The options for the original question have been well covered and the OP seems to have settled on a reasonable solution for his situation. And each vessel situation is different.
The key to successful fire fighting has always been to detect and get it out quickly. A boat fire that burns more than 10 minutes is unlikely to be able to be controlled. (an old statistic that I cant find the reference) Do not hesitate to pull manual bottles. Do not hesitate to "make a mess" with dry chemical. Use the entire extinguisher, once triggered even for a short blast, the powder will prevent the valve from fully seating and the gas can continue to seep out rendering the extinguisher useless in as little as a few minutes. So use it up and be sure the fire is beat down. Then grab another one...
I have a 1994 Express cruiser with twin gasoline engines, so a greater fire risk.
Boat was OEM with a single Halon bottle in the ER and no auto shutdown. I have added a second Halon bottle in the opposite corner. (I picked it up at an excellent price.) They are not synchronized but give me a "second chance" for suppression and spread out the sensor heads.
As has been pointed out in an engine room shutting down blowers, engines and generators quickly is crucial to effective flooding suppression. I now have a Fire Boy style control that is powered when any of the above are active and shuts down all of the above if a bottle triggers. And has the engine over-ride safety to restart if needed. The control is just a set of relays and not very expensive.
Shutting down all engines also shuts down any fuel pumps that may be adding to the fire regardless of how it may have started.
AND disconnect batteries since they are the primary source of electrical energy for fires. I have battery switches for each engine and a seperate for the house bank (VHF-DSC, GPS) so I have no hesitation about killing both engine electrical. And my switches are not in the engine room.
We should never depend on any one solution for all situations. I to have detectors and manual fire extinguishers grossly exceeding Coast Guard "minimums". A 5BC next to the galley, a 10BC at the cabin entry and a 20BC near the helm just in case.
Then there is the "most likely" causes, BoatUS indicated that 50% of all boat fires are electrical in nature and 80% of those start in the shore tie socket, the 30 amp twist style. Upgrading some high risk components is more effective then any reactive solution. I upgraded to IEC309 standard 120/240V 60A rated pin and sleeve commonly seen style in 100 amp marine service.
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Halon 1301 is an excellent flooding agent, it is no longer manufactured due to it being a CFC (chlorinated) agent. But it is recycled during bottle maintenance and is still readily available for refill if required.
What is Halon and How Does it Work?
Pat Hughes
USCG retired