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Author Topic: efficient paddle wheels?  (Read 10094 times)

Offline andy

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efficient paddle wheels?
« on: May 01, 2008, 06:47:20 PM »
Hi,

some months ago I found in the web a text, which said, that paddle wheels are more efficient in giving the power into the water instead of propeller- systems.

My opinion: I think, this is possible. When I was on paddle STEAMERS, these boats ave a great acceleration because of their big torques of the steam engines with very low revs.

Disadvantage of paddle wheelers I think is in the waves of the oceans and the danger of damage of the wheels, and the wider ships. So the propellers made their way.

A big nonsense I think are the diesel- converted paddlers, which need for the fast running diesel engines big gearboxes or hydraulic systems to reduce revs and to increase the torque. A steam engine connected with paddle wheels is a kind of a symbiotic technical system. So in Switzerland in 2003 an old steamer (Montreux) was reconverted from Diesel to steam again, and is economic!

http://www.dlm-ag.ch/index2.htm

Who knows technical books or mature works or so to this theme? Or your good based opinions and arguments ?

Andy

Offline kno3

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Re: efficient paddle wheels?
« Reply #1 on: May 01, 2008, 10:21:44 PM »
Interesting. I'd like to see an efficiency comparison of propeller versus paddlewheel too.

Offline Bierjunge

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Re: efficient paddle wheels?
« Reply #2 on: May 01, 2008, 11:42:02 PM »
Gentlemen,

Although I didn't find any reliable sources, please allow me to post a few own thoughts. Please note that's always dangerous to generalise like "a paddler/propeller is more powerful" or "faster" or "more efficient", because it's always a matter of what you compare. Boats with the same size/weight, or even the same hull? With the same engine performance (or even the same engine)? Or with the same tractive force?
Most vessels are designed and optimized for one specific purpose, and arbitrarily tethering two of them together to see which one outpulls the other (as suggested in a different thread) doesn't make any sense at all.

I would strongly suggest to clearly distinguish between Thrust, Efficiency and Speed.

Thrust:
We all know that paddlers in fact can produce quite a strong thrust (and thus initial acceleration). If high thrust at shallow draft is needed, paddlers continued to be the choice well into the middle of the 20th century, for example diesel driven sternwheel towboats in the US. In open water however, a propeller tug of the same thrust can be built much more compact.

Efficiency:
Both for paddlers and for props, energy efficiency has something to do with slip: If the circumferential speed of the paddle wheel is twice the boat's longitudinal speed for example, half of the energy is wasted.
If you tether the boat to a post, slip is 100% and efficiency 0%, but the thrust is the highest.
For a moving  boat, slip is basically a function of cw * A of the hull (that's why we make the hull as sleak and streamlined as possible) compared to cw * A of the floats (that's why we make the floats as large and anti-streamlined as possible).
Efficiency is further compromised by splashing and churning of the wheel. That's wasted energy not put into propulsion. Of course, a feathered wheels is more efficient (less splashing) than a rigid one.
Every paddle wheel however is acting at the surface, the border between air and water, and therefore involved in some surface effects and splashing, at least as soon as higher velocities or loads are considered. A high power paddle drive simply cannot be as energy efficient as a well desgned propeller drive. Sorry about that, guys.

Speed:
Increasing splashing and centrifugal effects driving the water out of the wheel also limit the velocity of paddlers.
We all know that there have been pretty fast paddlers, but there's no question that prop boats can be much, much faster.

A big nonsense I think are the diesel- converted paddlers, which need for the fast running diesel engines big gearboxes or hydraulic systems to reduce revs and to increase the torque. A steam engine connected with paddle wheels is a kind of a symbiotic technical system. So in Switzerland in 2003 an old steamer (Montreux) was reconverted from Diesel to steam again, and is economic!

I'm an epigonic steam enthusiast as well. But even if you take inevitable losses of gearboxes or even hydrostatic drives into account, a steam engine (even not the new swiss engines) could never reach the energy efficiency of a modern diesel drive. Sorry again.
So if a paddler is wanted, for reasons of aesthetic appearance or shallow draft, I totally agree that a steam direct drive is way more authentic and appealing. But a diesel drive could be more energy efficient. We can't turn back time.

Regards, Moritz

Offline kno3

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Re: efficient paddle wheels?
« Reply #3 on: May 02, 2008, 03:01:11 AM »
Hello!

I have just visited the Technical Museum in Berlin, and guess what they had on display: a demonstration model with a paddlewheel propelled hull and a propeller powered hull tied against each other. Both floating on water, with electric motors. With the push of a button the viewer could start them.
The result was that first the paddlewheeler would pull the the other ship, but after a few seconds the propeller ship would pull the paddlewheeler back.

Reading the description attached, I think they intended to demonstrate that the paddlewheels have more initial thrust but then the propeller gets the upper hand.

But I did notice a problem: the paddlewheels, while feathering, were set in the wrong position so the paddles wouldn't enter the water at a vertical angle...
« Last Edit: May 02, 2008, 07:03:29 AM by kno3 »

Offline mjt60a

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Re: efficient paddle wheels?
« Reply #4 on: May 02, 2008, 04:16:09 AM »
I don't think the paddles are supposed to enter at a vertical angle (ie. 90 degrees to the surface) are they? since they are moving more-or-less downwards at that point, they would produce no forward thrust at all (though also least resistance to the engine) If they enter at 'some' angle (say 20 degrees off vertical) they would provide at least some driving force while moving downwards...
I bet someone here could clarify this  :-\
Posted by Mick.
(.....gonna need a bigger boat.....)

Offline kno3

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Re: efficient paddle wheels?
« Reply #5 on: May 02, 2008, 07:04:23 AM »
Well, maybe they don't have to be exactly vertical, but those were off by a lot. Clearly not entering the water the right way.

towboatjoe

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Re: efficient paddle wheels?
« Reply #6 on: May 02, 2008, 11:40:15 PM »
A riverman once told me that sternwheelers are efficient up to around 500 hp, but the screws are more efficient above that.

Offline kno3

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Re: efficient paddle wheels?
« Reply #7 on: May 03, 2008, 07:13:53 PM »
Hello!

This is a picture of the demonstration model of paddlewheel vs. propeller I was describing earlier.

Offline mjt60a

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Re: efficient paddle wheels?
« Reply #8 on: May 03, 2008, 09:09:54 PM »
I see what you mean, the wheel looks too deeply immersed, also the centre of the feathering mechanism looks MUCH to close to the centre of the shaft for it to work correctly, they may as well have used a simple paddlewheel with the floats bolted to the spokes!
There was a similar test performed on the TV series 'Men of Iron' as I remember...
Posted by Mick.
(.....gonna need a bigger boat.....)

Offline kno3

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Re: efficient paddle wheels?
« Reply #9 on: May 03, 2008, 09:31:53 PM »
At a closer look, I have noticed that the shape of the hull at the back, where the propeller is, looks pretty bad from a hydrodynamic viewpoint. So I guess the propeller could lose some just because of this.
anyway, it wins the race in the end.
« Last Edit: May 04, 2008, 06:32:00 AM by kno3 »

Ivor Bittle

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Re: efficient paddle wheels?
« Reply #10 on: May 07, 2008, 01:36:01 AM »
 You could try reading my website in the website section.
Ivor Bittle

Offline kno3

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Re: efficient paddle wheels?
« Reply #11 on: May 07, 2008, 06:22:48 AM »
Very interesting website, congratulations!
I am reading the steam boilers section right now, very informative.

Offline Lars Schanz

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Re: efficient paddle wheels?
« Reply #12 on: May 09, 2008, 02:01:33 AM »
This had been done in real scae a fewe years ago: During a comparison In march 1845 between screw propellor driven HMS Rattler and paddle-steamer HMS Alecto the Rattler conclusively beat the Alecto in a series of races with the other ship and then won a tug-of-war contest in which the Rattler towed the Alecto backwards at a speed of 2 knots. Both ships were of comparable size and shape - Alecto even had the stronger engine.

Best wishes Lars

 

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