Friction and blocks, interesting reading

http://hem.bredband.net/b262106/Boat/Blockfriction.pdf

A report done by Martin Schöön in Sweden. Some of the conclusions are a bit surprising, but from a consumer perspective I like them <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Interesting. I'd really need to see more about the test setup and methodology before taking it to heart, though.

To many variables to really reach a usable conclusion (other than big lines have more friction). It is interesting none-the-less. I would like to see the difference between a line like maffioli vs. a composite core like V100 (I know, it's obsolete but it's all I can think of at the moment).

I love the durability of the line we have on the I20 but the cover is so thin and the core so thick that we really don't see much benefit from tapering it and the new 10:1 blocks we have don't work all that much better than the larger diameter 8:1 blocks.

Interesting practical investigation! I have tried to translate the numbers to what is of interest to some of us, in order to get an idea about the numbers. The conclusion is that if you have a 8x Block system (58mm blocks) an go from a 10mm to a 5mm line you get approximately the same force reduction as if you keep your 10mm line and switch to a 10x block system. The reduction is about 10 % in each case.

So what are the actual mechanical advantages of the tiny (3 mm?) mainsheet used on Marstron A-cats? Obviously it is tapered way up to the handled part.

What information are you missing?

/Martin

I'd like to see a diagram of your apparatus.

What did you actually measure? How did you compute friction from what you actually measured?

How did you control the line speed (I would think that friction is fairly dependent on line speed)?

I've been trying to think of how one could set up such an apparatus using a scale as the measurement device. I can imagine one way to do it, but it doesn't seem very reliable to me.

Please don't take this the wrong way. I realize this isn't a formal, published paper.

What's wrong with Fig. 1 on page 1?
Details I have left out: The knot used to form the loop,
the thin line and knots used to attach the hook of the
electronic scale to the line loop and the stuff I used
to create the different loads (the 174 N load was created
using a lead-acid battery). I left these out since I could
not figure out how this information could make any difference.


The force needed to pull the line loop through the blocks.


I have not computed the friction. The data shown in Fig. 2
on page 2 is the average of five measurements of the force
needed to pull the line loop through the blocks: raw data
with some low-pass filtering.


I state in the last paragraph of section 2 that I had no
means of measuring the speed. The speed was low---the kind
of line speed you would use when trimming sails---and
reproducibility is down to muscular memory and the limited
possibilities to actually vary the speed by much.


It is published if not in a peer reviewed, scientific
magazine. I have pestered some people with the manuscript
before publishing though.

I guess none of my papers in peer reviewed magazines have
ever been read by as many this soon after me writing them.
:-)

/Martin

Is this right:

You have a loop of line running through two blocks, one block directly above the other. The top block is fixed, and there is a dead weight hanging from the bottom block. This is shown in Figure 1.

Another line is attached to the loop at some point. The other end of that line is attached to an electronic scale. You hold the electronic scale in your hand(s) and use it to pull the looped line through the blocks while keeping the pulling line tangent to the looped line. As you pull, you try to maintain a constant rate of displacement, typical of line sheeting on a boat. As you pull, you record five load measurements from the electronic scale. The average of these five measurements is what is reported as the friction.

This second paragraph is what is not shown in Figure 1, and can't be figured out from your paper (at least I couldn't). I'd really expect to see this sort of explaination in a published, peer-reviewed paper.

You got it!


Well, whenever I expand this 'research' there will be a new edition...

/Martin

PS What's your field of research?

That's a better setup than what I was thinking of. The only problem I can think of is that the geometry of the pulling line would be tricky. I think you would have to keep it a little off from vertical to keep it from rubbing on anything. Maybe you could use a rigid puller with a crook in it to avoid that (sort of like an upside down "L").

A couple of observations:

The 58mm plain block had slightly more friction than the 47mm ball bearing block for the same line diameter. The gap also widened at higher loads.

Figure 4 would probably tell a different story if you used the 415N load level rather than the 174 N. I expect the ball bearing block would look better. (for US readers, 415N is 93 lbf, 174N is 39 lbf)

It would be really nice if you could compare a ball bearing block to a plain bearing block of the same sheave diameter. Actually it would nice to see that comparison at several different sheave diameters. Also, it would be nice to see a comparison of a new ball bearing block to a used one.

/ Just a Civil Engineer who sometimes plays with dirt in the lab.

After thinking about it more, did you have any problem with the bottom block and dead weight swinging?

It happened but then I dismissed the reading and stopped the swinging before resuming measurements.

/Martin

I opted for the 174 N data because the I have no 415 N data for the thinnest line that give us total friction very close to the pure block friction. I have attached a 415 N plot. As you see there is still little difference between the ball bearing block and the plain bearing blocks.

Do you sponsor me?
:-)

/Martin

It's not plotted in log-log space like Figure 4 (not that that's a bad thing), but it looks like the ball bearing curve is shifted down by about 20%. Hard to say for sure with only 3 data points, though.

Shifted down compared to what?

/Martin (just back from an extended weekend in Paris)

Well, if you plot a curve through your plain bearing data and another curve through your ball bearing data, it looks to me like the ball bearing curve will be about 20% below the plain bearing curve. In log-log space you'd see fairly straight lines instead of curves.

But you only have three ball bearing data points, so you can't make a firm conclusion. And based on your test setup, a 10-20% difference is probably just above the limit of detectability anyway.

Your point that line diameter and sheave diameter are the dominant factors seems pretty valid. However, I don't think you have good support for saying "there is hardly any difference between plain bearing blocks and ball bearing blocks". The fact that the 47mm ball bearing block outperforms the 58mm plain bearing block indicates that the opposite may be true.

Aha, I wasn't sure. It could be that you compared different graphs too.

Yeah, more data points would be nice. I'll try to work something out but I am not prepared to invest in ball bearing blocks. I do have some more line dimension but not of the same type of line which might work against me.

And today I found an error in Fig. 4. I'll see if I can find time to up-dated the report this weekend. The error has some impact on my conclusion so I'd better double-check :-)

/Martin

I have updated my report. No high-load measurements yet:
http://hem.bredband.net/b262106/Boat/Blockfriction.pdf

/Martin

This is consistant with a British report 5-8 year ago.

Interesting. Do you remember where it was published?

/Martin

The online copy was published by madforsailing.com. I lost the link when they went from free to subscription.

The surprise in the British study was that plain bearing blocks had lower initial friction than ball bearing blocks The theory was the plastic balls formed a temporary flat spot under static loads and required extra force until the ball bearings assmed a more rounded shape.

I am a madforsailing.com subscriber and I have searched their web with little luck. I find an old (late 2001) debate on this and some of it may be interpreted as referring to a published test, a test I simply can't find. I will write them and ask.

/Martin

well done Martin

your study post of the week in sailing anarchy

http://www.sailinganarchy.com/index_page1.php

think you won a t-shirt

I don't know about you, but I have ball bearing blocks for two reasons :

-1- the run smoother when very little load is placed on them, at least that is what I think; this is attractive for the light wind days.

-2- They don't jam up as easily with sand and salt. I have a few slide bearing blocks on my boat (no ball bearing) and indeed they do easily jam up with dirt and salt and are harder to get unstuck.

Wouter

Oops

/Martin

-1- is my experience too, even when in my 'lab' but the recorded numbers tell me a somewhat different story.

-2- Salt and sand are non-issues where I live and sail these days. When still in my native Gothenburg, salt was a problem and it really payed off to rinse (all types of) blocks and lines.

/Martin