Why use mainsheet systems described as 5:1, 7:1, or 9:1 rather than 4:1, 6:1 or 8:1?

These systems are quite common (see attached picture as example).

However, since it is only the blocks on the object which is moved (i.e. the boom) which contributes to increasing the mechanical advantage, these systems are actually 4:1, 6:1 and 8:1, respectively.

All systems which terminate the fixed end of the sheet line on the boom necessitates an extra length of rope and a block, without increasing the mechanical advantage. Hence, even if the sheet line must be pulled 5 inches for each inch movement of the boom (5:1), the mechanical advantage is 4:1.

Please correct if I am wrong!
I assume that the boom must be considered the moved object and the block on the traveller the fixed point (I know the theory of relativity, however in the moderate speeds obtained in sailing, I assume Newtonian mechanics and empirical measurements may be trusted).

Stein

I'm afraid you are incorrect. Each fall of line contributes to the overall mechanical advantage of the system regardless of whether it terminates on the "fixed" or the "moving" end (in fact, the two are moving relative to one another, so they are both moving). But regardless, a simple test: flip your blocks upside-down. You will find that you get exactly the same purcahse with the blocks in either orientation.

sm

Those blocks listed as 8:1 are 8:1. Do a load diagram...if you pull 8 inches and get 1 inch of movement than it is absolutely 8:1 - no way around it.

In hindsight, I put these block load arrows pointing the wrong way for a static diagram - but I think you get the point.






Just wait until you try to sort out a cascading system.

picture

You must both be right; your explantions seem highly plausible. It must be correct that advantage depends on ratio of (rope length pulled):(distance moving of boom).
My simple living-room experiment must be inaccuarate. And the general rule that only blocks on the object moved contribute to advantage, must be incorrect.

Thank you!

Stein

PS: I stick with my 10:1 setup for my Taipan 5.7, though.

Yeah - I think you are getting conturbulated thinking that the kinetic system (moving) is different than the static. All loads must be equal. To quicly determine the purchase of a (non-cascading) system, you can count the running lines between the two stationary points. 2 lines is 2:1, 3 lines is 3:1, 4....and so on.

If you have a cascading system, you separate the two systems and multiply them against each other. A lower block setup with 4 runs at 4:1 that attaches to a cascading system with 2 runs at 2:1 is a total of 8:1 purchase.

So just for practice, this one is 2 x 4 = 8, correct?

It looks like the black is a cascade doubler and the red on the left is 4:1.
The red-right is ineffective, turning only.

Assuming that the black and the red lines that just end at the boom are simply attached to the boom - yup. 8:1...the question is though....WHY all the complication?

I think is was Rolf that posted this as a Tornado setup. Why, you ask? Small diameter lines through the blocks, I'd guess. No taper needed. Maybe less total weight by using small blocks? Hidden mechanisms keeps the rookies staring? Easy to hide an illegal 10 or 12 setup? <img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" />

This drawing was done by Macca, I think.
It shows a low-friction, lightweight and very clean setup used on many top-notch Tornados.

There are no rules on how much purchase you can have on any sheet on the Tornado. Would be a silly rule in my opinion.

But this one has another line contributing to total boom movement...the black line on the extreme left. So won't is be 8:1 + 1:1 = 9:1?

Correct !

Wouter

Kind of like "Where's Waldo?"....I missed one.
The biggest, darkest line on the page!

9:1 is is!

I've been wanting to setup this exact system on my T...but I have not zero'd in on a double-block that is slim enough to fit into the Marstrom boom section. Plus, I have an outhaul system running inside already (that I quite like & use), so it would need to be removed/replaced with some other setup.

Right now I run a traditional 57mm harken carbomatic triple lower unit up to two carbo doubles (one with becket) hanging off the boom with spectra. I also put a 40mm single on the lower unit & run the tail back to the upper double's becket. Gives 9:1. Works pretty good.

The advantages are:
1. much lighter overall (blocks & 3mm vectran line)
2. Excellent easing (cascades are generally lower friction)
3. Lower cost can be achieved since blocks are small & cheap compared to 57 mm Harken Quads etc. (though I've seen several version of this running custom made blocks with carbon side plates...$$$)

Some disadvantages are:
1. can be a headache to work out
2. Can jam up inside boom if not setup right
3. Maintainance more difficult


Lots of top T guys run this setup, but it is not required to get to the top of fleet...Charlie Ogletree & Johonny Lovell run standard harken quads <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

I have often thought this wold be a good way to go. I do not understand what turns the section where in the diagram the line goes from red to black... ??

some pics, someone can link them so they appear in the posts but i have never worked that part out <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

This pic is the turning block at the front of the boom, the thin rope turns around this block and attaches to the floating block in the next pic.

this is the floating block, we have a new systems that allows us to change from 9:1 to 12:1 in seconds, but more on that when i put the boat back together and take pics. way to complex to explain here <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Many thanks for the photos as they will prove very useful to me. What I am having trouble understanding is how the larger sheet exits the boom at the sharp angle. is there a through block there?

Okay, have understood. : (