Ah, I don't know how to put this politely, but
there's been lotsa discussion about hull speed and
horsepower calculations, and while none of it has been
outright wrong (except for that one website calculator
we've all dissed), most of it seems to me to have
missed the point by a large margin. People have been
asking questions (and nothing wrong with that) and
others--mainly the teckies and engineers--have been answering.
But everybody's missing the point that this stuff is
much more of an art than a science, and in the realms
we've all been discussing, the answers really ought to
be--and are acceptable--when they're nothing more than
"seat-of-the-pants" guesstimates and WAGS.<br><br>Hull speed: First,
hull speed is not "variable," as somebody said. It is
a hard-and-fixed number, period. It is 1.34 times
the square root of the waterline length. There is no
other acceptable answer. But here's the problem--half
the people discussing it still don't know what it
means even after they come up with a figure. First off,
it is purely a theoretical number, and that should
be your first clue that we're dealing with voodoo
and black magic here, not science. Maybe 90 to 95
percent of all naval architects agree about hull
speed--but a few don't even believe such a thing exists
(Nils Lucander, with whom I have corresponded on many
occasions in the early 1980s, is the granddaddy of the
"there is no such thing" school of thought). <br><br>For
those who believe, hull speed is the theoretical
maximum your displacement hull will go; the formula
derives from Froude's observations and calculations
regarding the distance between waves (that part isn't
theoretical; no one has ever seen a wave move faster than 1.34
times the squareroot of the interval between ocean
waves). The theoretical part comes when naval architects
adapt this calculation to boats; most say you can, a
few say you can't (without getting into lotsa
theory). Also, the number of people who have lied and/or
greatly exaggerated their boat's hull speed probably
exceeds the tales of bass fishermen about the size of
their fish and the tales of men in the locker room
about...well, their prowess. <br><br>Be that as it may, most of
the present discussions involve relatively smaller
boats--30 feet and under, many well under 22--and
relatively small horsepower--in single digits. It is
basically pointless to try and calculate these kinds of
numbers with any precision--it's like calculating the
cost of birdfood for your canary every month. 37
cents? 39 cents? 40.684 cents? yeah, you can do it--but
why bother? similarly, why bother to calculate the
hull speed and horsepower for a 16 foot canoe---3.87
knots? 4.112 knots? 4.38769324 knots? It's false
knowledge--it's not useful for anything. You can't buy a 1.67HP
motor, or a 2.11398 motor, and if you're buying a
Minn-Kota or MotorGuide, shame, shame on you if you pick
the 27 lb. thrust instead of the 31 lb. thrust,
thereby saving yourself $7.36 at the K-Mart sporting
goods section. That's not naval architecture, it's not
engineering, it's not science, and it's not art. It's amateurs
using numbers they don't understand, and deluding
themselves into thinking they've made rational engineering
decisions.<br><br>Ya wanna know how to pick horsepower? One horse for
every 750 lbs. Round upward to the nearest whole
number, and double it if you're nervous. Keep the
calculator in the drawer until you're in the 20-30
horsepower range and have to think about which diesels are
available. Boat motors are like computers: buy the biggest,
toughest, most powerful one you can afford; no one sits
down and calculates that they "only" need 57 mhz of
clock speed and "only" 14.8 megs of RAM. Better to have
a big one running at half speed than a small one
running at flank speed.<br><br>End of sermon.<br><br>Bill
