How can you row a boat with a paddle?

1. Dec 29, 2009

sameeralord

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!

2. Dec 29, 2009

willem2

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.

3. Dec 29, 2009

pallidin

Which also explains why the boat turns.
An imperfect, yet functional "pivot point" is created.

4. Dec 29, 2009

DaveC426913

Last edited by a moderator: Apr 24, 2017
5. Dec 29, 2009

pallidin

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.

Last edited by a moderator: Apr 24, 2017
6. Dec 29, 2009

DaveC426913

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.)

7. Dec 29, 2009

turbo

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.

8. Dec 29, 2009

rcgldr

and higher water pressure aft of the paddle.
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.
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.

9. Dec 29, 2009

D H

Staff Emeritus
Last edited by a moderator: May 4, 2017
10. Dec 29, 2009

turbo

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. Dec 29, 2009

sameeralord

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. Dec 29, 2009

DaveC426913

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.

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. Dec 30, 2009

Integral

Staff Emeritus
Nit picky nit picky,

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

14. Dec 30, 2009

DaveC426913

Granted. But did someone say otherwise?

15. Dec 30, 2009

rcgldr

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.
Yes, except water is much denser than air, so you get the same force with much less acceleration of the same volume of water.
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. Dec 30, 2009

Integral

Staff Emeritus

17. Dec 30, 2009

Ranger Mike

does this wry cynical jaded sense of humor come after you have reached 2000 posts??

18. Dec 30, 2009

DaveC426913

Ah. That would explain why a page search for 'oar' didn't net any results.

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. Dec 30, 2009

Ranger Mike

Dave C ..good one!
you have to have a sense of humor to do what we do...otherwise would go NUTS!!!