Go with CO2. Halon is fine for computer rooms and other sensitive areas, but good old CO2 is still the standard on commercial and naval vessels. When CO2 is discharged in your engine room - the fire WILL go out and so will the engine. Your engine will not be damaged because as far as the engine is concerned, the air supply was removed. (You can also use it cool beer down real quick) But be careful - if you are in the engine room and the CO2 is discharged - not only will your engine stop running - so will you, you will only have a few minutes to get out before you pass out and die from lack of oxygen! Detroit Diesels have an air cutoff butterfly valve as standard, for other engines, you can buy a butterfly air shutoff and install a Morse or Teleflex cable to it and run the cable to the helm station. Make sure that the butterfly valve is sized for your size engine. Halon is popular because the canister is as lightweight as dry chemical (powder), but does not make a dusty clean-up nightmare after discharge. Also, all the major boating outlets market Halon - where you must go to a commercial fire suppression equipment supplier for CO2. The tanks are very heavy and of the same construction as oxygen and compressed welding gases and should be installed, plumped and "tagged" by your supplier. Brian-