This past wknd a buddies Merc was turning over too slow to fire so he attempted to jump the cranking battery from one of his 3 deep cycles. When he attached jumper cables it arched and melted the negative post. Is it not possible to jump from a single battery in a 36 volt series. I have jumped mine off before but it's a 24 volt system.

Did he go crank to deep or deep to crank?

Yes the 36v system can be used as long as you attach to only one battery.

should work as long as you just use one battery like Scott said. maybe he had his posts mixed up.

Must have had something hooked up wrong.
Last three Legends that I've had has a jump switch that when in jump position combines one of the 12v trolling batteries and the starting battery.
On older boats that did not have the switch I've used jumper cables plenty of times.

Yup as long as you use just one battery. 24v or 36v doesn't matter. We had to do this to get in on Saturday

He hooked up properly to a deep cycle and then to positive on cranking battery but when he went to hook up negative on cranking battery ZAP.....we double cked connections and everything was right....charger showed a full charge on all batteries of his Legend that morning....we just went to town for a new battery.

you got to make sure that he hooked the negative and positive to the same 12 v of the three trolling motor batteries.
If he hooked the negative to one and positive to another then he could have been sending 24 or 36 to the cranking battery.

There has to be user error in the actions and/or facts presented about the incident. Electricity doesn't have different sets of rules it operates by in any circumstance. If the jumper cables WERE hooked up correctly for the existing battery system, there would be no melted terminal - that is an absolute certainty.

"Something" wasn't correct about the circuits created when the jumper cables were added. For instance, does the boat have a wire connecting the starting NEG to the NEG terminal of the #1 TM battery? (I don't advise this, but some boat owners have such a connection.) If that "Common NEG" cable exists, then the only battery that can be used to jump is the #1 TM battery. Connecting to #2 or #3 will melt something or blow up a battery if there is a "Common NEG". In fact, it will do exactly what you have described as to when the sparks can be expected.