Double the Horsepower and
Double the Speed
By Robert A. Musch
from Messing Around In Boats)
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.