Block Sizing....

[Belliure 50]

Hi All, hoping the hive mind can help me out with some advice...


Previous owner of our Boat had a random menagerie of Blocks deployed throughout the running rigging. There isn't 2 that are the same brand or size. Ive got Lewmar, Wichard, Barten and some other weird things, all are at least 20 years old and need to be replaced.


Is there some sort of formula or rule of thumb for Sheave diameters and SWL that I can use to calculate correct block sizes?

Vangs, outhauls, reefing are all 8 to 10mm dia lines.

Mizzen sheets, Mizzen Halyard, and Staysail Halyards are 10mm diameter.

The Main & Genoa sheets are 14mm. Halyards are 12mm.
Main sheet is mounted center of boom.



Big heavy boat so I need to ensure I have the right sized gear.



Thanks in advance.

[fxykty]

Most efficient way to determine your loads is to pay an experienced rigger to provide you the working load numbers for each control. It will be less than an hour of their time. Once you know the working loads you can match blocks and shackles and the like to the working loads and the rope sizes. It will also confirm the rope sizes.

I’m surprised that your reefing line diameter is similar to vang line diameter - there is way more load on the reefing line and I would expect similar size to the mainsheet. That’s just a single example where your existing equipment is not a good guide.

You can also estimate working loads by inspection of the original components such as bolted tracks, turning blocks, deck sheaves, pad eyes, chainplate and shackle pin sizes, etc - basically all the bits that your blocks and lines attach to. You will need to carefully measure diameters (a good set of digital callipers), then use Google to find tables of strengths for the material (most likely 316 SS), or such as for a track from the manufacturer’s website. This will likely overestimate some components, but you could consider that better safe than sorry.

You can use wire and rope diameters as well to estimate working strength, but with rope especially you may not have the original anymore and it can be very difficult to identify the kind of rope for accurate strength values.

Note about working strengths, AKA safe working load or mean working load. These are usually 50% of breaking strength for 316 SS and around 20-25% for rope. Each manufactured product should have both values specified. Respect these ratios - don’t be tempted to go undersized to save money as the next time you drop off a wave and the loads spike you will break stuff.

[leecea]

Some companies have guides online but not sure how useful they are for you. Here is one:

https://www.harken.com/en/support/se...at-blocks-372/

[Jammer]

Main sheet has likely been upsized for ease of handling rather than load.


Most blocks are designed for a fairly narrow range of line sizes, so that within a particular manufacturer and design, you will only have the choice of two or three possible sheave sizes. That gives you a place to start.

[Bowdrie]

An old insult from the age of sail.
"Large ropes and small blocks to you".
When in doubt go bigger.

[Jim Cate]

Originally Posted by Bowdrie

An old insult from the age of sail.
"Large ropes and small blocks to you".
When in doubt go bigger.

I heard it as the sailors curse:

A sailors curse I wish on thee
Large ropes, small blocks,
Less wind than sea.

And I agree that larger is better, usually for strength, always for ease of use and longevity of cordage.

Jim

[Tcubed66]

Barton marine has a great chart on sizing blocks.