Technical Information on Block Systems
Purchase Ratios / Purchase Systems
(this is often referred to as the Purchase of the System)
The mechanical advantage of the system is always slightly less than the velocity ratio or purchase due to friction losses. In an ideal system (100% efficiency) the mechanical advantage = velocity ratio.
Block Loadings and Human Input
For example a small boat of 20ft (6.0m) jib could be easily trimmed, whereas a 50ft (15.2m) cruising yacht may require 5,000 lbs (2,250kg) of trimming force in a blow, but regardless of boat size the trimming engine is the same: a human being.
Today small crews often operate large yachts therefore efficient use of manpower for sail trimming is imperative.The average forces a man can exert are as follows-
And so, since human force is limited to a small pulling power, and the loadings sails create can be enormous, a mechanical advantage must be applied through Block Systems and Winches, to allow a sailor to correctly control sail trim.
Mainsheet system showing losses due to friction
Cascade System used on Kicking Strap
These are ideal for applications where fast response with relatively small movement is required, giving high power purchases, as each purchase acts as a multiplier.
Coarse and Fine Tune Purchase Mainsheet Systems
Fine Tune System.
Alignment of Blocks
Safe Working Loads on Blocks and Footblocks
There is another factor involved in block loading. This is the change of angle the rope turns through - the angle between the line of entry and the lines departing direction after passing around the sheave. Once the line starts to bend around the sheave, the total load on the sheave starts to increase until at 180 degrees the maximum sheave loading is reached, ie. see from the chart if a rope turns 180 degrees around the sheave and the rope has a tension of 10lb (4.5kg) the total sheave load is increased by a factor of 2 = 2 x 10lb (9kg) total sheave load.Deflection Loads
Choice of Bearing
Genoa Sheet Deflection Through Genoa Cars
The effort required to tow a Genoa car under load changes dramatically with the vertical sheeting angle. A No 3 blade jib will have a large sheeting angle, up to 70 deg. with the correspondingly high tow load of 65% of the sheet load. A no 1 Genoa may be as low as 40 deg. with a tow load of only 25%. When specifying the tow purchase care must be taken to match the equipment with the anticipated loads, use the following table as a guide.Genoa Car Towing Loads
Offset Loads on Mainsheet Cars
Mainsheet towing loads
As a guide, mainsheet towing loads are 25% of the total sheet load.
Snatch Block use
Where a snatch block is attached in a situation which will not permit full movement, such as through some toe rails, a shackle must be used to ensure full articulation.
Rope Clutch Holding Loads Explained
The holding load of each clutch is listed for the two main line sizes the clutch was designed to hold , the maximum and minimum listed are dependent on the line type used.
Rope clutches can handle one line size below the nominal range but at a reduced holding load, this can be usefull for more lightly loaded applications such as control lines and down hauls. In virtually all cases the larger the line the better the holding capacity, so where the ultimate load is required the larger of the designated line should be used.
These graphs show the range of holding loads that can be achieved, a good quality hard cored line will hold better than a softer line, in some cases this may be higher than the rated Safe Working Load of the clutch but in all cases the line will slip before the Breaking Load is reached.
D1 Rope Clutch
D2 Rope Clutch
Ad by Google
The information on this web site has not been checked for accuracy. It is for entertainment purposes only and should be independently verified before using for any other reason. There are five sources. 1) Documents and manuals from a variety of sources. These have not been checked for accuracy and in many cases have not even been read by anyone associated with L-36.com. I have no idea of they are useful or accurate, I leave that to the reader. 2) Articles others have written and submitted. If you have questions on these, please contact the author. 3) Articles that represent my personal opinions. These are intended to promote thought and for entertainment. These are not intended to be fact, they are my opinions. 4) Small programs that generate result presented on a web page. Like any computer program, these may and in some cases do have errors. Almost all of these also make simplifying assumptions so they are not totally accurate even if there are no errors. Please verify all results. 5) Weather information is from numerious of sources and is presented automatically. It is not checked for accuracy either by anyone at L-36.com or by the source which is typically the US Government. See the NOAA web site for their disclaimer. Finally, tide and current data on this site is from 2007 and 2008 data bases, which may contain even older data. Changes in harbors due to building or dredging change tides and currents and for that reason many of the locations presented are no longer supported by newer data bases. For example, there is very little tidal current data in newer data bases so current data is likely wrong to some extent. This data is NOT FOR NAVIGATION. See the XTide disclaimer for details. In addition, tide and current are influenced by storms, river flow, and other factors beyond the ability of any predictive program.