If you don't periodically re-tension your tramp then you should read this.
Your trampoline is the first and foremost means that holds your hulls and beams together (the forestay bridle does pull the rig together on a beat or reach, but not on a run or a broad reach, and never "evenly"; the rear beam clips will ultimately prevent this forward inward force from the bridle pulling the hulls outward at the stern if the tramp tension doesn't; there will be some inward pull from the windward shroud on a beat or reach, and from both on a broad reach or run - but again uneven). The tramp has to be a well-designed piece of kit to fulfil this function. A Laser Centre or Class Approved trampoline is made so that it will pull both hulls and both beams inwards when you tension up at the rear beam, making the cat "stiff". This means that there will be little free play if you lift a hull up - eg by the bow - the whole cat will move after just a fraction of free play. That lack of free play helps stop the beams from slopping around in the hull mouldings, restricting wear in the mouldings that will eventually prevent the cat being stiff.
Of course, if your cat is new or well looked after the beams will fit tightly in the hull mouldings you will find there is next to no free play with no tramp tension (good test for an older boat - lots of free play will mean there has been wear in the hull mouldings. I once asked Steve Sawford about repairs - not for mine - and this experienced boat builder/repairer had limited advice on how a guaranteed "square" repair might be made).
You really want to keep your cat stiff for performance reasons, especially pointing ability when beating to windward. The way to do it is to tighten your tramp as hard as you can, and check tramp tension regularly, re-tensioning when slackness is discovered. Tensioning positively limits the extent that the beams can slop around in the hull mouldings, limiting the wear and tear that will eventually contribute to your cat's ability to compete with newer boats, and maintain its value!
So don't sail with a loose tramp, especially when there is significant wave action.
I have the first (and at time of writing the only) non-Laser centre trampoline bought through the D15 Association as "Class Approved" and equivalent to a Laser Centre one. It was the third attempt - the first 2 tramps had to be returned as being "unfit for purpose" due to the maker having tried to cut corners in design as compared to the standard tramp. The one I have is a little long but this does stop the sheets disappearing through a gap between tramp and rear beam! I've had it since mid 2001 and I remain pleased with it..
Unfortunately, having at last made a tramp up ok, this maker decided to pull out due to the hassle proving too much. Our Association Committee had then to seek another maker (I last saw a tramp late in 2002. I rejected it out of hand as being "unfit for purpose" due to a design fault of the variety described below - corner cutting yet again by a prospective supplier!).
As well as my own experience beta-testing Class Association tramps, I have seen trampolines on D15/D18s where owners got a local sail-maker to make one, because they weren't worried about being "out of class" for racing (their local clubs turned a blind eye as neither the sailors nor their cats were usually competitive). They were cheap, but not properly made. As a result, they could not (either slightly oversize or made with the warp/weft parallel/right-angles to hulls and beams, or whatever) be properly tensioned up to pull the hulls and beams tight and so make the cat stiff. The beams could slop around in the hull mouldings, causing wear and progressive lack of rigidity, etc.
So how is a trampoline made up so that it can fulfil this "stiffness" function? The main tramp area will consist of 2 "triangles" of material, joined soundly together at the material hem and with this join reinforced/protected with a strip of material (I think it's the same as toe-straps). This join goes diagonally across the tramp, so that the warp and weft of the main tramp material will be at 45 degrees to the hulls and beams. Clearly the triangles are approximately 45 degree right angled, and the nature of the cuts required may mean quite a lot of the material needed to make the tramp will be waste - adding to cost.
The front and sides are sewn round boltropes of appropriate diameter for the tube sizes in the front beam and the hull tracks, or all but a few inches of these 3 sides of the tramp. This great length is clearly strong at the sides and the front. Because of the 45 degree warp/weft, when the tramp is tensioned up this diagonally-laid material pulls the hulls in as well as the beams. If you find this notion difficult, find a bit of net (ie material with "holes") and see what happens when you pull it lengthwise or sideways as compared to pulling 45 degree diagonally across the holes. In the latter case the net will pull in strongly at the sides whereas pulling along the warp or the weft will hardly pull the sides in at all. It's this pulling in effect on the hulls that makes your cat stiff and prevents the beans slopping around in the hull mouldings.
But critical in the tramp design is at the rear. A doubled piece of suitable material (the same as for the main tramp seems ok, but Laser Centre tramps use a closer weaved and more flexible material) must be strongly sewn on to the diagonal laid main area with its warp parallel to the hulls. The aft end is left as a loose fold to take the tramp tensioning tube. Thin slots are cut through this fold for the tensioning line. It is ESSENTIAL that this rear piece of material is laid "square" so that it can take up the strain with as little as possible "sideways" strain on the slots through which the tensioning line is threaded.
An unapproved tramp maker may try to cut corners by omitting this rear square laid material (this has happened at least twice in tramps that our Committee have tried to get made up by prospective "Class Approved" suppliers). Such a tramp is basically "unfit for purpose" because the tension that you must put on the tramp will pull the material apart at the cut slots due to the 45 degree angle of warp and weave. The trampoline tensioning tube will develop a "shallow scallop" shape, bent forward between the slots where there material retains strength and back towards the rear beam between the slots where the material is weakened by the slots. The tramp material will start to break at the end of the cut slots, loosening tramp tension. When you re-tension the scallop effect will increase further, and the next unbroken warp/weave joint will fail. There is then less remaining "strong" material between the slots. The scallops deepen. And again and again as you re-tension to keep your cat stiff. Such a tramp becomes seriously distorted and weakened in just a few outings, and it was my reckoning that such a tramp wouldn't last a season - at the time my home club was in the Thames estuary at Sheerness, with a notorious short chop as the usual wave action - testing conditions both for sailing and for poorly made tramps!
I haven't mentioned:
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side reinforcing next to the hulls, especially at the "cut-aways" at the front of the hull tramp tubes up to the front beam |
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the handles for underneath (for grabbing when righting to prevent the cat flipping from c90 degrees over through 180 degrees to 90 degrees over the other side. |
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the position of eyelets for toe-straps, trapeze shockcord etc |
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the sizes of the bolt ropes |
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the size and position of the tramp bag |
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etc etc. |
Note that if you have to replace the hull trampoline tracks on an older boat (eg due to one breaking if the rivets fail) you can now only get "Dart 18-type" tracks as fitted to new boats. These have a larger diameter "tube", so you'll have to get your tramp remade with a thicker bolt rope, or get a new one.
So if you are tempted to have a tramp made up, consider carefully whether you will get a tramp which will do the job properly, or whether you risk getting something that will be less than satisfactory, probably "Out of Class" for racing and prove poor value for money.
George Stephen
1594
28 March 2003
