A hardwired CO marine rated detector near the nav table at calf level on a fused breaker --I turn it on when underway it draws next to nothing. Its about 3 feet from the engine at mid point. The other is a off-the shelf First Alert fire and CO combo-runs all the time of two AA's I keep it on a shelf near the V berth where we sleep. It goes off twice a season when I desulfinate the batteries for some reason, the gases trigger it. Its a good test. As coastal vacation cruisers we spend a lot of nights at slips and some at crowded anchorages and I worry about other boats running something near me and the fumes being sucked or drifting in. I had two beloved neighbors die with their grandchildren in the house across the way 10 years back. I'm crazy about this.