Anchor Scope Calculator
Not the sounder reading. Add the tide and the bow.
Work out Anchor Scope. Not the sounder reading. Add the tide and the bow. Names the misconception directly.
Rode to veer
42.5 m
5:1 on 8.5 m · swing circle 103 m across
The depth that matters is the water under you at high tide plus the height of the bow roller above the water. Anchoring to the sounder reading alone is the classic mistake — at 5 m of water with 2 m of tide to come and a 1.5 m bow, the real figure is 8.5 m and the rode needed is 70% longer. Scope works by keeping the pull horizontal at the anchor. Above about 15° from horizontal the shank starts lifting and holding power falls sharply, which is why short scope fails suddenly rather than gradually. The swing circle is what has to be clear before you drop, and it is measured from the anchor rather than the boat. Neighbours on different rode types swing differently in a wind shift, which is how boats that anchored politely end up alongside each other at 3am.
How the Anchor Scope Calculator works
Rode to veer from depth, tide still to rise and bow roller height, with the swing circle, the angle of pull at the anchor, and what chain and rope each contribute.
Also known as: how much chain should i let out · anchor scope ratio 5 to 1 · swing circle at anchor · do i count the tide for scope
Frequently asked questions
What depth do I use for scope?
Water depth at high tide plus the height of the bow roller above the water. At 5 m with 2 m of tide to come and a 1.5 m bow, that is 8.5 m — and 70% more rode than the sounder suggests.
What scope ratio should I use?
Around 4:1 in light conditions, 5:1 to 7:1 normally and 10:1 in a real blow. Chain tolerates less than rope because catenary keeps the pull horizontal.
Why does scope matter?
It keeps the pull at the anchor horizontal. Above about 15° from horizontal the shank starts lifting, and holding power falls sharply — which is why short scope fails suddenly rather than gradually.
What is my swing circle?
The horizontal reach of the rode plus the boat's length, measured from the anchor. It is what has to be clear before you drop, and neighbours on different rode types swing differently in a wind shift.
Does chain really hold better than rope?
In moderate conditions, through catenary — its weight sags the rode into a curve that keeps the pull horizontal. In a strong blow the catenary straightens out and the advantage disappears, which is what a snubber is for.
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