Boat Forum / Building / January 2005

Good question!  I would have assumed that they would get thinner but
3/8" halyards on a 25' boat seem pretty overly large.  I'd pick 5/16"
for halyards and 3/8" or 7/16" for sheets.  Breaking strength isn't a
problem, but ease of fitting through sheaves is important.

Evan Gatehouse

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The loads on a 25 ft boat are not that high so anything 5/16 or
greater should have enough strength.  To minimize weight aloft I'd go
with a smaller, low stretch line for the halyards, preferably Spectra.
For the sheets, get something that feels comfortable in your hands,
probably 3/8.

Just a curiosity: did you try with a metric gauge????????  Moreover, can
you make a slightly more precise measure: 1/16" indeterminacy is quite
large.

Daniel

Braided line tends to run larger than the nominal diameter (Samson 7/16",
for instance, measures 1/2"), and I've never seen a synthetic line intended
for use as running rigging measure below. So your line measuring betweeh
5/16 and 3/8 is undoubtedly 5/16, and the other is undoubtedly 3/8.

Old line retains its original diameter, unless it's subjected to strains
near breaking strength.

HTH,
Tom Dacon

Having bought the wrong size many times over the years, I always take a
piece of the old line to the store just to make sure.

Old lines under tension get smaller.

With todays composite lines, you can probably go smaller.
Do you have lock-offs or cleats?

If the hardware is original. the minimum size of lines should be the
maximum size recommended for the sheave.  You NEVER put smaller diameter.  
Two good reasons: 1. Safety 2. handling ease.  larger diameter is easier
on the hands and grips better on the winches and is held better in the
cams.  One further reason is that in an emergency ALL lines may be needed
for jobs other than the ones the do normally.  Always make certain that
the lines FIT the hardware and if the hardware is not original and looks
a bit light or is smaller than another piece that is original and is in
line with the suspect piece...replace the suspect.

Just because some new material allows for a decrease in line size does
not make it acceptable for any job that matches its load capacity.  A
1/4" of kevlar/dacron might very well match your main halyard load
rating...but it won't match the hardware, and it sure as hell won't match
your hands!

My boat is metric- everything on it. My, it's been interesting.

In most cases you want the mazimum amount the sheaves will take but you can
expect the lines to run a bit slicker with one size smaller.  Check the
blocks.
I  splice or jacket up to 7/16 which is the smallest size that is
comfortable in my hands.
Rope strength doesn't mean as much as it used to since lines of much smaller
sizes than we use
have plenty of strength for the job.
It's unlikely to undersize the lines on most boats where strength is
concerned with the quality of the lines we have today. Unless you do some
seriously bad rerigging.