T&T: Fuel Usage

[Robin Brueckner]

"..A proper Displacement hull is much more fuel efficient then a
semi-displacement or planing hull at lower speeds..."

I'm going to disagree with the above statement (emphasis on "much")  as
an exaggeration but would like to see some actual slow speed test results
among some different boats. Anybody got actual comparisons for different
hull styles or the same displacements and speeds? 

A long thin hull is absolutely the most efficient, but not the most
comfortable nor spacious. The relatively small displacement and greater
length of this style trounces the three categories above. A displacement
sailboat would also trounce a displacement power boat in efficiency
because most often the sailboat total displacement is much smaller. For
example, a 40 foot sailboat usually has a much smaller engine than a 40
foot "trawler" because is displaces much less volume, yet many can cruise
at six or seven knots.

I'd guess maybe there could be a 5% to 15%  difference based solely on
hull shape...no bulbous bows...among the three traditional
categories...but I don't KNOW.

I spent about an hour searching "hull speeds" but never found any data
for different makes of production power boats of any type.

For example, is a single engine Grand Banks a lot less fuel effcient than
a same length and same displacement Seaton (or Thompson or Nordhaven)
trawler travelling at the same speeds? The Grand Banks is really more of
a planing hull and the Seaton (or Thompson) a round bottom deep draft
boat by comparison. I SUSPECT  that for two such similar sized single
screw boats, efficiency is NOT  all that different.

I THINK total displacement is likely a larger factor than hull shape at
slow speeds, but that's a guess on my part not a statement of fact.

FACTS:

At slow speeds frictional resistance between water and hull  IS the
primary opposing force while over speeds of about 1.0  speed/length ratio
wave making drag becomes predominant.

(Speed/Length ratio is the speed in knots divided  by square root of
length waterline..) In other words, a slow moving hull makes little
waves, waves grow as does speed.

And since power increases as roughly the cube of the hull speed,
comparing a six knot "trawler" with a 9 knot "planing" hull is totally
misleading. That's where a lot of misconception arises.

Rob Brueckner

Hatteras YF

New Rochelle, NY

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