I. HISTORY AND DEVELOPMENT 



In the course of the development of the facilities of the 

 Hydrographic Office to provide oceanographic data for use in naval 

 operations, a need arose for direct recordings of wave action in 

 deep water. An instrument previously developed by the University 

 of California Fluid Mechanics Laboratory (Folsom, 19hS and 19hl t 

 and Warner, 19k7) and called the 50-Foot Spar Buoy Damping Disk 

 System, consists of a spar buoy whose upper portion is graduated 

 in light and dark bands for recording the rise and fall of the 

 water surface against the buoy on moving picture film.^ The vertical 

 motion of the buoy is dampened to a mathematically determined extent 

 by a large metal disk suspended below the buoy. 



The Beach Erosion Board has developed another type of instrument 

 called the step-resistance gage (Caldwell, 19l*8) for making direct 

 recordings of wave action in relatively shallow areas where the 

 instrument can be fastened to a pier or any other fixed structure 

 extending into the water. Upon conferring with members of the 

 Beach Erosion Board staff, it was deemed practical to install a 

 modified step-resistance gage in a spar-buoy assembly or wave 

 staff (as it is called by the Navy) for use in deep water where 

 the benefit of fixed structures is not available. A wave-staff 

 assembly was provided by the Hydrographic Office, and a step- 

 resistance gage was installed at the Beach Erosion Board. 



The instrument is shown in figure 1. It consists of 3 water- 

 tight sections of 3-inch diameter, 12-foot long aluminum tubing, 

 coupled together with 1-inch threaded pipe to form a 36-foot staff. 

 Below this, at a distance of 20 feet, is suspended a 3-foot diameter, 

 corrugated, metal damping disk. The staff and disk are weighted, 

 the staff floating vertically with 6 feet exposed in calm water. 

 The effective measuring length of the staff is 12 feet. However, 

 the vertical motion is such that under certain wave -frequency 

 conditions, waves up to 20 feet in height can be measured. 



The step-resistance gage is mounted in the upper 12-foot section 

 of the staff and consists of a series of 36 contact points spaced k 

 inches apart, as shown in figure 2. Each contact point is an auto 

 tire valve stem with the actual valve removed, A wire is run through 

 the inside of the stem to its end, which is soldered over to form a 

 contact surface with the water. The valve stems seem particularly 

 suitable for the purpose because they are rugged, light, and easily 



1 The principal disadvantage of this method is the time and effort 

 required to analyze the resulting film strip frame- by- frame for wave 

 height and period. 



