Page 199 - Introduction to Naval Architecture

RESISTANCE 185

may be running in the region of mixed flow. The ship obviously has
turbulent flow over the hull. If the model flow was completely laminar
this could be allowed for by calculation. However this is unlikely and
the small model would more probably have laminar flow forward
turning to turbulent flow at some point along its length. To remove this
possibility models are fitted with some form of turbulence stimulation at
the bow. This may be a trip wire, a strip of sandpaper or a line of
studs.

Frictional resistance experiments

William Froude carried out the first important experiments in the early
1870s, using a series of planks with different surface roughnesses. He
tried fitting the results with a formula such as:

where /and n were empirical constants.
He found that both/and n depended upon the nature of the surface.
For very rough surfaces n tended towards 2. The value of/reduced with
increasing length. For smooth surfaces, at least, n tended to decline
with increasing length.
Later his son proposed:

in conjunction with/values as in Table 8.1.
The /values in Table 8.1 apply to a wax surface for a model and a
freshly painted surface for a full scale ship.
Within the limits of experimental error, the values of /in the above
formula, can be replaced bv:

2
where JRf is in Ibf, / in ft, S in ft and Fin knots, or:

2
where Rf is in newtons, /him, Sinm and Fin m/s.