I dredged out an old article of mine about glide because it was mentioned here as “important”. I have taken out most formulas and edited to make it shorter. Please excuse any typos.
To paddlers, “glide” is the distance a boat travels after you quit paddling or between strokes. It is believed that if the boat travels a longish distance or seems to hold its speed well then it has “good” glide and ergo must be a “fast” boat. On the other hand, an inefficient boat will slow down quickly and not coast so far. Sounds good but is it?
Among the world champions in the glide department are the fully loaded VLCC’s (short for Very Large Crude Carriers). Fully loaded at over 400,000 tons they can glide for miles. Jet skis, on the other hand don’t glide well at all and come to a rapid stop once we turn off the power. Neither of these boats are very good designs in canoe terms and yet the one is relatively slow and glides nicely and the other is relatively fast but glides poorly. The VLCC is designed for low speed travel and glides by virtue of its huge mass. The jet ski, despite its speed when planing, is poorly designed for low speed and stops like it has brakes. The same principles apply to canoes as to these extreme examples. Only the degree varies.
The resistance of a canoe is comprised of two major components, friction and wave making. Typically, at speeds above S/L 0.90 (the Speed/Length ratio is the speed of the boat in knots divided by the square root of the boat’s waterline length) the resistance comes primarily from wave making. Wave making increases roughly with the third or fourth power of speed (how fast depends on the design). So if you double your speed the wave making resistance can increase by a factor of eight or even sixteen. Below S/L ratio 0.60 most of the resistance comes from friction that increases with the square of speed. Fortunately, wave-making resistance is not significant below S/L 0.60.
How quickly a canoe slows down between strokes depends upon the resistance at the time the power is removed, how rapidly that resistance changes with changes in speed and the mass. A boat with low resistance in the low speed range will “glide” “better” than one with high resistance in the low speed range and vice versa and a boat with high mass will hold its speed better than a boat with low mass.
Some canoes low resistance a low speeds while others have lower resistance at higher speeds (relatively speaking). Major factors in this difference are wetted surface and prismatic coefficient (volume of the boat divided by the volume of a body having the same maximum cross section area and same length. For a constant displacement, a longer boat has higher wetted surface and consequently higher frictional resistance. The prismatic coefficient differs considerably between boats and some are more efficient at high speeds while others are more efficient at lower speeds. None is more efficient at all speeds.
As if that is not enough, another speed-robbing factor is the angle of yaw or the angle the boat makes with the intended course. All boats yaw a little but some more than others. Yaw can add as much as 5% to the resistance over that of a boat traveling straight ahead. Obviously, a yawing boat will seem slow while a straight traveling boat will seem fast. Similarly, two big paddlers will coast a lot farther than two lightweights in the same boat.
So "glide" does not provide a useful measure of performance unless you have a lot of other information not always available to the paddler and once you have that information you have no need to concern yourself with "glide" since you already know what the boat will do.
Of course, we have the common problem of perception. Red boats, apparently, always glide better than other colours.
