Scale effects on planing craft

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ARCHLEF

SCALE EFFECTS CN PLANING CRAFT

by

N

J.B. Hadler and W.L. Moore

Naval Ship Research and Development Center

Periodically, investigators at model basins have examined the problem of scale effects upon the trim of planing craft when tested ¡n the

"free.-to-trim" condition but relatively liti-le of this is ¡n the open literature

where the significan of the problem can become apparent.

The earliest published efforts were made in the early 1930's, in

Germany, on seaplane floats. Both Sottorf In Reference (I) and Schmidt

in Reference (2) studied scale effects by making comparisons of _geosims

with full-scale seaplane floats. Their work clearly showed that there

was the expected scale-effect, on the viscous drag and that it could be

easi ly corrected by the usual means. _{The effect of this viscous drag}

correction upon the trim, however, is much smaller and in the opposite

direction to the difference ¡n the trim measured ori the geosims and the

full-scale floats.

After World War Il, the David Taylor Model Basin started a scale effect investigation using a geometric series of models based upon the PT-8 hull.3 The results of the trim measurements made upon the five

models of the series versus volume Froude number _{are shown in the attached}

figures fran Reference (3). The models range in size frani 16.7 to 5.6 ft.

In the displacement región, there is good agreement _{among the various}

models. _{In the planin.g regime, though the data show some inconsistencies,}

the smaller models trim significantly higher than the _{larger ones. When}

it ¡s considered that this series spans less than a 3 to I ratio, whereas

Lab. y. Scheepsbowk

Technigsch00t.

most ful I-scale to model ratios are much larger, we shou ldn't be too surprised at the magnitude of the discrepancies we may experience when making correlations.

The hydrodynarnic mechanism which Is causing _{this scale efféct on}

trim is not clear. lt is upoh this pain-t- that we must concentrate _sorne

of our small craft research if we _{are to properly predict the performance}

of planing craft from model tests.

REFERENCES:

(I) "Scale Effect of Model in Seaplane-Float _{Investigations," W. Sotiorf,}

NACA Tech. Memo No. 704, Apri I 1933.

"The Scale Effect ¡n Towing Tests with Ai rplane Float Systems, Rudolph Schmidt, NACA Tech. Memo No 826, May 1937.

"Planing Boat Scale Effects on Trim and Drag (1MB _{Series 56),"}

W.L. Moore and F. Hawkins, NSRDC Tech.. Note 128 _(1969).

2

5

4

2

Figure 3 - Change of Trim vs. Froude Number for Geometrically

Similar liodels of Different Lengths

Symbol Model Test LWL(Ft)

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1° by Stern

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Figure 4 - Changé of Trim vs Froude Number for Geometrically Similar Modela of Different Lengths

Symbol Model Teat LWL(Pt)

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Design Displacement

Zero Trim