I hear a lot that tramp tension is very important for boat stifness, but i don't realy get hoq it works, could you please explain me how it works?

Your opening statement is true. Keeping the tramp tensioned will result in a stiffer boat that will improve through the water performance.

Tensioning is pretty straight forward. First loosen all the tramp lines. Get a tape measure and with a friend make sure the boat is square. Measure from the port bow to the starboard stern. Do the same from the starboard bow to the port stern. If the measurements are diferent move one of the hulls forwards or backwards until both measurements are the same.

Now it's time to tension the tramp. There are tools available to do the job but vice grips work fine. Start at the bitter end of the line and pull it as tight as possible through the closest grommet. Work your way through all the grommets until you get to the other end of the line. Tie some half hitches and you're all set.

Recommendations: Use non-stretch line so you don't have to repeat every time you take the boat out. In the past some used large diameter bungee. It works but seems to wear faster than good line.

Think about double grommeting. This will give you an additional purchase to stiffen the boat.

Take your time doing the job and you'll have a stiffer boat.

Hopefully a thought experiment will help you understand how a tighter tramp increases stiffness. Imagine that a catamaran was molded in one piece out of concrete, including the "tramp" between the hulls; or carved out of a solid block of wood; or laminated out of carbon fiber in one piece. Pretty easy to imagine how stiff any of those structures would be, right? The concrete boat would be a so heavy that all you could use it for was a neat reef, the wooden boat would be lighter but still heavy and need waterproofing, the carbon fiber boat might be fantastic but VERY expensive. Because our tramps are made of flexible cloth, they introduce flex into the total structure. The looser they are, the more flex there is and the tighter they are, the less flex there is. Does that add any clarity? BTW, Some beach cats actually have a solid "tramp", like the wooden or glass Shark catamaran.

Most tramps I have hung out with were quite loose, but they all increased the stiffness of my mast...

Imagine a rectangular frame with hinges in all four corners. Fit it with two diagonal wires ("X"). When both wires are tight the frame is stiff and keeps its shape, remove them and the frame will fold on the hinges.

The tramp in a cat is very similar to the wire diagonals in a rectangular frame, it increases the stifness of the rectangle formed by hulls and beams.

A tight tramp also pre-tensions the pair of hulls and the pair of beams towards each other, helping keep each pair together under stress.

Great...

Has anyone actually tried wires to stiffen a boat? (Of course, that wouldn't be class legal.)

A fleet menber showes up at a racw with his Hobie 16 wired together. He said it did make it stiffer. We let him race but did not count his score. He had them off for the next race.

I've used both 1/2" bungie and 1/8" spectra/vectra .... both work well. The advantage w/ the bungie is that is self-adjusting/re-tracking. The spectra/vectra I need to periodically re-tighten the lacing .....

Try either .....

Harry
TheMightyHobie18/P19MX

I call BS on the whole tramp tightening issue.

From an engineering stand point, and woven cloth laced up as a trampoline cannot provide enough tension to have any significant affect on overall platform stiffness. We all know even a super tightened tramp will still give & deflect lots when we crawl over it...thus there is always more stretch to be found from the cloth.
Any half decently made boat will have the hull/beam joints hugely more stiff in the fore/aft direction than what any section of cloth & line can provide....so the example of a square with hinges at each corner is not a good one. I'm baffled by the suggestion that a bungie/shockcord laced tramp can have any merits at all let alone platform stiffening. I once sailed an TheMightyHobie18 with bungie lacing...bounced all over the place trying to crawl across during tacking in rough conditions.

For performance it's hull racking that we're interested in controlling...the tendency of the hulls to rotate in the vertical plane relative to each other. This is primarily dependant on beam twist characteristics and to a lesser extent hull & deck stiffness. Having a near infinitely high tensile/low stretch material as a tramp would reduce racking...as the diagonals would have near zero elongation. However, real world tramps are stretchy woven cloth...grommets/bolt ropes etc will pull out long before the cloth reaches it's max stretched dimension.

One effect of trying to achive high tramp tension in hopes to stiffen the platform is that the tramp/grommets/hul tramp rails/lacing points will fail much earlier as you crawl around on the super tight material. It needs some give to provide a better vertical component to the force counteracting your body weight.

That's a lot of big fancy words... But go sail a Hobie 16 with a loose tramp and a Hobie 16 with a tight tramp and feel the difference.
I think this may be the key...

Mike,

I tell you what .... just let me wrap you up in stretched shrinkwrap .... and let's see what happens. It's the same theory w/ the 1/2" bungie cord that you stretched when you lace the tramp. The tramp material (vinyl) and the bungie lacing will come to a equilibrium between them and depending on how much you stretch the bungie when lacing, the load can be quite high which acts to stiffen the "platform". This is the same principle as "pre-stressed" concrete which is used in bridge construction and many other places ....

And a H16 isn't manufactured to the same tolerences as a Marstrom Tornado ..... of course it doesn't cost anywhere near as much either .....

Harry
H18Mag/P19MX

PS: .... and yes I've sailed on a Tornado several times ... a sweet ride ....

Search and read "Law of Sines". Its aplication on tight ropes or tramps is quite simple:

A tight stay keeping the mast up is, however, easily deflected by an orthogonal force. Likewise, a tight horizontal tramp is easily deflected by our vertical weight.

Unlike the stay, the tramp is not the only thing holding the boat in one piece, but a tight tramp makes a difference on horizontal plane stiffness.