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Subject: TWL: Re: Kedith's fomula
LRZeitlin@XXX.XXX
Date: Sat Nov 02 2002 - 12:14:48 EST
In a message dated 11/2/02 5:03:50 AM,
trawler-world-list-request@XXX.XXX writes:
<< Keith's formula does not give accurate hp predictions. In fact the
predictions it gives are routinely less than 70% of the true hp required in
the real world. Nordhavn's data for a repersentitive 24 hour run at 6.29
knots Keith's formula would predict a fuel burn of 1.435 gph while the actual
burn was
2.08 gph. The formula I normally use predicts a burn of 2.045, which is
within a couple percent of what the Nordhavn actually burned.>>
Bob,
What constant are you using in Keith's formula? I suspect you were using the
one given in Skene which computes power requirments for speeds measured in
statute miles per hour, not knots. That would explain why your calculations
for Nordhavn's fuel consumption differ substantially from mine. Using your
info, and Keith's formula as stated in Skene, 8th ed., but with a constant
adjusted for speed in knots, Nordhavn should have been using an average of
33.64 HP during that portion of the run. It should have consumed 2.02
gal/hr., a difference of less than 3% from the actual consumption. That's an
order of magnitude less than the error you state that the formula estimates.
You can't accept any of the maritime formulas on blind faith but Keith's
formula seems to be one of the more reliable ones for approximating power
required to move a displacement hull. It considers LWL, displacement, speed,
and power. Inherent in the calculations are assumptions made about propeller
efficiency, specific hull configuration, units of measurement, and sea
conditions. These are included in the scaling constant. In our marine
engineering work, we generally measure the power requirements of a similar
hull under known conditions and use it to establish the scaling constant. The
formula (and several more complicated variations) are then used for estimates
of power requirements as loading is increased or decreased or dimensional
changes are proposed. Nothing beats actual tank tests or full scale prototyp
es, but using a computer is a lot cheaper.
If you can cite any scientific study for the 30% underestimate of power
requirements in Keith's formula as correctly applied, I would like the
reference.
For those of you who want to work the calculations for yourself, Keith's
fomula is:
KTS = (LWL)^.5 x C x ((HP x 1000)/D)^.33
LWL is designed waterline in feet, at rest.
D is displacement in pounds.
C is a scaling constant varying from 1.1 to 1.5. It corrects for measurement
units, variations in hull form, and propeller efficiency. The constant used
in my calculations is 1.18 and is based on experimentation with trawler-like
full displacement hulls.
This provides a good estimate of the power required for trawlers of a given
length and displacement, moving at a specific speed.
Larry Z
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