Marathon Inertia

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Conor Buckboy
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Marathon Inertia

Post by Conor Buckboy » Sun Jan 29, 2017 10:13 pm

Hello, I'm having somewhat of an argument with one of my paddler friends. The subject is whether a lighter boat or heavier boat will be more efficient in a simple long distance paddle. He thinks the lighter boat while I think a slightly heavier boat will be better. I think this due to newtons law of inertia, basically the more force on an object, the more force is needed to change the state of said object. In kayaking terms this could come about where a heavier kayak will hold its momentum and carry its speed more (due to inertia) because it's harder to stop, as such the paddler could use a slower stroke rate/ save energy with less powerful strokes.
So basically to narrow it down my friend is arguing a lighter boat because it requires less force to move, while I argue a heavier boat because it requires more force to stop. Forget about portaged or sprints, we are simply talking about cruising speed.
Surely the lighter boat will require a higher constant power from the stroke to stay at the same pace otherwise it would lose speed quicker, whereas a heavier boat can slow down the rate to let the boat run and cruise.

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Re: Marathon Inertia

Post by jamiemagee » Sun Jan 29, 2017 11:01 pm

But the heavier boat would be lower in the water and hence have a different hull shape in essence. Surely that would slow the boat down. It's also often recognised that length of the boat has an impact on the speed it can go.

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Re: Marathon Inertia

Post by twopigs » Mon Jan 30, 2017 8:35 am

Quick Monday morning answer .....

Keeping the hull shape constant and the amount of the hull in the water constant then the lighter boat will be faster for the same amount of work done. The acceleration of the lighter boat will be greater than that of the heavier boat and the friction - or drag will be less. This also demands that the paddler mass and work done are identical.
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Re: Marathon Inertia

Post by PaulK1 » Tue Jan 31, 2017 10:27 am

Just as the heavy boat has more inertia to help it keeps its speed between strokes, that extra inertia makes it harder for you to increase the speed of the boat with each stroke.

Also, you can't magically have a heavier boat that sits the same height in the water as a lighter boat, it has to displace more water in order to float, so it has to increase the drag.

On the other hand, adding a couple of kilos to get a stiffer construction might be beneficial, but that trade off is hard to guage. But purely based on weight it can't help.

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