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Commercial Fisheries News
Volume 36 Number 7
March 2009
Pot hauler configuration critical to rope wear
In anticipation of requirements to use sinking groundlines on lobster trap trawls in most areas, the Massachusetts Division of Marine Fisheries and the Atlantic Offshore Lobstermen’s Association have worked since 2003 to identify durable sinking line and to understand the causes of groundline degradation and failure.
Our previous research focused on the rope itself as the main determinant of durability. We assumed that damage to sinking ropes was caused primarily by abrasive particles of sediment. During those tests, however, we discovered that seemingly small changes in the pot hauler sheaves made a significant difference in rope wear.
We had our test ropes examined microscopically by a rope-engineering firm and the analysis indicated that sediment abrasion was not the major cause of rope wear. As a result of those observations, we undertook a study to determine whether different hauler sheaves, splitters, and hauler adjustments affected rope wear.
We expect these projects to point to ways to reduce the costs associated with the use of sinking groundlines, particularly for offshore operations. This article is intended to provide information to fishermen that will allow them to improve the serviceability of sinking groundlines through modifications and adjustments to lobster trap haulers.
22 configurations
We used a rope-wear simulator equipped with an offshore lobster trap hauler to reproduce and accelerate the wear that groundlines experience in the field.
The simulator puts an endless loop of groundline under tension that is comparable to that experienced when hauling traps in depths of 175 fathoms (1,250 pounds). The groundline drops from the hauler into a tank of water with sediment in the bottom.
The rope was allowed to relax for a few seconds and then pulled along the bottom of the tank and up into the hauling mechanism again. It takes approximately three hours to run the groundline through the hauler 250 times, which we consider to be the equivalent of about five years of hauling in actual use in the offshore lobster fishery. Each of these is considered a “rope run.”
We tested 22 distinct hauler configurations. During the period Jan. 29, 2008 through June 6, 2008, we carried out 124 rope runs on the hauling simulator.
We catalogued, photographed, and shipped all of the rope samples to Southwest Ocean Services in Houston, TX for break-testing. The results of the majority of our treatments show statistically significant differences in rope deterioration depending on hauler configuration.
Sheaves
Common offshore hauler sheaves are machined from an 18" steel disc and have a standard surface angle of four degrees, which produces an angle of eight degrees between the two sheaves. Most have a bevel at the outer edge, which provides additional width between the sheaves for splices and knots. We used 16" machined steel sheaves with a constant four-degree surface angle to represent the standard offshore hauler set-up on the simulator.
We were surprised to find that seemingly small changes in hauler sheaves and hauler set-up could bring about significant differences in rope wear.
For that reason, readers should be aware that factors other than those we identified may play a role in the differences in rope wear we observed.
Similarly, because machining practices vary from machine shop to machine shop, we can’t say that our results with a set of machined sheaves will be identical to someone else’s experience with a different set of machined sheaves.
The fact that these small differences make a big difference in rope wear means that standardization of hauler specifications, such as surface smoothness, would lead to better overall performance.
The most noteworthy potential for a reduction in rope wear through hauler modification was demonstrated by the increase in residual breaking strength of sinking groundlines tested with 17" Hydro-Slave stamped steel hauler sheaves compared to rope tested with the 16" machined steel sheaves that we considered our standard hauler set-up.
We believe that one important factor in the difference that we observed is the surface smoothness of the stamped steel sheaves compared to the machined hauler sheaves.
Standard machining leaves circular ridges that are easily visible on the surface of the sheaves. A microscopic view of a machined steel surface reveals jagged peaks that look like they were designed to eat rope. Machine shops may or may not sand or grind the sheave surface after machining.
Rope make-up
Another finding of our research is that sinking groundline constructed from blended polyester and Polysteel© showed greater loss of breaking strength compared to floating rope constructed of straight Polysteel© when used in our machined steel offshore trap hauler.
The greater loss of strength for the sinking rope occurred with and without sediment in the simulator tank. It appears that certain characteristics of some blended fiber ropes lead to a loss of strength caused by the hauling process.
The prevailing wisdom has been that the shorter service life of sinking ropes is the result of sediment abrasion. That belief has guided efforts to improve the durability of sinking groundlines. Our discovery that sinking groundlines deteriorate faster than floating ropes, even in the absence of sediment, points to other approaches that might improve the service life of sinking groundlines.
Sheave difference
Interestingly, the difference in rope wear between floating and sinking rope was not evident when we tested the rope with the stamped steel hauler sheaves.
This indicates that at least part of the superior service life that fishermen have experienced with floating rope can be achieved with sinking rope through hauler modifications.
We can’t say that all of the improvements we saw in the laboratory can be accomplished in the commercial fishery.
Our preliminary field-testing of machined steel sheaves with a variable surface angle indicated that further design changes are needed on our prototype to provide adequate grip on knots and splices.
Enough for offshore?
The feasibility of using the 17" Hydro-Slave stamped steel sheaves in the offshore lobster fishery will require further field-testing and/or reports from fishermen who use those sheaves on large boats in deep water.
Offshore hauler sheaves are typically 1-1/4" thick when they are new and are generally retired when they are still more than 1/2" thick. It’s not unusual for offshore hauler sheaves to suffer from stress cracks around the bolt holes. The 17" stamped steel Hydro-Slave sheaves are 3/8" thick when new.
Representatives of the company that manufacture Hydro-Slave haulers believe that their sheaves are capable of being used in the offshore lobster fishery and say they have never had reports of sheave failure. The new hagfish trap boat Meridian has a 17" Hydro-Slave hauler, which should provide a good test of its deepwater capabilities.
Sheave spacing
Our tests of the effect of sheave spacing on rope wear indicated that wider sheave spacing did not cause more rope wear for machined steel sheaves, but it did for stamped steel sheaves.
The difference in rope wear with sheave spacing may be related to the smaller angle between the stamped steel sheaves as the rope rides deeper.
The constant angle of the standard machined sheaves means that the rope is squeezed into the same V regardless of sheave spacing. So our conclusion that wider sheave spacing produces more rope wear only holds true for variable angle sheaves.
Rope/splitter angle
Our hauling simulator is fitted with a load cell that measures the pressure exerted on the splitter by the rope. Although the load cell has proved troublesome, our testing indicates that the pressure the rope exerts on the splitter is less when the splitter is modified to reduce the angle at which the rope impinges on it.
Our modified splitter lifts the rope out of the sheaves more gradually than the standard splitter. If you look at the angle at which rope hits a standard splitter in a trap hauler, it’s almost at right angles.
We reduced the angle at which the rope hits the splitter by cutting a reverse curve into the leading edge of the splitter. A comparison of residual breaking strengths after testing showed that sinking rope retained more strength with the modified splitter than with the standard splitter.
We also found that the pressure on the splitter is higher when machined sheaves are spaced more widely apart. We would expect higher pressure on the splitter to create a potential for more rope damage, particularly as sharp edges form on the wear-groove.
Polyurethane sheaves
Our experiments with polyurethane sheaves were based on the assumption that a resilient surface would do less damage to the rope.
We have had success with polyurethane sheaves in the inshore lobster fishery off Cape Cod. Those sheaves were manufactured and field-tested with funding from the National Fish and Wildlife Foundation and the International Fund for Animal Welfare.
We found that polyurethane sheaves provide good grip on the rope, which may make it possible to keep the rope riding at a wider sheave angle. We don’t have a polyurethane formulation that provides acceptable sheave wear under offshore hauling conditions. However, we believe that more research may find a polyurethane sheave that will last long enough to make it cost-effective to replace sheaves more often if it saves on rope wear.
Polyurethane splitter
Our comparisons between a polyurethane splitter and the steel splitter did not show any improvement in rope wear, but both splitters were in good condition.
The advantages of the polyurethane splitter may be more evident as the splitters wear to the point where sharp edges form along the groove in the splitter.
Polyurethane splitters have the additional advantage that the wear groove can be easily removed with hand tools, without the risk of creating pits and snags that sometimes occur when steel splitters are filled by welding and grinding.
We are hopeful lobstermen can use the results of this research to improve the service life of sinking groundlines through modifications to their haulers.
Complete details on our testing procedures and results can be found in our final project report, which is available online at: <www.mass.gov/dfwele/dmf/programsandprojects/ritwhale.htm#right>.
Dick Allen
Dick Allen is a Wakefield, RI-based fisheries consultant and a former lobsterman. He can be reached by e-mail at <rballen@cox.net> or by phone at (401) 789-1463. To learn more about Allen’s many projects, visit his web site at <www.FisheryConsulting.com>.
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