Tom Collins is correct that CO is lighter than air. But, what's best in a house with 8' ceilings is not necessarily best for a boat with 6' ceilings, and the detector manufacturer's instructions are intended for use in residential living spaces, NOT ON BOATS. Some detectors specifically state that are NOT suitable for RVs and Boats, but that's likely just for liability avoidance. Most sleeping locations on a boat are lower than living locations. True for most aft cabins and most fwd berths. Placing the CO detector at mid wall height, but slightly above the level of a sleeping person, minimizes nuisance trips while yet protecting sleeping people. Yes, hydrogen from batteries will alarm propane, CO, and CO2 detectors. And, some smoke detectors. Personal preference how you do this, but smoke detectors high and CO detectors mid wall is a suitable compromise for both. Smoke detectors should be the dual function variety with both visible smoke photocells and radioisotope ionization detectors. Wherever else you put 'em, PLEASE MOUNT ONE ON THE OVERHEAD OF THE BOAT'S ELECTRICAL CLOSET. The electrical closet of a boat is a closed space with poor ventilation. If a fire gets started in that space, it will come to human attention very late in the process. If an overheating connection in the electrical closet is detected in the ionization phase, it's basically a non-event. If it isn't detected when it's still in the ionization phase, it's quite likely to progress to an open flame. If it's detected EARLY in the flame stage, you MIGHT save the boat, but it's definitely an event. Else... Well, at least you have time to get out and save yourself and your crew... Jim Peg and Jim Healy, living aboard Sanctuary http://gilwellbear.wordpress.com <http://gilwellbear.wordpress.com/> Monk 36 Hull #132 MMSI #367042570 AGLCA #3767 MTOA #3436