Trampolines on Catamarans: Look Before you Leap
by Charles Kanter, AMS
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The majority of production catamarans are built with trampolines as a foredeck. This represents a sea change from the earlier models where most catamarans had a solid foredeck. The topic of which system has what advantages is explored elsewhere in my new book, "Cruising In Catamarans." I must convey trampoline safety findings to you that are becoming apparent as the fleet ages.
Chuck Kanter has over 20 years experience as a yacht surveyor specializing in multihull vessels, sailing vessels and small craft. He is a recognized authority and featured speaker on cruising in catamaran sailboats. Check out his website: www.sailcopress.com
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How they are used
First, we must review how trampolines are used. With a day sailor, most of its sailing life is spent with one or two people sitting on the tramp. They roll around on it and seldom stand or jump on it with only bare feet. It?s average storage life is under cover or rolled up and stored away.
Conversely, on a cruising cat, trampolines have constant use by people standing up and walking on it, thus you have many more people, often of great individual weight, giving it higher point loadings over considerably greater spans. Jumping on it just for fun is not restricted to children! When not being used it stays exposed to the elements, lying horizontal in the blazing sun and exposed to various sorts of air pollution especially in urban areas and near airports. (photo: A well used trampoline)
My observations lead me to believe there are three inherent weaknesses in the trampolines I see constructed. There are simply too many people falling though trampolines not to bring up this vital safety issue.
Rule of thumb:
A trampoline should be strong enough to support twice the weight of the entire crew.
First item is geographic location. The further south you go, the more the sun affects your trampoline. Boats stored in the tropics deteriorate at an alarming rate from the sun and trampolines, because they are horizontal, are especially vulnerable.
Second, is the manner in which trampolines are connected to the vessel. Many are connected by lashings or shackles to ordinary plastic sail slides. White sail slides are not a good connector though they look snazzy and do not chafe either the metal extrusions or the lashing lines. They are simply not strong enough for the shock loads and are susceptible to sun damage.
(La Forza's trampoline is woven from one piece of 2" Nylon belting, hot glued and sewn with sun blocked Teflon thread. It is framed with 1/2" PVC outdoor electrical conduit with a 1/2" safety rope running through it. It was built by MARE & Co. back in the 1980's It is lashed to stainless fittings bolted through the hull with 3/8" line and tied from the center of each span. It is kept covered with a sunbrella cover when not in use)
The lashings themselves must be oversize, not for strength but for protection from sun damage. Lines smaller than quarter-inch are theoretically sufficiently strong but strength disappears quickly under the savage tropical sun because of the line?s small diameter. On the trampolines themselves, metal grommets pressed through material eventually create stress points that distort the material and pull out leaving the lashings pulling directly on the material.(Maine Cat 30 trampoline. Note nice promenade with anchor chain groove and split trampoline)
Third and perhaps most important are materials and methods of construction. I suggest that any trampoline that relies on stitching alone for its integrity is not going to give you satisfactory service. My observation over the past two decades is that trampolines that are constructed by folding back the materials around a reinforcement of some type that distributes the load and has several rows of stitching over a wide area of material will give reasonable service. That reinforcement can be rope or solid material such as stainless or aluminum bars or PVC pipe. I feel that the very best trampolines have the fewest parts or the KISS (keep it simple system) principle. From my observation, the most fool proof trampolines of all, are cargo nets woven from a single piece of two-inch belting. That way, there is only one joint in the entire trampoline and that is usually joined in the center of the trampoline. After weaving and shaping, the best cargo net constructed tramps are then hot-glued and sewn with UV resistant thread at each crossing. This construction is far from the cheapest but it is not the most expensive either.
(Catana trampoline way past its prime)
The heavier the materials, the longer they last. Two-inch wide belting materials stand up better than thin mesh. Knotted fish net was meant for catching fish. It is overly stretchy, uncomfortable to walk on barefoot and does not last very long. However, it is easy to attach strongly and cheap to replace.
If you must drag anchors, anchor chains or a dingy across your trampoline, it must be made of material rugged enough to handle it. The point loading from your feet while handling those items is enormous. For that type of usage, a cargo net construction is almost a must.(Privilege charter boat trampoline about to become a heavy lawsuit. Examine carefully, note the failure mode of the netting)
I examined several failed trampolines, all with the same MO. That MO was that the owners failed to observe any failure until there was catastrophic failure.
Remember, racer-sailor-author Rob James was lost at sea when he fell through a trampoline? I have had a serious accident myself when I fell through a trampoline in Marsh Harbor, Abaco, Bahamas while approaching a dock on the last leg of a delivery from Jamaica. I have outlined a couple of procedures for all you owners and especially charterers to do the next time you board a trampoline catamaran. These procedures will require you to use a magnifying glass and get on your hands and knees, but it is worth the exertion in the end.
1. Connections first. Take your glass and examine the sail slides or other connecting devices be they wooden strips with drilled holes or eye-bolts, U-bolts or whatever. If bolted through the glass, check for cracking and crazing at the penetrations. Check not only the fittings, but the lashing rope. Look for fraying in the rope. Small stuff, unless almost new, is not up to the job. Quarter-inch line is the smallest diameter you should tolerate. This has nothing to do with theoretical breaking strength but with susceptibility to both chafe and sun damage. Cored line is susceptible to internal damage that is not readily noticeable. Perhaps I am a bit old fashion, but dear old three strand rope has the advantages of having wear quickly and easily noticeable and being inexpensive to replace.
2. How is the trampoline strung? Individual lashing or shackles are the least vulnerable to catastrophic failure and windings or continuous line are the most vulnerable. If you are using a continuous line, it should be secured and started in the middle of each side and spiraled in opposite directions. That way, if there is a failure, only one half the span is lost rather than the entire side of the trampoline. For instance, you are on the leading edge of the tramp, the one lashed to the crossbeam. You are changing sails or weighing anchor or some other activity that has your full attention. Suddenly a lashing snaps under the load. If it is properly lashed from the center, only one half the span will fail perhaps saving you from injury.
3. Sewn tramp with grommets. Check the grommets to see if they are stretching the fabric enough to unseat the grommets. A little stretching is normal. If the holes are elongated enough to have space behind the grommet, that is signal of the end of useful life. Many tramps have several rows of sewing. Check this sewing with your glass to assure yourself that the threads are still intact and contiguous. A dead giveaway are the ends of stitches sticking up looking like a row of mini-prairie dogs. Again, at or close two the end of useful life. The material itself can generally be gauged by comparing the underside to the top for the difference in deterioration. Any broken strands are a dead (and I do mean dead) giveaway that you should not be cavorting on said trampoline.
Examining trampolines, we find three basic types, fishnet, woven strapping and sewn fine mesh materials. Each has its advantages.
Fishnet is painful to walk upon barefoot, uncomfortable to lie upon or attempt to sleep upon, has very poor longevity but is dirt cheap and easily replaced. There are different types, sizes and grades of fishnet. Some, like the coated knot-less variety, more suitable than just plain knotted net. In fact some knot-less coated nets actually are very good, however, they still have the disadvantages of excessive stretch and toe catching hole size.
Woven strapping is by far the strongest and has the best potential longevity. The larger the straps the greater the longevity. Two-inch Nylon or polypropylene is the most common size. They can be either lashed directly or with end-bars such as seen in photo of La Forza?s trampoline or fastened in other ways.
Fine mesh materials can take many forms. There is the ultra-fine polypropylene mesh and the various grades and textures common to day sailors. The materials themselves are excellent. They are the most comfortable and have reasonable wear characteristics though they are the most expensive. With these trampolines, construction is everything. All of these trampolines rely upon stitching to maintain integrity. If they are built so that they rely solely on the stitching, they may be dangerous from the git-go. You can tell if this is the construction method by inspecting the joint between the edging material and the base trampoline material. If the trampoline material ends abruptly and the edge material begins and is separate and distinct, then the trampoline is relying only on threads and may be questionably constructed no matter how many rows of stitching there are. Sometimes this construction is difficult to observe because the edging is a folded over piece of material with the grommets through it. The fold is then slipped over the raw edge of the base material forming a sandwich with the base material in the middle and then sewn. This method looks very strong at the outset, but it still relies solely on stitching.
If they are built so that the base material is a direct part of the sewn edging, most likely they will give you good service. That infers the base material is inside the folded sandwich and any grommets or fastenings go through all the layers including the base material.
Some multihull owners voice concerns that the fine mesh materials do not rid themselves of water fast enough and will not allow a buried bow to shake itself out sufficiently fast and/or keep the bow buried excessively. Depending upon the boat, the loading and the usage there is a certain validity to this concern.
Trampolines should be protected from rough use and excess wear. Some vessels are built so the trampolines must have the boat?s anchors, anchor chain or even dinghies dragged across them. Other designs carefully avoid this wear by providing central utility walkways often referred to as promenades, for that purpose. The central walkway also provides a structural component for larger boats .I have seen bights of chain, flukes from anchors and other damaging items hanging through trampolines. This is detrimental to safety, ease of use and longevity.