304 H. R. Chaplin 
You have got to think of the places you can’t get to any other way, the marshes, the bogs, 
the ice flows, the out of the way places where you can’t land an aeroplane. These are the 
places where I believe the hovercraft will really take root and where we shall gain the 
experience we need to allow us ultimately to build big ocean craft. 
L. Landweber (Iowa Institute of Hydraulic Research) 
The following comments were written before the contents of the present paper were 
known. That can be justified, however, by noting that these comments will serve as a 
bulwark to the title of Mr. Chaplin’s presentation which they supplement by describing the 
work in this field that is being done at the Iowa Institute of Hydraulic Research. These 
comments were written by Lawrence R. Mack and Joachim Malsy: 
Increasing interest in the possibilities of ground effect machines hasled many organiza- 
tions in several countries to undertake basic studies of ground effect phenomena or studies 
of the means of practical utilization of these phenomena or both. During the past two years 
the Iowa Institute of Hydraulic Research, with the financial support of the Office of Naval 
Research, Contract Nonr 1509(03), has conducted both analytical and experimental investiga- 
tions of the behavior of an annular-jet nozzle in proximity to solid and liquid surfaces. The 
results of the first year’s work were presented at the Princeton Symposium on Ground Effect 
Phenomena in October 1959.* Since then a thesis describing an experimental study of an 
annular jet moving over water has been completed;' the results of this study, together with 
certain results for a stationary jet over land obtained preparatory to the over water experi- 
ments, are contained in a forthcoming report to the Office of Naval Research.* It is the 
intent of this discussion to summarize briefly these results. 
A rigidly supported 7-inch-diameter annular nozzle discharging air vertically at different 
rates through a 1/8-inch gap was towed at different altitudes and speeds (including zero 
velocity) over initially quiescent water. Chosen combinations of these three variables led 
to 18 runs, for each of which the configuration of the water surface in the vicinity of the 
nozzle and the pressure distribution on the base of the nozzle were determined and plotted 
in the form of contour drawings. 
The measurements of water-surface configuration were obtained by means of a capaci- 
tance wire and a point gage and were supplemented by stereo photographs as an aid in pre- 
paring contour maps. All stationary cases show a deep annular depression, caused by the 
impinging jet sheet, about 1.2 nozzle radii from the center line of the jet. The water surface 
under the nozzle base was considerably higher than what would, under the assumption of 
hydrostatic pressure distribution in the water, correspond to the pressure within the base 
cavity, a ridge at about 0.7 nozzle radius actually projecting above the still water surface. 
Forward speeds produced a considerable change in surface configuration from that of the 
static case. A forward ridge, or bow wave, was clearly discernible in all runs. With higher 
jet momenta and especially with higher speed, this bow wave seemed to split into two 
separate ridges, each situated in front of the nozzle about 45 degrees from the direction of 
* Lawrence R. Mack and Ben-Chie Yen, “Theoretical and Experimental Research on Annular Jets 
over Land and Water,” Proc. Symposium on Ground Effect Phenomena, Princeton, Oct. 1959, pp. 263- 
284. Also available as Reprint No. 164, State University of lowa Reprints in Engineering. 
t Joachim K. Malsy, “Experimental Investigation of an Annular Jet Traveling over Water,” M.S. 
Thesis, State University of Iowa, Aug. 1960. 
¢ Lawrence R. Mack and Joachim Malsy, “Experimental Studies of an Annular Jet,” Iowa Institute 
of Hydraulic Research Report to the Office of Naval Research, Sept. 1960. 
