Hi Group, I have an actual boating question as opposed to all the
political crap on the list now. On the average what kind of gal/hr
usage would I be looking at with either a single V-8 or twin V-8s and
probably Volvo drives, probably small block Chevy at say between 10 to
15 knots cruise in a 27' to 30' flybridge cabin cruiser? Thanks, Jim

--

On Sun, 16 Dec 2012 10:32:22 -0600, "James"
wrote:

Hi Group, I have an actual boating question as opposed to all the
political crap on the list now. On the average what kind of gal/hr
usage would I be looking at with either a single V-8 or twin V-8s and
probably Volvo drives, probably small block Chevy at say between 10 to
15 knots cruise in a 27' to 30' flybridge cabin cruiser? Thanks, Jim


You are asking what is essentially an unanswerable question.

The fuel consumption of a small block chevy will depend on:

Carbureted or injected engine
Even the model of carburetor will effect fuel consumption.

Displacement:
Small blocks were built in sizes ranging from 265 cu. inches right up
to 400 cu. inches.

Rpm that the engine is operated at.

And,

LWL of boat

Weight of boat

Type of hull - planing, semi-planing, deep hull displacement, shallow
hull displacement, etc.

Weather: wave pattern, prevailing wind during voyage, etc.

Current/tide: Going up the river - or down the river.

Condition of bottom - smooth and hard bottom paint? Marine growth? and
so on.

Basically a internal combustion engine's fuel consumption is a factor
of the horse power produced and the RPM at which it is being produced
but unfortunately the usual installation lacks the instrumentation
necessary to measure these values.

The usual method is to fill the tanks and then make a fairly long trip
and than top off the tanks to determine how much fuel was used to
arrive at a gallon/hour figure. Which would be an approximation and
will vary considerably depending on conditions.
--
Cheers,
Bruce

On Mon, 17 Dec 2012 08:43:50 +0700, Bruce
wrote:

On Sun, 16 Dec 2012 10:32:22 -0600, "James"
wrote:

Hi Group, I have an actual boating question as opposed to all the
political crap on the list now. On the average what kind of gal/hr
usage would I be looking at with either a single V-8 or twin V-8s and
probably Volvo drives, probably small block Chevy at say between 10 to
15 knots cruise in a 27' to 30' flybridge cabin cruiser? Thanks, Jim


You are asking what is essentially an unanswerable question.

The fuel consumption of a small block chevy will depend on:

Carbureted or injected engine
Even the model of carburetor will effect fuel consumption.

Displacement:
Small blocks were built in sizes ranging from 265 cu. inches right up
to 400 cu. inches.

Rpm that the engine is operated at.

And,

LWL of boat

Weight of boat

Type of hull - planing, semi-planing, deep hull displacement, shallow
hull displacement, etc.

Weather: wave pattern, prevailing wind during voyage, etc.

Current/tide: Going up the river - or down the river.

Condition of bottom - smooth and hard bottom paint? Marine growth? and
so on.

Basically a internal combustion engine's fuel consumption is a factor
of the horse power produced and the RPM at which it is being produced
but unfortunately the usual installation lacks the instrumentation
necessary to measure these values.

The usual method is to fill the tanks and then make a fairly long trip
and than top off the tanks to determine how much fuel was used to
arrive at a gallon/hour figure. Which would be an approximation and
will vary considerably depending on conditions.

===

Bruce's comments are all correct but it is possible to make some rough
estimates based on the most popular small block configuration and
usage.

By far the most popular small block engine is the carbureted 350 cubic
inch version (5.7L). That engine is typically rated at about 260
horsepower and cruised between 3200 and 3400 RPM. Under those
conditions each engine will usually burn about 13 gallons per hour.

I've owned two different boats with single small block V8s, and a
third boat that had a pair of 454 big blocks. The general rule of
thumb is 1 gallon per hour for every 10 horsepower actually developed.
Actual horsepower varies as the square of RPMs so you can get an
estimate by dividing cruising RPM by maximum (wide open throttle) RPM
and then squaring that number to get the fraction actually produced.

For example: 3400/5000 = .68

..68 squared = .46

..46 times 260 rated hp = 120 actual horse power

120/10 = 12 gph

Those "rule of thumb" estimates work out to being very close over a
wide range of boats and conditions. On my old boat with the two
carbureted big blocks (a relatively heavy 33 ft sportfish), each
engine was rated at 350 hp and we ran them at 3200 RPM, developing
about 50% of rated power. Typical fuel consumption was 35 gallons per
hour. A friend of mine has a 38 ft express cruiser with a much newer
pair of fuel injected big blocks. He also burns about 35 gph.