Seattle, Washington. The original collector 

 was lost due to failure of the support cable 

 during early sea trials, but the trials showed 

 that certain changes were needed to make the 

 unit durable and reliable. Among these were 

 a heavier cable and linkage elements, heavier 

 hoses, enlarged hose-cable terminal assem- 

 blies, a rigid bridle, and a smaller pump. The 

 earlier hose, which was 2-inch I.D. thin- walled 

 vinyl tubing had the advantage of transparency, 

 but it stretched excessively with internal 

 pressure and was also susceptible to the 

 development of weakpoints and subsequent 

 small leaks at sites of folding and abrasion. 

 The earlier chain bridle, though simple to 

 fabricate, fouled too easily during setting of 

 the collector. The earlier pump, which had a 

 1-inch diameter intake, frequently overloaded 

 the electrical system and stopped operating. 

 It required power in excess of that obtainable 

 from the 1/2-hp. motor, and a larger motor 

 would have required power in excess of that 

 obtainable from the vessel. 



Though the above changes have made the 

 collector and hose-cable assembly reliable 

 and durable, they do not make the instrument 

 capable of sampling at various depths. This 

 will require, in addition to further changes 

 in the collector and hose-cable assembly, 

 very considerable improvements in the winch. 

 The winch in its present configuration does 

 demonstrate that a hose of considerable length 

 can be reeled in and out while water is being 

 pumped, but the unit is not strong enough to 

 sustain the load that a longer hose and greater 

 depressing force would produce, nor could it 

 accommodate a hose much longer than that 

 now being used. 



Sampling at various depths will also require 

 continuous depth-of-tow information. The in- 

 clusion of reliable depth sensing and tele- 

 metering apparatus will be a priority item in 

 the design of future models of the system. 



The only difficulty with the filtering unit 

 was the appearance of pinhead rust spots on 

 the filtering screens after the two funnels 

 together had been exposed to nearly 600 

 cubic meters of sea water (approximately 100 

 hours of sampling). These were patched with 

 epoxy to forestall the development of holes. 

 It is probably unreasonable to expect longer 



service than this from stainless steel screen- 

 ing as delicate as that used here. Future im- 

 provement in the filters should include some 

 simple way of replacing these surfaces at 

 intervals. Monel screening might well elimi- 

 nate this problem, but it is not available in 

 mesh sizes as small as that used here. 



The temperature sensing and recording 

 apparatus incorporated into the filtering unit 

 should also be improved in the future. The 

 electronic components do not perform reliably 

 enough for sustained operations, and the loca- 

 tion of the sensing element at the filters 

 rather than in the collector is questionable. 

 It may well be that the temperature increases 

 as the water moves from the collector through 

 a few hundred feet of hose to the sensing ele- 

 ment. Submersible temperature sensing units 

 and various kinds of telemetering equipment 

 are available so that a satisfactory solution 

 should be a matter of integrating a unit with 

 the desired performance characteristics into 

 the existing system or some later model of it. 



SUMMARY 



1. A towed plankton pump and shipboard 

 filtering system for sampling small zooplank- 

 ters has been designed for use aboard the 

 Bureau of Commercial fisheries vessel 

 Black Douglas. 



2. The system consists of (1) a collector 

 towed by a suspension unit consisting of a 

 hose through which runs a steel cable for 

 support and an electric line to power the 

 pump, (2) a winch, and (3) a filtering unit 

 composed of a watermeter, a double-throw 

 valve and two filtering funnels. 



3. The collector, which contains a 1/2-inch 

 bronze centrifugal pump and an hermatically 

 sealed, capacitor starting, 1/2-hp. A.C. motor, 

 is mounted in a frame bridle suspended from 

 a terminal hose-cable assembly made of 

 galvanized pipe and fittings. A 43-pound homo- 

 geneous depressor is suspended from the 

 bottom of the bridle. 



4. The hose is a 2-inch I.D. single- jacket 

 firehose and both the 1/4-inch stainless steel 

 support cable and the neoprene covered power- 

 line extend to the deck of the vessel through 

 the hose. 



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