CO2 is definitely the way to go in my opinion. Non-toxic, no cleanup and effective, especially in a confined space. CO2 in not carbon monoxide so if you find yourself below and short of air, just go above, breathe and you'll be fine.
BTW, you mentioned an alarm. CO2 alarms are very expensive and generally are only found in spelunking applications. You don't need one.
CO2 extinguishers can also come in very handy if your diesel engine decides to "run away." If you can't COMPLETELY seal off the air intake just empty the extinguisher into the air intake and you will have saved your engine from destruction. A friend of mine was a diesel mechanic in the Marines and had an engine run away on him. He said he never before heard "metal scream". When they couldn't stop the engine they ran away. When the came back the found a piston rod embedded in the ceiling of the hangar.
Dry Chemical is far superior to CO2 as a fire-fighting agent. I have fought fuel fires with Halon, water, foam, CO2 and dry chemical. The best by far in an enclosed space is Halon. CO2 will work well in an enclosed space but is not as effective as dry chemical due to the manner in which they extinguish the fire. CO2 displaces/replaces the O2 starving the fire. Halon and Dry Chemical interrupt the chemical chain reaction shutting the fire off. Any agent if improperly used will be less effective or ineffective.
Remember that the fire and products of the fire are far more lethal than the effects of the agent you're using -- put the fire out then worry about side effects of the agent used.
Having fought more shipboard fires and a few aircraft fires than I'd like to admit (in the 20 years I was in the Active Navy), I can tell you many can change extremely fast. A leak in a battery can cause an explosion with a flip of a circuit or an electrical fire can start a Bravo fire (Fuel, oil, etc.) These all change the techniques required to put the fire out.
Your points are well taken and valid. I think I will add a CO2 option but since we already have ample dry extinguishers aboard, keep them, too.
I have never dealt with a fire in an enclosed space on a boat. I hope I never do. I imagine the possibility and it seems a starting point would be helped if I could flood the space with CO2. It could all happen quite fast. Even retrieving a second or third dry chemical extinguisher will take precious time. The engine space on my boat connects to the lazarette so considerable volume of CO2 would be helpful in slowing things down, at least. It would reduce the rate of involvement of secondary materials while the source could be addressed.
CO2 is only effective in suffocating the fire, you need to remove one of the other legs of the fire triangle or it will re-ignite as soon as the CO2 dissipates. Any engine compartment fire has the potential to be a type a, b, and c fire (structure, fuel and electrical).
Turning off your power at the main switch in a electrical fire immediately shuts down the ability to communicate outward that you are experiencing a fire, and has a 50/50 chance of making you fight the fire in the dark - nevertheless in all cases it should be done immediately - even if you are sure it's a fuel fire. That's because the fuel fire may have caused an electrical fire.
Only when the fire is out should you restore power, and do that with a extinguisher in your hand.
Fiberglass fire blankets are great tools for firefighting on a boat, if kept handy. You can use it as a shield and approach the fire, then snuff it, and keep it snuffed.
As has been discussed, PKP (chemical) is a nasty bit of work to clean up.
Beats burning to death though. Depending on space / budget and and risk aversion levels you should keep every option you can, and know how to use them.
I also gained way too much experience fighting fires in the Navy; including duty as the repair party officer for repair 1 forward on the America the night the JP5 blew and took three lives, and again the night the bridge burned.
Thanks for your voice of experience. Clearly having as many options and understanding of procedures is crucial. Power off might get overlooked in the excitement. Good point!
This has been a very helpful thread for me. My original plan of simply taking a fully charged 20 pound CO2 extinguisher that sits almost empty in my shed, aboard, and figuring out a simple way to direct its discharge into the engine space, has evolved in important ways.
I will be changing the bilge blower arrangement I now have. I haven't tackled automating the extinguisher by using a solenoid of some type that is controlled by the high temp alarm but that should be fairly straight forward. Having it at the ready even with manual control will be a plus. I will cover the fuel lines with Firesleeve and make the fuel shutoff more accessible or solenoid.
None of these are difficult things to do and the changes will make a lot of sense. I needed another project to keep me busy while TD is on the hard. NOT! Ha!
When I now think of the difference these mods will make if I ever do have a fire; compared to what that might be like without them I get a cool feeling!
An added important aspect for your fire fighting/prevention efforts is ensuring you have a way to cut the electrical power, particularly to electrical devices in your engine space. Aboard ship, the vast majority of fires (55%+) are caused by electrical devices gone wrong/shorted, which is a real possibility in an engine space with vibrating machinery, sharp edged metal objects and plenty of wiring easily chafed through. (We, unfortunately, discovered this when a copper foil grounding strap chafed its way through a power cable and created a heck of a problem.) Wiring needs be well secured and armored/chafe protected and one needs be able to cut the power entirely without opening the space. (A good discussion of the matter is presented at BoatUS: Seaworthy). One is also wise to have a fire fighting port into the engine space through which one can discharge a fire extinguisher to avoid having to open the space and allow air/oxygen to reach the fire which can exacerbate an already difficult situation.
Further, having once had to deal with a fire aboard due to a malfunction in a kerosene stove burner, a valuable fire fighting aid is a fire blanket. In our case that happened to be a handy damp wool blanket (from a cold wet night passage) that suppressed the fire until we got the fuel source secured and the extinguisher discharged. An actual purpose made fiber-glass fire blanket would have been better and we now carry a sizable one on the inside face of the locker door opposing the galley for easy access.
__________________ "It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."
...CO2 will work well in an enclosed space but is not as effective as dry chemical due to the manner in which they extinguish the fire. CO2 displaces/replaces the O2 starving the fire. Halon and Dry Chemical interrupt the chemical chain reaction shutting the fire off.
I am under the impression that the dry chemical extinguishers worked through the same mechanism as CO2 and halon, i.e., they deny the fire oxygen by either the blanket of gas (in the case of halon or CO2) or by a blanket of solid particles suffocating the fire (in the case of dry chemical). Both will be putting a cold substance on the fire because of the expansion of compressed gas, but I believe the temperature contribution is not their main mechanism of action.
Can you explain the differences in the mechanism of action further? I am especially interested in hearing more about the underlined section.