Pretty much every anchor manufacturer has a table of recommended size anchor based on boat weight or length. I have always selected an anchor one size bigger or at least at the upper end of the size chart of what’s recommended. You also have to factor in the conditions you are typically anchoring in, depth, sail area of your boat, etc. Too many variables to just have a straight ratio.
Back in my sailing days I used a 1/1 ratio: The storm anchor had the same holding power as the displacement of the boat. Massive overkill, but I sure slept good. (A Fortress FX 55) The holding power in good sand was 16,000 lbs, my boat, a CSY 33 Cutter had a 16,000 lbs displacement when new, a bit more when fitted out for cruising, but close enough) We had a long thread on this forum about anchors after the big sailboat drag in the Keys and the owners had a hard time salvaging it and in the meantime slept on the boat with a shotgun to keep looters away. Not sure how it ended, but I saw the boat floating in Key West 2009, quite banged up. Forgot the boat’s name, 156’ ?.
That was a long interesting read. This brings me to another question. I flew around Gulf of Mexico a lot after Katrina/Rita. (I’m a helicopter pilot) I saw many a yacht floating around that had broke free from there moorings. Most of them were completely unhurt. I also saw quite a few that were tied to a dock with major damage. It almost seem like being out in open water was better. Of course, I wouldn’t want to be on a yacht in a cane, but I wonder if it would be better to take it out and anchor, go back to shore with a small boat and come back after the storm. What about these docks where the yacht is tied in the center of four or more poles. It seems like it could move around, but not hit the poles causing damage. I’m looking doing something like this on my personal dock. I won’t be there most of the time to loosen or tighten ropes. Seems like it would be good even for the tide changes.
In many areas people tie boats into the mangroves and bayous with multiple anchors and lines to trees or roots. You can tie boats between pilings but they need to be far enough apart for the boat to stay off the pilings as lines stretch. The pilings need to be tall enough to make sure boat does not end up on top of them with storm surges. It can work but it really depends on the conditions especially the dock location and how much fetch you have
Let me ask you this. I’ve flown over big work boats (180’) at oil rigs during a storm that were bouncing and rolling in the waves. I know I wouldn’t want to be on one of them, let alone a 45’ boat. We were a 100 miles off shore. I’m guessing the waves were about 30’ from valley to crest. How big can a wave be in a bay far from the mouth? I feel like my bay is safe because it’s more than 15 miles to an opening, but it’s still a couple miles across, so kind of big water.
All depends on the bay, its shape, the opening etc. even if waves don’t come in from the opening, a 2 miles fetch can cause 2 to 3 footers with 20/25 kts wind.