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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 01/29/2004 : 21:09:07

From the [url="http://www.harken.com/"]Harken[/url] Tech Tips...

Tackles are systems of blocks and lines that increase pulling or lifting power. Cranes using single-pulley wheels were invented 3,000 years ago and compound tackles, with several wheels, date back to 400 B.C. It is said the Greek mathematician, Archimedes, devised a compound pulley capable of hauling ships ashore! Today's sailors use tackles in many sail adjustment systems, including vangs, cunninghams and mainsheet.

To determine a tackle's mechanical advantage, count the number of short line lengths in the tackle, or the number of times the line goes over a movable block. For example, a single block with becket divides the line into three parts for a mechanical advantage of 3:1. Ignoring friction, this lets you pull 90 lbs (41 kgs) with only 30 lbs (14 kgs) of effort. To determine the correct number of purchases for your system, calculate your line load and divide by pulling power, 50 to 60 lbs (23 kg-27 kg) for an average adult.

Remember that the more purchases you have, the more line you will need to move the block. (For example, to haul the sheet 1 foot (2.5 cm), you must pull 3 feet (7.5 cm) of line.) Also remember that the more times your lines run over a sheave, the more friction increases. Harken ball bearing blocks with large diameter sheaves help keep friction to a minimum and direct more pulling power to the task at hand. When increasing purchase, a slight reduction in line diameter also reduces friction.

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 01/29/2004 : 21:14:42

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">

Like I said, maybe I am confused....and I was...

So Frank, here is/are my question(s): I have the same configuration. Fixed double on the end, becket and double on the sail. The line starts on the becket goes to the end, back to the block on the sail, to the end of the boom, to the last block on the sail THEN.... goes to a block that is mounted on the boom with a diameter 2 times the size of the blocks on the sail.
1. Does that last block make my purchase 5 to 1 or is it still 4:1 with the direction only being changed? and
2. Does the fact that the last block is bigger in size do anything in this configuration?

dw
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
The cheek block only acts as a fairlead, it does not add to the purchase. The size helps the line make a more gradual bend and so it helps to be a bit larger. Pulleys are a member of the wheel and axle family which in turn is a member of the lever class. The length of the lever from the fulcrum is the radius. Big door knobs are easier to turn. However the work is being done on the axle, not on the device turning the wheel. In our case the line is just changing direction and doing it without gaining any mechanical advantage.

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skipn809
Navigator

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111 Posts

Response Posted - 02/01/2004 : 11:00:34

Boy, you guys know everthing!!!!
I bet all the knowledge in the world can be found right here on this site.

Seriously, I have been wanting to go to a Strictly Sail show in Chicago for a couple of years. Maybe next year. (Retirement, or at least semi-retirement looms.) Is it a good show? Any recommendations on where to stay?

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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.