86 NOTES FROM THE MODEL BASIN. 
that the effect of the rough surface of the models as cast is to reduce the maximum 
efficiency as compared with the smooth model from about 72 per cent to about 63 
per cent. 
When a vessel lies for even short periods in waters where fouling occurs 
rapidly there is frequently a considerable accumulation of barnacles and other sea 
growth on the propeller. In order to inwestigate the effect to be expected from the 
worst probable condition of propeller surface, propeller No. 285—that is, the bronze 
propeller with smooth finish—was varnished, and while the varnish was still soft 
there was applied a covering of granulated cork such as is ordinarily used in ship 
work for cork paint. The appearance of the smooth-finished model and of the same 
model covered with granulated cork is shown in Figs. 5 and 6, Plates 66 and 67. 
The results of the test are shown in Fig. 8, Plate 60. 
From this it will be seen that the effect of the roughening of the surface is to 
reduce the maximum efficiency from about 72 per cent to about 36 per cent. If com- 
pared on the basis of the same thrust, the prejudicial effect of the rough surface will 
be even more pronounced. 
AIR RESISTANCE OF A SHIP’S MODEL. 
During the trials of the U. S. S. Neptune it was found that a considerable varia- 
tion of power was required to maintain a given speed, depending on the direction 
and force of the wind. As the Neptune is a collier with a large number of coaling 
booms for rapid handling of coal, the top hamper presents a large area for the wind 
to act on, so an investigation of the air resistance seemed desirable. 
As there was available an “Exhibition Model” of the ship to a scale of %4 inch 
to the foot, on which all top hamper, including rigging and deck obstructions, were 
carefully fitted to scale, the resistance of the model was measured in the wind tun- 
nel at the model basin. 
On Figs. 9 and 10, Plates 70 and 71, is shown the appearance of the model as 
suspended in the tunnel, and on Fig. 11, Plate 72, the results of the test. 
From model experiments in water it is estimated that the vessel, which is 542 
feet in length over all by 65 feet beam, with a displacement of 19,340 tons at a 
draught of 27.5 feet, would require about 3,850 effective horse-power to attain a 
speed of 14 knots. 
From the curves on Fig. 11, Plate 72, it will be seen that with a wind directly 
ahead with an absolute velocity of 30 miles per hour and velocity relative to the 
ship of about 46 miles, the additional effective horse-power to overcome wind resist- 
ance would be about 770, or 20 per cent of the power to overcome the water resist- 
ance alone. This comparison leaves out of consideration any increase in water re- 
sistance due to waves. If the power developed by the engines were not increased, 
this additional resistance of the wind would cause a reduction in speed of about 1.9 
knots. 
In a general way the results of the test in the wind tunnel confirm the results ob- 
tained on trials of the vessel. No exact comparison, however, is possible, because of 
lack of accurate observation of the velocity of the wind at the time of the trials. 
