Re: What size rope to use for different boat sizes?
First off, remember that it is only 'rope' onland. Once you put it on a boat it becomes 'LINE'.
I've seen several places that post recommended sizes, for example:
BOAT US
http://www.boatus.com/boattech/dockline.htm
<20 ft: 3/8" line
20-30 ft: 1/2" line
30-40 ft: 5/8" line
>40 ft: 3/4" line
According to Rope inc:
http://www.ropeinc.com/choosingdockline.html
Boats under 20 feet generally use 3/8" line:
20 feet to 30 feet use 1/2" line
30 feet to 40 feet use 5/8" line
40 feet to 70 feet use 3/4" line
http://powerboat.about.com/od/smallboatseamanship/ss/Boatline_7.htm
The following is an approximate guide:
Line Diameter
Boats under 20 feet = 3/8"
Boats 20 to 30 feet = 1/2"
Boats 30 to 40 feet = 5/8"
Boats 40 to 60 feet = 3/4"
According to BoatUS:
http://www.boatus.com/boattech/casey/21.htm
Line Diameter Boat Length
3/8" up to 25'
1/2" up to 35'
5/8" up to 45'
3/4" up to 55'
7/8" up to 65'
I use 1/2 braideed dockline on my 28 ft boat (7,500lbs) and 5/8" braided for my bow anchor line. Keep in mind that 3 strand and braided will have differently breaking strengths for the same sized line. The write-ups in powerboat.about.com as well as BoatUS goes over details about different types of line and material as well.
If you search on some combination of "Boat Line Size" or other such phrases you should find many resources that discuss this topic. In the case of variation in recommended sizes, just look at what the majority are recommending. If one says 3/8" line and five others say 1/2" line, then 1/2" line is probably the right answer. At the end of the day, nobody hurt their boat by placing line too large on it, PROVIDED that the line properly fits around the size of the cleats installed on the boat. Keep in mind there is such thing as line too large for the cleat.
Yes Boat US has it posted with slightly different information in two different places, go figure.
I hope this helps