Paddling Science – If I lose weight, will I be faster?

Paddling  sports such as standup (SUP) paddleboarding, kayak or surfski amongst others can be thought of as moving a hole through the water. This hole is created as a result of water displaced by the hull. The total weight of the paddler(s), plus the weight of the canoe and gear, equals the weight of the water displaced by a loaded hull – this is Archimedes Principle (“Eureka!”). So if the weight of the paddler(s) increases, the amount of water displaced increases. Since the weight of the displaced water equals the density of water times the displaced volume of water (submerged lengthx widthx height, a cubic dimension), we see that to first order weight is proportional to the length of the hull cubed.

As you paddle your “moving hole,” water is continually rushing in behind the hull to fill the hole it just left behind. This is why wake riding works: you are essentially pulling along a chunk of water as you paddle, which a waking canoe can sit in. The mass of this moving hole increases in proportion to the amount of water displaced by the hull, which is linearly proportional to the weight of the paddler(s). From Newton’s Second Law, the force you exert to accelerate a canoe is proportional to the mass you wish to accelerate. So, if your weight increased in the off-season by 5%, then you must apply 10% more force to accelerate your C-1 to the same cruising speed as last year. Why 10%, and not 5%?

Because you must accelerate not only your (expanded) mass, but also the increased mass of water entrained by the hull – a heavier paddler displaces more water, which you have to pull along with you. So that 5 pounds you gained from all that Christmas cheer will actually cost you around 10 pounds on the water the next time you catch the lighter paddler that always smokes you. A 10- pound weight gain will cost you around 20 pounds, etc. And since the hull slows down ever so slightly between each and every stroke, you are slightly accelerating the hull with every stroke just to keep it at cruising speed. Makes you think twice about that second helping of stuffing…

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