Page 207 - Practical Ship Design

Powering I I73

Taylor’s units are: V in knots, L in feet, and A in tons of 2240 Ibs. R, and R, in lbs.
The R, /A values for the actual BIT are then obtained by interpolation.
The skin friction R, is calculated on the basis of
RflA = f S V‘.83 (6.35)

Taylor used a value off= 0.00904 as derived by Tideman as applicable to a 500
ft ship of steel construction (riveted construction, clean and well painted) and
suggested taking a value of S from the following formula:
s = CJAL

The coefficient C has an average value of 15.4 in its original units of tons, feet and
feet squared which becomes 2.6 for units of tonnes, metres and metres squared.
Taylor’s wetted surface formula remains a widely used one and a modified SI
unit graph of the coefficient plotted against midship section coefficient and BIT is
reproduced as Fig. 6. I.
In their day, Taylor’s methods gave quite an accurate estimate of the resistance,
although some variables that are now thought to have a significant effect are
missing. For example, no account is taken of the position of the longitudinal centre
of buoyancy and no distinction is made between single-screw and twin-screw
forms, both of which are generally recognised as influencing the resistance. By the
same token, the power estimate gives little help towards designing lines to
minimise the power required.
A much improved presentation of the Taylor tests is given in “A Reanalysis of
the Original Test Data for the Taylor Standard Series” by M. Gertler published in
DTMB Report 806 of 1954.
Several tanks to this day present the ratio model resultRaylor prediction as an
indicator of quality.

6.6 AYRE’S C2 METHOD

This was presented by Sir Amos Ayre first in 1927 and revised in 1933 and 1948 in
papers to the North East Coast Institution of Engineers and Shipbuilders.
Ayre’s formula is

(6.36)

This is very similar to the Admiralty coefficient formula, except that the
formula is for EHP and not SHP, and the index of the displacement is 0.64 and not
213. Contours of C2 are plotted on a base of VI& for a range of LlA”3 from 10 to 30.