Next question, I will be buying 100m rope and then chain option is 10,20 or 30m.
It is important to understand how an anchor and chain actually works.
Knowing this one can look at the depth, know where the tide is, change of depth, and as drop down have a clear picture in ones head of what is happening and what has to happen.
The chain is not about length or size , individually. Rather a combination of both in regard to current, wind etc.
Think of it like this for example
you can say drop a 50kg weight onto the top of the anchor then the warp off that... all the weight does is hold the anchor 'arm' down on the seabed flat... the harder you pull, the more it digs in.
As soon as that 'arm' rises, it then twists the anchor claws up, and it releases from the sea bed.
And example is very large ships , in a storm, anchor up, then side a big weight down to hold the chain and anchor flat on the sea bed.
A big mistake is often ppl find they cant get a grip on the sea bed, so the go out get a bigger anchor... the problem is usually
1/ Chain not laying out flat on the sea bed
2/ Drop the anchor to fast, chain tangles and cant roll over and dig in, and chain cant hold it down.
3/ The warp from the end of the chain is to short, and hold the chain up, taking weight off the anchor arm
A 5.5m in current and good wind in same direction, with 3m x 6mm then 3m x 8mm then another 3m x 6mm, just doesnt move.
Get into shelter for over night and the chain doesnt even get to straighten right out past the 1st 3m.
For a 6m and going 10mm of short link.. that a damn heavy lot of chain...10m would do just about everything yu need.. drop another couple meters on
Also what is important is putting out enough warp, so at the point of the warp and chain join, you have a reasonable angle.. not close to vertical
100m of warp on a 6m boat... which has excellent potential to head out wide to some 60+m deep... I have 130m in the 5.5m.. I would be suggesting say 150.
I also carry a spare anchor and 6m 8mm chain... IF loose an anchor next to a reef in say 30/ 50m water, and intending to stay out over night, or need to moor up bad weather whatever, still have more than enough warp to put out at steeper angle, to do so safely