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Anke and I are building our next boat, and writing about it at ABargeInTheMaking.blogspot.com. Access to the net comes and goes, so I'll be writing in fits and spurts.

Please feel free to browse the archives, leave comments where you will and write, and I'll respond as I can.

Fair winds!

Dave and Anke
triloboats swirl gmail daughter com

Friday, December 30, 2016

Mechanical Advantage: It's a Matter of Leverage



It's a matter of leverage.
-- Cap'n Jack Sparrow from Pirates of the Caribbean


Mechanical Advantage: It's a Matter of Leverage

Katy Burke, in The Handbook for Non-Macho Sailors, points out that the veriest foredeck ape can be plucked from deck and flung across smoking water by the merest shrug of the sea.

For millennia, sailors have fought physics with physics. The mass and momentum of natural forces vs. the leverage of mechanical advantage. They might not think of it as such, but the humblest sailor and the great Archimedes are of a kind, brain amplifying brawn to shoulder the burden at hand.

Simple machines – the inclined plane, the lever, the wheel and axle, the cam – are reliable, easy to build, maintain and repair, and cheap! These spread effort over distance (or, one can also say, over time).

Mechanical advantage or purchase is the proportion by which one's effort is multiplied.

For example, a halyard with 2:1 advantage (two to one) takes twice as much line, and twice as long, as the same halyard with no advantage. That sucks, you say? The 'advantage' is that, at any point, we only need apply half the effort albeit for twice as long.

This is like making two easy trips to carry in the groceries, rather than getting it all in one, heavy go. Same groceries, same overall effort expended over twice the distance and time. But at any given point, we're not staggering under the load.

The astute reader will note that there is some overhead to this... you're carrying your body as well as the groceries, so doing it twice takes a toll. Friction can, too. There are diminishing returns to advantage.

The inclined plane (also wedge and screw) spreads lift over its length (advantage = height to length). A block and tackle spread lift out over length of line rove between blocks (advantage = the number of moving lines through the block attached to the load). A lever spreads out the length of throw(distance between up and down) at the lift end over that at the handle end (advantage = throw at load to throw at handle).

Clear as mud? There are many excellent sources which cover the physics and uses of simple machines far better than I can do here. I'll list two of my favorites, below.

Simple machines, or combinations of them, underlie all the manual tools on board. From halyard to handy-billy. From winch to windlass. From sweep to jack. Simple machines working for you to provide mechanical advantage over the forces we face.

As you explore their possibilities, here are a few tips and things to keep in mind:

Consider safety. Though mechanical advantage help us tame large forces, they are still large forces! They can get out of hand. Even watered down, they can pinch, crush, break or strain. Consider setting up as fail-safe as possible, and, where possible, stay out of harms way.

Consider your body. We are the draft animals hitched to these simple machines. Our bodies work well if used in accordance with anatomy and within our limitations. Line up with effort, use the right muscles for the job, avoid effort while twisted, don't jerk and respect your limits!

Consider ergonomics. Is line thick enough to get a grip on? Are handles well shaped for hands, with enough clearance for fingers? Do you find yourself stooping or on your knees to use gear?

Consider your leads. Leads (literally, angles along which lines are led) allow one to line up for an effort with ergonomic efficiency. Is footing good and plenty? Is the body well positioned for an untwisted effort? Is there enough elbow-room for the effort? Simply improving the leads can make a difficult or impossible effort easy.

Consider stowage. Is stowage close at hand? Easy to access in a timely fashion? Secure? When stowed, are decks, gangways and leads clear?

Consider arranging enough advantage for the least physically powerful member of the crew. It is downright dangerous to face a task for which one has not the strength (or weight). Something's gotta give... something that must goes up too slow, down too fast or neither at all. A shoulder sprains or finger breaks. Sufficient advantage may slow the job, but it gets done without trauma.

Consider that you can (generally) lift more than your own weight. Once you've lifted your feet off the ground, that's it for effort applied. But if you arrange your tools so that you are lifting from your feet (with your legs), you can generally exert a fair amount more than you weigh.

Consider work stoppers. Cleats, pawls, dogs and stoppers can take the load off to catch your breath or tend to an emergency. Something at hand, quick to make fast and fast to free can be a big help!


I'll end this section with a cautionary quotation:

When you combine ignorance and leverage, you get some pretty interesting results.
― Warren Buffett


* * * * *

By combining simple machines, we can accomplish any job on a typical cruising sailboat. We can raise sails and sheet them. Raise and lower anchors and masts. Load and unload cargo or deadweights. Move the boat over water or land(!).

As basic and essential as knots, simple machines are vital tools in the sailor's kit.





Two helpful books books introducing simple machines. Both very accessible (not a lot of math or physics). Both written by sailors, with sailors in mind.

A Handbook for Non-Macho Sailors by Katy Burke -- Apparently out of print, but worth it's weight!

Moving Heavy Things by Jan Adkins -- Focused on Mechanical Advantage



6 comments:

  1. Good post! Happy New Year to you and Anke!

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    Replies
    1. Likewise to you and Laurie... to a New Year on the water!

      Dave Z

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  2. Happy New years to you and Anke as well. I have just finished the inside layer of my main cabin roof, and have next week off so I intend to get some more accomplished before I return to work. Are you sailing yet, or still just floating? I received PJR for Christmas, and am reading up on sails.

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    Replies
    1. Hi Dennis,

      And a Happy New Year to you!

      I don't know about you, but closing off the cabin is a red letter day! For the first time, the space becomes very real.

      We probably won't be sailing till spring. 8/ Winter's reverted to something approaching normal, so our glues have all quit setting when it's dry, and we're rained out when it's wet. So puttering and doing catch-up.

      Happy reading...

      Dave Z

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  3. This post inspired me to do some more work on my counter balanced mast!

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    Replies
    1. Hi Alan,

      Phil Bolger liked a mast that could be raised or lowered with one finger. My brother's MARTHA JANE wasn't quite that easy, but close. Whatta sweet deal!

      I'm still trying to track down details from Thames River Barges... one hand would use a winch to drop and raise hurking masts while the other at the helm 'shot' under a bridge. The whole maneuver (lower and raise) had to be completed before they lost way AGAINST the current!

      Wow.

      Dave Z

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