151 



the lifting cable but of sufficient strength to lift the concrete base, holds the two 

 halves of the structure together. A lifting line of 3/8" wire rope is attached to 

 the top of the pipe framework and to a marker buoy. If an attempt is made to 

 lift the tripod while it is covered with sand the shear pin will fail and only the 

 pipe framework need be lifted to recover the instrument. 



The concrete base of the Woods Hole tripod is provided with a cavity di- 

 rectly below the tripod in which cable can be coiled. A sufficient amount of 

 cable is stored in this cavity to reach the surface of the water, allowing the in- 

 strument to be removed without lifting any cable which might also be covered 

 with sand. 



Typical tripods used by the Waves Research Group of the University of 

 California are usually 5 to 7 feet in height with the instrument located 4 to 5 feet 

 above the base. Scrap metal or cast concrete blocks are used to increase the 

 tripod weight. The lifting cable is attached between the top of the tripod and a 

 marker buoy. 



Marker Buoys For Sub-surface Pressure Gages - In the past, the methods of 

 attaching the lifting line and marker buoy have been very unsatisfactory. Usual- 

 ly the tripod was lowered to the bottom by a 3/8" to l/2" wire rope and a buoy 

 attached to the lifting line after the tripod was in place on the bottom. To pro- 

 vide working cable to pass over the hoisting frame and attach to the winch, the 

 lifting cable length was normally made twice the depth of the water. Several 

 difficulties arose from this type of installation. The continual working of the 

 cable due to the buoy following the surface waves weakened the cable and eventu- 

 ally caused failure. To reduce this action a snnall buoy with buoyancy just suf- 

 ficient to remain afloat under the weight of the cable was used. These small 

 buoys would still break the cable under the action of large waves, especially 

 after several months of exposure of the cable to salt water. 



Marker buoys installed according to Coast Guard specifications using 

 chain between the anchor and the buoy, certainly would last a longer time but 

 would require larger boats to make the installation and a larger tripod to serve 

 as an anchor. Typical Coast Guard specifications for a small open-sea type 

 buoy would be as follows: 3rd class special nun buoy, 656 pounds; 3/4" chain; 

 maximum water depth, 14 fathoms; chain length 2| times water depth; ZOOO 

 pounds concrete block anchor. An installation of this type should be serviced 

 once each 6 months (paint buoy and check chain). The chain should last between 

 1 and 2 years, depending upon the annount of wave action. 



Greatest wear of the chain occurs between the links that touch bottom 

 during wave troughs at low tide and the links that are lifted off the bottom during 

 wave crests at high tide. This wear is due to the rotation of each link as it is 

 lifted. Additional chain may be lifted off bottom during storms and high winds, 

 but the percentage of time is small and the wear of these links is not as critical 

 as that wear caused by normal wave action. 



By using a chain whose entire length cannot be lifted by the buoy, a rela- 

 tively small anchor can be used. The anchor serves only to prevent the chain 

 from being dragged along the bottom during large storms. A long chain also 

 prevents any snapping action caused by the buoy lifting all the slack out of the 

 line. This snapping action is probably the greatest cause of cable failure when 

 small, round buoys are used with wire cable lifting lines. 



The inconveniences of handling larger anchors, buoys and cable, as nor- 

 mally used by the Coast Guard, have prevented their use. A small, light 



