Digging through my stash of batteries, I found the ones you see. A few minutes with the scales gave me this:
4 AA's in holder- 101g
Yellow 4AA pack - 84g
Slingshot battery - 129g
Rechargeable PP3 - 42g
So it looks like 9 volts of rechargeable battery it is then. Best of all, the lump is physically smaller than the AA set in it's holder so I can move it back a bit in the boat. Let/s hope this works.