# How much Push or Pull do you need ?

Discussion in 'Propulsion' started by Canada Bob, Dec 22, 2011.

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### Mr EfficiencySenior Member

Talking about psi is the wrong way to look at it, it is a case of the magnitude of a propulsive force, rather than the concentration of it, required to match the resistance at a given speed.

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I hear you Mr E, but this project of mine is about psi, can't say too much at this time, have to try it out first, but... Thanks to the feedback above I just found the link below, and it gives me a good idea of what I need to achieve.

http://www.alltrollingmotors.com/

Thanks again...

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### Mr EfficiencySenior Member

Force= Mass x Acceleration . The thrust imparted to a motor vessel is achieved by accelerating a mass of water in a direction opposite to the desired direction of travel, it could involve ejecting high pressure water out the back of the boat, but the velocity change and mass involved will determine the thrust.

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Appreciated, thanks for that, I'm a good few steps further on now than I was an hour ago.

I guess the above is how a rocket works, that clarifies the picture for me...

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### Mr EfficiencySenior Member

Now, how did those putt-putt boats work we had in the bath as kids ? Certainly had me intrigued !

6. ### Submarine TomPrevious Member

Not that simple CB.

But, if you must have a formula: the number of paddlers equals the square of the speed you wish your raft to achieve in knots.

When you're ready to tow your vessel with an inline scale, do it beside the boat in clean (undisturbed) water.

-Tom

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Hello ST... few things ever are that simple but thanks to the feedback in here the waters aren't quite as muddy. Wish I could say more about what I have in mind, but I want to do a bit more work on it now, time now to knock out a prototype to see how things go.

Thanks for the formula, so if I needed to move this theoretical canoe at 5 knots {through calm/still water} how many sq inches of paddle would I need in the water to make progress ?

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### DCockeySenior Member

Drag of a boat depends on hull shape. Rough estimates are available for some types of hulls but there is no universal rule for all hull types.

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Appreciated, the thing is with this little project of mine is, not how efficient the method of propulsion would be, or how fine the hull needs to be, it's whether this idea of mine would work at all, if it does provide propulsion then I could look to improve and refine the details.

Wish I could say more, but if it works I'd be looking for IP Rights on it.

10. ### Submarine TomPrevious Member

CB,

248.3 sq inches

-Tom

11. Joined: Apr 2009
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248.3 sq inches...

Right then, I guess that torque comes into this somewhere ? to turn a paddle that size ?

The thing I'm trying to get at Tom is, {say} I had a paddle {or some other device} with an area of 250 sq inches, and then applied 250 lbs of torque, would that result in 1 lb per sq inch {leaving out friction etc etc}.

Thanks again.

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### DCockeySenior Member

Torque is a rotational force about some point. More common for a canoe paddle is to talk about the force at the blade.

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### DCockeySenior Member

Are you sure it's not 248.4 sq inches?

The actual answer depends on how fast you are willing to paddle.

14. ### Submarine TomPrevious Member

Oh yes, I'm certain.

For 5 knots.

-Tom

1 person likes this.

15. ### Submarine TomPrevious Member

You're welcome.

I have no idea.

I thought you wanted to move it at 5 knots.

-Tom

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