The View Up Here

Random scribblings about kites, photography, machining, and anything else

Trapeze Wires – Pull Up or Pull Down?

Posted by Tom Benedict on 08/05/2012

Unfortunately I don’t have pictures to post with this. And without the pictures this topic is a lot more confusing than it should be. (Trust me… I’ve been researching this one for DAYS!) This has to do with trapeze wires on sailboats and how they’re rigged. The question boils down to this: Should the bungee on the trapeze wire try to pull the trap ring up or should it try to pull the ring down?

For those who haven’t sailed on dinghies or catamarans, here’s a hand-wavy description of why you’d want trapeze wires and how they work:

When wind blows on a sailboat’s sails it propels the boat forward. It also tries to blow the boat over. This is true of every sailboat. And every sailboat solves this in much the same way: You make the boat as stable as you can, and finish up the rest with movable ballast. On larger monohulls this is handled through the use of rail meat (otherwise known as “crew”). On smaller boats like dinghies and catamarans, you don’t have the excess crew to order out on the rail, so you have to handle movable ballast some other way.

When a small boat starts to heel over from the force of the wind acting on the sails, the skipper has two choices: de-power the sails or move crew weight outboard. De-powering the sails means you’re not extracting as much speed from the wind as you’d like. So it’s better to move crew weight outboard. If you have straps on the deck of the boat – or in the case of a catamaran, on the trampoline – the crew and skipper can hook their feet under the straps and hang their butts off the side of the boat. If they have strong stomachs (and hey, what sailor doesn’t have a strong stomach?!) they can extend their bodies even further. The further out they can position their strong stomachs, the less the boat will heel.

But their legs and feet are still on deck. Wouldn’t it be better to get their entire bodies off the side of the boat? This is why some classes of small boats started using trapeze wires. These are wires that hang down from a fitting on the mast and extend down to either side of the boat. The wires are fitted with hardware that let the sailors hook onto them with a harness and hike out on the side of the boat, suspended by the wire. Because their entire bodies are hanging off the side of the boat, the righting forces are much higher. And because the righting forces are much higher, the boats can either take on more wind or they can run larger sails. Either way they get a speed bonus.

Without some way to hold the wires in place when not in use, though, the boats would be death traps with these flopping wires hanging overhead. The trapeze hardware on most boats includes a bungee cord that tries to pull the wire back to a parking position when a sailor is not hooked in. And this is where the fun begins…

There are two broad stroke approaches to this. The first, which is common on dinghies, is to tie the bungee to the bottom of the trapeze ring so it tries to pull the ring down. (The trapeze ring is what the sailor hooks their harness into so they can hike out on the trapeze wire.) What this means is that when you are hiked out on the wire, you stay attached to the ring. But when you come back in off the wire, say when you’re tacking the boat, the ring will try to slip off the hook on your harness. This makes tacking smoother because you never have to worry that you’re still attached once you come in.

But it can make hiking out on the wire more of an adventure since the ring is actively trying to disengage from your harness the entire time. Think of this scenario: A boat is tacking and the crew slides across the deck to the new upwind side. They grab the trapeze ring and hook it onto their harness. Then they position their feet (the ring slips off their hook without their noticing) and they push off from the rail… And they promptly go overboard.

Catamarans take the opposite approach: The ring is rigged in such a way that as the bungee is pulling the trapeze wire back to its parking position, it’s also trying to pull the trapeze ring up. When the crew hooks in, they have to pull the ring down to their harness. The bungee continues to provide an upward pull, which tends to keep it hooked into the harness. Hiking out on a wire is straightforward: Your trapeze ring is constantly trying to stay engaged, so you know you’re safe just to push off and hike out.

But when you come back in off the wire, the hook will try to stay engaged. It takes an intentional act to disengage the trapeze ring. This can slow down a tack, or even cause a blown tack if the ring becomes entangled. Since tacking on a catamaran is such a tightly timed affair, this can make or break a race.

Which brings us to our dilemma…

After a lot of hemming and hawing, Rydra and I decided to install trapeze hardware on our P-Cat. Some parts of the hardware were already there, so clearly the boat had been rigged for trapeze wires in the past. But most of the other hardware was gone. Initially we decided to get adjustable systems that let you change the height of the trapeze ring on the fly. But completely by accident we wound up with systems that were set up for a dinghy: they pull the ring down. I posted my concerns to The Beach Cats forum and got some good feedback. So I picked up enough hardware to let us re-rig the trapeze hardware for a catamaran, which pulls the ring up.

Now I have to decide what to do. In looking through the hardware, we should have enough bits and pieces to rig a system similar to the one on the Prindle, which used a fixed height ring that you could only adjust when the system wasn’t under tension. We should also have enough hardware to rig a completely adjustable system with a 2:1 mechanical advantage, which will let us change the height of the ring while we’re hiked out. By putting things together in different ways, we should be able to build a pull-up or a pull-down setup for every configuration.

In short, with all the bits we’ve got we can do practically anything we want. (Boat rigging Legos!) Right now I’m planning to set up fixed systems similar to the Prindle. Once we re-learn the basics, we can start experimenting. Eventually I’d like to wind up with the 2:1 adjustable systems installed on all four trapeze wires. But what that system will look like is anyone’s guess.

This should be a fun summer!

– Tom

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