Battery Cables & Bow Thrusters

A lesson for all to learn from and remember.

Recently here a luxury vessel had a small, (fortunately contained) fire in the bow thruster area. The end result was damage to the bow area and burnt out batteries etc etc. A fire at sea can ruin your entire day!

The interesting thing was this. The positive and negative cables from the batteries (series/parallel) were crossing each other at the terminals of one of the batteries. Now whether the cable became hot from excessive use or the the insulation wore through (or both) the end result was that there was a direct short in 0000 battery cable from the cross over point to the two terminals, with all the usual drama to be expected when 500+ amps gets out of the box.

Remember to be aware of crossing cables where this circumstance could be repeated, I do not think the abrasion side of the story would be correct as the boat is less than a year old, but the insulation heating up and becomming soft, leading to the copper wire touching could be a very real scenario.

Let's all be aware of this sort of problem and learn from others mistakes. I am sure we have all done similar things in the past without realising the consequences.

 

Plastic insulation and "Cold" flow??

There are different types of plastic used in wire insulation, and some are rated for higher temperature. There is also the type THW? which has an additional harder-plastic coating on the outside that is temperature and abrasion resistant.

Is is well known that plastic will 'cold flow' which is slow movement and shape change with constant mechanical pressure. Of course this happens faster with 'less cold'!

I recently had a lighting fixture explode and drop a glass lens and bulb and socket right on the dining room table of my 'new' apartment here in China. It was an elaborate fixture, but done with the wrong low-temperature cable. The wire insulation 'cold flowed' after 1 month, the wires shorted and the flash / over current burned totally through the wires and dropped the lens and lamp and socket. Fortunately we were in another room when we got the loud Pop, then Bang! and all the lights went out.

NOT anywhere near as scary as unexpected smoke onboard!

I think we need to think about 2 things here: (1) The correct higher-rated temperature and abrasion type cable, and (2) mechanical restraint of cables with as many cable ties and cable attachments as needed to assure that they do not move against each other under shock and vibration.

Many years ago I was a test engineer on US Navy jet fighter planes. Every cable was tied or laced, and attached to retainers every 6 inches or so. No insulation moved and rubbed together. Maybe we need to consider offshore class boats in a similar way.

 

Interesting point about the cold flow Terry, ta.
Cable rating was 105deg, which is the norm for battery cable in Chinese boats, and elsehere too for that matter.

New apartments in China, as you well know, are built for a price, they are erected in 8 to 10 minutes, often by entirely unskilled labour (there is so much development they cannot get skilled people), and the end result is what you witnessed. We have the same problem in boatbuilding, same results naturally.

 

I would say that the cable is insufficient, no way should a cable get that hot.

Incidentaly I use the same cables for the generator start as the anchor windlass. It is extremely unlikely that I would be using both at the same time.

If I had a bow thruster I see no reason why the windlass/ gen cables coudnt be used for that too.

My bridge windlass sw was in the on position ( dont know why but it was ) When I turned on the batt one day I did not hear the clunk or the groans of the windlass trying to pull on an already parked anchor. After 20 minutes I had smoke bellowing from the forward hatch, the damage was extensive, but not to the cables, they were re-used. The windlass and its soleniod were a melted mess. I now have an alarm on the solenoid.

A fuse would have done nothing as I have discussed this with an dc elecrical engineer. If you fused to 300 amps 280 will not blow it. 280 amps will do the same damage.

 

Yeah I agree on all points Frosty, the cable may or may not have got "that " hot at all, but because they were crossed, the situation allowed the direct short from the crossover point to the single battery terminals.

A fuse would not have blown, as you say.

We are supposed to fuse a circuit at the power source, yes, but that is not actually at the batteries, it is in line outseide the battery box (as usual).

This is a tricky circumstance, often done in actual fact, but obviously (in hindsight) wrong.

Not anymore with my boats anyhow!

 

How to handle this one???

Hey Frosty, please tell us how / what type of alarm you have added..

This is the kind of thing where some smart power control approach could have a "Current times time" alarm set, that would alarm when the amount of energy put into the windlass was 'too much'. Whatever you might decide that was

 

I would expect a simple pietzo quartz beeper would do, it will just drive you mad while the motor is in operation, after that the power is cut off and so is the bloody noise. Such as a reversing alarm from a car would do the trick.

 

Personally I prefer to be the fuse on my winch. I am well aware of how hard it is working and give it a rest occasionally. I cant imagine anything more anoying than a fuse blowing just when you need it the most. If its labouring I let it rest and pull the boat up on the engines, then pull again.

The alarm is a simple 12V beeper bought from an electrical shop,--less than a dollar. It is sited under the control panel and will alarm if either station winch switch is thrown.

Some kind of timer would work,-- alloying all current to flow but say 5 seconds. Simply release the switch and switch again.

I have seen something like this in 240V for starting big motors in stages, it could be times to 1/2 second.

 

OK, so the Alarm sounds any time the winch is running, much like the backup alarm on trucks and busses. Is that right??

 

Yes exactly. When the primary circuit of the winch solenoid is activated the alarm sounds.

Its is also beneficial for other uses apart from warning me that the solenoid is energised, it would give me an indication of diagnosis. If the winch was to fail yet the alarm was working I would know imediately that it was a winch motor failure, or solenoid,-- but certainly not the primary circuit which is usually the case.

This along with an ammeter showing DC draw and a battery voltage indicator I can determine exactly what is going on.

I strongly believe that these high ampage pieces of equipment need careful consideration and you can quite easily loose your boat as I nearly did.

 

Cause of Fire

This thread is very interesting in trying to understand what initiates a fire on board and how we can avoid it.

Only 2 weeks ago the boat on the mooring next to mine cut loose in a storm and was drifting onto the rocks.

It is reported that the owner noticed this and immediately raced on board to avoid going aground. Apparently, when he tried to engage the screw the engine stalled due to some rope getting caught in the screw. What happened then is vage but apparently smoke was seen coming out and eventually the whole boat caught fire. The boat ended up on the rocks totally razed to the ground.

Since then I have been trying to deduce what started it. To me I would think the owner continued to force the starter motor without disengaging the engine from the stalled shaft and in the panic he did not realise he was just producing heat instead of rotation.

regards

 

seems to me likely that loaded the clamps could have been loose, melted down, sparks, , gas, bomb
properly sized cables never get warm hardly, intermittent load, let alone hot nuff to melt

 

Any high amperage device should be fused at BOTH ends of the cable both positive and negative wires.... HF SSB radios, bow thrusters, windlass should all be installed this way.

 

Why?

I have never seen this recommended. Why would you do this??

If the fuses are the same rating, and one blows, you have to search through both ends to find it.

The only way this makes sense is IF you had a large value at the source end, to protect against a cable short circuit, and a load-appropriate fuse at the load end. But usually this is done at the source end, protecting against both faults.

Sometimes a larger fuse and cable is used to feed a sub-panel with more than one fuse for multiple loads, but that's a different situation.

 

I would never fuse heavy ampage equipment. I notice that you mentioned SSb with bow thruster and windlass. SSb maybe but windlasss no'

A windlass could take 200 amps. Besides the annoying replacement or re setting to fuse /trip, If you fused it at 180 it will not blow untill you overload by 180 amps .

A windlass will haul slack chain with a small 20/30 amps but will quickly load to 200 or more. How can you fuses that?

20 amps is more than adequate to start a fire and it will continue to supply under that 180 amp value.

Personally I feel a windlass should be protected by a timer and should not be able to operate longer than say 10 seconds without re engaging.

You don't fuse a starter motor do you?