Island Time 1,211 Posted April 12, 2015 Share Posted April 12, 2015 Here is an example of reaving the same blocks the right and wrong way; The easy way to tell whats right is in the top pic there are 2 lines pulling the cart, and in the bottom pic there are 3. The bottom pic is the correct method, providing a 3x advantage. The top pic has only 2x... Quote Link to post Share on other sites
rigger 47 Posted April 12, 2015 Share Posted April 12, 2015 On a good number of vessels I have worked we have rigged to advantage or disadvantage depending upon the best lead. Though if a 3 to 1 is needed for the job and a lead from the standing part is required then a single and double block might get used, but if 2 to one will do then two single blocks rigged to disadvantage is fine. Saying correct method or wrong or right really depends upon the overall situation. ie: HMS Victory the standing block is a double, sure if you rigged it the other way you would get 4 to one rather than three to one but where would the crew work it? With a tack tackle where you want to pull the tack out to the cats head (if so rigged) rigging to advantage would mean the person hauling from outboard of the cats head - pretty impossible for the crew to stand in in midair. Hope the above is not confusing. On a prod how is tack line rigged? Quote Link to post Share on other sites
rossd 16 Posted April 12, 2015 Share Posted April 12, 2015 Yes have wondered about that with my halyards They exit the bottom of mast which would be OK if going to the cockpit but they dont. They come up mast to the winch . Always seems the effort is going in the wrong direction. . Quote Link to post Share on other sites
Island Time 1,211 Posted April 12, 2015 Author Share Posted April 12, 2015 Yep, fair point Rigger. I was just trying to illustrate that the same gear can be used to get more or less purchase, depending how it is rigged. I thought some of the users might find this interesting. Like all things boating, there are some situations where you may not want to rig, or cannot rig to advantage as you have ably illustrated. I like the pic of the Guns!! Quote Link to post Share on other sites
rigger 47 Posted April 12, 2015 Share Posted April 12, 2015 All good IT. Just thought it worth another perspective. Might not be around for a little while - Off to China today to test out a new boat, no sails unfortunately just 6000+hp Quote Link to post Share on other sites
Black Panther 1,568 Posted April 12, 2015 Share Posted April 12, 2015 You have a terrible job don't you? Quote Link to post Share on other sites
Kevin McCready 83 Posted April 12, 2015 Share Posted April 12, 2015 Having had a career back and forth to China, yes it is a terrible job. I keep away these days because of the pollution, OHS and food safety issues. One site I was on had people packing powered asbestos into brake block moulds with their bare hands outside in the wind! I left within 20 seconds. The model was the same sort of mum, dad and the kids backyard operation that got the Japanese car industry started all those years ago. The same operation used the outsourcing model to supply a huge part of the world's grinding disks. These days I sit comfortably at home or on the boat and do translations about benzene glues killing workers and pregnant girls in Nike shoe factories or slave labour in Apple factories. Oh, and I try, though it's really hard, not to buy Chinese. Quote Link to post Share on other sites
SloopJohnB 323 Posted April 12, 2015 Share Posted April 12, 2015 Here is an example of reaving the same blocks the right and wrong way; Rove to advantage.jpg The easy way to tell whats right is in the top pic there are 2 lines pulling the cart, and in the bottom pic there are 3. The bottom pic is the correct method, providing a 3x advantage. The top pic has only 2x... Another disadvantage....... The load will run over you. Quote Link to post Share on other sites
Kevin McCready 83 Posted April 12, 2015 Share Posted April 12, 2015 from wikipedia: if the load is to be hauled against gravity, then there is a benefit to reeving the block and tackle to disadvantage, because in this case the weight of the individual can offset the weight of the load. On the other hand, if the load is to be hauled parallel to the ground, there is a benefit to reeving the block and tackle to advantage, because the pulling force is in the direction of the load movement allowing the individual to manage obstacles. Quote Link to post Share on other sites
NevP 0 Posted April 13, 2015 Share Posted April 13, 2015 Another disadvantage....... The load will run over you. Or if my thinking is correct, you just stand on the cart/trolley and haul from there and you're back to 3:1 advantage. Quote Link to post Share on other sites
Island Time 1,211 Posted April 13, 2015 Author Share Posted April 13, 2015 Another disadvantage....... The load will run over you. This is part of the IQ test!! Next issue is if you stand aside, how much effect does the angle of haul have on the purchase?? Quote Link to post Share on other sites
NevP 0 Posted April 13, 2015 Share Posted April 13, 2015 This is part of the IQ test!! Next issue is if you stand aside, how much effect does the angle of haul have on the purchase?? Trick question. None. All the last turn does is change direction, it doesn't affect the advantage. If you remove the last turn through the block completely you still have 2:1 purchase, you reduce the friction due to the final sheave, and you don't get run over. Quote Link to post Share on other sites
Island Time 1,211 Posted April 13, 2015 Author Share Posted April 13, 2015 Lets make this a bit clearer - how do you work out the loads on the last example here? Quote Link to post Share on other sites
John B 106 Posted April 13, 2015 Share Posted April 13, 2015 Dad, do you know the piano's on my foot? You um it son , I'll play it. 1 Quote Link to post Share on other sites
ScottiE 174 Posted April 13, 2015 Share Posted April 13, 2015 you can start by redrawing it at its correct location for it to be in static equilibrium Quote Link to post Share on other sites
Island Time 1,211 Posted April 13, 2015 Author Share Posted April 13, 2015 I never claimed to be an expert Scottie, and I did not understand your point? Happy for you to submit a new drawing (ok I admit it's rough!) if you feel I've missed something important? Take it as given that all the lines are parallel except the one marked 45 deg.... Quote Link to post Share on other sites
NevP 0 Posted April 13, 2015 Share Posted April 13, 2015 My bad. I thought we were talking about the tackle rove to disadvantage (configuration 2 here). In which case the angle of the final lead has no effect. In configuration 3 the angle definitely does have an effect. I think what Scottie is saying is that in configuration 3 the hanging weight will NOT hang vertically from the top block because of the the side force from the 45 deg pull. However, I think the original trolley example would actually have a straight pull similar to your diagram due to it's wheels keeping it on track. So the two scenarios are not identical. At this point we need some vector algebra and that's taking a little more thought. Quote Link to post Share on other sites
John B 106 Posted April 13, 2015 Share Posted April 13, 2015 45 degrees = 1/2 what it would be if it was vertical. That'd be fair wouldn't it? Quote Link to post Share on other sites
Island Time 1,211 Posted April 13, 2015 Author Share Posted April 13, 2015 Agreed NevP, but exactly how much offset to show was too hard!! So, lets say in the 3rd case then the 10KG load is on a friction-less track that can only go straight. JohnB, nope, it's more than that. The question is, how much more, and how do you work it out? There is a relatively simple method, that I just learned, and I thought some others would find it useful... Quote Link to post Share on other sites
NevP 0 Posted April 13, 2015 Share Posted April 13, 2015 45 degrees = 1/2 what it would be if it was vertical. That'd be fair wouldn't it? That was my first thought, but that's not correct because there are two vertical vectors and one at 45 deg so it would be closer to 1/3 of 45 deg (15 deg). But even that isn't quite correct when you add the vectors the pull is more like 15.14 deg. And then as previously mentioned the vertical lines won't be exactly vertical because of the side pull... So at that point I gave up. Have just read IT's latest reply. So we could prob use 15 deg to calculate for practical purposes. But sounds like you may have a simpler method IT. Quote Link to post Share on other sites
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