Don:

Interesting method -- I haven't seen it in any of my reading. (I
should say I've been to your web site, so I assume that you know what
you're talking about.)

Perhaps you can help with the following facts, then. (For background,
see www.mvFintry.com). The Royal Navy made it very easy to measure
fuel consumption on the Fleet Tenders (and, I would guess, on its
other vessels as well) with a special three tank and sight gauge
permanent setup. As a result we have good fuel consumption figures at
various speeds. Assuming that fuel consumption is a good proxy for
horsepower (ignoring the typical small increase in specific fuel
consumption as engine speed decreases), we have pretty good numbers
for the horsepower required at various speeds.

Base point is 325hp at 10.5 knots, S/L 1.21. From there down to seven
knots, your cube formula gives a higher horsepower than the fuel
numbers, topping at 144% high at nine knots. At seven knots (S/L
0.81) the two curves cross, and below that the cube rule falls off
much faster than the fuel consumption would indicate.

Am I correct to guess that the cube formula applies in the "sort of
well behaved" area where wave making dominates, but not overwhelms,
from around S/L 0.70 to 1.20? Above that, it fails low and below
that, friction is more important and not very speed dependent.

Is there an explanation for the 44% error? Given that cube formulas
are sensitive to starting point, it could well be that the full speed
horsepower is less than 325, which would change the SFC number from
the 18.55hp-hr/USG used.

Am I missing something?

Jim Woodward
www.mvFintry.com




"D MacPherson" wrote in message ...
Lloyd:

There is a quick way to guess at the speed increase for a modest repower,
given the following assumptions.

Assuming,

a. the existing engine is producing full power and it has a properly matched
gear and propeller,

b. your new engine will also have the proper gear and propeller,

c. the vessel is of a displacement type (i.e., not of a planing craft form,
as a significant immersed transom changes speed-power relationships),

d. your operating speed is below a speed-length ratio of about 1.2 (this
keeps things well-behaved),

e. the loading of the boat (draft, displacement and waterline length) will
not change, and

f. your speed data is reliable,

then, you can scale the expected speed increase by the cube-root of the
power increase. For example, if you were going 6.5 kts with 21 hp, and your
new power is 31 hp then your speed increase is the cube-root of 31/21, which
equals 1.14 (or a 14% increase in speed). So, your expected speed (with all
of the above assumptions) is 7.4 kts. In our experience, this offers a
pretty reasonable answer (if not a wee bit conservative).

Regards,

Don

Donald M. MacPherson
VP Technical Director
HydroComp, Inc.
http://www.hydrocompinc.com
tel (603)868-3344
fax (603)868-3366