Page 263 - Practical Ship Design

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- The choice of material used for the blades; a strong material permits the use
of slender blade sections giving a higher efficiency.
- The number of blades; propellers with fewer blades generally have higher
efficiency; those with more blades have the advantage of producing smaller
pressure pulses and less vibration.
- Blade area; blade shape
- Distribution of pitch;
- Blade rake and skewback.
A well known statement about propellers that is both comforting and disturbing, is
that it is very difficult to design a really bad propeller but equally difficult to design
a really good one. In practical terms this means that a naval architect can expect to
get a propeller whose efficiency is within about 2% of the currently accepted best
possible for the design conditions -but will find it very difficult to get that further
2% that will give a really fuel efficient ship.
Probably the most common fault with propellers is that they are not matched to
the engine and are either too heavily or too lightly pitched (see the next section).

7.5.4 Specibing propeller design conditions

The design of a fuel efficient ship can only be achieved if amongst other things the
propeller design conditions favour fuel efficiency, a question that will generally go
back to the specification and contract.
If these require a high trial speed using full power there is bound to be a
tendency to design the propeller specifically to meet this. This is likely to result in
the propeller being over pitched for the service speed which in turn may mean that
the engine will not be able to develop full power within the limit of permitted
cylinder pressure and is almost certain to result in to the engine developing
excessive pressure and with it excessive cylinder wear.
A form of specification which, whilst recognising that the speed must be
measured on trial, gives the designer the best incentive to design for service
conditions is:
- On trials in deep water and in fair weather conditions, with the ship newly dry
docked and loaded to a draft corresponding to a deadweight of “D” tons, the
service speed of “R’ knots is to be obtained with the machinery developing
not more than “H” S.H.P., thereby demonstrating that when operating at the
service power “S’ there is a margin of power of M = (S - H)/H to maintain the
same speed “K” in service conditions of weather and fouling.
- If trials are run in ballast, the speed “K” is to be obtained on a reduced power
calculated from the tank test results to represent an equivalent performance.
For many ship types acceptance trials can only be run in ballast.