Masthead31_Sept2014.pub - page 5

The Masthead
Sept. 2014 Page 5
First To Fly
Continued
hull s—i n many vari ants—though the term i s se ldom used i n general adverti si ng .)
Herreshoff , you see ,
was work ing from
the prac tical si de . A
success fu l (poss ib ly
the
most
success fu l)
and very busy boat
bui lder and
des i gner , he a l ready
knew that if you
t r ied to make a boat
go very fast, it wou ld
l if t i ts bow and si nk
i ts stern. I t seemed
c lear to h im that a
wi de f l at s tern wou l d
cont rol th is
tendency . It a lso seemed obvi ous to him that a ll owi ng some smal l inc li nat ion of the hul l would
cause the water to st ri ke it f rom underneath and l ift it some . By 1887, the Herreshof f des igned
Now
Then
—an 85 footer with double -wedge form—made the passage f rom Newport to New York Ci ty
averag ing
21.2 knot s. Th is is a speed length rati o of 2.3, whi ch i s not qui te full y pl ani ng , but darn
c l ose .
Sl ender Sl im
Still, most fast boat s of the day (here and in Europe) were long and narrow wave piercers, rather
than sk immers . There ’ re ac tual ly some good arguments for these u lt ra -s lender , round -bilge speed -
s ters . One is
that they can
be run at
nearly full
speed i n ver y
rough water
because the ir
s lender hu ll s
cut the waves ,
reduc i ng
pound ing
great ly .
Another is
that they’re
very effi cient ;
they need far
l ess power to
achi eve full speed, and full speed coul d be qui te fast i ndeed! The
Ursula,
was about the las t of
these long s lender racers . Designed and built i n Eng l and by S. E. Saunders , she was l aunched in
1908. Forty-nine feet overall (and exactly the same on the waterli ne), she we ighed 11,800 pounds ,
and was a mere 6- foot 6-i nch beam—slender i sn’t the word !
Ur sula
was cl ocked at 35 knot s , pow-
ered with a single 760-hp gas eng i ne . Thi s i s a speed - length rati o of 5, which accordi ng to some
means she was pl ani ng .
The Rat i o of Speed
What is speed -length rat io? Simp le . Take any boat’s length on the waterl i ne , in feet , and get the
square root of that . In
Ursu l a
’s case the square root of 49 feet i s 7. Then di vide the boat speed , in
knot s , by the result . Agai n, for
Ursu l a ,
we get 5—a speed-length ratio of 5 (35 knots
÷
7 = SL Rati o
5) . The important thi ng here is that convention says that any boat goi ng faster than an SL Rati o of
3 i s p lan i ng , regard less. Thi s , though , is too c rude a way of l ook ing at i t.
I ndeed, a light fl at bottom ski ff , may wel l plane at a S/L ratio of just 2.2, while a heavy deep-vee
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