A boat hoist will be unable to lift its maximum capacity if the slings being used are not the ones rated for use with the machine to achieve maximum load capacity. Occasionally, marina owners will substitute alternative sling sets that may be sufficient for lifting smaller vessels to preserve the life of the larger slings. Be sure to verify that the operator is using the proper size slings before allowing him or her to lift your boat. Each sling should have a sewn–on leather tag that shows its rated capacity. Ask what size and type of slings will be used for hauling your boat. Slings wear in numerous ways, and some reasonable multiple should be used to add a significant element of safety. I will only allow my boat to be hauled out if the mobile boat hoist and sling capacity is twice my vessel's displacement weight, and I insist that the yard use twice the number of slings needed for the displacement weight. Is this overkill? Absolutely!
Manufacturers of boat hoists typically specify which brand of slings should be used with their machines to ensure safe lifting. This specification is extremely important from an engineering standpoint, and deviation from this standard introduces a dangerous variable into the lifting situation. Having your boat lifted with a boat hoist using generic slings presents a serious risk to your vessel.
Sharp edges in or around a vessel can cause sling failure when lifting a boat, unless the slings have been properly padded prior to lifting. If not properly padded, sharp edges can cut through the webbing and result in failure of the sling. Keel pads (located at the bottom of slings) protect the slings from abrasion caused by the boat keel. Chine pads are recommended and offer additional padding for the sling in other areas that come into contact with a sharp edge. Failure to use padding on slings should be a red flag to a boat owner that the operator is cutting corners and not taking every precaution possible to ensure a safe lift.
Synthetic web slings can be damaged by cutting, snagging, pinching, crushing, overloading, etc. The slings should be inspected by the operator prior to each use. Slings need to be replaced regularly, particularly if they are exposed to harsh environments. Old or worn out slings will have weakened strength characteristics, which could result in a sling failure. Some manufacturers take extra precautions to ensure that boat hoist owners can identify when a sling needs to be replaced. Marine Travelift specifies slings for its equipment that have "red guard" warning yarns woven into the slings. This red yarn becomes visible only when the sling is wearing out. Fundamentally, any exposure of the red guard warning yarns on a Marine Travelift machine is sufficient cause to remove a sling from service and destroy it. Red guard warning yarns are the most critical indicator to look for when inspecting slings that are currently in use on a Marine Travelift machine. These are distinct wear indicators, and any sling exhibiting these red guard yarns should be replaced before lifting a boat. Beyond these red guard warning warns, there are several telltale signs that indicate a sling has been damaged or abused and needs to be replaced, as described below.
A tensile break is characterized by a frayed appearance at the point of damage. Such damage is caused by loading the sling beyond its allowable capabity.
A cut is easily defined by a clean break in the webbing structure of fibers and usually results from the sling contacting a sharp object or unprotected edge of a load. Such damage may be found anywhere in the body or eyes of a web sling.
Cuts And Tensile Damage
Tensile damage can be seen clearly when a sling shows signs of fraying at a place where the strap has been cut. This fraying shows that a strap has been used after being cut, which causes the strap to weaken overall. Cuts can severely reduce lifting capacity, and continued use may result in the sling breaking when lifting a load far below the rated capacity of the sling.
The most common abrasion damage occurs when a sling slips while in contact with a load during a lift, or when being pulled from under a load. Abrasion is characterized by frayed fibers on the surface, which expose the "picks," or cross fibers of the webbing that hold in place the load–bearing (lengthwise) fibers. Further abrasion at this point will expose the red guard warning yarns (on a Marine Travelift sling) to signal the inspector or sling user that both serious damage and loss of lifting capacity has occurred. Any damage to load–bearing fibers should be viewed critically.
Although polyester webbing has considerable resistance to certain acids, any contact with acids should be avoided. Nylon should never be exposed to possible contact with an acid. Metal sitting on slings should not be exposed to any acid or corrosive liquid.
In addition to the above, there are other indicators of water, such as inability to read the sling's labeling. Also, exposure to ultraviolet light causes deterioration in the sling fabric's strength, which can be assessed by observing both the graying of the sling's color and the stiffness of the sling's fabric. In addition, the pinned–type slings used by Marine Travelift reportedly may experience excessive wear if the port and starboard segments are allowed to drag on the ground when separated.