Bedienungsanleitung Navman Diesel Fuel Flow Sensors (Seite 15 von 39) (Englisch)

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Diesel fuel flow sensor kit Installation and Operation Manual

NAVMAN

The propeller is the final link in the chain that

decides if your vessel is a peak performer or just

another boat. The best hull and the strongest

engine will be wasted unless your propeller is

the right size and shape.

From the fuel consumption curve, you can

very simply work out how well your propellers

are working. The performance of a propeller

is measured by its slip factor, and this section

describes how to calculate the slip factor of

your propeller.

Propeller size

First of all, a few basics on propellers. Propellers

are described by a series of numbers - nearly

all propeller people seem to prefer inches to the

metric measurement. For example if a propeller

is described as 24 x 21 then:

The diameter is 24 inches,

The pitch is 21 inches. The pitch is the

theoretical distance the propeller would

move forward through the water for each

full turn.

Slip factor

In reality, because water is a liquid and some

of it will sneak around the edge of the propeller

blades, the boat will not move forward by the

pitch for every turn of the propeller. This is called

the “slippage” or slip factor. The amount of slip

depends on many things - shape of hull, any

rudders, struts, keels etc near the propeller.

Calculating the slip factor

It is very useful to work out the slip factor for

your propeller.

You will need to know:-

A boat speed and RPM setting. Use the

boat speed at maximum rated engine

RPM:

a Either use figures you have taken

from the fuel consumption table (see

section 3).

b Or run the boat at the maximum rated

engine revs and note the revs and

boat speed through the water.

The ratio of the gearbox(es) on your boat

- this will be a number typically between 1

and 3.

The pitch of your propeller in inches.

First calculate theoretical speed:

1 Take the engine RPM and divide by

the gearbox ratio. This will give you the

number of revolutions per minute the

propeller turns.

2 Multiply this number by the pitch of your

propeller in inches. This gives the number

inches forward per minute, if there was no

slip.

3 - Multiply this number by 60. This gives the

number of inches forward per hour.

4 - Divide this number by 72912. (72912 is

the number of inches in a nautical mile)

As an equation it is:-

Theoretical speed =

RPM x pitch x 60 .

Gearbox ratio 72912

The answer is the theoretical speed, for the

chosen RPM, if there was no slippage. The

units are nautical miles per hour (knots).

Because of slippage, the actual speed you

were able to achieve is less than the theoretical

speed.

To find the slip factor (as a percentage):

1 Take the theoretical speed you worked

out above.

2 Subtract the actual speed of the boat to

get the difference in speed.

3 Divide this by the theoretical speed.

4 Multiply by 100 to express it as a

percentage.

As an equation it is:-

Slip factor (percentage) =

Theoretical speed - Actual speed x 100

Theoretical speed

4 Measuring your propeller’s performance

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Navman Diesel Fuel Flow Sensors Seite 15

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