Page 265 - Practical Ship Design

Powering I1 223

There are a number of formulae for f, w, and qr, most, if not all, of which were
derived by regression analysis. An examination of these shows:
(i) That thrust deduction f is reduced if the water flow to the propeller is good
and the clearances from the hull are relatively large; aspects of design
which should in any case be observed to avoid the possibility of propeller
induced vibration. Favourable factors are:
relatively small propeller diameter
relatively large LIB ratio
relatively fine Cb
Lcb relatively far forward
In practice a designer is unlikely to be influenced by these considerations.
(ii) That wake fraction w, depends primarily on the block coefficient with
which it increases linearly, indicating that water is entrained to a greater
extent by a full ship than by a fine one, and on the ship’s length and the BIT
ratio.
(iii) That relative rotative efficiency qr increases primarily with increasing
propeller diameter relative to ship length and to a lesser extent with increase
in block coefficient.

Although all these formulae may enable a reasonably accurate prediction of hull
efficiency to be made, they give disappointingly little guidance on how to improve
hull efficiency. There does not for example appear to be any size effect such as is
given by the & in Emerson’s QPC equation. This is made even more surprising
when the same reference then goes on to give a formula for QPC which has the N
& term plus a number of other refinements.

7.7 TRIAL AND SERVICE ALLOWANCES

7.7. I Trial power
It is important to remember that power estimates cannot be relied on to be 100%
accurate. Grigson’s statement that the errors in the ITTC friction line can cause an
underestimation of power by as much as 7% certainly suggests caution. There is
also the question of shell finish and what standard can be relied on.
The margin of power (over the best estimate) which it is wise to provide should
depend partly on whether the ship is reasonably similar to ships which the shipyard
has already built and partly on the penalty invoked for non-compliance with the
trial speed. The author generally allowed 5% initially in the power given to the
marine engineers but was not sorry when a bigger margin arose, as it generally did,
when the main machinery was selected.