This subject is timely for me right now, because I’m finally in the process of looking around for an outboard engine to provide auxiliary power for my sailing skiff. I know, I’m a blow-boater and have called power boats ‘stinkpots’ more than once in my life, but now I’m thinking about sailing on bigger water on the sounds of North Carolina and that makes it a safety issue. Years ago I had a tiny little outboard to power a daysailer sailboat. At the time all I could afford was a rebuilt Sears Gamefisher with a whopping 1.2 horsepower. No, that 1.2 hp is not a misprint. It ran well but sounded somewhat like a kazoo with hemorrhoids and was vastly underpowered for anything but very calm conditions. The boat I have now is a little heavier and I’m going to be sailing on bigger water so an increase in horsepower and dependability is essential. I’ve settled on one, I think, but the selection process has been long and that is as it should be.
Engine selection should be based on what the boat is or might be used for. An outboard engine to power a small sailing skiff like mine to get me off the water when it’s glassy calm and I see black storm clouds on the horizon will be very different from an inboard auxiliary to power a 150 foot yacht commanded by the rich and shameless. An old rule of thumb is that the engine should be powerful enough to give a speed of at least 2 knots (that’s 2 nautical miles per hour) against a Force 5 wind with the weather shore at least 2 miles distant. A simple old formula for estimating that speed in knots is to take the square root in horsepower, multiply that by 15 and divide by the beam (that’s the width) of the boat in feet. That is for a displacement hull (one that goes through the water), not a planing hull (one that rides on top).
One final word of advice on boat engines, and this goes to inboard engines especially, be sure to check the oil level every time before you run it. Just like a car, if the level is low you may well have a leak and that could lead to engine seizure. But for a boat engine if the level is high, it’s entirely possible that water has found its way into the oil sump. In that case you could crack the cylinder head, break a piston or both, just by cranking it.
All in all, marine engines are pretty durable things, they have to be in order to do what they do. Just be certain to perform your preventative maintenance (PMs), including flushing the engine to get the salt out of it and mothballing procedures at the end of the season so you won’t have to pull your arm out of the socket cranking it the next year.