"a" wrote

Even more aviation: you could not maintain altitude: couldn't get up
on to planning speed.

But 5 knots? My 25 foot swing keel O'Day (think of the keel as a
vertical wing) could do that in a reasonable breeze and it
was NOT a fast boat.
s
Not surprising, to me.

Every boat has a "hull speed" than can be calculated, relating mainly to
length, that applies in a strictly displacement mode. The longer, the
faster, generally. The O'Day 25 (used to have one) had a hull speed of
about 6 1/4 knots. To go faster you would have to apply enough HP (LOTS
more HP) to get up on plane, and I suspect the one engine in the power boat
did not have enough power to get it over the hump. Also consider that a
considerable amount of one engine's HP would be consumed by rudder drag,
since the rudder would have to be nearly 45 degrees to keep the boat going
straight.. Also, the 5 knots mentioned is probably not terribly accurate,
since the accuracy of a power boat's "speedometer" is not very good at low
speeds.

It hurts my head to think about what parts of the two engines were
common, and which were different.

Not that bad, I think. Camshaft, crankshaft, oil pump, water pump, harmonic
ballancer, distributor, alternator.

_ Jim in NC