40 J. D. van Manen 
1. Encountering waves of length and height in which it is not designed to operate. 
Statistical or spectrum analysis of the sea conditions in the operating area can provide 
much data to guide the design of the craft as regards lift, stability, control, and response. 
It can also provide an assessment of the probability of meeting isolated severe waves, but 
these cannot be predicted in the true meaning of the word. 
2. Impact with such isolated severe waves, or even waves over which the craft is 
designed to operate if, for instance, a defect develops in the control system. In this con- 
nection, it should be noted that while models of the size normally associated with ship tank 
work can be used to measure the external forces, an assessment of the stresses induced by 
these forces is a matter for the structural designer. To relate the stresses measured in the 
full scale craft directly to those measured in a model, the latter would need to be of the 
order of 1/2 to 3/4 full size. So far as ] am aware, no attempt to carry out investigations 
of this nature has yet been made, but presumably data available from flying-boat investiga- 
tions would provide valuable guidance. It is significant also to add that the structure of 
hydrofoils and GEMs can hardly be regarded as “robust” in comparison with displacement 
or planing craft. 
Having been forced to land in severe conditions from any such cause as | or 2, or due 
to engine failure, the question arises: Can the craft withstand the normal bending and shear 
forces imposed in a seaway, or the impactive forces due to slamming? The answer to this 
question also has yet to be resolved but in the absence of data on the subject, it is fairly 
sound to assume that present designs would need to be strengthened. 
It is realized these observations may be regarded by some as being very pessimistic. 
At the same time, it is necessary to place the possibilities of high performance craft in their 
proper perspective in relation to what can already be achieved by displacement craft in 
unexpected severe weather conditions. The simple fact is that the planing or displacement 
craft can successfully sustain any sea condition, and proceed, even though the speed may 
be considerably reduced. Actually the reduction of speed is not so serious as is sometimes 
implied. Comparisons can be odious and figures can be exaggerated in the absence of fac- 
tual data. Nevertheless it is my opinion that, for instance, size for size, the planing craft 
will still be capable of proceeding at nominally high speeds in conditions when the hydro- 
foil or the GEM is brought virtually to a standstill. In such conditions it is the planing 
craft which has an overwhelming advantage. Incidentally it must not be forgotten that a 
planing craft to carry 50 to 60 passengers at speeds exceeding 50 knots in calm water now 
offers no design or constructional difficulties. 
The conclusion to be drawn from these few comments on this one aspect of the possi- 
bilities of high performance surface craft is that there exists a real need for investigations 
to determine more precisely their limitations in irregular sea conditions. This entails not 
only the advancement of theory and model experiment but also correlation with full scale 
results in similar conditions at sea—a long and arduous task following the lines of what is 
already being attempted for displacement craft. Costly and time consuming though the task 
may be it is quite essential to carry it out if any real comparison is to be made between the 
orthodox and unorthodox type of craft. If it is not carried out then the risk must always 
remain that a craft designed for some specified state of sea will meet more severe condi- 
tions and suffer serious damage or even loss, and as so often happens with new ventures, 
this could cause a serious setback, if not cessation, in development. 
