Page 247 - Introduction to Naval Architecture

232 PROPULSION

and:

Hull efficiency
Using the thrust deduction factor and Froude's notation:

Now TV A is the thrust power of the propeller and RV S is the effective
power for driving the ship, with appendages, at V s. Thus:

Using Taylor's notation, P E = (&r) (I - t)/( I - u^).
In terms of augment of resistance (l-t) can be replaced byl/(l-fa).
The ratio of PE to P T is called the hull efficiency and for most ships is
a little greater than unity. This is because the propeller gains from the
energy already imparted to the water by the hull. Augment and wake
are functions of Reynolds' number as they arise from viscous effects.
The variation between model and ship are usually ignored and and the
error this introduces is corrected by applying a factor obtained from
ship trials.
The factors augment, wake and relative rotative efficiency are
collectively known as the hull efficiency elements.

Quasi-propulsive coefficient (QPC)
As already explained, this coefficient is obtained by dividing the
product of the hull, propeller and relative rotative efficiencies by the
appendage coefficient. If the overall propulsive coefficient is the ratio of
the naked model effective power to the shaft power:

The propulsive coefficient = QPC X transmission efficiency.

1
The transmission efficiency can be taken as 0.98 for ships with
machinery aft and 0.97 for ships with machinery amidships. The
difference is due to the greater length of shafting in the latter.

DETERMINING HULL EFFICIENCY ELEMENTS

Having debated in qualitative terms, all the elements involved in
propulsion it remains to quantify them. This can be done in a series of