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Length and the CommonPaddler

By John Winters

In Partnership With Swift Canoe & Kayak

2394 Highway 11 North RR#1Gravenhurst, Ontario Canada

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Scribd Online Database Series

December 2010

2010 Swift Canoe & Kayak/John Winters. This document is not to be printed, distributed, sold for profit, or used inany capacity outside the Scribd document database without the expressed written consent of Swift Canoe & Kayakand/or John Winters

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Those who preach the Gospel of Sensible Length exhort, for the most part, aninattentive audience. Paddlers persist in buying the longest boats they can find. "Hang theweight. Hang the resistance. Hang the sluggish handling. Hang it all, just give me a big

boat." A less tenacious (read possibly wiser) person would throw in the towel and peddlelong boats like a good boy but Momma didn't raise a quiter so here goes again. Thosewhose prejudices are rock solid needn't read further. The rest of you might find thisinteresting.

The commonly voiced "technical" reason for longer boats is that they are easier to paddle. The reasoning evolves from a misreading of the great pioneer in hydrodynamicsWilliam Froude who first proved the importance of length in reducing wave makingresistance. Since Froude was a scientist he assumed his audience would read the entire

book and discover that wave making resistance was but a part of the whole that includedfriction and a few other bit players. He could not anticipate that laymen and diletantswould miss the point and elevate wavemaking and length to celebrity status. Meanwhile,

paddlers show their disdain for frcition (which accounts for the greatest part of kayak resistance at any speed) by letting thier bottoms (boat) become waterborn corn fields of scratches, gouges, and worse.

Correcting years of misinformation is difficult in this case because the technicalexplanations fill chapters of naval architecture books. The reading is dull and ponderousto all but the most rabid boat fanatic. It is far easier to expound a half truth than explain acomplicated reality. What I will to do is sidestep all the technicals and cut right to the

bottom line - the quantifiable effect of varying length. The method used is precisely thatused by naval architects to determine the most efficient length for a new design butwithout the incomprehensible language and math.

Pressing forward, let's imagine a basic kayak design in its virgin state. The shapeand form parameters have been determined but we want to be absolutely certain the boatwill have the lowest possible resistance at cruising speed. In this case we will set cruisingspeed at three and one-half knots. Now, we can stretch the boat while retaining its formcharacteristics but we cannot retain the same wetted surface. In fact, the wetted surfacewill increase roughly by the square root of length. This is just a clever way to say that, if we increases length by four the wetted surface will double. We can prove this with somefancy math but I promised to avoid that so we will prove the point with child's play. Takea block of Plasticine and roll it into a round ball. Cut the ball in half and you have theunderwater shape of a boat similar to the one sailed by the wise men of Gotham. Itsvirtue is that it has the least possible wetted surface for its volume. Its vices are have highwavemaking resistance and no directional stability. Now form the same plasticine into ahalf cylinder. You still have the same volume but now the surface area has increased. If you were infinitly patient and careful you could actually make a body of infinite surfacearea without changing the volume!

A boat hull is slightly more complicated as is the task of determing the effect of stretching the hull. In the days before computers (BC) it meant umpteen hours of calculation. Today we can stretch the boat, calculate the surface area, estimate the

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performance using a performance prediction program, and graph the results for a varietyof hulls with a few clicks of the mouse. What we might get is shown in Figure 1.In the graph, Rr is the wavemaking resistance, Rf is the frictional resistance, and Rt is thetotal resistance. The imporatnt curve is Rt and it dips down to a low at around 15' andthen begins to rise. So, if our target cruising speed is 3.5 knots our "best" length is 15'.Since people have limited horsepower and exertion reduces it further, the appropriate

bias is to favour less rather than more length. However, if the intended paddler is strong,the designer might hedge towards the long side to provide lower resistance when the urgeto sprint is strong. If our target speed was three knots, the low point on the curve willshift to the left implying an even shorter boat and vice versa for a higher target speed.Clever readers will note that only .03 horsepower is needed to maintain 3.5 knots but thatisn't the reality. Maintaining a steady rate of 3.5 knots requires acceleration to a higher speed with each stroke to offset the loss in speed as the boat slows down between strokes.In addition, stroke and paddle inefficiencies drain away energy. The result is that weexpend more effort than is required to maintain a uniform cruising speed. How muchmore? Depends upon how you paddle, your stroke rate, your paddle, and emotional state.

Now, this is a simple version of the process. readers of an earlier article about the effectof prismatic coefficient on performance will know that, if speed is constant, reducinglength increases the Speed/Length ratio and the higher the speed length ratio, the higher the required prismatic coefficient. The interesting thing about this is that variations in

prismatic coefficient have their greatest effect at higher Speed/Length ratios whichfurther favours a shorter boat. If we adopt the best prismatic coefficient for each length,the shortest boat in our sample is 1.5% more efficient while the longest boat doesn'tchange.

No doubt you have already anticipated another important point. Most of our paddling isat cruising speed or below. Few of us spend many hours sprinting at top speed and, whenthe wind howls, even top output won't produce high speeds.

2010 Swift Canoe & Kayak/John Winters. This document is not to be printed, distributed, sold for profit, or used inany capacity outside the Scribd document database without the expressed written consent of Swift Canoe & Kayakand/or John Winters