Battery Cables?

Hi
My Dad left me a 16' bow rider with an early Nissan 90 hp. There was no batteries in it, just a bunch of cables.
I bought 2 new batteries but can't figure out how to connect the 2 batteries together.
1 positive cable goes to the motor, one negative cable goes to a ground. Now I am left with 2 extra cables that are just laying there with nothing attached to either end of them. I don't know where these to lose cables hook up.

he may have had the two batteries hooked in parallel....there is no need for a second battery to start and run a 90 hp engine...the 2nd battery should be used for aux stuff like a trolling motor etc....i am of the opinion that hooking two batteries in parallel with no isolation switch is a no-no...i like my ignition battery to have no additional load except possibly the bilge pump...so i would roll those spare cables up nice and neat and put them in the boats storage...

Thank you papyson
I took the battery out of my truck last night and hooked it up using just it, but starter would only spin and not engage, so I figured it took the 2 batteries for enough juice to spin the starter and make it engage. One of my boys is suppose to buy 2 batteries today for it.
It has 2 batteries on he left rear, I figured they were for all the other things.

i would address that problem instead of going to two batteries....make sure the battery is fully charged and reads 12.7 or so without a load..then hook the meter up with the negative lead on a good motor ground..hook the positive lead up to the positive post on the battery...with the starter spinning you should read in excess if 10.5 volts..go from there and decide if you have too much voltage drop or a starter problem..

another little test is to have the motor in neutral with no fuel feeding it....then take a jumper cable and go from battery positive to the positive post on the starter...it should engage and start turning the engine..

look at the starter and make sure the gear will go up and down with your finger...if you decide it needs oil only add a drop or so..tou do not want oil down in the starter..

Thanks I'll try that. If I engage starter with a screwdriver then it well start.

I don't hardly know anything about working on anything, but I'm trying, would like to go fishing in Dads boat.

Freddie

If the starter spins but does not engage, the starter bendix probably needs attention. Maybe a drop of oil as mentioned. Also check/clean/tighten all battery and ground cables, from the battery, to the solenoid, to the starter motor. The NS90A will only require 1 battery to start/run. A group 24 battery will get you by, but a better setup is to run a group 27 or 31 if it will fit in the battery box.