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ModMMax 12-06-2006 11:56 AM

How to size rope
The broker who sold us our boat suggested that we buy a spool of 3/4" rope and cut it into the variety of lenghts needed for different docking/rafting/slip situations. We went to a marine store and looked at a spool of 3/4" rope that was sitting beside a spool of 5/8" rope. The 3/4" looked huge. I'm sure the 3/4measurement is when the rope is stretched with rated load, but man, it looked like it would be impossible to knot and/or attach to cleats. The 5/8" looked like it would be difficult to work with but not as bad as the larger size. I could use some advice about purchasing the 5/8" rope as opposed to the 3/4". My boat is a 30,000#, 47' cruiser and yes it is high and will catch a lot of wind. I know there is no definitive answer to this since conditions exist that will cause even the 3/4" to fail, but generally, will the 5/8" rope be adequate? Thanks in advance for your thoughts.

btrayfors 12-06-2006 12:03 PM


Buy the 3/4" rope. Your boat size, weight, and high freeboard can put a huge strain on docking and anchor lines.

The 3/4" rope should work very well on the cleats on your boat, and on dockside cleats.


pigslo 12-06-2006 12:05 PM

All of your rope manufacturers have info online as well as a chart at the West Marine store. As I recall 3/4 is perhaps the minimum your craft needs but you can look it up. I think you can be too small, but not too big. One of the considerations on docklines is that they be made of nylon for stretch.

Sailormon6 12-06-2006 12:54 PM

There are numerous charts that recommend the correct size of lines for various sizes of boats. Consult those charts and follow their recommendations. Size does matter. Docklines should stretch, so that they absorb some of the shock when the boat rolls and draws up short on the docklines. That isn't terribly important normally, but when that 30,000# boat is bucking and rolling in it's slip during a hurricane or other severe storm, docklines that don't stretch enough can rip the cleats out of the deck. Oversized lines don't stretch as much as lines of correct size.

camaraderie 12-06-2006 01:14 PM

Mod...3/4 inch has 4500 lbs. higher breaking strength than 5/8 in 3 strand nylon. If you have trouble dealing with 3/4 inch 3 strand then I would suggest spending a bit more and getting 3/4 inch double-braid nylon which is MUCH softer and more supple and easier to tie AND even provides more breaking strength. 5/8" is just not enough for your boat. ... I know you can do about .30 cents a foot bettr on the price but I think sailnet will match prices.

T34C 12-06-2006 01:22 PM

I tried the West Marine on-line product advisor for dock lines. It came back with 3/4", for both 3-strand and braided line, for a boat your size. To me that means that is the min. line size I would recommend. While it may be larger than you are used to, go with the 3/4.

SVDistantStar 12-06-2006 01:45 PM

What about a 1972 Pearson 36 sailboat. It displaces 13,500lbs. My current lines range from 1/2'' to 3/4''. The last owers cut up most of the long dock lines so my longest line is about 25'. Ive been thinking of getting a spool of rope and cutting some nicer docklines(esp now since ive got whipping down).

T34C 12-06-2006 02:25 PM

I use 3/4" 3 strand dock lines on my Tartan 34 that displaces 11,500. Slightly over sized, but haven't had one break yet.

hellosailor 12-06-2006 03:17 PM

Max, as pigslo mentions all the rope makers have sizing charts. They'll also gladly work with you on the phone or by email.

The size of a rope isn't everything, a better quality fiber or different manufacture can make a thin rope stronger than a thick one. You need to figure out the loads you expect on your lines, and then pick lines that can safely carry that load. "Working" load and breaking load being two very different things, also.

Dock lines should have some stretch in them, so they can absorb the snaps and jerks of bouncing around at the dock. And in time, they will wear and need replacing, so buying a larger spool and putting half away may actually be cheaper in the long run.

Don't be surprised if the lines you select dont' fit properly under some cleats and hardware--it is not uncommon for hardware to be sized smaller and cheaper than it should be. If you can't fit the proper lines under the hardware--replace the flimsy hardware, too.

Some folks secure a boat, really secure a boat, every time they dock, figuring you never an be sure what weather will come through before you get back to the boat. Others are more casual about everyday versus storm prep. That's a choice you can make too, there's no need to use docklines that will hold you in a major hurricane all the time. (You just need to be sure you can and will get back to the boat as needed.)

And of course, if you're out cruising and tying up in other places, it never hurts to have some extra lines stowed. Some old ones that you don't mind using on tarred pilings, some longer ones if you need to float in tidal ranges, etc.

sailingdog 12-06-2006 05:02 PM

Generally, cleats should be 16x the diameter of the rope you're putting on them. So, 8" cleats are generally good for 1/2" line, and 12" cleats for 3/4" line. While this may sound excessive, this guideline generally leaves the cleats large enough to put two lines of that diameter on them without trouble... and the cleats can generally handle the load from the lines without breaking, provided they're mounted and backed properly.

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