How can you row a boat with a paddle?

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SUMMARY

The discussion centers on the mechanics of rowing a boat with a paddle, specifically addressing how the displacement of water creates low pressure and propels the boat forward. Participants explain that as the paddle pushes against the water, it generates an aft force, which in turn creates a forward force on the boat due to Newton's third law of motion. The conversation also clarifies that paddling on one side of the boat induces a torque that causes the boat to turn, as the forces exerted are misaligned with the center of mass.

PREREQUISITES
  • Understanding of Newton's third law of motion
  • Basic knowledge of fluid dynamics, particularly pressure differentials
  • Familiarity with canoeing techniques, specifically the J-stroke
  • Concept of torque and its effect on object movement
NEXT STEPS
  • Research the mechanics of fluid dynamics in relation to boat propulsion
  • Learn about the J-stroke technique for effective canoeing
  • Study the principles of torque and its application in steering boats
  • Explore the differences between paddling and rowing techniques
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Canoeists, boaters, physics enthusiasts, and anyone interested in understanding the mechanics of water displacement and propulsion in aquatic environments.

sameeralord
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Hello everyone,

When you go on a boat and use the paddle and displace water molecules you create low water pressure right? Then why does the boat move forward? I know water would flow from high to low but how can you specfically know that water would fly from high to low in forward direction, can't it happen in any direction? Also why does paddling one side make the boat turn? Thanks! :smile:
 
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sameeralord said:
Hello everyone,

When you go on a boat and use the paddle and displace water molecules you create low water pressure right? Then why does the boat move forward? I know water would flow from high to low but how can you specfically know that water would fly from high to low in forward direction, can't it happen in any direction? Also why does paddling one side make the boat turn? Thanks! :smile:

The boat moves forward, because as the paddle pushes against the water, the water also pushes against the paddle, the paddle pushes against your arm, your arm pushes against the rest of your body, and the rest of your body pushes against the boat.
 
Which also explains why the boat turns.
An imperfect, yet functional "pivot point" is created.
 
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pallidin said:
Nice! I knew it was possible, just didn't know how. Thanks DAVE.
Fascinating! STEP 1 demands this technique to be applied at the rear of the canoe.

Yep. When single-handing a canoe you always sit at the stern. It's also a narrower beam there, making it much easier to reach the water with your paddle.

(With two in the canoe, the one in front provides the most power but no steering; the one on the rear provides some power but is responsible for steering.)
 
DaveC426913 said:
Yep. When single-handing a canoe you always sit at the stern. It's also a narrower beam there, making it much easier to reach the water with your paddle.

(With two in the canoe, the one in front provides the most power but no steering; the one on the rear provides some power but is responsible for steering.)
Canoeing is the ultimate exercise in mechanics, and not always for the reasons that one might think, though Dave has given a nice hint with the leverage argument, above.
 
sameeralord said:
When you go on a boat and use the paddle and displace water molecules you create low water pressure right?
and higher water pressure aft of the paddle.
Then why does the boat move forward?
You exert an aft force on the paddle, coexistant with you also exerting a forwards force on the boat. The paddle exerts an aft force on the water, coexistant with the water exerting a forwards force on you. The foward force you exert on the boat coexists with the boat exerting a backwards force on you. The boat exerts a forward force on the water, coexistant with the water exerting a backwards force on you. If the forward force exerted by you on the boat exceeds the backwards force exerted by the water on the boat, the boat accelerates. If the forces are equal in magnitude over time, then the boat moves at constant speed.

Momentum is conserved within a closed system, so the linear and angular momentum of the earth, water, and the boat with you in it are conserved. If the boat is accelerated forwards from rest, then the water in the lake or river and/or Earth are accelerated backwards by a very tiny amount.
I know water would flow from high (pressure) to low (pressure) but how can you specfically know that water would fly from high to low in forward direction, can't it happen in any direction?
It could, but the paddle imparts a force in an aft direction, causing the water to accelerate in an aft direction, and the waters momentum tends to cause it to flow in that direction. Note that the paddle induces an aft flow, while at the same time creating a low pressure zone fore of the paddle, and high pressure zone aft of the paddle. If not for the presence of the paddle, the flow would be forwards from the high pressure zone aft of where the paddle was towards the low pressure zone forwards of where the paddle was. However, with the presence of the paddle, the flow can't go through the paddle (just around it), and so the paddle induces a flow as well as a pressure differential.
 
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  • #10
If you are a fly-fisherman on little ponds, you'd better learn every variation of that stroke, as well as braking maneuvers so that you can get in place for a drag-free presentation of dry flies during hatches... or you can go home with no fish.
 
  • #11
Right! Thanks for all the answers. I understand Newton's third law at work here but if you displace air molecules don't other air molecules come to fill the space. Doesn't the same thing happen with water. I think I'm talking about buoyancy force. Also I still don't understand why paddling one side make it turn?
 
  • #12
sameeralord said:
Right! Thanks for all the answers. I understand Newton's third law at work here but if you displace air molecules don't other air molecules come to fill the space. Doesn't the same thing happen with water.
Air is so easily pushed aside that it is rather uselss as a medium for paddling. No so with water. Water is much denser than air and, unlike air, is incompressible.


sameeralord said:
Also I still don't understand why paddling one side make it turn?
What don't you understand? If you take any object and apply a force to it that is off-centre of the centre of mass, it will move one side and not the other - turning it in a circle.
 
  • #13
Nit picky nit picky,

You paddle a canoe with a paddle, you row a boat with oars.
 
  • #14
Integral said:
Nit picky nit picky,

You paddle a canoe with a paddle, you row a boat with oars.

Granted. But did someone say otherwise?
 
  • #15
sameeralord said:
If you displace air molecules don't other air molecules come to fill the space.
Yes, but those air molecules have to accelerate to fill the space, and since the air has inertia, the pressure is reduced in the zone where the "space" (or void) is being created, and the reduced pressure is what draws the air towards the "space" being created by the moving paddle.
Doesn't the same thing happen with water.
Yes, except water is much denser than air, so you get the same force with much less acceleration of the same volume of water.
I still don't understand why paddling one side make it turn?
The water exerts a forward force at the paddle and a backwards force at the center of the boat. The misaligned forces result in a torque force that yaws the boat, and the boat's hull is designed to resist movment perpendicular to the direction it's facing, so it ends up changing direction as it yaws.
 
  • #16
DaveC426913 said:
Granted. But did someone say otherwise?

Thread title?
 
  • #17
does this wry cynical jaded sense of humor come after you have reached 2000 posts??
 
  • #18
Integral said:
Thread title?
Ah. That would explain why a page search for 'oar' didn't net any results.


Ranger Mike said:
does this wry cynical jaded sense of humor come after you have reached 2000 posts??
It's part of PF's value-add. If the poster wanted nothing but a short, dry description he would have consulted an encyclopedia.
 
  • #19
Dave C ..good one!
you have to have a sense of humor to do what we do...otherwise would go NUTS!
 

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