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Subject: TWL: Re: Kedith's fomula
From: Bob Walters (bobandching@XXX.XXX)
Date: Sun Nov 03 2002 - 11:32:40 EST
When it's all said and done, we're saying the same thing even though we're
getting to it via a slightly different way.
However, you might want to check your math. Using the formula you quote and
your scaling factor of 1.18 I get a hp required of 30.09, only 89% of the
number you come up with.
By the way, formula you quote is slightly different from what I know as
"Keith's Formula" , which is :
HP=D*(KTS/(12(LWL)^.5)^3
Converted to look like your version, I believe the scaling factor would be
1.2, which is quite close to your 1.8 and the exponent would be 1/3 which is
close to your .33, but far enough off to produce significant differences.
No matter, really. If it works for you, that's good.
It appears as though you use a more pessimistic calculation for the
relationship between hp and fuel burn than what I use, so some of our
assumptions offset each other.
As they say, the proof is in the pudding because process I used for the
Nordhavn came up with a predicted fuel burn of 2.045, you came up with 2.02
and the actual burn was reported at 2.08 gph, all pretty darn close and well
within any of the sort of measurements one might expect on a boat with
ordinary instruments like a well calibrated Flow Scan fuel flow meter, a
good clock, calibrated knotmeter, and a properly functioning GPS.
So depending on what scaling factors one elects to use and how one tailors
the exponents of various formulas we use to predict fuel burn, two people
can come up with significantly different numbers, but in the end if we're
talking about fuel burn and everyone winds up with answers that closely
match the real world, then I don't have a problem with how anybody gets
there.
So It appears that both you and I, along with jillions of other folks, can
come pretty close to calculating fuel burn as if we use realistic
assumptions.
Nevertheless, if one is comparing well designed, displacement, pleasure
boats I truly believe that claims of amazing fuel economy are pipe dreams
no matter if they come from bad measurements or bad calculations. I do not
believe they are the result of one modern engine being 30 % better than the
next or one hull form being 30% better then the next.
And I still believe that careful measurements of the fuel burn for a Krogen
49' Express Yacht at "real" speed of 8 knots in "real" water will show a
fuel burn significantly above the claimed burn of 2.5 gph.
Bob Walters
> 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
>
> Larry Z
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