How to use the anchor scope and rode calculator
- Choose meters or feet, then enter the water depth where you plan to anchor.
- Add any tide still to rise, the bow roller height above the water and the scope ratio you want.
- Optionally add the boat length in the advanced options to include it in the swinging circle.
- Read the length of line to let out, the distance to the anchor and the radius and area of the swing.
Formula
Height H = depth + tide to come + bow roller height
Rode (line out) = scope ratio x H
Reach from bow to anchor = sqrt(rode^2 - H^2)
Swinging radius = reach + boat length
Area swept = pi x radius^2
This treats the rode as a straight, taut line from the bow roller to the anchor. A line that hangs in a curve covers less distance.
A calm anchorage and a bigger tide
In 4 meters of water with a 1-meter bow height and a 5:1 scope the height is 5 m, so 25 m of line is let out. The reach is sqrt(625 - 25) = 24.5 m, and the circle swept has an area of about 1,885 square meters.
With a 2-meter tide still to rise, a 7:1 scope and a 10-meter boat, the height is 7 m, the line out is 49 m, the reach is 48.5 m and the swinging radius is 58.5 m. In feet, 20 ft of water plus 1.5 ft of tide and a 3 ft bow with a 5:1 scope needs 122.5 ft of line.
What scope means
Scope is the ratio of the length of line let out to the height from the seabed up to the point where the line leaves the boat. A larger scope makes the line lie flatter along the seabed, so the pull on the anchor is more horizontal and it holds better. Because the tide changes the depth, the depth you use should include the rise expected before the next low water, otherwise the boat may end up with less scope than planned.
The common values quoted by boaters, such as 5:1 for light conditions and 7:1 or more for storms, are examples and not rules. Conditions, the type of anchor and chain, and the bottom all matter, and manufacturers and local guides give better advice for your boat.
Swinging room
The boat can swing in a circle around the anchor as the wind or current changes. The radius of that circle is the horizontal reach plus the boat length, so more scope takes more room. This matters when other boats are close. The area figure helps compare how much space different scope choices need.
For other boating figures, see the hull speed calculator and the tide rule of twelfths calculator, and the distance to the horizon calculator for how far you can see from the deck. The pythagorean theorem calculator shows the reach calculation on its own.
Assumptions and limits
- The line is straight and taut from the bow roller to the anchor, which gives the largest possible reach.
- Scope is measured over the height from the seabed to the bow roller, including any tide still to rise.
- The scope ratio is your choice; the page does not recommend one.
- Wind, current, chain weight and bottom type are not modeled.
Frequently asked questions
Why include the bow height in the scope?
The line runs from the anchor on the seabed up to the bow roller, not just up to the water surface. Measuring only to the surface would give less line than the ratio intends, especially on boats with a high bow.
Should I add the tide?
If the tide will rise after you anchor, yes. The scope has to hold at high water, when the depth is greatest. If you anchor at high water and the tide will fall, use the current depth.
Why is the reach less than the line length?
The line goes up as well as out. Only the horizontal part counts for reach, which is the square root of the line squared minus the height squared. With a large scope the two are nearly equal.
Does a chain change the answer?
Heavy chain lies along the bottom and makes the pull more horizontal, so a given scope holds better. The figures here assume a straight line, so a chain-based setup may have slightly less real reach.