Block Loads

Tedd

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Jul 25, 2013
745
TES 246 Versus near Vancouver, BC
I'm planning to buy some new blocks and I'm wondering how to determine the load rating I need. Can anyone recommend sources of information on the loads that halyards and sheets carry, or any other sources of guidance on sizing blocks?
 
Jan 4, 2010
1,037
Farr 30 San Francisco
Well I think Harken has a calculator. Blocks always seem oversized to me. How hard can you pull on the rope? 50 lbs? If so then a block that turns the rope 180 degrees has 100lbs of force to deal with.. factor in some shocks etc and maybe you look for a block with a SWL of 300 lbs done.

Genoa lead is maybe different. Now you have a 46x winch with 50lbs of push 2300lbs times 2 on a turning block plus some safety margin.
 
Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
Yep, Harken has a calculator: Harken Sailboat Hardware and Accessories

When sizing blocks, sometimes the load is not the only consideration. In some applications a larger line is easier to handle, so the block needs to be large enough to handle the line for example lines like the topping lift and outhaul don't have high loads, where a ¼ inch line can handle the load, a 5/16 inch or ⅜ inch line is easier on the hands.
 

Tedd

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Jul 25, 2013
745
TES 246 Versus near Vancouver, BC
Thanks, everyone! That Harken calculator is handy, and, interestingly, it gave a number for my boat not very different from John's estimate.

The SV-SecondStar.net site led me to marinehowto.com, which seems to have lots of great information on a wide range of subjects.
 
Oct 22, 2014
20,992
CAL 35 Cruiser #21 moored EVERETT WA
marinehowto.com
Yes Tedd. Our very own wizard. MaineSail is a great resource. You can find his info here on the forum as well.
As for a shameless plug. Check out his Bed-it Butyl Tape. An essential for sealing up the leaks you may have experienced on anything you had to screw down on to your boat deck. I am a user and I get no benefit from this recommendation.
 
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SG

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Feb 11, 2017
1,670
J/Boat J/160 Annapolis
The issue of block sizing for sheets because the are often subject to shock loads. In the “old days”, before high strength lines, you err protected because the diameter of the line that the block could take usually provided a measure of inherent safety.

I have seen on snatch block “blow up” on a genoa. It would have really injured someone if someone had been in-line with it. It caught a lifeline and bent a stancion. Pretty loud bang.

I think that the usual weak point is probably the shackle or twisting on the shackle.
 
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capta

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Jun 4, 2009
4,766
Pearson 530 Admiralty Bay, Bequia SVG
Blocks always seem oversized to me. How hard can you pull on the rope? 50 lbs?.
50#'s? With a boat weight of around 77,000 pounds, I'm guessing it's a tad more than 50#'s in our case. At any rate, I've seen dozens of even properly sized blocks explode (there is no other word) over my sailing career. lol
 
Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
The SV-SecondStar.net site led me to marinehowto.com, which seems to have lots of great information on a wide range of subjects.
Thanks for looking at the site. Yes, MaineSail, aka RC, aka Compass Marine, is an incredibly valuable resource. His forum here on SBO well worth following.
 
Jan 4, 2010
1,037
Farr 30 San Francisco
Bigger boats make you stronger? From one perspective I have a mainsheet with 6:1 I can pull at 50lbs so the tension in the line is 50lbs. The pull on the boom is 300 lbs. On a line with a winch I can push or pull 50lbs onto the winch times the winch power 15x 40X whatever and that is the tension I can put into the line. Shock loads like when a spinnaker collapses and refills with a bang well that is another story. With all the modern high tech zero stretch lines and zero stretch sails shock loads are getting worse. Lots of lines have a minimum suggested radius which might size the block.
 

Tedd

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Jul 25, 2013
745
TES 246 Versus near Vancouver, BC
It's not just shock loads though, John. Once you tie off the halyard the load is depended on the aerodynamic loads on the sail -- nothing to do with how hard you pulled on it, unless you pulled on it harder than the wind does.
 
Jan 4, 2010
1,037
Farr 30 San Francisco
sure but I have never had a situation where I couldn't control the mainsheet. i.e. so much sail that once uncleared the mainsheet pulled harder than I could. Look in the end you should go with the recommendation of the professionals rather than some random dude on the internet.
 

SG

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Feb 11, 2017
1,670
J/Boat J/160 Annapolis
Maybe a modestly strong women, but for many of you, if you put your back and legs into it, I suspect that you'll be putting more than 50#'s into pulling a line ;^)))
 
Jan 4, 2010
1,037
Farr 30 San Francisco
Ok but no more than 200 so why is the SWL on my mainsheet 4000lbs??? I think stout twine should do it although tough on the hands. New England ropes (I think) has a nice video where they take a modern hi-tech rope the size of your pinky and load it up to 14klbs before it breaks, incredible, then they ask "now what happens if we drop 200 lbs through 6 ft with this rope?" well it snaps. No stretch = brittle, then they introduce their arborist ropes.
 
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Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
Ok but no more than 200 so why is the SWL on my mainsheet 4000lbs??? I think stout twine should do it although tough on the hands.
As you note, ¼" dynema may be more than strong enough, but tough on your hands.

A couple of thoughts about the high SWL on your mainsheet. SWL are calculated by stretching the rope in a controlled environment until it breaks and then taking some percentage of the breaking strength as the SWL. A key factor is that the line is stretched in a straight line.

When a rope is knotted, as when tied to a block or shackle the line becomes much weaker, depending on the knot it can be as much as 50%. See this page for test data Knot Break Strength vs. Rope Break Strength A simple google search yields other sites with similar data.

One of the reasons that knots reduce line strength is that when a line goes around a turn, the forces on the line are not uniform across the diameter of the line. The fibers on the outside edge of the turn are under greater tension than those on the inside of the turn. The repeated tensioning of those fibers weaken them over time. As the individual fibers and yarns stretch and relax they generate heat causing them to lose strength. This is one reason dock lines fail. And why high load blocks have large diameters.

Finally, just exposure to the elements weakens line. UV exposure, dirt, and salt all contribute to the demise of line.

When these factors are considered, your 4000# SWL line has an effective SWL considerably below its rating.

Splices instead of knots, reduced exposure to the elements and dirt, and large diameter blocks help to preserve the SWL of any line.
 
Jan 4, 2010
1,037
Farr 30 San Francisco
Well I go with what MS says but I couldn't find your link. What frustrates me is sailing ropes aren't rated for shock in any way like for instance a climbing rope. So stronger but lower stretch may be more vulnerable than an old bungy cord.
 

SG

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Feb 11, 2017
1,670
J/Boat J/160 Annapolis
If you search rigging shock loads you'll see that real "SHOCK" loads operate differently in a non-elastic environment. Repeated "fatigue" type loadings of low stretch lines aside, they don't really have that much vulnerability unless you go extreme. By example:

If you got a 1/8" Vectran line as a halyard, you're going to face issues with cutting (or your hands), abrasion, ability of stoppers, cleats, and winches to make handling it reasonable. If you simply stick within normal ranges, you're likely not to have issues.

I think the bigger issues with extreme downsize of lines to take advantage solely of the elongation resistance (stretch) or working tensile strength of the line is that abrasion, stress on fittings (cutting), and other unintended issues arise.

Force = mass x acceleration still governs. If you have huge negative accelerations due to a very stiff line, you'll increase the consequences no only on the whole system, but individual components in ways that you didn't anticipate. If you're loading-up a system which has built-into it some "give" (like a nylon spinnker", then having a Vectran sheet is less of an issue on the loads at each end.
 
Jan 11, 2014
11,321
Sabre 362 113 Fair Haven, NY
Well I go with what MS says but I couldn't find your link. What frustrates me is sailing ropes aren't rated for shock in any way like for instance a climbing rope. So stronger but lower stretch may be more vulnerable than an old bungy cord.
I don't think MS has opined on line size and block size. The link is just a link to his forum.

Most lines on a sailboat are not subject to shock loading. Most are subject to a constant strain or tension. The most vulnerable lines to shock loading are the traveler control lines, mainsheet, jacklines/tethers, and spin sheets.

Drew Frye has done research on using dynamic climbing ropes for travelers for Practical Sailor, he may have posted some of the info on his website (Sail Delmarva) as well as on PS. He is an occasional visitor to SBO and posts under Thinwater. Search his posts to see if he has written anything on this.

One issue with testing line for shock loading is coming up with a standard. It is pretty straight forward for climbing ropes, drop a specified weight a designated distance and see how times it takes to break the line. Climbers also count the number of times they fall, I don't think sailors count the number of accidental jibs.

The issue for shock load testing, I would think, is coming up with a standard. The shock load in an accidental jibe in 10 knots of breeze is going to be much lower than in 20 knots. I suppose there a standard could be developed but how would a sailor evaluate it? In climbing the falls are counted and the rope retired at or before the rated number of falls. It is also pretty easy to know the forces involved. The climber knows his/her weight and the distance he fell. Without some sort of load gauge on the rigging, how would a sailor know how much force was exerted in an accidental jibe?

I have been intrigued with the notion of using dynamic climbing rope for the traveller. Unlike a halyard, static stretching is not much of an issue for a traveller, and unlike a traveller dynamic stretching is not an issue with a halyard.