Page 218 - Introduction to Naval Architecture

204 RESISTANCE

FULL SCALE TRIALS
The final test of the accuracy of any prediction method based on
extrapolation from models must be the resistance of the ship itself. This
cannot be found from speed trials although the overall accuracy of
power estimation can be checked by them as will be explained in Chapter
9. In measuring a ship's resistance it is vital to ensure that the ship under
test is running in open, smooth water. That is to say the method of towing
or propelling it must not interfere with the flow of water around the test
vessel. Towing has been the usual method adopted.
The earliest tests were conducted by Froude on HMS Greyhound in
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1874. Greyhoundwas a screw sloop and was towed by HMS Active, a vessel
of about 3100 tonf (30.9 MN) displacement, using a 190ft (58m)
towrope attached to the end of a 45ft (13.7m) outrigger in Active. Tests
were carried out with Greyhound at three displacements ranging from
1161 tonf (11.57 MN) to 938 tonf (9.35 MN), and over a speed range of 3
to 12.5 knots.
The pull in the towrope was measured by dynamometer and speed by a
log. Results were compared with those derived from a model of
Greyhound and showed that the curve of resistance against speed was of
the same character as that from the model but somewhat higher. This was
attributed to the greater roughness of the ship surface than that assumed
in the calculations. Froude concluded that the experiment 'substantially
verify the law of comparison which has been propounded by me as
governing the relation between the resistance ships and their models'.
In the late 1940s, the British Ship Research Association carried out
full scale tests on the former Clyde paddle steamer, Lucy Ashton. The
problems of towing were overcome by fitting the ship with four jet
engines mounted high up on the ship and outboard of the hull to avoid
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the jet efflux impinging on the ship or its wake. " Most of the tests
were at a displacement of 390tonf (3.9MN). Speeds ranged from 5 to
15 knots and the influence of different hull conditions were investi-
gated. Results were compared with tests on six geometrically similar
models of lengths ranging from 9 to 30ft (2.7 to 9.1 m). Estimates of
the ship resistance were made from each model using various skin
friction formulae, including those of Froude and Schoenherr, arid the
results compared to the ship measurements.
Generally the Schoenherr formulae gave the better results, Figure
8.13. The trials showed that the full scale resistance is sensitive to small
roughnesses. Bituminous aluminium paint gave about 5 per cent less
skin friction resistance and 3.5 per cent less total resistance, than red
oxide paint. Fairing the seams gave a reduction of about 3 per cent in
total resistance. Forty days fouling on the bituminous aluminium hull
increased skin frictional resistance by about 5 per cent, that is about \