mast tang for trapeze

Your hobie shouls have the trap wires attached the same as my H17, using the same shackle as the shrouds. the order should be trap wire, shroud, forestay, shroud, trap wire.

I hope this helps

thanks,

thats how i'll probably end up doing it. it just sees like a lot of stress on that one tang. i'm predicting critical equipment failure for this one. (and my girlfriend going overboard when that tang pulls out)

engineering student? first off, don't overthink it. this is the way a H16 is rigged. end of story. but since I also have a tendency to overthink things......

if that tang pulls out you'll have more to worry about than a MOD. but the tang won't fail. the tang isn't under any more appreciable load than without the trapeze. it's the shackle that experiences the change in load.

think through the free body diagram of the shackle.... the leeward shroud is generally slack, so ignore it. there's the tension of the headstay, which is countered by the tension of the mainsheet/sail/rig. there's also the tension of the windward shroud. all of these tensile forces are resisted by the compression of the mast. the tang transmits the rearward tension of the rig and the compression of the mast.

now add in the tension of the trapeze, assuming that the crew is trapped out amidships..... the load of the windward shroud is reduced, since this acts in basically the same direction. the load of the headstay increases slightly to accommodate the additional rearward pull of the trapeze.

There is some additional compression in the mast which is transmitted through the tang, but a couple of hundred pounds of crew doesn't matter much when it's already under a couple of thousand pounds of simple sailing stress. I suppose there's also some torsional stress caused by the eccentricity of the rotatating mast, but it's negligible. furthermore, this added stress on the tang's fasteners is fundamentally shear stress, not tensile stress. they'll be fine.

Very good points. However, my main concern was really the combined downward components of the forces applied by the shrouds, forestay and my trapezing girlfriend, being counteracted by the reaction force upwards at the base of the mast causing a shearing force on my rivets. I probably wouldn't even be considering that as a problem except that the rivets on my mast base recently sheared while stepping. so now i'm questioning the integrity of all the rivets on my old boat before i apply any additional load. it's nice to hear some engineering jargon though, all this sailing slang is not my native language.

If this was a common failure you would hear more of it. If you do not trust the rivets, why not replace them with new stainless ones. If the holes are worn, sleves are available in the murry's catalog. If the rivets are goog i doubt it would ever fail.

By your mast tang not having a seperate hole for the trapeze wires, it is an old set up. I do not know about the old mast tang, but the one on my mast has twelve 3/16 rivets around a curved surface. There is no way that one can pull straight out on all of the rivets at any given time. This is a very good design. Even when you drill the rivet heads, you have to take a chisel or a punch to pop the rivet loose from the tang. If your rivets are not loose, go sail, have fun, and do not worry. I have more problems with loose screws on my boat, which I have been putting Lock Tite on. The most important thing on any piece of equipment is to take care to inspect it and keep it in good working order.

If you are worried about this, why not find out what the sheer strenght of a 3/16 SS rivet, multiply it by twelve, and add the dynamics of pulling on the rivets from different angles at the same time.

You should be fine - You should have 2 shackles 1 for the shrouds (2 shrouds) and 1 for the trap wires (4 trap wires). You can get hobie manuals online at:

http://www.hobiecat.com/cats/partset.html .

Good luck.