Anchor rode and scope calculator
Scope is the ratio of rode length to the depth from the bow roller. Too little and the anchor pulls up instead of digging in.
Rode to let out
53.5m
- In feet
- 176
- Depth from the bow roller at high water
- 10.70
- Swing radius
- 63.4
- Circle you need
- 126.8
- Absolute minimum at 3:1
- 32.1
- Weight of that much 8 mm chain
- 118
- Angle of the rode at the bow
- 11.5 degrees
Show the working
- Charted depth = 6
- Expected tide rise = 3.5
- Bow roller height = 1.2
- Scope = 5
- Boat length = 11
- Depth from the bow roller at high water = 6 + 3.5 + 1.2 = 10.7
- Rode to let out = 10.7 * 5 = 53.5
- Swing radius = sqrt(max(53.5^2 - 10.7^2, 0)) + 11 = 63.4191
- Circle you need = 63.4191 * 2 = 126.838
- In feet = 53.5 * 3.280839895013123 = 175.525
- Weight of that much 8 mm chain = 53.5 * 2.2 = 117.7
- Angle of the rode at the bow = 53.5 > 0 ? deg(acos(clamp(sqrt(max(53.5^2 - 10.7^2, 0))/53.5, -1, 1))) : 0 = 11.537
- Absolute minimum at 3:1 = 10.7 * 3 = 32.1
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- rode = (charted depth + tide rise + bow roller height) x scope
- swing radius = sqrt(rode^2 - depth^2) + boat length
Always use the depth at high water, not the depth when you anchor. This is the single commonest anchoring mistake.
Chain catenary provides much of the holding. In strong wind the chain straightens and effective scope drops.