Ship Speed _ 
The fact that surface vessels have not especially in mercnant vessels, is an im- 
undergone the same degree of evolution in portant factor as indicated in Fig. 4 which 
speed as have various other means of trans- shows the reduction in speed computed from 
portation does not detract from the consid- drag alone, and the average voluntary re- 
erable economic and military importance of duction experienced by a number of merchant 
greater speed to the Navy and merchant ships transiting the North Atlantic. 13 The 
marines. A part of the reason for this slow computed reduction due to drag at the upper 
evolution is that although the speed of any limit of sea state 6 was 14 percent, whereas, 
ship is basically dependant upon its design the actual reduction amounted to 56 percent 
characteristics, it is further limited by of the sea state 0 speed, 
such factors as currents, winds, and most 
important of all, by sea state which serves As a result of many actual measurements, 
to reduce speed through the following effects: James 13 has been able to construct ship speed 
(1) added resistance due to wave reflection, versus wave height curves for several mer- 
especially in short waves; (2) rolling; (3) chant ship classes. These curves give average 
pitching; (4) indirect effect of added re- speed reduction values for head, beam, and 
sistance to propulsion; (5) propeller racing; following seas and are presently being used 
and (6) voluntary reduction in speed to ease to construct Least Time Tracks in the Navy's 
severe motion of the ship. Ship Routing Program, Table 1, taken from 
these curves, Shows the effect of head and 
It is difficult to correlate the speed of a following seas of various heights on the 
given ship with sea state, For one thing, speed of several classes of merchant ships. 
the period of the waves and the frequency Of interest here is the fact that following 
of encounter must be considered along with seas also cause a consideration reduction 
wave height. Too, the degree of voluntary in speed, 
reduction in speed varies with captains and 
conditions, Voluntary speed reduction, 
TABLE 1. THE EFFECT OF WAVE HEIGHT AND HEADING ON SHIP SPEED 
SHIP TYPE _ 
Wave |T5-S-12A | T2-SE-A2| T2-SE-Al| VC2-AP3 | VC2-AP2 
Ht. _ SHIP SPEED (in knots) AND HEADING 
(feet) |H*¥ Fe pe HF Hae 
20 1825) 18.5 pore 
10 Vian lea5 (CREE 
Tae Il.1 
* H - Head Seas, F - Following Seas 
231 
