Block size for a davit

Simply put, the ratio of a pulley system is found by counting the number of falls from the moving block, so if it has three ropes that move then the ratio of weight load is 3:1
Tackles can be roved to advantage or not by how the moving rope falls.

 

You are neglecting frictional losses which can be significant as daiquiri has explained. The ratio of the load to the tension at the free end of the rope will always be less than the number of falls from the moving block because of friction.

 

In the end I gave the 60mm sized blocks to my sister. She flew out to Fiji today to continue her traverse of the Pacific on the boat she is crewing.

I think the 50mm blocks would have been adequate being triples. They are also very slippery despite being plain bearing models as some people have remarked. I will find out in a couple of weeks how easy it is to lift the dinghy and report back here for reference.

Good point about water in the bottom of the dinghy. There's always a bit sloshing around.

Thank you all for your help and advice. All very useful

 

Average block efficiency with modern ball bearing blocks is 93-95% not 80-90%. High end blocks like Harken Carbos have such high efficiency it's has become difficult to test it (a recent PS test indicated 105% efficiency on their load cell).

If you are still using 80% efficiency blocks then I would recommend replacing them with blocks that work.

 

With due respect, your efficiency figures are way too high. The 93-95% efficiency is an ideal value claimed in some sales brochures and possibly attained in some lab-condition tests, but not backed by field test data. Some blocks seem to be able to attain 93% in operative conditions, but most of them seem to fall in the 80-90% range, based on field test data.

Back in that other thread about pulleys and blocks (which, once again, I'd suggest you to read) there are data about measurements done by Joakim, which show an average sheave-block efficiency of approxim. 90% on his rigging.

A comprehensive set of test data can be found in these sites, which show that single block efficiency varies anywhere between 75% and 90%, with a maximum measured value of 93%:
http://www.bwrs.org.au/sites/default/files/PulleyReport/PulleyReport-w.html
http://hem.bredband.net/b262106/Boat/Blockfriction.pdf
The most important factor which governs the sheave efficiency is the sheave/rope diameter ratio. Anything below 12:1 will give efficiency figures below 90%, regardless of the type of bearing That's because the biggest part of friction is created between rope strands, not at the sheave axle.

On the normative side, Class rules for lifting appliances prescribe the 95% efficiency figure for plain-bearing sheaves and 98% for roller-bearing sheaves, but these figures relate to greased steel-wire ropes and at least 18:1 sheave/rope diameter ratio. The friction in nylon or PE ropes with 6:1 sheave diameter ratio is way higher than that.

We use 95% efficiency figure for commercial davits with steel wirerope which have to comply with Class rules, but even then we routinely have to multiply the resulting pull force with a safety factor in order to not undersize the actuators. Marine environment is harsh, performance-degrading and unforgiving for any equipment which doesn't make use of substantial safety factors in every aspect of their design.

 

Do you mean the friction is higher, not lower?

 

Yes, you are right. It is a typo.

The friction is way higher for a 6:1 diameter ratio (than for a 18:1 ratio).

 

I did read the article (be wouldn't open), and I remember the thread (but reread it anyway). Nothing in them backs up your assertion, except using a stretch test on line of unknown line age and quality to determine the likely load applied to derive friction losses on a 42:1 cascade, with blocks of unknown age, service life, and construction. I don't consider this a reasonable test. The article I could read didn't test a single block I would consider representative of modern leisure marine hardware.

If instead you look at recreations hardware, which is generally much better, the friction losses go down.

http://www.practical-sailor.com/issues/37_30/features/Mainsheet-Control-Systems_11051-1.html

 

I am aware of the article in the Practical Sailor Magazine, but at no point they clearly say what load was tested in order to obtain 24 to 33 lb of pull. We can presume they have applied 100 lb, but that's a guess. In that case, a physically impossible 105% efficiency indicates a serious flaw in their measurements and a uselessness of that table for engineering purposes. If we have to choose which paper is less useful as an argument, among all the data given these two threads, I have no doubts that it is the one in the PS magazine.

I believe that we have a dynamometer somewhere in the workshop, so I'l see if I can set up a simple rig and try to measure a couple of blocks we have in the warehouse.

 

Well, to be honest, the test described in the paper by Martin Schöön (linked in my post #20 and the other discussion which you know well by now) is fairly well done and rappresentative of modern blocks, arranged in a straightforward manner (not in 42:1 cascades). He has tested in a proper way Ronstan RF 469, Rutgerson 50, Lewmar Synchro 50, O.H. mod. 00403 and Harken 2607 blocks. The results are consistent with the common knowledge about physics of sheaves and ropes. They might be numerically surprising, but that's what science is all about - disrupting the established dogmas.
If you want even more data and arguments than that, you'll have to dig hard to find them.

 

Edit - The comments below expand on daiquiri's statement in post #20 The most important factor which governs the sheave efficiency is the sheave/rope diameter ratio. Anything below 12:1 will give efficiency figures below 90%, regardless of the type of bearing That's because the biggest part of friction is created between rope strands, not at the sheave axle.

Losses in block systems have two separate causes. One cause are the losses which occur in the bearings/bushings due to the rotation of the sheave. These losses is a function of the load on block and bearing/bushing only, and are virtually independent of the diameter and construction of rope running over the block and the size and design of the sheave.

The other cause is friction within the rope and between the block and sheave. This is a function of the load on the rope, the diameter and construction of the rope, the overall diameter of the sheave, and the shape and size of the groove in the sheave. These losses are virtually independent of the losses from rotation of the sheave. They will still occur even if there are no losses in the bearings/bushings.

My guess is claims of block efficiency in advertising are based on minimum losses in the rope.

Also the losses in the block increase the pull required to lift an object, but decrease the pull required to control the descent of an object.