T&T: Fuel Usage..hulls, engines

[Robin Brueckner]

Al's post regarding two different fuel usages got me thinking further
about hull and engine configurations. His post was not the fuel
efficiency comparison I was seeking between (just) hull types as in his
post one boat was twin screw and one single screw.

There are several  related issues  which after all my years boating I
either don't know or maybe forgotten the answers:

(1) If you have an identical hull and one is twin engine and one is
single screw, what's the typical difference in fuel consumption at the
same cruise speed?  (Same engine types would be the best comparison.)

(2) What's the typical difference in efficiency between the three hull
types...displacement, semi and planing?

On the first, I'd guess (but don't know) there's maybe a 15% or 20%
difference, likely a bit higher with twins in common practice as the twin
screw boat would frequently cruise faster.

Anybody know figures for the same boat?....Grand Banks, for example, made
single and twin screw models of some boats....somebody must have data...

On the second, my original post on the subject, I'm asking about the
hulls themselves, not differences due to engine configurations. Did I
guess 5% to 15% there?? Anyway, if Al's post of 4 and 2.7 GPH  are in the
ballpark at 8 knots, that's a total difference of 2.7/4 or ...0.68  32%
less fuel use for the single and more efficient hull...my silly guesses
from above would range between 20% and 35%, not too shabby for a first
approximation..

 But I still have no idea about the proportional causation...Arild's post
suggests more of the difference may be do to hulls...definitely a
possibility...

For the record, on the Hatteras Owners Website ( all "planing" type
hulls) several people including myself, posted about running a twin screw
boat on a single engine (one engine off) . I did not have flow scan data,
some did. I think a summary would be that while there was appeared to be
some  gain in efficiency running on one engine it was (a) minor, (b)
likely due to a slower cruise speed, (c) a general pain because many
transmissions cannot be allowed to rotate when not running. Then we got
into a discussion of "freewheeling" or locked prop as most efficient...If
I recall there was contradictory evidence from different owners using flo
scan data (fuel usage) data...so maybe different props perform
differently or maybe somebody's data was off....

And to repeat an earlier post, since power rises as roughly the cube of
speed, slowing down a bit is still the easiest way to save fuel as long
as your engine is running hot enough to burn fuel cleanly.

Rob Brueckner

Hatteras YF

New Rochelle, NY

------------------------------------------------------------------------

Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it
now.