There is always much discussion about how much rode to let out for a given depth of water. The consensus is always ‘more is better’ – but how do you actually know how much rode you have already let out?

A rode counter is the obvious answer but other methods are to mark your rode using coloured whipping line at regular intervals, use cable ties, plastic ribbons, chain markers or even paint on a chain rode.

Alternatively, you can employ a little trick, as demonstrated in the photo (above). All you need to know is approximately how high off the water your bow is (B). Multiply this by five for a 5:1 scope, and you should see that much rode, when it is fully stretched out and taut, between your bow and where it enters the water (A).

The basis for this is some simple school geometry (intercept theorem): The ratio between two sides of a triangle stays the same regardless of the size of the triangle as long as the angles do not change.

The ratio of the amount of rode you see above the water (A) to the bow height (B) is equal to the ratio of the length of rode you have let out (R) to the total height of your bow above the bottom (H), and either of these are equal to the scope. So, if A/B = 25ft/5ft = 5/1, then you indeed have a 5:1 anchor scope!

Viewing the vertical depth and the outstretched rode (stretched taut) as two sides of a triangle, all you need to know is how high off the surface of the water your bow is. Say it is 5ft: multiply this by five if you want 5:1 scope, and you should see the resultant length of 25ft of rode stretched out between your bow and where it enters the water. This means that you will actually have 25ft visible above water and enough scope underwater regardless of the actual depth.

Now you finally have a practical application for the maths you were required to learn in school.

There are a couple of things to be aware of. This method works best, of course, for a rope rode. An all-chain rode with lots of weight tends to sag unless conditions stretch it taut.

The other point is that this model supposes that the seabed is horizontal. Naturally, the depth of the water under your boat is likely to vary to a lesser or greater degree as you move away from the anchor’s location. However, this does not affect the scope as it relates to the height of your bow over the seabed at the point where your anchor is: hence the validity of using a horizontal seabed in this model!

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