Fuel Usage..hulls, engines
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
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now.
Putting aside number of engines, HP, hull shape, prop size/pitch...all of
which are hard to control, just look at the wake. Is it not correct to say:
The smaller the wake, the more efficient the hull? (Assuming same waterline
length and speed.)
Watch a Krogen 42 and a GB 42 side-by-side at 8 knots...GB wake is
substantially larger. It takes more energy to create the larger wake.
Bob
Robert Calhoun Smith Jr in DC
M/V MARY KATHRYN
Hatteras 58 LRC
National Harbor
Near Washington, DC
-----Original Message-----
From: RC Smith Jr
Putting aside number of engines, HP, hull shape, prop size/pitch...all of
which are hard to control, just look at the wake.
Is it not correct to say:
The smaller the wake, the more efficient the hull? (Assuming same
waterline length and speed.)
Watch a Krogen 42 and a GB 42 side-by-side at 8 knots...GB wake is
substantially larger. It takes more energy to create the larger wake.
Bob
REPLY
Very true. It is something that got buried by all the marketing hype
surrounding the Trawler craze and leavened by the desire to have all the
luxury comforts aboard.
My very first boat was unusual. It was double ended with a canoe stern.
It had a single barn door rudder 36 inches high but also had twin screws.
To make life more interesting, when the original Thornycroft diesels
(original 1936) finally died in 1956, they were replaced with a pair of P6
Perkins but both turning the same way. Very interesting handling.
Point being the hull was more like a Colin Archer Tahiti ketch than a power
boat. Beam was 11 feet a hul llenght on waterline was 48 feet.
At seven knots I could cruise through the harbor anchorage without causing a
distubance. I had about a 6" or 8" wake at full throttle and doing 10.5
knots. Total horsepower was 125 HP both engines combined. At full throttle
and doing 10.5 knots the engines burned 3 imperial gallons per hour by
actual measurement.
A yacht builder in Mission BC told me why they always built semi
displacement hulls even if the owner only planned to cruise at hull speed.
A flat sectioned hull mold is easier to modify and add or subtract hull
lenght than one with curves.
You can blame the builders and the marketing people for creating a market
acceptance of less efficient hulls, because it improves their profit margin.
My hat's off to builders like Krogen who have stuck to their guns in
maintaining a more efficient hull shape despite popular trends. But I see
that there is also a Krogen express hull with flatter hull sections aft.
Oh well.
cheers
Arild
"Arild Jensen" 2elnav@netbistro.com 1:36 PM 4/30/08 >>>
My hat's off to builders like Krogen who have stuck to their guns...But
I see that there is also a Krogen express hull with flatter hull
sections aft.
Oh well.
Reply:
I've spoke to the Kadey Krogen folks at the boat show and have asked if
Krogen Express was a related company. The answer was "No, the company
is not related". It seems there must be some kind of connection there
but they don't seem willing to talk about it.
Dale Klahn
A couple John & Betsie T. bought the Krogen Express line from Kadey-Krogen a
few years back and re-tooled the mold from a 48 or extended 53 versions to
the current 52 foot model. They are built in the same yard as their cousins
Kadey-Krogen. How ever they market separate and are indeed separate
companies.
Stuart
-----Original Message-----
From: trawlers-and-trawlering-bounces@lists.samurai.com
[mailto:trawlers-and-trawlering-bounces@lists.samurai.com] On Behalf Of Dale
Klahn
Sent: Wednesday, April 30, 2008 10:50 AM
To: T&T T&T
Subject: Re: T&T: Hull efficiency axiom... Kadey Krogen/Krogen Express
"Arild Jensen" 2elnav@netbistro.com 1:36 PM 4/30/08 >>>
My hat's off to builders like Krogen who have stuck to their guns...But
I see that there is also a Krogen express hull with flatter hull
sections aft.
Oh well.
Reply:
I've spoke to the Kadey Krogen folks at the boat show and have asked if
Krogen Express was a related company. The answer was "No, the company
is not related". It seems there must be some kind of connection there
but they don't seem willing to talk about it.
Dale Klahn