Purchase and block friction measurements and calculations

Discussion in 'Sailboats' started by Joakim, May 10, 2013.

  1. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    Ludde won 2 Hobarts..yes he does tell a good story
    ( one line honours I think )
     
  2. patrik111
    Joined: Sep 2003
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    patrik111 Junior Member

    Hi
    If this is true, are not a large amount of blocks, also from renowned manufacturers, significantly too wide? I seem to en up with wire blocks if i want to satisfy the mentioned ratio. Does anyone know where one could find narrow sheaved blocks for lower load applications? (wire blocks tend to be high load and in many cases heavy). It seems the line diameter rather than load should govern geometry, and that load carrying ability should be a separate consideration.

    Please post if you know of light, narrowsheaved blocks suitable for loads say up to 200daN

    ///patrik
     
  3. jehardiman
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    jehardiman Senior Member

    If you mean sheave dia/ rope dia is too small, yes. If you mean that the groove width is too large for the rope dia, that all depends. Modern low-stretch linear core ropes need to be able to flatten out to prevent from overloading the outer fibers and unloading the inner fibers, hense the shallow wide U seen on modern hardware. This is very different than wire or braided construction where the helix angle provides compliance and where the rope needs to be supported on the sides during bending to prevent crushing of the strands, that called for the old round bottom U or V shape.

    From a hardware point of view, having large sheave diameter only helps with free running multi-part falls. For the most part, this is not the case in yacht rigging where weight and size, rather than free running parts is needed. For the most part, those things that need to be free-running, halyards, sheets, etc are mostly single part. As I pointed out in post #4, you can always just wait for the load to work through the falls then tighten up and the load will pull the falls out.
     
  4. patrik111
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    patrik111 Junior Member

    Thanks, i was refering to sheave dia vs rope dia (sheave width)
    The area of use for such sheaves would be on small multihulls, where loads, especially on sheeting and downhaul, tends to be both high and needing to run freely.
    I am looking for solutions that will significantly reduce the need for winches.

    Very interesting about the groove shape, anywhere i can find some further reading?

    Thanks
     
  5. jehardiman
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    jehardiman Senior Member

    Sampson say that the groove for modern synthetics should be at least 10% greater than the diameter, bu that is braid construction.

    http://www.samsonropecatalogs.com/home/100239.pdf

    I need to see if I can find a direct reference to linear core.

    New England recommends 10% larger grove for both Sta-set linear core and Regatta double braid, but the Sta-set has a breaking strength 15% higher for the samedia. If you used a Harken Carbo block, you would need a 75mm block to get anywhere near the breaking strength of a full wrap on a 6mm line of either of those.

    http://www.neropes.com/Resources/06NER_marine_final.pdf
     
  6. Joakim
    Joined: Apr 2004
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    Joakim Senior Member

    Just to update I made some new measurements with a bit changed backstay system. Two years ago the first block in the cascade (not the one attached to backstay) failed causing the second 5 mm dyneema to fail. It was Harken 304 Harken Sailboat Hardware and Accessories https://www.harken.com/productdetail.aspx?id=4262&taxid=414 and it has only plain bearing for high loads. The hole in the sheave had worn out badly causing the sheave to get stuck.

    I replaced it with Lewmar 60 High Load ball bearing block and a bit later I also replaced the dyneema going through the Harken 75 ESP block attached to backstay to 10 mm dyneema. So now I have 75 ESP at backstay -> 10 mm dyneema -> Lewmar 60 HL -> 6 mm dyneema -> Harken 304 -> 5 mm dyneema tripple Harken Carbo29 -> 6 mm polyester rope lead to cockpit sides.

    A few days ago I made a bit more accurate measurement of the true efficiency of the system. I used a very high load (10 tonnes) load cell I happen to have access to and a small one (40 kg). The high load one is very accurate, but has only 5 kg resolution at these loads. It is quite long, thus I can't put it between the backstay and the tackle without replacing the backstay with a shorter one. Thus I used main halyard and tensioned it enough to slacken the backstay.

    First I connected the load cell to tripple Carbo29. I measured the load I got by pulling the line 40 kg. When I pulled from starboard cleat I got 120 kg, port cleat 145 kg and directly at the carbo29 block at the lower end of the tackle 155 kg. 6:1 tackle had only 3:1 force ratio at STB, 3.6:1 at PORT and 3,9:1 at the tackle.

    Then I connected the load cell to Lewmar 60 HL First I pulled 40 kg downwards from Harken 304 (2:1) and got 70 kg. Then 40 kg downwards from Carbo29 tripple block and got 140 kg. The 4:1 cascade works at 3.5:1, but this is rather low load for it. Then I pulled 14 kg at PORT cleat or 16 kg at STB cleat to get 155 kg. At 40 kg pull I got 350/410 kg (STB/PORT). 24:1 works at 9-11:1 depending on load and which end of the line is pulled.

    Finally I put the load cell to ESP 75 thus where the backstay is. I got 335/360 kg (STB/PORT) with 20 kg pull. I got some noises from the attachment between ESP 75 and load cell and didn't dare to try higher loads. At this load the actual force ratio is 17-18:1 for the 48:1 tackle. Much better than the earlier measurements. It seems ESP 75 and the 6:1 Carb029 are the worst parts.

    I also found out that 350 kg is about the maximum I can pull to the main halyard with Lewmar 30 winch and 10" handle. 350 kg was measured at the end of the halyard. Probably there was considerably more near the winch.

    The block test I referred earlier can be downloaded here: Testberichte - Info & Service https://bootsport.sprenger.de/info-service/test-reports.html

    These test reports compare several non-boat blocks:
    https://pdfs.semanticscholar.org/dd2e/045dacd2c8c2b9cb4064a0ae3a563e8d03a1.pdf
    http://itrsonline.org/wordpress/wp-content/uploads/2014/09/McKently2011_ITRSPresentation.pdf

    Most tests were done with 1/2" or 11 mm rope similar to ones used as sheets or halyards in boats. None with plain dyneema core. Note that steel wire had clearly superior efficiency to any rope and even very good efficiency with a very small block.

    There were several very good efficiency blocks using sealed ball bearings. Why are these not used in boat blocks? I would imagine rescue blocks also may see salt water. Instead we are offered extremely expensive roller bearing blocks for high load applications. Is it due to salt water tolerance or weight? Or no reason at all?
     
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  7. philSweet
    Joined: May 2008
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    philSweet Senior Member

    Let's look at that 6 part line again. To get 3.6 to 1 out of a 6 part tackle with one extra turn, I'm going with pull of P, .87P after the turn, and 85% efficiency at each wrap on the pair of triples. If the top sheaves are 1, 3, 5; and bottom are the outside sheaves of an identical triple 2, 4, then we get loads as follow 1 - 1.61P, 3 - 1.163P, 5 - 0.840P and 2 -1.36P, 4- 0.988P. Total for 1, 3, 5 is 3.6P.

    Think about the upsetting torque on those triples. As currently reeved in the photos, there is a load imbalance on the top outboard sheaves of 21% of the total load on the top block , and on the bottom, it's 16% of the load on the bottom block. That causes a lot of misalignment between the sheaves and the line. This can be mitigated. It may seem a bit odd, but one of the falls off the top triple should come from the center block, not both of them from the end blocks. This reduces the cocking moment about in half for both line pulls. Switching to a double block on the bottom would help on the bottom. An even better solution is to use a double on the top with a single on the becket under it. The single runs crosswise to the double. Having the sheaves more square to the lines can improve that 0.85 efficiency quite a bit.

    - like the top end of this - Palan d'├ęcoute de grand-voile avec 2 poulies-winche et 2 taquets coinceur https://www.marinepro.ch/fr/poulies-winch-70-a-80mm/8848-palan-d-ecoute-de-grand-voile-avec-2-poulies-winche-et-2-taquets-coinceur.html
    [​IMG]
     
    Last edited: May 19, 2019
    Joakim likes this.
  8. Joakim
    Joined: Apr 2004
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    Joakim Senior Member

    Interesting. I had calculated the forces on individual sheaves, but not really thought about moments. I have to try rerunning the line. I haven't noticed clearly bad angles of tripple block, but vang has same blocks and sometimes freezes with the tripple loaded only on one side. The line there is much stiffer.

    Most doubles with becket have the becket under one block only, not centered. How much of a problem would this be?

    One option would be just to reduce the 6x to 4x. Probably would still deliver 2.5-3:1 force ratio and I would only need to pull half the line.

    I may also try Sprenger roller bearing blocks. They don't have that high working load (460 kg for 50 mm), but they are very cheap, about half the price of Carbo 57 and the ball bearing varient (only 250 kg working load) had lower resistance than Carbo 57 in the test.
     

  9. philSweet
    Joined: May 2008
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    philSweet Senior Member

    Yep, you want a centered becket arrangement. Stupid how few of these exist. You can use strops in a pinch, but a solid drop is what helps to pull the block straight when the load isn't balanced. So strops defeat the purpose of going to a double up top.

    Ronstan RF35212 solid center becket. The same double without the becket can be stropped through the center hole.

    [​IMG]
     
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