I've always used half inch anchor line which is certainly overkill for a 21' boat but I was diving in some deeper spots and leaving nobody in the boat so I wasn't taking any chances. I don't dive much anymore and the line is just about had it. Is there a boat weight to line size formula to calculate appropriate breaking strength? My anchor is a fortress rated for boats to 48'.

I have a 21 that weighs 4000 lbs or so, and use 3/8" anchor line. I have been anchored in some pretty big rips such as sow and pigs in 6 footers and have never broken a line. I also pull my anchor under power and have bent a few anchors but no line breaks.
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I'd keep the 1/2 inch line because it's easier to pull up than a thinner diameter rope
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I'd keep the 1/2 inch line because it's easier to pull up than a thinner diameter rope
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I feel the same way about anchor rodes. I don't concern myself so much with breaking strength. The smaller diameter cuts into your hands when pulling the anchor. I prefer the overkill of the larger diameter of 1/2".
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I'll second the 1/2" ,,,
I pull with a 20" ball,,, shackle to line, power forward , tie to stern cleat, power up and beyond anchor local and the ball brings the anchor up, pull er in

Tog fishing !

As others said stay with the 1/2" line....fine for your boat and much easier on the hands.

There are line rating published for the lines itself. BUT THE IMPORTANT THING TO KNOW is CHAIN and scope.

THE CHAIN IS THE KEY TO ANCORING.

For you boat I would have 20ft of 5/16" chain. This extra weight creates what they call Cantenary. Acts like a bowe (buffer) as to not jerk to line and anchor, letting the anchor line lead flater and not up to the boat.

Scope, have enough scope (extra line) as to notbe pulling up at too much of an angle.

For your boat I'd carry 200ft of rode minimum.

Thanks everyone. Seems to be a huge swing in pricing out there. Any good sources? I'm finding pricing from $1.5 per ft to $45 for 200'. I'm assuming somewhere in the middle is good?

I will be splicing a wye with loops for cleats down to an eye to shackle to the main rhode, etc. Don't want the cheap stuff that sucks to splice.

There used to be a shop in Fall River, MA that sold factory seconds on braided nylon line. A lot of the guys running lobster traps bought from them as well as a lot of the quahoggers. I used to use 3/8" for my 18' skiff and carried 300' for raking purposes and never had a problem with it and I'd haul back on the line at times and it didn't cut into my hands if I did it right.
Ron

Don't go to Homies or Lowes and get the 3 strand nylon. That stuff never seems to be wound tight enough.

With all boat stuff this is the order of the best most of the time......
Defender, Hamilton, Jamestown,...West

1/2" 3 strand at Defender 0.75 or 1.02 depending on brand.

As others said stay with the 1/2" line....fine for your boat and much easier on the hands.

There are line rating published for the lines itself. BUT THE IMPORTANT THING TO KNOW is CHAIN and scope.

THE CHAIN IS THE KEY TO ANCORING.

For you boat I would have 20ft of 5/16" chain. This extra weight creates what they call Cantenary. Acts like a bowe (buffer) as to not jerk to line and anchor, letting the anchor line lead flater and not up to the boat.

Scope, have enough scope (extra line) as to notbe pulling up at too much of an angle.

For your boat I'd carry 200ft of rode minimum.

This is spot on. 1/2 inch, chain, scope.
I scored two complete anchor set ups at yard sales this spring. Now if I could just find time to get out on the boat......

3rd in agreement with Jon.
Chain is key.
Have as much chain as the length of your boat.
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3rd in agreement with Jon.
Chain is key.
Have as much chain as the length of your boat.
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Thanks. Your avatar is hilarious BTW. Says it all. If you could only have Deval pointing pack in a jousting position, it would be complete.