# How much Push or Pull do you need ?

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

1. ### Submarine TomPrevious Member

CB,

If you can't give us the whole picture then we can't give you very good answers.

10 pounds thrust on a 1000 pound canoe... 2 knots maybe.

Jets propulsors? Come on, spill the beans or hire a Marine specialist on contract.

To quote PAR, and active member on this forum (who's too smart to get involved in this thread): "How long's a piece of string?"

We just don't have enough information.

Tow it and you'll know your exact drag/thrust requirements.

Is this a company fun paddle you're building?

-Tom

P. S. Personal Message me and we can correspond privately by email if you like.

2. Joined: Apr 2009
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Location: Halifax, Nova Scotia

Right then, in an attempt to clarify the picture a little, here goes...

To give you guys a better idea of the concept {without describing the actual thing} the following sort of describes my line of thought.

Back to the canoe thing then, if you can imagine that at the stern of this square ender we have a hole {yea, I know, not a good thing to have in a canoe}, let's say a 2" dia hole, through which goes a 2" dia {non rotating} shaft, it's sort of just a push rod for all intents and purposes.

At the end of this shaft is {let's imagine it's} a 10" x 10" piece of 1/2" thick plywood {could be anything almost but let's stay with ply} nailed to the shaft, vertical to the shaft, I'm sure you guys get the idea...

To start off the shaft {say 2ft to 3ft long} is inside the canoe, and the "push plate" {the 10" sq ply} is right next to the stern of the canoe...

What happens if I push the rod ? {say for 2ft} ? let's say that I exert a force of 20 lbs.

I'm only interested in this initial stroke, not the secondary/recovery stroke, I have that possible counter action covered.

My thought is that, you can't compress water, so it would essentially be like pushing against a solid {am I right} ? and that by pushing against a solid {and being afloat} my boat is going to move forward, I'm pretty sure that would be the resulting action, {but what do I know}

One part that I can't get my head around though is, what difference would it make if the "push plate" was twice the size, or half the size and still subject to the 20 lb push ? what would be the result of changing the size of the plate ? if you kept the same force, speed and length of stroke ?

Thanks again guys for your feedback, it's valued and appreciated.

3. ### Submarine TomPrevious Member

There is only one way to find out. Try it. I'd make it round, hinged in the middle for the recovery stroke. Your power stroke should progressively accelerate.

Water: Non-compressible yes, solid no.

BTW a propeller is a way better device.

-Tom

4. Joined: Apr 2009
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Location: Halifax, Nova Scotia

Hello Tom,

I'll be working on this project the next couple of months, a bit short of water at the moment as it's -10C round here {in Nova Scotia} right now. But having said that I'll make a couple of scale models first to test the concept that I have. As for the hinge, yea, that would work, but the thing that I describe is just descriptive of the principle of pushing against water. The actual application that I intend to use would be quite different, sadly I can't describe it {in public} as if it did work I'd loose Intellectual Property Rights by describing it, other than in its simplest form.

As to water being a "solid" {sort of} the fact that it isn't compressible is the thing that could make my idea work.

No doubt that a propeller is by far a better means of propulsion than what I have described, but the description does little more than consider the forces that may be involved.

The thing that instigated this little project Tom was watching all the vessels that come and go from Halifax, container ships, passenger liners/cruise ships, military ships and submarines, fishing boats etc etc etc. As I was watching one of the ships head out I wondered how long it would be before "something" would make propellers obsolete, no doubt something will, nothing is forever, someone will "dream" something up, and when we see it we will wonder "how come we didn't think of that", hence the quest that I have set myself

5. ### Submarine TomPrevious Member

Good for you Bob, carry on.

-Tom

6. Joined: Apr 2009
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Location: Halifax, Nova Scotia

Thanks again Tom for your input, wish I could say more, even though like most projects many of them finish up as just a pile of sawdust, but, you never know, folks do stumble on extraordinary things, and I've had a good run with inventions so far {Patents awarded in 34 countries over the years}, none of them related to boats though, hence my need for some input and guidance in here, all of which I greatly appreciate.

One last question for you Tom, regarding the size of the push plate, what difference would it make if the 10" x 10" was 20" x 20" or 5" x 5" ?

Thanks again.

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### LandlubberSenior Member

...all I can say is that if I leaned with my back on my old boat (160foot island trader), she would leave the wharf, I mean just resting not trying to push. I was surprised to find that out the hard way, cos the lives were slack......

8. Joined: Apr 2009
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Location: Halifax, Nova Scotia

Hello Landlubber, thanks for your input, maybe you can help me on this one...

Using the concept of a push plate outlined earlier, which method would provide the better method of progression, a slower longer stroke, or a {much} shorter faster stroke...

No matter how I tried to rephrase that it didn't come out any better, so I'd best quit while I'm ahead

9. ### Submarine TomPrevious Member

Well, I think the answer to that is pretty obvious isn't it?

Again, (and for the last time I might add) the answer all depends on many variables which you are not in a position to disclose.

Your best bet would be to try these different sizes and document the data for further analysis. However, your question may be hypothetical in which case trials wouldn't be of much benefit as they wouldn't apply directly to your design. Although, if, in your vail of mystery you think they might, then I would make it so. But only on a full moon and if the humidity is above 59% relative.

-Tom

10. Joined: Apr 2009
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Location: Halifax, Nova Scotia

Hello Tom...

Looks like I have upset you, that wasn't my intention, I regret that, and I offer my apologies to one and all if any of my comments innocently cause offense.

I valued the feedback, I wasn't disrespecting anyone by not being able to fully disclose the details of my little project, I guess that life has taught me to be cautious.

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### Mr EfficiencySenior Member

Apart from the dubious efficiency of this push-plate idea, you have to contend with the pulse-like intermittency, and its effect on the occupants of the boat, which is clearly not desirable.

12. ### Submarine TomPrevious Member

Bob,

You haven't upset me in the least and I am sorry you were left with that impression, it's unfortunate.

What you have done is frustrate me a great deal. You ask all these questions which can't be answered really.

"How long is a string?"

Here we are in a place that wants to help people but can't because they can't give away their project.

I offered to help in private but you haven't taken me up on that. Your call.

I hope it all works out for you.

-Tom

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### LandlubberSenior Member

...i would expect that big and slow would be better than flat lot and small, same as a prop works, big and slow really is powerful.

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### MechaNikSenior Member

I wouldn't talk in Lbs. Simply put there is resistance opposing the movement of the vessel through the water (drag) and you measure this in Newtons. As you go faster this resistance will increase.
To apply this force there will need to be a pressure applied over a certain area, such as your paddle. Pressure=force/area so pressure x area(blade)= force(Newtons) needed to be applied parallel to opposing drag force.
The power therefore needed to be applied would be the force x distance(m) per second the paddle would have to move for said speed. This is basically only for movement parallel to the flow and no loses, if you wanted circular motion you would need to would in rad/s and account for the vector loss for unparalleled flow, this is why they choose massive paddles wheels with minimal water penetration.

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### pistnbrokeI try

How do you go from rep 10 to zero ...easy post a pointless thread and waste everyones time

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