[PCW] fuel economy

[Tim Askins]

I'm currently running a production built Venturer 44 with Yanmar 6LP-STE 300
hp engines designed by Crowther and built in Austrailia. At 6-8 knots we are
cruising in a nearly "no wake" mode and burning 1.5-3 GPH in "typical" (Is
there such a thing?) conditions. With 600 US gallons on board that gives us a
2,000 NM +/- range, well beyond the target of 1000 NM. The beauty of the cat
hulls is that at at my preferred daytime, good weather (6ft or less and below
25 knots)cruising speed of 18 knots we are still only burning 16-18 gals/hr
with No bouncing ,pounding or listing and no stabilizers. If I need to make
harbor before dark or beat a squal I can bump it up to 24 knots and burn 30-35
gal/hr. Like the Manta this boat also experiences the "No Man's Land"
speed/fuel consumption ratios in the 10-15 knot range. This boat, also like
the Manta has the accommodations of a typical 55 ft monhull- 2 queen
staterooms and a quarter berth and 2 full heads. Engine room space is tight
but adequate. The cockpit is enormous.

Many of Malcolm's boats experience better results. Orren Byrd's Tennant
designed "Hummingbyrd" 62ft LOA had a range of 2000 NM at 18 kts on 15000 gals
and double that at 8 knots. But keep in mind slender hulls are really 12:1
beam/length ratio or higher, with the optimum being 14-16:1, and this is only
one of the many factors that determine fuel efficiency in a cat. Construction
weight, full load dispalcement, engine size, transmission selection, wheels,
fuel capacity and superstructure wind resistance all contribute. There's
always a compromise depending on the owners ultimate goals of
speed/distance/economy.

For instance,if you only wanted to go 8-10 knots (like a monohull) in the
Venturer 44, one could change the transmissions and props and optimize the
boat to perform in that range, but it would be practically impossible without
going to much smaller engines-say 50-75 HP per side. Then we'd have extra
engine room space, change the required amount of fuel on board, reduce full
load displacement, change the boats moments and balances and we have
re-designed the boat.

I think for me the bottom lie is the cat hull and Malcolm's CS Form in
particular offer options for speed, range, fuel efficiency, comfort and
accommodations far beyond anything you'll get in a monohull.

Cheers
Tim Askins
M/V Arriba
Charleston,SC




> Date: Thu, 10 Jan 2008 22:19:55 -0800> From: john_r_holbrook at yahoo.com> To:
power-catamaran at lists.samurai.com> Subject: Re: [PCW] fuel economy> > Hi
Robert, > > Thanks for the response. The bit about wetted surface resistance
vs wave-making resistance and speed is new to me and makes sense. I was under
the impression that some power cats were very fuel efficient and capable of
crossing the Atlantic, i.e. 2000 nm range in the 10 - 12 kn range. > > Yes my
boat is not efficient at 10 kn and will not reach 12 kn but a 1000 nm plus
range is the deciding factor for me. You mention that the manta will travel at
higher speeds reasonably efficiently, i guess that is subjective and depends
what type of boats you are comparing it with. > > >From a different
perspective with fuel prices increasing the resale value of boats in 5 years
may be greatly influenced by their fuel efficiency. > > For the record the
Manta looks like a great boat and that fact that I'm using it as a benchmark
here is because it is what data i had available for this genre of boat. > >
Cheers> John> Seahorse 52> Singapore> > >
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