Home page About us Instruction Guiding Locations map Gift vouchers Prices Articles Contact us Accommodation Links	Stealth Part 1: The importance of (in)visibility to the trout fisher Flyfishers understand that they should take care to make sure they are not seen by trout. This page explores the question "What is a safe distance?". If we apply a bit of maths - and some common sense - it turns out that a useful "rule of thumb" is: multiply by 6 your height above the water, and stand at least that distance away from trout you are targeting.
Because of the laws of refraction, fish cannot see any part of an object which lies below an angle of 10 degrees to the water surface at the edge of their "window". The red triangles in this diagram show this "blind spot" extending outwards all around the edge of the trout's window. [ Diagram adapted from Fig. 6 in Professor L T Threadgold's book "Dry Flies: An Improved Method of Tying" see Ref#2 ] The next set of pictures apply this law of refraction to show the effect of standing upright and crouching on an angler's visibility to trout . The head and upper body of the standing angler would be visible to a trout lying beside the clump of midstream vegetation on the right hand side of the photograph. (The triangles in these picture were created using GIMP see Ref#3. The calculations were made using an online calculator see Ref#1.) But if the angler crouches, only his head  would be visible to a trout lying beside the clump of midstream vegetation on the right hand side of the photograph. We can begin to answer our question: What is a safe distance? Angler is: Height of angler (in feet) Distance angler invisible to trout (in feet). Based on calculation made with aid of Ref#1 Standing 6 feet =>34 feet Crouching 4 feet =>23 feet Things to bear in mind: This doesn't mean you need to have 23 or 34 feet of fly line outside your rod tip, because: the leader is probably 9 feet or longer your rod is probably 7 feet or longer Assuming your leader straightens out, and you hold the rod straight out in front of you with the tip at water level: if you are standing, you might get away with casting just 18 feet (34-(9+7)) of fly line if you are crouching, you might get away with casting just 7 feet (23-(9+7)) of fly line But of course these assumptions are very unlikely (for example, your may want your leader to contain slack to avoid drag etc.) so you should cast a few extra feet of fly line to be safe. In addition, the depth at which the trout is lying also has an important effect. So far I have assumed that you are casting to a trout lying close to the surface: A trout lying 6 inches below the surface has a small 5 inch window (Shown on the far right of the diagram below). But if the trout is 6 feet beneath the surface (far left in the diagram below) its window has a radius over 5 feet. So the radius needs to be added to the "rule of thumb" introduced at the top of this page, because the law of refraction operates from the edge of the trout's "window". Or to put it another way, a deep-lying trout is closer to you (in optical terms) than you think. Image from Ref#4 References: Right Triangle Angle And Side Calculator Professor L T Threadgold. "Dry Flies: An Improved Method of Tying", Swan Hill Press, Shrewsbury, 1998 GIMP the GNU Image Manipulation Program How Does a Trout Catch a Fly?: Marinaro's "Edge of the Window Theory"
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