Harry Krause wrote:

My recollection is that BOATING magazine's "cruise speed" for planing
boats is the throttle setting that produces the best fuel economy while
the boat is on plane. Typically, it is somewhere around 70-75% of the
engine's top RPM on a boat with a gas engine.


and Gould 0738 commented:

Actually that would not be correct. The greatest fuel economy with a planing
hull boat (eliminating the "dead idle" option) is achieved just after the boat
reaches plane.



I think you are both correct but some tweaking of Gould's comment is
required. Engine load and fuel flow rate won't be minimized simply by
getting the boat on plane. Once on plane, proper trim of the boat via
load distribution or engine, outdrive or tab trim will often result in
an increase in RPM and speed for a given throttle setting. Properly
trimmed and on plane, the ideal cruise speed (with best fuel economy)
may be several knots or mph higher than the boats natural planing speed.

Diesel engines are easy. The manufacturer's recommended cruising RPM on
my boat is 200 below maximum RPM. Simple.

Eisboch