Join Date: Apr 2006
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For the same reason we park in the driveway and drive on the parkway: To confuse alien invaders and illegal immigrants.
Ron, they are absolutely great. The early ones (20 years ago) were problematic because when vented improperly they could give off enough carbon monoxide to kill you while you were showering. Look for one with CO detection (or get that anyway) and make Real Damn Sure you follow the instructions for ventilation. "Proper" marine ventilation probably will mean that both the intake and exhaust are outside the boat and you may have to run some ductwork to get them installed properly.
Outside of the US, a very big piece of the world market for home hot water heaters has been using these for a long time now. The technology (including installation precautions mainly) is way better than what scared sailors 20 years ago.