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National Advisory Committee for Aeronautics, Technical Notes - An Analysis of the Main Spray Characteristics of Some Full Size Multiengine Flying Boats

An analysis is made of available full-scale and model data on the
height of the main spray at the propeller plane of multiengine flying
boats. It is found that, with fixed CAD and Lf/b, increasing the
beam will increase the spray height at a. slightly greater rate than
would. be expected by Froude's Law.

By adjusting the data to a common beam, a. good correlation of spray
height and forebody length/beam ratio is obtained. This correlation in-
dicates a very appreciable reduction in. spray height can be obtained. by
increasing Lf/b.

Appropriate combination of the correlations provides a design chart
which will enable determination of the necessary hull height to keep the
propellers clear of the main spray. This chart is specifically for mul-
tiengine flying boats of generally conventibnal design.

The increasing size and weight of modern flying boats has empha—
sized the absolute necessity of obtaining good water performance in new
designs. In the past, a great deal of trouble has been caused by spray.
Propellers, flaps, and tail surfaces have been seriously damaged by
impact with spray in relatively smooth water, and vision through the
windshield has been badly obscured by spray thrown up the bow in rough
water. It would be very desirable to have means for predicting the heigat
of the spray as affected by the load and dimensions of proposed designs.

It is pointed out in reference 1 that spray has two principal
origins: (l) The "main spray," which exists in smooth and rough water,
originates from the rear 80 percent or so of the forebody; and (2) the
"bow spray ," which normally can be avoided. except in rough water, orig-
inates from the forward. sections of the forebody.

Parkinson (reference 2) correlated. some full-scale experience on
main spray in terms of the forebody length/beam ratio and a spray "inten-
sity" constant. However, since his correlation does not include spray
height, it would. be possible to design a hull having a low value of the
spray-intensity constant and unknowingly locate the propellers so that
they were operating in the spray.