When to use thimbles instead of blocks?

[tolachi]

I'm curious when you would consider using a thimbe instead of a block. Apparently Moth's, 18' skiffs, and even some big boats use thimbles instead of blocks in parts of their vang purchase. I've also seen them used very nicely instead of blocks as twings and barber haulers. I'm wondering what criteria you would consider to determine whether it is better to use a thimble instead of a block. I'm guessing that low travel high load applications are one. I love how simple and failsafe this solution is. No moving bits, nothing to explode. Clean. See below for an example. All thoughts welcome.

[Brian Duff]

your on the right track.

that bare spectra line (photo) helps things as it is so slippery itself.

any short travel application might be alright without a turning sheave, as long as it doesn't need to move much or at all once loaded.

add backstays, runners, lazyjacks, topping lifts, etc.

[Brion Toss]

Hello,
You're talking lizards and dumbsheaves here, which is to say that the concept of a no-moving-parts purchase goes back a long ways, and that it might be helpful to look at antique applications to inform contemporary ones. While it is true that high-load, short-travel doesn't always need friction reduction (deadeye lanyards, for instance), there are instances where relatively low loads, over somewhat longer distances are also done sheaveless ( sending topmasts up, for instance). I think of it in terms of how much bother the friction is; it seems likely that sheaves are the product of frustration, an invention that, complex and difficult to make as it is, is worth it for the labor it saves and/or the efficiency it gains.
And this is part of a larger progression. No purchase at all is simpler than a purchase through a lizard, and the lizard is simpler than a sheaved block, and the block is simpler than a multipart purchase, and the purchase is simpler than a hydraulic setup, which represents a complete departure from the previous tactics. And alongside this progression are parallel progressions in material weights, densities, elasticities, durability, etc., and all of those are entertained as possibilities to the extent that they seem worth the effort/expense.
Therefore what we are talking about is good ol' design, whereby we consider what loads we have to deal with, what resources we have available, and then go about selecting as we think proper. So the more you know about the advantages and limitations of lines through thimbles, the more likely you are to make the best use of them.
On a practical note, one limitation of the setup in your photograph is that, given sufficient run, load, and time, the line will wear a groove in the thimble, increasing the friction and threatening to saw right through. And the process can be invisible, so lift the line often to check for wear.
Fair leads,
Brion Toss

[tolachi]

Originally Posted by Brian Duff

your on the right track.

that bare spectra line (photo) helps things as it is so slippery itself.

any short travel application might be alright without a turning sheave, as long as it doesn't need to move much or at all once loaded.

add backstays, runners, lazyjacks, topping lifts, etc.

Thanks Brian. At this point I am thinking that the application must be either low travel or low deflection. Single braid spectra seems like an important part of the equation.

I am guessing that in addition to the unexplodability of the thimble you would encounter some cost savings with larger applications. However, for dinghy's and small keelboats that I sail on it actually doesn't seem to be cheaper than a small harken wire block.

[Ian McColgin]

I make a combination of quarter lifts and lazy jacks - called locally LazyIans but really LazyLifts - and for rigs with light (aluminum or hollow wood) booms with sails up to about 500 square feet I find that a simple thimbled eye is far more suitable than the over-engineered high windage clutter of blocks we see in some pre-made lazy jack systems that are rigged to disadvantage so they can't be turned into LazyLifts anyway.

In general it seems to me that any situation where you don't need to haul under load may well be great for a thimbled eye.

G'luck