I give up <faceplant>
Just don't plan on qualifying for a captains license with masters endorsement. IIRC this stuff is on the test.

If they ask the difference between gun and luff tackle, any idiot can repeat the material on wikipedia.

But this isn't either (as I've drawn it), so you have to think about it.

Now, if my FBD is wrong, I'd like to hear why. If you don't believe the result, that's your problem.

By the way, what I said about the last loop on the luff is also correct: it's exactly as I've drawn the FBD.

You drew an approximation of a gun tackle configuration not luff - on the left side. It is shown rigged to advantage. Apparently "any idiot" cannot decipher the drawings on the Wiki pages. I'm done trying to educate you.

I don't know anything about sailing terms, but I do understand how mechanical advantage works, and rogerstg is dead wrong about calling a truckers hitch a theoretical 2:1. If there was no friction it would be a 3:1 pulley system.

I think his sailing experience is confusing him, because I looked at the gun tackle picture, and it seems like it is used for hoisting something (a gun maybe?) into the air.

So in the interest of brevity, I present you with my masterpiece:

http://img40.imageshack.us/img40/2107/truckerhitch.jpg
If you look closely, both diagrams have identical setups. The difference is which end of the rope is the load end. A truckers hitch is usually cinching something down towards a fixed object, so the top is the load end.

Back on the original topic, I used something like the truckers hitch with some 3mm para cord to fix a broken spoke on my rear cassette side out in the middle of nowhere. The spoke snapped at the hub, so I bent the rim piece up into a sharp hook. Then I tied one end of my cord to that loop, threaded the other end through the empty hub hole and then back up through the loop and down again (making a 3:1 pulley). I loosened the nipple out as far as it would go, then cinched the cord as tight as it would go and tied it off. (I think I used overhand knots, but a figure 8 hitch might stay tighter). I retightened the nipple, tuned up the surrounding spokes a bit and kept on riding with less than 1mm wobble in the rim.

Classic. And what is the "advantage" of "rigging in advantage"? (Small hint: it gives you an extra helping force on the load!)

Anyway, my terminology might be wrong, but the analysis is correct. I'm not saying it's one or the other -- just referring to the pictures as an explanation.

Regardless, my point was that w.r.t. the load on the lower pulley, the last loop over the top pulley in the luff doesn't add anything (really, it doesn't!). But I guess only understanding the physics and not knowing the terms makes means you can still go home feeling high and mighty about your sailor's license.