THE EFFECT OF SIZE ON THE SEAWORTHINESS OF HYDROFOIL CRAFT 
A. Hadjidakis 
Aquavion Holland N.V.,. The Hague 
The pitching movements and the vertical accelerations of hydrofoil craft 
with surface-piercing foils are studied in a very simplified way to show 
the effect of size as a function of wave parameters, and speed or Froude 
number. Considerations are given to variations of static pitch angle, 
heave, natural frequency, and damping ratio—the latter based on a line- 
arized equation of motion. The analysis indicates that at wave lengths 
of 2/3 and 3/2 of the length of the hydrofoil craft, extreme values of 
vertical accelerations are to be expected, which decrease rapidly with 
increasing lengthofthe craft, or decreasing speed, which leads to the con- 
ae that the seaworthiness increases with the length of the hydro- 
oil craft. 
INTRODUCTION 
The seaworthiness of hydrofoil craft is sometimes doubted, while the comfort offered to 
the passengers is often considered to be insufficient, when going on a seaway [1-7]. The 
purpose of this paper is to give in the most simple way some insight into the behavior of 
hydrofoil craft on sea, and to determine the most unfavorable conditions, which are compared 
for craft of different sizes. 
This, of course, needs some explanation. The high cost involved in constructing big 
hydrofoil craft makes it necessary to obtain experience regarding seaworthiness, stability, 
and comfort, as well as structural loadings and other technical aspects, by means of scale 
models or small prototypes. It is therefore desirable to prove that the results obtained in 
this way can be interpreted for craft of greater size. 
Today a hydrofoil craft does not present problems on relatively smooth water. Its 
behavior under unfavorable conditions is thus decisive for its suitability for passenger 
transport at sea. 
Therefore it is necessary to gather the existing knowledge which may contribute to a 
prediction of the properties of a big hydrofoil craft, based on the known performance and 
behavior of a small craft, so that a project for a seaworthy passenger ferry can be started 
in full confidence. 
The following considerations and calculations have been formulated in a general way, 
making them valid for a large number of existing hydrofoil systems. However, they are not 
valid for systems which cannot be compared to a spring-and-mass system. Thus they do not 
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