Page 221 - Introduction to Naval Architecture

206 RESISTANCE

of 1 per cent per day. The results indicated that the interference
between skin friction and wave-making resistance was not significant
over the range of the tests.
18
Later trials were conducted on the frigate HMS Penelope by the
Admiralty Experiment Works. Penelope was towed by another frigate at
the end of a mile long nylon rope. The main purpose of the trial was to
measure radiated noise and vibration for a dead ship. Both propellers
were removed and the wake pattern measured by a pitot fitted to one
shaft. Propulsion data for Penelope were obtained from separate
measured mile trials with three sets of propellers. Correlation of ship
and model data showed the ship resistance to be some 14 per cent
higher than predicted over the speed range 12 to 13 knots. There
appeared to be no significant wake scale effects. Propulsion data
showed higher thrust, torque and efficiency than predicted.

EFFECTIVE POWER

The effective power at any speed is defined as the power needed to
overcome the resistance of the naked hull at that speed. It is sometimes
referred to as the towrope power as it is the power that would be
expended if the ship were to be towed through the water without the
flow around it being affected by the means of towing. Another, higher,
effective power would apply if the ship were towed with its appendages
fitted. The ratio of this power to that needed for the naked ship is
known as the appendage coefficient. That is:

Effective power with appendages
the appendage coefficient =
Effective power naked
Froude, because he dealt with Imperial units, used the term effective
horsepower or ehp. Even in mathematical equations the abbreviation ehp
was used.
For a given speed the effective power is the product of the total
resistance and the speed. Thus returning to the earlier worked
example, the effective powers for the three cases considered, would
be:

(1) Using Froude.

Total resistance = 326 700 N
326 700 X 15 X 1852