NiMH Battery Question

I've got a V1 Impulse that's been running well on 2 6 cell NiMH packs (yeah, remember those heavy things?), but am looking for a bit more speed. I don't have my documents from when I got the boat with me, particularly the ESC instructions, and have had some issues getting a solid answer online as to the question of the input voltage range.

My thought would be to replace one of my (failing) sets of 6 cell NiMH packs with a pair of 7 cell packs of the same capacity to pair with my remaining good 6 cell packs to run a 15 cell setup. I haven't had any major heat/electronic issues so far even running props up to the M445 with the twin 7.2 volt packs (14.4 volts in total), but I'm concerned about jumping up to add another cell to the system (15.6 volts) if I'd be exceeding any limitations or outright frying the ESC without even running the boat even on the stock prop or a Grim 42x55.

Any input on the possibility or experience running higher voltages through the 60 amp ESC or should I just be picking up another set of 6 cell packs to replace the duds?

LiPOs are not an option due to the difficulties I'd have safely storing/charging them as I'm currently operating on a boat with a few hundred gallons of fuel aboard.

P.S. I have evaluated using 2-7 cell NiMH packs instead of mixing cell counts though I haven't seen anyone do it successfully and ProBoat advised against it.

if you want more out of the boat,and you cant use lipos,then I'd recommend updating your esc from the version 2 impulse.then you could run 2 8.4v batteries with no worries.if that's not suitable,then just grab a new pair of 7.2v nihm's and enjoy.i wouldn't try pushing the stock esc too much if you don't have the funds to upgrade it in the first place.

I'm going to keep my eyes out for a cheap V2 ESC, or just look into some third party units that could handle the increased load. Thanks for the suggestion!

How does it run on the NIMH packs? What do you lose or gain vs lipo ?
Steve