"the boat is to be a present for my son. He works abroad for long periods and I reckon a lead acid
would go flat beyond recovery."
In that case an SLA (especially the types produced to power alarm and emergency lighting system
s) would be your best bet. They can be left on trickle charge indefinitely. Two 6V in series might be better than one 12V to help with trimming out the boat. I use a pair of 6V 3Ah (about 600gm each) in my 135cm destroyer and 107cm submarine
. They run for hours if I don't keep the pedal to the metal all the time. 😉 Back home I stick 'em on the trickle charger
and leave 'em until the next outing. They've lasted my several years so far. Much longer than my NiMhs.
An NiMh looses approx 1% of it's charge per day. So, assuming it starts fully charged, in 3 months or less it would be in Deep Discharge and have joined the Dodos😭
Cheers, Doug 😎