Relationship between Rope and Block sheeve diameters

Lucky Duck

Lucky Duck

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I have read in a couple of places that you should aim to use a block which is 7 times the diameter of the rope in order to minimise the effort expended in turning the rope through the block.

In a couple of places this would lead to a block size a bit too big for where I would like to use it or a rather large price tag.

As far as I can tell this rule doesn't seem to be universally quoted (for example I couldn't see it written in the Harken catalogue), could I wisely ignore it and rely on the maximum rope diameter given by the block manufacturer?
 
Brent Goose said:
I have read in a couple of places that you should aim to use a block which is 7 times the diameter of the rope in order to minimise the effort expended in turning the rope through the block.

In a couple of places this would lead to a block size a bit too big for where I would like to use it or a rather large price tag.

As far as I can tell this rule doesn't seem to be universally quoted (for example I couldn't see it written in the Harken catalogue), could I wisely ignore it and rely on the maximum rope diameter given by the block manufacturer?
Click to expand...

Try the rope manufacturers' information. e.g.

http://www.marlowropes.com/technical/ropecare-advice.html
 
Brent Goose said:
I have read in a couple of places that you should aim to use a block which is 7 times the diameter of the rope in order to minimise the effort expended in turning the rope through the block.

In a couple of places this would lead to a block size a bit too big for where I would like to use it or a rather large price tag.

As far as I can tell this rule doesn't seem to be universally quoted (for example I couldn't see it written in the Harken catalogue), could I wisely ignore it and rely on the maximum rope diameter given by the block manufacturer?
Click to expand...

You take the block makers suggestion for max rope size then decide yourself based on your knowledge of the system whether you want to adopt his reccomendation and suffer an early failure of a highly loaded rope system or follow the rope makers specifications.

This is what Marlow have to say on the matter.

http://www.marlowropes.com/technical/ropecare-advice.html

Basically if your system is of secondary importance and lightly used then ignore the 8 to 1 rule . However in the case of highly loaded systems then even 8 to 1 might not give an ideal sheave size especially for certain rope materials.
 
Pick a block that has a SWL that covers what you want.

Select a line that has 2mm diameter less than the block manufacturer's max recommended size

Double check the SWL of the line.

Then you'll find you have a block and line that are good enough for the job, run without excessive friction and are considerably smaller than the rope manufacturer recommends. :)
 
Thanks for the responses.

I see Marlow recommend 8 times line diameter. I would be interested to know just how many people follow that advice as I don't recall seeing many boats with 80/90mm blocks at the mast for their halyards.

lpdsn said:
Pick a block that has a SWL that covers what you want.

Select a line that has 2mm diameter less than the block manufacturer's max recommended size

Double check the SWL of the line.

Then you'll find you have a block and line that are good enough for the job, run without excessive friction and are considerably smaller than the rope manufacturer recommends. :)
Click to expand...

That was where I was going.....
 
Brent Goose said:
Thanks for the responses.

I see Marlow recommend 8 times line diameter. I would be interested to know just how many people follow that advice as I don't recall seeing many boats with 80/90mm blocks at the mast for their halyards.
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The 8 x line diameter is to get maximum breaking strength of the rope. Halyards will mostly be sized for minimal stretch and to be good to grip with hands.
So in most cases halyards are over sized.

The block manufacturers operate with less diameter in their recommendations.
 
nigelmercier said:
Surely it also depends on the wrap round the block?
Click to expand...

You mean the angle between the two parts of the rope?
It will affect the numbers a bit.

There are several parameters to consider when matching a sheave with a block.

The sheave diameter determines the bending radius for the rope.
To not weaken the rope the bending radius should be 8 times the rope diameter or more.
The reason for this is that a tighter radius will give uneven load on the yarns in the rope (inside vs outside of the bend)

The width of the sheave must also accommodate the rope diameter so you avoid friction sides of the block.
A typical block will have specs like this
Sheave Ø 60 mm
SWL 1150 kg
BL 2100 kg
Rope Ø 6-14mm

Double braid polyester
6 mm BL 1150 kg
8 mm BL 1800 kg
14 mm BL 5000 kg

So in this case a rope of 8 mm or less would within the rope manufacturers range, and the rope will be the week link.
If if go for the 14 mm rope the BL of the rope is twice the BL of the block, but since the sheave/rope 60/14=4.3 we have reduced the strength of the rope.
But we have a system sized for reduced stretch.
I don't know the factor used to calculate the reduced BL of the rope, but I think we can assume that the block is now the week link in the system.
 
Sheave diameter affects three three things:

1. The rope strength. With a sharper bend, more load is carried by fewer fibers (on the outside if the curve). A 1:1 bend radius generally reduces the rope strength by 50%.

2. Fateigue life. A sharper bend will decrease fateigue life. How much depends on material. Kevlar was particularily vulnerable to this.

3. Friction. A larger sheave diameter will reduce block friction loads - approximately linearity with sheave diameter.

Factors 1 & 2 are big deals with elevator cables, but not so much with sail boat halyards which are usually sized for stretch and thus way over strength and today usually made from materials with long flex fateigue lives. Factor 3 is perhaps more important for sail boats.
 
Our best blocks here, Conrads, have the sheave and shell dimensions shown in this article -- http://www.woodenboatfittings.com.au/articles/conrad-blocks.pdf

Edited to add -- These of course are traditional wooden blocks, not the stainless-type most of you chaps seem to use. On the other hand, the relationship between line diameter and sheave diameter is no different -- the smaller the radius of the sheave, the greater the stress on the line. The numbers given show an adequate relationship between the two. (The block shown has a SWL of over 2 tons.)

block-d.gif
Mike
 
Last edited:
LadyInBed said:
Can't see much truth in the size to diameter theory
Mainsheet blocks can have different diameter.

644.jpg
Click to expand...

It is true and it have been explained at least twice in this thread
http://www.ybw.com/forums/showthrea...-Block-sheeve-diameters&p=4604093#post4604093
http://www.ybw.com/forums/showthrea...-Block-sheeve-diameters&p=4604141#post4604141

But as I tried to explain in my post there trade offs to be considered.
In your example of the fiddle blocks you will most likely find that the BL of the blocks is less than the BL of the max rope size for those blocks.
Fiddle blocks have one good feature that it will give a more balanced pull than a tackle made by two double blocks, so in many applications we can live with reduced BL of the rope.

The reduced BL of the rope due to the smaller sheaves wont matter here as it will still be more than the BL of the blocks.
On a 4:1 mainsheet like this you will normally size the rope so you can hold on to it, this will give a over sized rope with regard to BL.
 
It's certainly true that the internal friction in the rope is significant when running around a small block.
Dinghy mainsheets used to use 4 inch blocks in the 70's but these went out of favour for various reasons.
These days most people use thinner rope and wear gloves if it's windy.
I think the structure of the rope also changes things.
Rooster sell a sheet rope where the strength is in the sheath not the core, that is superficial a bit stiff (hence does not tangle as much) but runs through blocks well in light weather.

Rinsing the salt out of rope helps too.
 
nigelmercier said:
Surely it also depends on the wrap round the block?
Click to expand...

Actually no. All the bending takes place when going onto or leaving the sheave the wrap between those points doesn't affect it.
Its always the rule with aircraft control system wires, you must use the same size pulley for a given wire size whether it's changing the route by 10 degrees or 90 or whatever.
 
Brent Goose said:
Is there something similar around for friction?

In one of the applications I am thinking about the line will be hand pulled so a load of between 25 to 50kg shouldn't get anywhere near the breaking strength of an 8mm double braid rope.
Click to expand...
Friction is more complicated (more factors that affect the picture)
- The bearing between the sheave and the sheave pin (type of bearing), there is big difference here with associated price difference.
- The clearance between rope and the sides of the block
- The design of the block
- The surface of the rope
I don't think sheave diameter have a significant effect on friction.

For low load lines with a low friction requirement ball bearing blocks is a good choice.
Are you moving a lot of rope in/out or just smaller adjustments?
 
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