How much anchor rope or anchor rode you need will depend on several factors.
The general rule I always hear is what is called a scope of 7:1, which means if you are in 1 foot of water, you would use 7 foot of anchor line, 10 foot of water = 70 foot of anchor line, etc.
Now this 7:1 scope is meant to cover everything up to extreme conditions where you are anchored in overnight or extreme conditions, such as major current, tide or weather conditions.
The higher scope is to create an angle of pull where the pull is just a few degrees off the bottom. To give you an idea, if your anchor is straight down under your boat, it doesn’t take much to pull it off the bottom, one small wake or wave will lift it. But the more scope you have, the more movement it takes to get your anchor off the bottom.
When you have more scope and your boat moves, it has more of a tendency to lift the rope, way before it lifts the anchor. When it does move the anchor, it is only lifting at an angle and not necessarily lifting it off the bottom.
So here’s an example of how much anchor rope you may need. I have anchored up in a spot in the river that was a little over 100 foot deep and only let out about 150 foot of rope, (1.5:1 scope), but there wasn’t any current to speak of since this was a very wide part of the river and was very calm with no wind, so the anchor was only being used more for weight and barely used for grab on the bottom.
In the areas that I typically fish and anchor in, I will have anywhere from 2:1 to 5:1 scope, depending on conditions, but you need to understand the conditions of the water and weather. It is better to error on the side of caution until you have a good understanding of the conditions. This is not an area where you want to make mistakes.
Whatever scope you use, you will want to make sure that you carry enough anchor line in preparation to maximum scope when possible. For me, I usually don’t anchor in anything deeper than about 50 or 60 feet and carry 300 foot of rope plus another 6 – 12’ of chain and if I do anchor in deeper water, I make sure I understand the limitations of a smaller scope.
So, you can see how much anchor rope you need will depend on many factors, but isn’t that difficult to figure out. Carry more than you need and error on the side of caution.