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Old Jan 30, 2012, 12:03 AM
Just call me "Mo"
mogogear's Avatar
NE PDX OR
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Question
Avg running RPM Cheddar Puffin

What would any of you say is the average operating rpm range of a Cheddar Puffin engine? Underway at scale cruising speed?

i.e. - 300- 400; 400-600 rpm or is it more like 800-1000rpm?

Thanks- I am working on my gear rations for my side-wheel tug..And I am shooting for an operating range of actual wheel rpm of 120-160rpm or so.

So I ( think) would love a 4:1 ratio and run the engine around 500 and get about 120 out of the wheels. A 6 tooth gear on the engine and a 24 on the drive shaft. But of course the gears I need are discontinued by SDI and now I am having to go up to say an 8 and a 36 ..But that throws in another set of changes at the bore on the 36tooth sprocket is HUGE - so an bore reducer would need to be employed -

Aint' it always something?

So to keep the bore where I like them and available I am looking at maybe a 3:1 ( 8 tooth / 24 tooth) that makes 500 engine rpm translate into 166 wheel rpm. That just seems low- but I could be way off on my thinking.

A lot of explanation for this but is 400 rpm a good constant run speed on a puffin?

Thanks for opinion or knowledge!!
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Old Jan 30, 2012, 06:09 AM
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zorrow's Avatar
United States, WA, Everett
Joined Apr 2004
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Hi Mogo, RPM of the engine will depend on the PSI output of the boiler. Possiblly consider the use of cog belt drive. Two advantages to this. 1. a zillion different combinations available. 2. very quiet in use. 3. easy to adjust for correct belt tension.
also consider that your engine will have a "sweet spot" as far as rpm goes. More rpm equals more torque.
Unless you are going for the "Miss triftway" look (major rooster tails) keep your final RPM low. Try not to load down the engine, Use higher pressures= a higher rpm on engine and lower final to the paddles
my 2 cents worth
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Old Jan 30, 2012, 09:49 AM
Just call me "Mo"
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Quote:
Originally Posted by zorrow View Post
Hi Mogo, RPM of the engine will depend on the PSI output of the boiler. Possiblly consider the use of cog belt drive. Two advantages to this. 1. a zillion different combinations available. 2. very quiet in use. 3. easy to adjust for correct belt tension.
also consider that your engine will have a "sweet spot" as far as rpm goes. More rpm equals more torque.
Unless you are going for the "Miss triftway" look (major rooster tails) keep your final RPM low. Try not to load down the engine, Use higher pressures= a higher rpm on engine and lower final to the paddles
my 2 cents worth

Thanks for all that- But I am not interested in belt drive- just a quirk on my part- I like brass and chain etc- METAL stuff Mere preference

and I realize there are a lot of variables-

This is just my first paddler and I have not been using a tachometer in t he last 4 years- so maybe someone will have a knowledge of AVERAGE operating RPM if there is such.

My Cheddar boiler will most likely be set to run around 2.75- 3.0 bar range with an attenuator on board to regulate the burner while idling and such

Thank you though for the note-- yes many things to consider!!
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Old Jan 30, 2012, 11:38 AM
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Somerset England
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Hi.
You could use more than 2 gears, thus keeping the size of the cogs down .
Build a simple reduction gearbox with say 4 cogs to get final drive .
I would guess that arround 400 rpm would be near to running speed , but have never checked it .
Dunc2504
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Old Jan 30, 2012, 11:55 AM
Just call me "Mo"
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Thanks Duncan--a reduction box is an option - but trying to keep this simple for my feeble mind.

I really was searching for something that won't exist till I just buy a set and see what happens. Then plot the next change of course based on what I find out.

As long as 400 rpm does not set off alarm bells like "Puffins never run well under 600 rpm- everyone knows that!!"

Time to spend more money....which I have done a lot of in the last few days--attenuator, fittings, new paddle wheel set, changed burner !

But it will be fun to open packages and get things installed!
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Old Jan 30, 2012, 02:47 PM
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Joined Mar 2008
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Actually Puffins runs well even at very low rpm's, like 100 or so. At least mine do...
I I were to make a paddlewheeler, I'd couple the steam engine directly, no gearing. That way it would use up less steam. A Puffin cylinder (11x11mm) is quite bg, and makes a powerful engine, but also a hungry one. No reason to have it spin fast when you need a low rpm for the paddlewheels.
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Old Jan 30, 2012, 07:19 PM
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It's easy to reduce the bore of a gear, just stick in short segments of K&S brass tube(s) as needed. They nestle perfectly. Heat them up to temper them, let cool, then install. That way, the gear's setscrew will provide enough clamping. Or you can CA the nest of tubes before sliding them onto the output shaft. Still nice to soften the brass set of tubes before CA-ing so the setscrew does not have to work too hard to dimple them.
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Old Jan 30, 2012, 08:31 PM
Just call me "Mo"
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Hello to see you Calin and Mr Brooks. Good input from you both. As for direct 1to1 gearing I only hesitate as I usually find mid-range on anything is less problematic as a general statement.

It would use a lot less steam that is for sure. Brooks the nesting picture tube picture is one that escaped me. Thank you.

I have gone with the 3:1 ratio and so ordered the 8nesting tooth sprocket and the 24. So that can give me theoretic cruising on approximately 300-350 engine rpm which is moderate in any book. With plenty of reserve for forward. And strong all reverse if needed.

We will see what happens .... Thank you with all reverse for your input
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Old Feb 20, 2012, 12:50 AM
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Operating speed for sidewheeler

I'm building a 62 inch scale model of a sternwheeler and have done extensive research and calculations on the paddlewheel rpm needed to achieve scale speed.
First, how long is your model? Second, what are the dimensions of your paddlewheels? Given your engine speed stated above of 300-350rpm and a gear reduction of 3:1, your paddle wheels will be spinning 100 to 116rpm. My concern was to build and size the steam engine to give me a maximum paddlewheel speed of 120rpm and a more realistic 60 to 80 rpm cruising speed. Any more than this and the paddlewheel becomes inefficient and splashes water all over. I chose a scale of 1/32nd to give me a larger paddle wheel (6.5" dia x 7.5" wide) slower paddlewheel speed with the intent to make it look more realistic. I am sure you know that in scaling speeds for model boats, a direct relationship between the size of the prototype and model doesn't work and that William Froude's formula is globally accepted as producing the most realistic appearance and an appropriate wake from the hull. That said, my boat should look good running at about 1. 7 knots. This should be achieved at a paddle wheel speed of 100rpm. My setup is using a scale model of the Gillette and Eaton non-condensing steam engine found in the full-size steamboat. It will have a bore of 5/8" and stroke of 2 1/4". This will connect directly to the paddle wheel via 10" long pitman arms, no gear reduction. You might want to run some numbers to see how fast you'll need to move your boat to achieve scale speed. Aside from scale speed calculations, you must know your model's hull speed. For my 5-foot model that works out to just under 3 knots. This is the approximate maximum speed potential of the hull. I'm happy at half that speed. This is easy to calculate: Knots roughly equals 1.34 X the square root of the waterline length. There are a lot of beautiful videos on YouTube of model paddle wheelers. There you can see examples of what you are trying to achieve.

Good Luck, Mike in Edmonds (See W.T. Preston Build thread on Scale Ships)
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Old Feb 20, 2012, 02:05 AM
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The Modeler's Fallacy of Froude :-)

Froude's work seems to be nearly universally misunderstood by model makers. What Froude tried to do was figure out how you could take a model boat speed and use it to estimate a real boat's speed. Not the reverse, which is the modeler maker's fallacy problem.

Nautical architects found that if they measured a 1/32 scale model max tow tank speed of 1 knot, they could not multiply that speed by 32 and successfully predict the max speed of the real ship as 32 knots. The real ship was found to move much slower. The model speed had to be adjusted downwards to get an accurate prediction of the real ship's max speed. But by how much? Froude figured out the equation that let nautical architects do that.

The problem with trying to run his equation backwards, i.e. to use the equation to set a "realistic" model speed, is twofold:

1) Water does not scale, it is what it is. If you want the model wake to look realistic, you will have to redefine realistic to mean something less than mimicry; that is, model realism will never be the same as "real" realism. Surface tension ensures that model boat waves will never break the same as a real boat's waves, for instance. No matter what speed you run, the waves won't look peaky, with copious white breaking crests, but will look lumpy with smooth crests (at least for models moving at hull speed, I'm not talking about models moving at planeing speed). This is why movies use CGI instead of tow tanks. They want to fool the audience into thinking it's watching something real, but water won't let them do that. You want realistic waves, you have to have a real ship...or a sneaky computer guy.

The wave form, in terms of frequency & scaled wavelength will be the same for models and real ships, but the wave tops will not be the same. So, define model wake realism as wave form/frequency/scaled wavelength , but eschew wave appearance, accepting lumps instead of peaks.

2)When humans look at something to judge it's speed, they use the size of the moving object. They look for how much time it takes a boat or plane to move its own length, for example. They don't use absolute speed: a Pitts Special biplane looks fast because it's small, a 747 looks slow because it's big, yet the 747 is actually traveling faster in absolute terms. In 2 seconds, the Pitts travels more hull lengths than the 747, and this fools the brain into thinking the Pitts is faster. This human eye&brain activity means that the human eye automatically scales the speed to the length, not to some esoteric formula of absolute speed, e.g. Froude's equation. So, the brain speed to run your boat for realism is to use its length as the gauge, i.e. it's scale. To make a 1/32 boat look real, it needs to cover the same number of model boat lengths/unit time as the real boat covers it's real boat lengths/unit time i.e.. the 1/32 model needs to run at 1/32 of the real boat's speed to look right to the eye&brain.

Actually, psychology gets in here too. I find models moving at the realistic speed (say 1/32 of real) to look slow. My 1/40 Aldebaran topsail schooner can travel at a scale 50kt [in terms of (boatlengths/sec) ]. The real ship probably rarely topped 20kts. While the Aldebaran looks fast, it does not look particularly unrealistic to my eye. Slowing the boat down to the realistic scale speed of 1/40 * 20 makes it look intolerably slow to me. Perhaps I am just so used to fast model airplanes, that I need to see fast model ships. After all, the people who rode railroads in the 1860's thought them dangerously fast, even though the average speed was on the order of 15mph, darn slow by today's standards. They were used to buggies moving at 7mph. If I was modeling back in the 1860's, maybe I'd need to see slow model boats to be comfortable.

So, you pays your money and takes your choices. If you want to run Froude backwards, there's nothing stopping you. It's not good math/physics, but 99% of the guys watching your ship will never know. If the resulting look of your model bothers you, then speed up, at least I'll be happy, both mathematically, and psychologically :-).
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Old Feb 20, 2012, 11:30 AM
Just call me "Mo"
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That why I like to surround myself with folks smarter than me- it takes all the pressure off...

Thanks to you both for the discourse !!

I was "shooting" for paddle wheel 100-120rpm " based mostly on what other learned folks on Paddleducks had gather in their knowledge base as to what looked right and what the bow wake developed like.

I was mostly interested if the Puffin engine liked a certain RPM range or had any unknown power curve problems. If it liked 300rpm - then 3:1 was the gearing- I would use it. If not, I would adjust my gearing to accommodate it and the end RPM target.

All good- SDP gears and chain in hand- attenuator coming - plumbing and gear fitting is about to commence..I have been fitting all sorts of superstructure and deck details and towing hook nonsense. Paint soon

Thanks again for the time and words and thought-
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Old Feb 20, 2012, 01:18 PM
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You also need to factor in some reserve power to have available to get the boat out of trouble and safely back to shore, especially if the wind suddenly increases or in a larger body of water wakes from larger boats.

I usually go for the LAR method for my model speeds, "Looks About Right".

Steve
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Old Feb 20, 2012, 06:23 PM
Just call me "Mo"
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Quote:
Originally Posted by steveciambrone View Post
You also need to factor in some reserve power to have available to get the boat out of trouble and safely back to shore, especially if the wind suddenly increases or in a larger body of water wakes from larger boats.

I usually go for the LAR method for my model speeds, "Looks About Right".

Steve
I am with you Steve-O- I know it when I see it - I am pretty darn sure that I have quite a bit more throttle in the Puffin to overpower a head wind or a head-ON! especially with the gearing mechanical advantage
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Old Feb 21, 2012, 10:58 PM
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LAR Method

The problem with LAR, at least for me, is determining what to build without some stakes in the ground/water upon which to base spending a lot of money and time. I guess if you want to dispense with the math, you still need to know where to start. It would be better to error on the side of too much power. You can always dial steam back to achieve the rpm needed to look about right. You cannot easily add power to a steam engine once built and installed, especially if flat out isn't cutting it. Where to start... That is what I had to deal with. At 8 knots, the real Preston moves 13.4 feet per second. That's a little over 12 seconds to move one boat length. The 1/32nd scale Preston moving at 1.7knots/2.86 feet per second will go one boat length in 1.74 seconds. I'll be curious to see what this looks like in terms of wake and paddlewheel/water interaction. Based on a previous live steam sternwheeler build of mine, a 5-foot model, I'm shooting for similar performance, but have more power at my disposal this time around. Running Forude's number formula backwards at least gave me a starting point to see if the steam engine I planned would turn the paddlewheel fast enough to achieve a realistic looking speed. I used this info to size the boiler and engine to achieve 100 to 120 rpm. There is a sizable fudge factor thrown in for slippage and paddlewheel efficiency. Many model steamboats have been built for us to examine to see what we like in terms of performance and looks making way. All this was my starting point before investing in a custom built steam plant
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Old Feb 22, 2012, 07:07 AM
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My experience is that power of a steam motor is dependent on boiler pressure. Maintenance of boiler pressure (in the face of demand from the motor) is dependent on effectiveness of energy transfer from fire to water. To increase power, increase energy transfer rate. If you want to kill performance, couple a big motor with an ineffective boiler&fire system.

We are offered internally-fired boilers, in the model business (fire travels through flues, not around the outside of the boiler). They are convenient wrt containing the burning heat of a fire (preserving our models from burning up), but are not effective steamers. The path of the flame through the boiler is just too short to allow for efficient heat transfer. You can tell this when you put your hand over the smoke stack and get burned. A really effective heat transfer setup will suck out almost all of the heat of the fire, making the stack temperature low enough you don't suffer an immediate burn. It's the difference between fire gasses exiting at 212degF (temp of boiling water) and exiting at 1000degF (temp of gas flame).

The simplest way to increase rate of heat transfer is to make a bigger fire. This is easy with a gas burner - just turn up the gas. If your gas tank is big enough, you are good to go. I found with G-scale locos, though, that making a longer flame would at some point escape the internal flue, enter the smoke box, and start melting metal. At that point, I started inserting various metal devices into the single flue. This slowed the passage of the hot gasses, allowing them more time to give up their heat to the boiler. An addtional efficiency came when the devices got glowing hot, introducing a radiant heat component, thus capturing energy that would have otherwise simply been lost out the smokestack.

A potentially easy way to make a fire more effective is to increase the heating surface area of a boiler. The simplest way, if not always the most feasible, is to convert the boiler from internal firing (flues) to external firing (pot). For the Midwest boiler, this increased heated surface area about 5x over the single flue's area. As you can imagine, this made a tremendous difference in the ability of the motor/boiler/fire system to produce useful power. It also lead to a great savings of fuel, since the added efficiency of heat transfer let us turn down the gas.

Keeping the fire heat from burning the model is a challenge of insulation&ventilation of the space around the fire&boiler. For an open hull model like A.J.Goddard, this was easily obtained - an insulated wrapper placed outside the boiler with about a cm of free space between wrapper and boiler was sufficient. For the enclosed boiler of the SteamTramp, this was achieved by lining the fire room with aluminum flashing, providing a big smokestack, and luck...err good engineering *grin*. Being able to turn down the fire (ceramic burners are great that way) also helped reduce heat damage potential.

The idea that a simple Midwest single-cylinder, single-acting, oscillator could run the 4' hull SteamTramp seems ludicrous on the face of it. Yet, by turning the single flue Midwest boiler into a pot-style boiler (still with its single flue), the oscillator easily moved the ship. The Goddards motor (Midwest oscillator) needed the additional help of gearing to turn the paddlewheel. But both ships run very nicely, in my opinion.

btw, the boilers of 1850's Mississippi and Missouri river steamers were return flue type - that is, half flue and half pot boiler style. About half of the boiler's outer surface was subjected to fire gasses. After licking the boiler's shell, the gasses were sent though large flues before exiting the smokestack. The "reverse" moniker came from the path of the gasses: first aft along the bottom half of the boiler, then forward through the flues.
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