I didn’t want to revive the recent thread on “How does a rowboat work?” But my brother just asked me:
Why are there multiple wakes formed at the bow of a canoe?
IANAP (I am not a physicist) but I might be able to comprehend a reasonably technical answer.
I would have said ‘bow waves’ for those and put the wake behind the canoe, but no matter.
I’d guess, on the basis of no evidence whatever, that one wave comes from the cutting edge of the bow, and another comes from the ‘shoulders’ farther out to the sides. If you’ve got more than two, I have no idea.
The underwater portion of any solid object being propelled through the water will displace water as it moves along.
The water pushed aside by the front (bow) of the object will form what is known a bow wave. The rest of the water displaced by the object will collectively form what is known as the wake.
As to why a canoe would form a particular shape of bow wave or wake, that is a result of the physics/fluid dynamics of the interaction between the canoe and the water which is well beyond me.
Here’s a discussion of sailboats and bow waves.
Also interesting is this hydrodynamics site, but it’s fairly technical.
I saw the answer in The Feynman Lectures long ago.
Here’s a similar question: when I throw a rock into a pond, why don’t I get a single expanding ripple? Why do I get concentric waves like a bullseye target?
Answer: as a wave-medium, water surfaces are highly nonlinear. They’re not at all like sound waves in air or EM waves in vacuum. The nonlinear math of “gravity waves” propagating on fluid surfaces has a strange effect. It causes any impulse-wave to become “chirped”, in other words the different frequencies in a brief impulse will propagate at different speeds. When you throw a rock into the pond, the impulse quickly becomes distorted. You get a fast-moving long wave, followed immediately by narrower and narrower waves which move slower and slower. As the impulse travels, it spreads out wider and wider into multiple ripples.
The same thing happens to the bow-wave of any boat. It doesn’t continue as a single “impulse”, instead the water mathematics decodes the Fourier spectrum of the impulse into an array of ripple-waves each having different wavelength. I think The Feynman Lectures even showed an example: an arial photograph of the waves surrounding a boat speeding along a river.
PS, the “rowboat” thread continues… but I figured out the key which answers nearly all my questions. The mysterious backwards flow cannot exist if the ring-vortex was sent out by a vortex launcher. The wider flows surrounding both the ring-vortex and the launcher-device are in opposite directions, so they annihilate each other. DOH!