Even though during my intermediate and advanced CYA trainings, I learned how to theoratically calculate the scope I need when anchoring, there are a few things I overlooked as a new boat owner and learn that theory is much easier than practice!
How to calculate your scope
How much chain do you need: calculate your ‘maximum scope’
To calculate the maximum scope, you will have to estimate the maximum depth you will anchor in (in the Bahamas/shallow water we used 10 ft).
I found a great image (shown below) that clearly laid out the different depth and offsets you need to consider when anchoring.
Then, take into account your draft (on the image below: surface offset + keel offset), for our boat it’s 4 ft.
Then the distance from the waterline to the bow roller (on the image: height of bow) also called freeboard, for us about 6 ft.
Considering a maximum scope of 8:1, the max anchor rode you need will be:
If you don’t go all chain you should always have at least 20 ft of chain on your anchor for avoiding any chafing.
How to calculate your scope when anchoring
Key metrics
To calculate the necessary scope, there are key metrics that you should keep in mind:
- Make sure you know the tide when you’re anchoring (if it’s low tide, in the Caribbean, add a tide offset of 3 ft)
- Don’t forget to add the freeboard to your calculation. The depth indicated by the depth sounder is not nearly enough (made that mistake before 🙂 )
- Know if your depth sounder is calibrated to show the depth below the keel OR if you need to add a keel offset
Keel offset (or not)
In fair weather with a scope of 5:1, the anchor rode necessary will be:
For a depth sounder calibrated below the keel:
(depth sounder reading + draft + freeboard + tide offset ) * scope = anchor rode length
For depth sounder not calibrated below the keel:
(depth sounder reading + keel offset + draft + freeboard +tide offset) * scope = anchor rode length
Your known variables
It looks like a lot of math to do before dropping your anchor! But rest assured you can make this easier by calculating your known variables beforehand. For us, we know that:
- draft + freeboard = 10 ft OR draft + freeboard + keel offset
- scope is at least 5:1
So we know we will drop at least 10 ft x 5= 50 ft of chain every time we anchor: Let me say this again: Every. Single. Time. At. Least. 50. Ft
Then add another 5 ft of chain per ft read on your depth sounder. So for example, if your depth sounder reads 5 ft, you will drop another 5 x 5ft = 25 ft of chain.
And then, if you anchored at low tide, add another 5 x tide offset = 15 ft of chain
So in total, you dropped 50 ft of chain to start, then another 25ft and then if it’s low tide another 15 ft: 50 +25 + 15 = 90 ft
How to mark your anchor chain
To make your anchoring process easier, it’s best to mark your chain in some ways so you actually know how much chain you’re dropping.
DIY anchor chain markers
A very inexpensive and easy way to do this is by spray painting your chain with different colours (ours goes like this: BOGPW: Blue –Orange – Green –Pink -White) every 25 ft . Not the best acronym to remember, maybe you can do better! We use standard acrylic spray paint cans and cover about 1ft of chain to make it easy to see (anchor can go down quickly sometimes!). The downside is that you will have to re-do it every year as it tends to wear of.
Painting the chain on the dock requires a lot of space!
I also used some neon zip-ties to mark the chain every 5 ft (which might be overkill but if I loose track then I still know how much chain I put out). So let’s say I have already 75ft out (Green mark), I placed the zip-ties as follow:
- 1 zip-tie at 80 ft
- 2 zip-ties at 85 ft
- 3 zip-ties at 90 ft
- 4 zip-ties at 95 ft (Pink mark)
Zip-ties are really inexpensive, will last a little bit longer than paint and go seamlessly through a windlass but, in my opinion, they are not as easy to spot as paint.
Painted chain every 25 ft and neon green zip-tie
Commercial anchor chain markers
A more pricey but long-term solution is to buy anchor chain markers. They act similarly to the zip-ties but don’t get damaged as quickly over time (at least that would be my guess!). They are available in different colours so you can alternate the colours the same way you would do with paint or with neon zip-ties.
Anchor chain markers
So now you think you’re good to go! Go sailing and spend the night at anchor. On good days, the swinging of the boat puts me to sleep and I wake up to the sound of the ocean and the surrounding wildlife. Stay tuned for my next blog post about ‘Anchoring tips’. Yes, there is still more to learn!
The diagram on scope is simply wrong. The holding power does not necessarily decrease with scope. As long as the chain pulls horizontally at the anchor shaft, the holding power is not impacted. What you MAY wanted to be stating is that a totally taut chain will have this effect. But the diagram suggests a chain with some slack and pulling horizontally at the anchor…
Could you suggest a diagram on scope and/or a link to a resource that would explain the holding power vs scope better? Thanks!
Well, there is no easy relationship between holding power of the anchor and scope. As long as the chain pulls horizontally on the anchor, holding power is maximal, whatever it is, and whatever scope you use. When you start pulling at an angle, holding power will decrease, but by how much, this will depend on the anchor design. Have a look at the free calculator at anchorchaincalculator.com and play around with the values a bit, and you will see.
For instance, in very deep water, like 40 m or so, likely a scope of 4:1 will still result in the chain pulling horizontally at the anchor – unless you are really facing a severe storm. And when the chain pulls horizontally, there is no negative impact on the anchor holding power at all! So, this simple table of yours just does not work!
Scope alone is a very poor metrics when it comes to anchoring. If you want a more sophisticated calculator for calculating the required minimal anchor chain length, try this one: http://www.anchorchaincalculator.com