Look after your nuts!

[Graham_Wright]

I experienced a couple of little problems during this week’s trip. I found my big alternator, although outputting 14.2 volts, was not charging the domestic batteries. The alternator is not grounded through the frame and the 0v connection to the main ground via a stud was losing 5volts due to a loose nut.
The second similar problem was at the starter motor solenoid 12 volt connection which is shared between the battery direct + output and the alternator (different standard one) as on a vehicle system. A resistive connection here generates heat and, in my case, a fairly serious fire with flames requiring two CO2 extinguishers, copious amounts of water and a lifeboat tow.
Hence - check your nuts!

 

[Graham_Wright]

As an addendum, no installed temperature sensor would have detected the fire (probably until it was too late).
I needed/need a flame detector - perhaps some sort of remote pyrotechnic device.
The fire brigade awaiting at Tenby harbour after our tow in, used such a hand held device to check for hot spots.
Maybe a fire bomb is the answer. I believe they don't leave a mess?

 

[Sandy]

Water Mist every time.

 

[Graham_Wright]

Water Mist every time.

What sets it off? Extinguishing was straightforward save for fume inhalation. Detection was accidental - I had just gone below for on-the-hour log entries and spotted flames - not even smoke.
If I had had earlier warning, the damage would have been less.

 

[Fr J Hackett]

Surely the fire would have originated from the alternator and not the starter motor.

 

[Graham_Wright]

Surely the fire would have originated from the alternator and not the starter motor.

No!
I'll take a photo of the starter end of the cable when I return. The end cap of the solenoid is no more but the experts on the Canal World forum have shown me the way to remove and replace the solenoid housing. They also suggest just the end cap of the solenoid will need replacing.
The solenoid is easy to remove in situ - the starter motor is not.
Lucas Type Starter Selenoid Cap for Massey Ferguson BMC 7109531013251 | eBay

 

[SaltIre]

Surely the fire would have originated from the alternator and not the starter motor.

If your nuts are on fire does it matter?

 

[Fr J Hackett]

Current only flows through the starter motor when it's energised so yes it's possible when you were cranking the starter motor the high current started the fire but I think that you would have had to be cranking for quite a time for it to develop but once the engine is running and the alternator charging or trying to output its charge then that is where there would have been a lot of heat generated.
I am no expert other may have a better understanding and explanation.

 

[Fr J Hackett]

If your nuts are on fire does it matter?

If your nuts were on fire I think water would definitely be of assistance

 

[rogerthebodger]

I have seen the battery positive cable connected directly to the start solenoid and the output of the alternator connected to the same point.

If the nut on the solenoid was loose the power from the alternator would heat that connection which could result in a fire as described

 

[Graham_Wright]

I have seen the battery positive cable connected directly to the start solenoid and the output of the alternator connected to the same point.

If the nut on the solenoid was loose the power from the alternator would heat that connection which could result in a fire as described

That was exactly the case.

 

[Graham_Wright]

To counter the breathing criticism; I have been on a fire fighting course and I am a diver. As long as you don’t breathe the CO2 or the fumes., it is ok. My hatch is close to the engine and duck and squirt worked for me.
I followed the CO2 with water.
I see no problem with water for electrical fires on a diesel engine.
12 or 24 volts won’t kill you. On a petrol engine it might be different with 25kv and a more combustible fuel that could be spread.
To add to the humour - on a hot day wearing just shorts (excessive for me!) I trod on a CO2 bottle trigger.
Straighten up the divide!

 

[Graham_Wright]

That was exactly the case.

No it wasn’t! See previous.

 

[westernman]

Surely the fire would have originated from the alternator and not the starter motor.

I have had a fire from a starter motor. Fortunately it could be easily put out,

It happens if the pinion does not disengage for any reason. In my case it was because there was a big bunch of keys on the same key ring as the engine start key and the key was stuck in the far right start/engage position and did not spring back to the run position.

 

[ylop]

As an addendum, no installed temperature sensor would have detected the fire (probably until it was too late).

That's a bold assertion. I suppose it depends what you mean by too late.

I needed/need a flame detector - perhaps some sort of remote pyrotechnic device.

I think a pyrotechnic would be a rather radical way of detecting flames! I think you mean pyrometer or pyroelectric? Those are actually just measuring temperature though - so I go back to your previous sentence...

What sets it off?

I assume they are set off the same way as auto powder and auto clean agent (halon replacement) extinguishers - when the temperature at the firing head gets too hot. Mine is triggered if the temp at the head exceeds 79 deg C and will fire in less than 10 sec at that temp. I note that you said you saw the flames - I'd be very surprised if those flames would not have made enough smoke to trigger a smoke detector (possibly even the heat before the flames would have) - i have 3 interlinked smoke detectors on board. One in the saloon near the galley for early warning that the toast is ready (the batteries are also closest to that one), one in the engine room to warn about things we can't see and one in the space where the rudder etc is because there's a diesel heater and engine exhaust in that space. If you do go for an automatic engine room cylinder two things to be aware of: 1. you probably shouldn't be able to see into the engine room anymore (or the effectiveness of the extinguisher will be diminished - unless its some sort of glass panel); 2. you probably need to be conscious that a sudden engine failure and strange noise should not immediately result in opening the engine hatch (or you let the extinguishant out and oxygen in).

 

[Graham_Wright]

I