution is a Rx battery pack and disconnect the BEC supply.
(May need to remove +ve wire from ESC(s) plug?)

Hi Will, Sounds good to me👍 I like the Action Electroncs stuff as well, have their dual ESC with rudder mixer which I want to try in my long thin destroyer to reduce the turning radius.
Was misled about the SLA by a previous post🤔
Makes sense re the BEC which I believe is 5V 1.2A. I prefer a separate RX batt anyway😉
BUT: if you run a 6V NiMh down to 5.6V you are close to the Deep Discharge zone which will seriously damage the battery and drastically shorten it's life 😲 Threshold voltage is 1.0V per cell, so take erratic control as a warning that the battery is flat and come home soon!
Look forward to your Sea Trials report. Cheers, Doug

Doug,
The 6volt battery is currently an NiMh battery not an SLA. The problem was that according to Mtroniks the minimum voltage to the ESC to adequately run the BEC connection is 6volts. That is why I was getting erratic control when the battery voltage was 5.6 volts. The action speed controller will work 2-12 volts and I am going to use a separate receiver battery as suggested by Dave Milbourn just to make sure.
Regards Will

hi Will,
I agree with replacing the SLA. if you don't want to go LiPo cos of chargers etc etc how about a 5 cell 6V NiMh? much lighter than the SLA
For the ESC switch to the Mtroniks Viper Marine Micro 10. That handles motors up to 550 can size, 10A motor limit, and runs off 4.8 to 12V 👍👍
I use these very happily on ships running 2 or 4 540s. A destroyer and a cruiser. Both about 54".

http://www.mtroniks.net/prod/Boat-Speed-Controls/Viper-micro-Marine-10.htm
You can get 'em about 5 quid cheaper here
https://www.cornwallmodelboats.co.uk/acatalog/Mtroniks-marine-Speed-Controllers.html
There are also 15 and 20A versions if you want to run both motors off one ESC 😉
Cheers, Doug 😎

I think it would be worth the risk of running the DC Motors at 7.2v (NiMh) or second choise 7.4v (Lipo) and get rid of the 6v lead acid battery.
The chance of damaging the motor is limited, the battery packs will be lighter than an equivalent lead acid, but will need to by a charger as well.
Always put a fuse in line rated 5 amps below that of the ESC.

Or buy two higher voltage motor?
only £6.99 each plus P&P link copy and paste into browser address:-
https://howesmodels.co.uk/product/graupner-speed-600-8-4v-motor/