Double the Horsepower and
Double the Speed
By Robert A. Musch

(Excerpted from Messing Around In Boats)
(click here for more information about MAIB)

My friend, Jim Lohr, owns a 14' Crestliner fiberglass motorboat powered by a 6hp Evinnide outboard. With two of us aboard it will do about 6mph because it just does not have enough power to plane. By himself, Jim says it will plane and do about 7mph. Since I own a 6hp Johnson outboard, we decided to see what the boat would do with two engines on the stem. We moved one engine aside and attached the other next to it with only inches to spare. The propellers were only 18" apart and the engines touched on turning, giving us about 15 degrees of turning radius. We knew we would be making wide turns.

The boat was launched on the Miles River on the Chesapeake on a calm August afternoon, and I took along my Etrex global positioning satellite receiver to accurately measure speed. After warming up the engines we smoothly accelerated up to 10 mph. Then the trouble began. The starboard mounted Evinrude would pull smoothly all the way to full throttle. However, the port mounted Johnson would pull well only to about half throttle before it would start to cavitate. Engine rpm would increase dramatically without any increase in speed.

Jim had his hands pretty busy adjusting throttles and steering tillers while trying to watch where he was going. He tried various combinations of throttle settings and engine alignment to get the highest speed, but the best we could do was 11.1 mph. I moved my weight forward and aft to trim the boat while videotaping the whole procedure. Then we went ashore so I could take photos from the beach. All the weight in the boat was moved forward for trim and he tried it by himself

The highest speed attamed was 13.3mph before that port engine would start over-revving. Why did the port engine cavitate? The three-bladed propellers were identical and turning in the same direction although they were very close together. Were they interacting with each other? It felt like the engines had a lot more power to give but the boat just could not use it. Does this boat simply lack enough planing surface to get up and go no matter how much horsepower is applied? Perhaps one 12hp motor would push this boat faster because there would be less surface protruding below the waterline. On the way back to the ramp we noticed that everything just seemed "right" with this motor combination at about 10 mph.

We didn't prove very much. Our wives would never imagine putting two engines on a small boat and spending hours trying to make it go faster. But we had a lot of fun running up and down the Miles River laughing the whole way, and that is the point of sim ply messing about in boats.