Dock lines

Is there a preferred ratio for dock lines to boat size, or is it just personal preference?

 

For a boat, you select a dock line based on the cleats or the "hand" of the line, generally about 3/8" to 1/2". Really, most lines that size will rip the average cleat right off a production boat before the line breaks. And never double up a line.

Ships are a whole different matter.

 

I am talking about boats in the 80ft range.
I hope I am not the only one who does not know this, but why never double up
your lines
Thanks for the reply

 

80 feet really isn't a boat. <<shrug>> Some recommend 1/8" per 10 ft. Really you should do a mooring calculation if it is 80 ft. Generally that is at SS 4,max storm surge current and 50 or 100 year return winds. See any of the good naval architecture texts for the calculation.

Anyway, the reason you don't double up the lines is because you halve the lines compliance so you double the spring constant, doubling the loading on the cleat. I always comment to the ship's bo'sn when I see a 90k breaking strength line doubled (or tripled) on a 35k max load cleat. Just not smart, because if the ship wants to move, you have to let it move and having the lowest spring constant is better than bringing her up short on a line.

 

Thanks. Makes sense about the doubling of lines.
About the dock line size. I was actually interested in the length

 

That depends on your cleat and fendering layout. If your length is 80' and your beam is ~20', you should have at a minimum 2 springs, 2 breasts, and a pair of night riders. Realistically you want twice that on hand. The springs should be ~120 ft, the breasts ~75 ft, and the night riders should be long, as long as practical, so they can be used as warps also. This is because for most large vessel piers, bitts and cleats are spaced ~50-60 ft and bollard spacing is ~100-120 ft.

 

That's a lot of "rope"
Thanks again

 

Not really, I carry ~300 ft of 3/8 nylon for a Catalina 22.