Masthead28_Dec2013 - page 33

The Masthead
Dec. 2013 Page 33
ack in 1913, my father was wondering what the most efficient shaft angle would be for the 60-plus-mph racing
Chelsea II
, which he and Harry Andrews had just built.
Chelsea II
was a single-step, 20-foot hydroplane, with an adjustable Monel plate on the step for trim control. She was
powered by twin 225-hp, overhead-valve, overhead-cam Triebert engines driving one prop thru a V-drive gearbox, which
Pop designed and built.
In the 1908 book
Speed and
Power of Ships,
the grandfather of
modern naval architecture,
Admiral Taylor, wrote that small
French hydroplanes were using
large shaft angles with good re-
sults. Admiral Taylor wrote that as
a result further research should be
done on the matter. My father did
the research as follows. He moved
the gearbox forward for the lowest
possible shaft angle, then amidships for an average shaft angle, and aft for the highest shaft angle. At all three shaft
angles the boat made 60 mph. The shaft angle made no difference. Pop reasoned, with his sixth-grade education, that the
slipstream from the high shaft angle was reacting against deeper less disturbed water while the low-shaft-angle
slipstream was reacting against shallow more disturbed water which reduced propulsive efficiency.
The above research was confirmed by Bill Garden, in his book, which showed his design,
We’ll Sea,
with 20-degree shaft
angle, which was very efficient. Understanding this has enabled me to design boats with engines further aft than my
competitor’s boats, and thus have more room for cabin accommodations. The down-angle gears, available for the past
twenty years, allow more shaft angle without causing engine-lubrication problems.
Westlawn graduate David Martin has spent a lifetime designing all types of boats.
His books,
Martin’s Yarns
and the
Book of Dave Martin Designs
are available on CD from
to purchase
Martin’s Yarns
on CD from
to purchase the
Book of Dave Martin Designs
on CD from
As Dave Sees It
What is the Most Efficient Shaft Angle?
By Dave Martin © 2012 By David P. Martin
The author’s father and Harry Andrews, in 1913, racing at 60 mph on the
Chelsea II
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