MacGregorSailors.com

One of my winter projects is to set up a pulley system inside the boom to adjust outhaul tension. One approach that looks good to me is shown in the following image:

Outhaul Plan A


Another is:

Outhaul Plan B


I have not yet figured out how to determine mechanical advantage ratios and could use some help. What are the ratios for these two plans?

Thanks!

In your first sketch, is the point to the left of your exit block an eye, or a block?

If a fixed eye, 4:1 reduction. Tension at cleat is 1/4 load.

Sketch B is 1:1. I wouldn't do it that way. Lots of hardware, no mechanical advantage.

Thanks for those responses. I am getting the same from other quarters.

Yes, Plan B has demonstrated to me that a rope going over a pulley can take advantage as well as give. Silly me.

As I understand it (and as evidenced by the lack of an additional block in the budget), there are only only 2 exit blocks and 2 internal blocks. The lines intersect at the left where I assume they are tied off individually to the bolt that holds the vang bale in place. I suppose that could be one line wrapped around the bolt, but that wouldn't make sense to me.

Another responder at the sailboatowners.com forum said he thought perhaps the bungee was an unnecessary complication. Thoughts? Also, thoughts about doing this with Dyneema line?

Thanks!

Stickinthemud57 wrote: ↑Sun Nov 06, 2022 6:30 pm Yes, Plan B has demonstrated to me that a rope going over a pulley can take advantage as well as give. Silly me.

As I understand it (and as evidenced by the lack of an additional block in the budget), there are only only 2 exit blocks and 2 internal blocks. The lines intersect at the left where I assume they are tied off individually to the bolt that holds the vang bale in place. I suppose that could be one line wrapped around the bolt, but that wouldn't make sense to me.

Another responder at the sailboatowners.com forum said he thought perhaps the bungee was an unnecessary complication. Thoughts? Also, thoughts about doing this with Dyneema line?

Thanks!

Actually, there are times when blocks are arranged to provide a multiplication of travel at a mechanical disadvantage. So, there are applications for arrangements where a block is used in the configuration to the right in sketch B (excluding the link between the blocks and everything to the left). Think a short throw hydraulic cylinder used to move an object twice the distance of its throw. You just don't see them paired up where one cancels the other, such as in sketch B.

The bungee is likely there to keep the lines from getting slack and tangled inside the boom. If you disconnect the outhaul from the sail, there is nothing to keep the lines and blocks from turning into a tangle. That appears to be why someone put a bungee in.

I have a very similar rig in my furling boom to tension the mandrel axis cable. But, I never have occasion to slack the assembly, so no need to worry about tangling. A stop knot on the line to the cleat would be a good idea, too. You will find working inside the boom a bit tedious. I did, anyway. You may be better at it than I am.

I'd try it with the bungee first. Just make sure you have enough stretch length for the travel you need.

Love dyneema. Don't see a downside.

There are some pretty smart guys over there on sailboatowners.com. I spent time there when our Forum went down. Dont get over there much anymore.

I miss Tomfoolery's input on this type of discussion. He always has valuable info to share on things mechanical. Hope he's ok.

I'm a mechanical engineer, and I double-checked my answers by rigging the configurations - to make sure I told you straight.

This is perfect, thanks!

I figure I can assemble this all on a 10' long 2x4, test it and tweek it, then figure the ways to get it in place inside the boom. I have shock cord of varying strengths, so I should be able to come up with something workable.

A few things to remember. The line you are going to use to adjust the outhaul should be sized by hand feel, not by strength (especially true for dyneema). Others can be sized by strength if desired. The first block after the exit block will need to move twice as far as the outhaul adjustment block. The outhaul adjustment block will need to be far enough fron the outer end of the boom to give you enough shock cord length to absorb the outhaul travel. You probably already know all of this, but thought I'd throw it out there anyway.

Modeling the system on a 2x4 before installing is brilliant!

Looking forward to seeing this come to life.

Good thoughts on the hand feel criteria for the human interface end! While Dyneema is great stuff, it is slippery.

I'll try to remember to document the process for posterity.

Here u go Boy,s been there & done that yrs ago , here,s my set up may save u some headaches
works great
viewtopic.php?p=274666&hilit=in+boom+outhaul#p274666
J
PS if the pics don,t come up I still have them on my desk top

I,m able to see most of them but u may not so guys let me Know & I,ll post them

J