There is always a lot of discussion around how much rode to let out for a given depth of water. The consensus is always “more is better”. But how do you actually determine how much rode you have let out?
To help determine the correct amount of rode to let out to attain the optimal scope, you can use a rode counter. You can also mark your rode using coloured whipping line at regular intervals, cable ties, plastic ribbons, chain markers or even paint on a chain rode.
Alternatively you can employ a little trick. All you need to know is approximately how high off the water your bow is (B). Multiply this by 5 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.
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 5 feet. Multiply this by 5 if you want 5:1 scope, and you should see the resultant length of 25 feet of rode stretched out between your bow and where it enters the water. This means that you will actually have 25 feet visible above water and enough scope underwater regardless of the actual depth.
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 = 25’/5’ = 5/1, then you indeed have a 5:1 scope!
So, finally you have a practical application for the math you were required to learn in school. Of course, this works best for a rope rode. An all chain rode with lots of weight tends sag unless conditions stretch it taut.
Note: this model supposes that the sea bed is horizontal. Naturally, the depth of the water under your boat will 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 sea bed at the point where your anchor is. Hence the validity of using a horizontal sea bed in this model! –
Thanks to Mike Pool for pointing out this possible source of confusion.
By Alex & Daria Blackwell
Alex and Daria Blackwell are the authors of “Happy Hooking – The Art of Anchoring.” It covers every aspect of anchors and anchoring in a fun and easy to read format with lots of photos and illustrations. It is available from good chandleries, Amazon & CoastalBoating.net