speed vs. hp

Bill Zeitler (WZeitler@compuserve.com)
Fri, 6 Nov 1998 18:02:09 -0500


- - - - - - - - - - - - - - - - - - - - - - - - - - -
West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
- - - - - - - - - - - - - - - - - - - - - - - - - - -
>Dear fellow sailors,
>I conducted some tests over a known distance to determine whether or not I
>could exceed hull speed in my P-15. With a 3.5 hp nissan, max speed of
5.3
>mph was achieved at 25% throttle. No increase in speed was measured at
50%,
>75% or 100% throttle. If one assumes a linear relationship between
throttle
>setting and horsepower, this seems to say that approximately 1 hp is
>sufficient to achieve hull speed in calm winds and no chop. However,
wind,
>current, and chop would necessitate having a reserve of at least 2 hp.
Has
>anyone conducted similar tests?
>Richard Dickerson, Ph.D., Diplomate, ABT
>Associate Professor
>The Institute of Environmental and Human Health
>Texas Tech U/TTU Health Sciences Center 5C200

Richard,

Although I sold my P -15 earlier this year I still am a die-hard " potterer
"....now in a 21 foot Bay Hen. The Bay Hen is a flat bottom, shoal draft,
_displacement_ hull. I think however that if one uses enough
horsepower, both a P -15 and a Bay Hen would get up and plane. Not
being interested much in speed I am yet always " messing about it boats ".

I have read that maximum hull speed for displacement hulls is 1.34 times
the square root of the waterline lenght. I bought a " Knot-Stick " "
speedometer " unit which is a calibrated disk at the end of a long elastic
line that drags behind the boat and causes a pull on a calibrated spring in
the handheld unit onboard. The waterline length of my Bay Hen is 18
feet. This would indicated I should have about 5.6 knots of speed max [
unless grossly overpowering ]. I have a 5 HP / 4 Cycle Honda outbaord [
which by the way I used on my P -15 with a reinforced transom...in that the
engine weighs about 62 pounds ]. I replaced the standard prop with a
lower pitch " workboat prop " in that my boat weighs about 1,000 pounds.

With the KnotStick set for max range of 0 - 9 knots, I began my experiments
in calm water.

At idle rpm I get 3 knots
At Cruise rpm I get about 6 knots
At Max rpm I get a _non-linear_ 7.5 knots.

By close metering of fuel [ using a graduated cylinder and measuring fuel
before the tests and fuel remaining after the various tests, I calculated
that at cruise rpms [ 6 knots ] I use from 0.25 to about 0.33 gallons [ US
] per hour. So my rear end will give out before my 3 gallon remote tank
is used up!

At least with _my_ hull I would guess if I put about 25 HP on I would
plane. I'm sure a P -15 would also...and I think others have done so in
their P 15s [ even under sail! ].

Enjoy your " messing about in your boat ".

Since I operate a lot on the Delaware River...tankers, freighters,
tug-boats, and some times a pretty good tidal current, I am comfortable
with all 5 HP as a safety factor. When I'm bucking the current even with
5 HP my speed made good over the ground is noticably slower ! My main
concern about speed is that I get home in time for dinner ! ;-D>

Bill Zeitler
Wilmington DE
A " potterer " at heart.
Ex- P -15 # 2060 [ X - " Just Wight " ]