I have a few LiFe 2500's in stock that will likely work for you. Can't see carrying much more mah than that.
You cannot compare MAH ratings of batteries of different chemistries.
MAH does not carry over like you think.
Yes you think it is a load versus time and all rated the same but it is not the case at all.
LiPo in a transmitter will outlast a NiMh of same capacity many times over.
Same if you take a 1800 LiFe. It would out last a brand new 1800 NiMh pack by a huge amount.
You couldn't even compare MAH going way back and referring to alkaline versus say NiMh as the NiMh would have about 4 times and when comparing NiMh to LiFe or Lipo it is even more than 4 times for same mah.
Look at lithium batteries for like motorcycles compared to lead acid or agm. For same exact mah rating the lithium has more than twice the power inside.
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NEVER choose a Futaba LiFe transmitter battery as they have shutdown built into them and are not suitable for receiver use. I would never use one in a transmitter personally.
FT2F2100B-V2 – 2100 mAh Transmitter LiFe Battery
Lithium Ferrite Battery for 3PV, 4GRS, 4PK, 4PKS, 4PKS-R, 4PL, 4PLS, 4PM, 4PV, 4PX, 4PX-R, 6J, 6K, 7PX, 7PX LE, 8J, 10J, 12K, 14SG, 16SZ, 18SZ, 18SZ 70th Anniversary (UBA0135) NOTE: This is for TRANSMITTER use ONLY.
DO NOT use that pack in your boat. Transmitter only and I do not even recommend these in transmitter.