Near Miss on a Boat Fire

[dutchman2858]

Had some excitement today in our 16' Starcraft. The starting battery was dead, so I pull started it and let the Mercury 30hp run while we motored around with the trolling motor looking for some smallmouth in a local lake. The fish finder went blank on me and while I was trying to figure out what was wrong my son hollered, "Fire!"
I whirled around and saw smoke pouring from the battery compartment. I turned off the key for the Merc and grabbed the fire extinguisher. No flames visible, so I let the smoke dissipate and examined the damage. The cord end to the power downrigger had melted and burned to a crisp. It was a pigtail assembly that simply runs from the battery so it can be plugged into the Cannon downrigger when in use.

It almost got REAL exciting...the gray hose in the picture is the fuel line.

No other apparent damage, except for a melted groove on the starter ground cable. I'm not sure what caused the overheat, but will have the battery tested tomorrow. I've never heard of an issue using an outboard to charge up a depleted battery, so am doubtful that caused the problem.

A fisherman on another site indicated that it was probably due to a bad ground causing too much resistance. Looking back I recall that the wingnut on the battery ground wasn't snugged down tight--he may be right.

[Amx]

That's NOT how you heat the boat in the winter.

[dutchman2858]

It was a bit chilly out there, AMX!

[Larry3215]

I dont buy the loose ground idea. Increased resistance will reduce current flow which will reduce heat - not increase it.

Were you using the downrigger when the fire started? You would have to be raising it or lowering it for power to be flowing in the pig tail. Other wise there would be no load and no current flow - so no heat.

I think you had something in the pigtail short out. Check the end where the rigger plugs in and look for corrosion deposits/build-up or anything that could cause the short. If the rigger was plugged in, you need to check it as well. But if the rigger cable didnt melt, then thats probably not the problem.

Its also possible that something was rubbing on the pigtail which could rub through the insulation and cause it to short. The rub through will be where the melting stopped. It could also be something crushing/compressing the cable and causing a short. If the cable got under the battery or between the battery and the case or was pinched by the battery lid for example.

Again, the point where the short is will be the furthest point from the battery end that is melted. If the entire cable melted, then the short was in the plug. If only half the cable melted, then the short is at the half way point.

Glad you didnt have more damage! That could have been really bad if the gas line had burned through!

[Larry3215]

One more thing - its hard to tell but it looks like there is some green corrosion on the wire where the insulation is broken near the plug end. It also looks like there may be some 'black wire' in other places - but that could just be from the heat.

How old was that pigtail? If there were cracks in the insulation, that would make a short more likely/easier to occur.

[dutchman2858]

Makes sense, Larry, thanks for the input. Downrigger was not on board, so no load on the cable. I looked over the remains of the cord carefully and noted what may well have been internal corrosion. The insulation was compromised so much by the heat that it was hard to tell if it had cracks in it prior to the overheat, but the last 8" or so was relatively intact, so your supposition that it was a cord failure seems probable. Cord had about 2 years on the boat, but since I bought a "new" older model Cannon Mag 10, I'm not sure the "static" age of the power cord.

Definitely something to keep an eye on in the future for trolling motor and electronics power supply cords also. Thanks again.

[Larry3215]

Yeah, pretty much any power leads coming off the positive post on the battery could have the same issues.

For future reference, you might want to consider putting a fuse on the pig tail right at the positive connection to the battery. That way any short in the future will just blow the fuse and not cause a fire. Im pretty sure thats in the instructions for the Cannon If Im remembering correctly, it called for a 60 amp fuse on my Mag 10's.

That's considered "best practice" when boat wiring. I do that for all my accessory power leads - including the one that runs up to the dash fuse panel if its separate from the engine control wiring. The engine control wiring leads will already be fused near the battery or inside the engine housing.

The only thing that will still be subject to this type of failure will be the main battery leads to the engine starter and they are much better insulated.

[Larry3215]

By the way, technically you should use marine grade fuse holders that are sealed properly and have marine grade insulation and tinned wiring - especially if you operate in the salt. Im too cheap to pay those prices so I get my heavy duty holders and fuses at the auto parts store and then water proof them myself.

[dutchman2858]

All good info...thanks! Knew about the fuse--got lazy. Won't ignore THAT requirement again!

[Bobber_Dogging_Gal]

I'm a big fan of inline fuses on anything. Glad it wasn't more serious. Nice barn door halibut in your photo. Where caught?

[dutchman2858]

Thanks, "Bobber"! Kodiak, AK. I lived in Kodiak for 20 yrs (40 total in AK) before moving to WA 6 years ago. The young lady is the daughter of an old friend of mine. I took them out on a blustery day to try for a halibut or two. We had to hide behind a couple small rock islands to get out of the wind in about 60' of water. It's a good location with currents running through the area bringing "food stuffs" to waiting fish...this one was right about 220#.

[MotoBoat]

This sounds like the power cable insulation was rubbed through somewhere along its length, and the unprotected cable inside was touching/grounding on metal! Since the downrigger cable was attached to the battery, there was current running through it. As mentioned, if a inline fuse was installed, it would have most likely blown. Instead, the power cord became a heating element, melting the protective coating!There should be a burn mark on the wire, and where that positive wire was touching metal or aluminum.

The other possibility is that the positive end that connects to the downrigger was touching metal or aluminum, since it was not connected to the downrigger. In that case, the fitting or plug would show signs of arcing.

I have placed the tip of a screwdriver on the positive side of a batter to see if it was dead. Not recommended, but when diagnosing why something electrical is not working. Isolating whether the item or battery is the issue. The screwdriver can become a welding rod, and heats up quickly. The better the contact, and charge level of the battery would control how quickly the heat builds up.

I see the in picture the plug has a cap on it, so that is not the culprit. But if it was submerged in water..........that could be a 3rd option.