Blocks: why is recommended maximum diameter rope, so slender?

[Greenheart]

This is a daft question, because I suspect it's like asking whether food is safe to eat, one day after the expiry date...

...but maybe there's some good reason I don't know about.

Looking at blocks for sale in the majority of chandlers, or from the manufacturers themselves, surprisingly few models are offered as being suitable for thicker sheets than 14mm...

...which seems surprising, when the same stores sell, for example, 20mm Marlowbraid line. Where do big-yacht owners find blocks to run this rope through?

I found a little old Holt Allen 4380 block in the back of a drawer, and it easily carries 10mm braided line without any problem, or even the slightest abrasive wear. Yet the website states that the thickest line recommended is only 8mm. Why would the maker limit its use?

Likewise, Barton's size 7 blocks...the sheaves are 20mm thick, so the space will be a clear 22mm, yet the advised maximum diameter line is only 14mm. Why? I certainly used a much smaller Barton size 4 with 14mm line, for years, without any noticeable wear...

...yet these limits seem to apply to all the manufacturers' products. Selden, Harken, you name it.

I prefer thick sheets - much less of an ordeal to keep a grip on, under tension. So naturally I'd need bigger blocks than are standard on dinghies...but is there a good reason not to use thicker line than the makers recommend, when there is clearly space to do so?

 

[Hazeltine]

Thinner rope runs a lot easier/ more freely, less drag so it's easier to play the sheet, that's the thinking in a little laser anyway, thinnest string you can use without shredding hands

 

[Twister_Ken]

Well, obvious, innit? To get you to buy bigger, more expensive blocks.

 

[Greenheart]

Well, obvious, innit? To get you to buy bigger, more expensive blocks.

That does indeed sound obvious...except that NOBODY actually sells the bigger, more expensive blocks...I can't find ANY source for them. Still, I hope you're right, Ken. Right then...18mm Marlowbraid, and Size 7 Bartons...

 

[Deleted member 36384]

There is a technical reason: the sheave is sized in both diameter, width and groove depth to support the sides of the rope so that it remains at full strength as it runs around the sheave. A larger rope is too wide on a sheave for a smaller rope and some of the support might not be optimum; as such load bearing capacity is reduced. The correct sheave size also reduces wear on the rope both internally and externally. On fibre rope the margin is probably quite wide on wire rope, especially at loads nearing the SWL, the margin will be much smaller to the extent that incorrect sheave size would cause the wire rope to fail.

An example of this is combined rope / wire rope halyards where the sheaves can be dual sized with a groove for the wire and a profile for the fibre rope combined.

Sheave dimensions for rope size or wire rope size is easily available on the Web.

 

[awol]

That does indeed sound obvious...except that NOBODY actually sells the bigger, more expensive blocks...I can't find ANY source for them. Still, I hope you're right, Ken. Right then...18mm Marlowbraid, and Size 7 Bartons...

You haven't really tried hard, have you. Harken 3199 takes 16mm line for a mere £800, Harken HC 8633 takes 25mm line for a trifling £1300.

Any block that takes a large diameter line will be designed to carry loads > the breaking strain of the line so stop being a woose, fit smaller lines and wear gloves.

 

[Greenheart]

There is a technical reason: the sheave is sized in both diameter, width and groove depth to support the sides of the rope so that it remains at full strength as it runs around the sheave. A larger rope is too wide on a sheave for a smaller rope and some of the support might not be optimum; as such load bearing capacity is reduced. The correct sheave size also reduces wear on the rope both internally and externally. On fibre rope the margin is probably quite wide on wire rope, especially at loads nearing the SWL, the margin will be much smaller to the extent that incorrect sheave size would cause the wire rope to fail.

An example of this is combined rope / wire rope halyards where the sheaves can be dual sized with a groove for the wire and a profile for the fibre rope combined.

Sheave dimensions for rope size or wire rope size is easily available on the Web.

Thanks, Old Boots. That certainly is a good answer. And, it seems to say what I'd hoped anyway...there's no good reason to read much into the recommended maximum, when the loads (in my use) won't ever approach what they were manufactured to withstand.

You haven't really tried hard, have you. Harken 3199 takes 16mm line for a mere £800, Harken HC 8633 takes 25mm line for a trifling £1300.

Any block that takes a large diameter line will be designed to carry loads > the breaking strain of the line so stop being a woose, fit smaller lines and wear gloves.

Thank you for pointing out the big blocks available - actually I had tried long and hard to find them - I don't know where you can have looked. Meanwhile, take your comfortless narrow-gauge cheese-wire sheets, and wrap 'em round your prop, "woose". (Translate, please?)

 

[Lakesailor]

Chandlers who carry blocks at those eye-watering prices are either too wealthy to need the shop or about to go bankrupt.
You'd only get those in for a special order.

 

[Greenheart]

Probably so. What's odd, is the fairly-widespread availability of rope, apparently too big for the blocks that are sold alongside.

 

[Twister_Ken]

Probably so. What's odd is the fairly-widespread availability of rope, apparently too big for the blocks on sale alongside.

Yerbut a lot of thick string we buy is not required to go around a block. It's for mooring, anchoring, towing, etc.

 

[Greenheart]

Yerbut a lot of thick string we buy is not required to go around a block. It's for mooring, anchoring, towing, etc.

Apologies for saying 'rope'...I meant sheet. Only ever sheet.

What kind of bloke spends £5 per meter of thick Marlowbraid, then uses it as anchor rode?

 

[Twister_Ken]

Apologies for saying 'rope'...I meant sheet. Only ever sheet.

What kind of bloke spends £5 per meter of thick Marlowbraid, then uses it as anchor rode?

A rich one?

 

[Greenheart]

A rich one?

Those block prices certainly must be the Park Avenue choice. (Block - get it? )

Actually I did find ash blocks, able to carry very large sheets, very pretty, and only 10% the price of the Harkens above. Enormous great things though, with a suggestion of the Onedin Line, which might look, what's the word...ludicrous, in a small boat.

 

[Tranona]

what's the word...ludicrous, in a small boat.

Just a ludicrous as 14mm sheets on a small boat!

If you are buying a boat big enough to need 14mm sheets you should have the budget to buy the appropriate gear to handle it.

If, on the other hand you are buying a more normal boat that needs sheets in the range of 8-12mm you should have no difficulty in buying or affording the correct size blocks.

 

[Greenheart]

Just a ludicrous as 14mm sheets on a small boat!

If you are buying a boat big enough to need 14mm sheets you should have the budget to buy the appropriate gear to handle it.

If, on the other hand you are buying a more normal boat that needs sheets in the range of 8-12mm you should have no difficulty in buying or affording the correct size blocks.

Only a matter of comfort. I found the narrow lines that came as standard on dinghies, decades ago, to be pretty vicious in a breeze. And they seem to have got narrower since, in the pursuit of performance and weight-saving. I finally used a 14mm mainsheet, on a Topper...

...it was very comfortable and a great improvement on the standard sheet. A very long way from ludicrous! But a block the size of a frying pan, hanging off a dinghy boom...that's ludicrous.

 

[Twister_Ken]

I use 14mm on mine. But that's with 67 m2 of sail.

 

[Seajet]

I'm with Dan on this one, I use 14mm sheets as they're comfortable to handle; maybe manufacturers are wary of larger rope sizes as that infers high loads, big boats ?

I used to crew a trendy state of the art International 14 dinghy, the jibsheets were so thin and highly loaded ( weight & windage reduction ) that it was impossible to sail without gloves.

I did work at a chandlery briefly, the attic store was full of big blocks etc for a large classic boat, the order had been cancelled after £10,000 worth of kit had been bought in for this 'prestige order'.

As Lakesailor correctly foretells, the business went bust, going downhill like a bungalow on the Cresta Run.

 

[Tranona]

Only a matter of comfort. I found the narrow lines that came as standard on dinghies, decades ago, to be pretty vicious in a breeze. And they seem to have got narrower since, in the pursuit of performance and weight-saving. I finally used a 14mm mainsheet, on a Topper...

...it was very comfortable and a great improvement on the standard sheet. A very long way from ludicrous! But a block the size of a frying pan, hanging off a dinghy boom...that's ludicrous.

If you are going to rig a lightweight dinghy as if it is a 40 footer then you need the gear to go with it!

 

[Greenheart]

If you are going to rig a lightweight dinghy as if it is a 40 footer then you need the gear to go with it!

On this occasion, I wasn't planning any rig-upgrades. But I see no reason, on a boat whose use wouldn't ever be influenced by racing pressures, why one shouldn't improve on the weirdly ascetic standards that so many sailors seem to regard as vital to their seagoing egos...

...to the effect that "you're not really there, unless you're in discomfort!". If I was in discomfort, I wouldn't want to be there!

It's not as if cheese-wire sheets convey information to the crew any better than broad, comfortable ones can. Why suffer?

 

[Searush]

On my dinghy mains I use the biggest ROPE I can get thro the little blocks.

I want it easy to hold & a tad of friction helps that, but the ropes & blocks are never loaded anywhere near their capacity. As long as it is free running enough to allow the sail out in a very light wind it will do. If too much friction for that then downsize the rope or upsize the blocks.

I am removing all the strands of thin braided cotton thread off my GP14, it is awfull stuff & most of it has breaks in the core anyway!