At this prop size both motor and fuse are about to blow up soon
It seems to me that the general setup is wrong for that style of boat, normally I´d guess you need like 100-150W for scale speed. So a 25A fuse should not blow up at any time.
Motors generally have too much rpm for brass propellers with a pitch rate of around 1.
With 30mm props maybe perfekt, but the boat won´t accelerate anymore.
50mm is definetly too big for a size 700 motor in direct drive, fully submerged I would stop at 40mm.
That would be the first change to make sure the fuses don´t blow up.
If you want a real economic drive train you should consider using the Robbe Power 755/40 motors (I´m sure somewhere available in the UK as well).
I´m using these in all my SAR ships around 1m length since years with brass props from 40-50mm and mainly 4S Lifepo4 batteries.