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





"Lloyd Sumpter" wrote in message
...
On Thu, 25 Sep 2003 15:17:23 +0000, Lloyd Sumpter wrote:


BTW: I see lots of equations, etc. for estimating top/cruise speed of
various boats with various engines. Anyone have anything I could use to
estimate theoretical cruise/top speed with these various engines?

Before I had a GPS, the M25 (21hp) could (I think!) push me at 6.5knots
WOT (I'm guessing my knotmeter might have been a bit optimistic). Now I'm
lucky to get 5.5, even with a clean hull. Any guesses what 26, 29, or 31hp
would get me?

Lloyd Sumpter
"Far Cove" Catalina 36