[PCW] Fuel consumption

[Malcolm Tennant]

There are currently a lot of figures flying around on the power catamaran
list for various types of boats and various power plants. I would like to
add a baseline to the figures being bandied about by adding to the mix the
figures for a vessel for which there exists very precise  data,
professionally measured, and also the means by which this data was obtained.

The power catamaran is a full displacement CS hull.  13.75m length overall ,
[44.29'] has an overall beam of 5m [16' 5"] and has a full load displacement
of 13,300kg [29,040lb]. Powered by 2 x 156kw[210hp] diesel inboard engines.
It has a top speed at full load displacement of 24.9knots and at a cruising
speed of 19 knots and uses 2.2 litres of diesel/nm at 19 knots . This is 41
litres per hour [a total of 10.83 us gals /hr].

There are some very important assumptions being made when making the
measurements [1] the engines reach their designed maximum rpm. [2] the
propellers are as efficient as they can be.

How was this data obtained?  The measurements were made by an independant
fuel consumption consultant company[ as they always are]. The boat was in
full load displacement [full fuel,full water,food and all the people] which
is the worse case scenario.  The measurements were made in flat water as it
is almost impossible to quantify rough water. The procedure was as follows:-
the fuel lines were cracked and a fully calibrated, certified, professional
fuel flow meter inserted into the system. The tachometers were calibrated
using a strobe light on the tail shaft.And the gearbox reductions were
recorded. It is important that both engines be checked because if the
propellers are contra rotating then it is possible that one gear box is
running in reverse and has a different reduction ratio. Which must be
determined to get the right engine rpms. The throttles were then opened
200rpms at a time until the maximum rpm is reached. Time on distance was
measured in each direction for each throttle opening.

It is very difficult to rely on tank tenders unless they are very carefully
calibrated. Given, that it is unlikely that your fuel tanks are uniform in
cross section, a dip stick also needs to  be calibrated.

It is a tedious and time consuming job obtaining accurate fuel figures but
it is the only way to be sure. It also serves as a check on the accuracy of
the vessels own fuel flow system which can be critically important on trans
ocean trips in adverse conditions.

Regards,

Malcolm Tennant.

Malcolm Tennant Multihull Design Ltd
PO Box 60513 Titirangi,
Waitakere 0642
NEW ZEALAND

ph +64 9 817 1988
fax +64 9 817 6080

e-mail malcolm@tennantdesign.co.nz
www.tennantdesign.co.nz
www.catdesigners.com