Scale Engine Speed

[William BeckParticipant @williambeck]

Hi everyone, new here and first question of I'm sure will be many.

I'm hoping to start building a scale model of a steam marine recip. Engine.

What do you think is the best calculation to get the rotation speed of the engine looking a true match for the real thing (78rpm full speed)?

Watching all of the exposed linkages moving around I absolutely love, and want to know what I'll be looking at is truthfull.

Thanks for any help

Will

[William BeckParticipant @williambeck]

How to work out a good scale speed for an engine

[JasonBModerator @jasonb]

Are you running on air or steam? Single cylinder or compound?

[William BeckParticipant @williambeck]

Hi Jason,

Will be steam, triple expansion. Still a bit of a pipe dream, especially with this coronavirus.

Will this affect the scale speed though?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by William Beck on 10/04/2020 15:35:42:

[…]

I'm hoping to start building a scale model of a steam marine recip. Engine.

What do you think is the best calculation to get the rotation speed of the engine looking a true match for the real thing (78rpm full speed)?

Watching all of the exposed linkages moving around I absolutely love, and want to know what I'll be looking at is truthfull.

[…]

.

An interesting question, Will

I think that [perhaps surprisingly] the answer might be 78rpm

A small-scale model should look just the same as the original would at a greater distance.

Therefore the ‘realistic’ speed remains the same.

MichaelG.

[awaiting contradiction from those who either know better, or just want to argue for the sport of it]

[SillyOldDufferModerator @sillyoldduffer]

Oh, bother – much as I'd happily pay £1 to have a 5 minute argument, I agree with Michael!

I believe a rotating wheel on a marine engine should turn at full scale rpm.

Is the confusion because full rpm applied to the wheels of a scale loco would be wrong because the resulting mph is scaled too fast, ie the distance travelled isn't correctly proportioned to the size of the engine?

I think. Not done the sums…

Dave

[JasonBModerator @jasonb]

Yes there is not calculation required to get 78rpm

However the smaller a model the harder it is to get to run at the same speed so it's not so much a calculation as to making the model well so that there is minimal drag so that it can turn over easily on minimal pressure and volume of steam.

It will also help if you can fit a working condenser, get the mass of the flywheel as high as possible which may mean using non prototypical materials if you need to keep the scale look correct

[Paul KempParticipant @paulkemp46892]

Scaling speed is an age old thorny subject as speed is distance over time and time you can't really 'scale'. If you wanted a scale 60mph for a half size model and you scaled 60 by half and a minute by half it would be 30 miles in 30 minutes, which is………. 60mph. RPM would be the same. You are straying into Einstien territory here!

Paul.

[Neil WyattModerator @neilwyatt]

Ideally rotational speed should be the same as full size.

A scale displacement hull will show (proportionately) the same wake as full size if the boat travels in proportion to the square root of scale.

So a model at 1:25 scale will look 'right' at 1/5 of full size speed. Works pretty well for planing hulls too. This also stops models looking like they are barely moving.

A bit of extra speed works for railway locos as well. My 3 1/2" gauge shunter (1:16) would have a scale speed of 3/4 (three quarters not 3 or 4) mph which might frustrate other rack users as the full size only does 11mph.

Neil

[John OlsenParticipant @johnolsen79199]

Just to add to the fun, a full size marine engine would not normally have a flywheel, other than that provided by the length of the shaft and the propeller. They tend to run very jerkily without all that connected. So a model might need a flywheel, which you might want to try to conceal. The "natural" speed of a small engine tends to be blurrily fast so you will need quite a good size flywheel to manage to keep it reasonable.

If you want to make it easier for yourself, you could look for either the Stuart triple or the Bolton triple. Both have been available as castings sets although I am not sure about current availability. The Bolton is quite an accurate model of a Sydney harbour ferry engine. I'm not sure if the Stuart is based on any particular prototype but it looks the part.

Lots of small steam boats do of course have flywheel, because the shaft and prop are too small to have any useful flywheel effect, and because a flywheel can be used to manually pull the engine off dead centre if (when!) it decides to stick. Dancer, for instance, currently being serialised in Model Engineer, has a disk flywheel about ten inches diameter with a three inch wide rim.

John

[Paul LousickParticipant @paullousick59116]

Hi William, " Will be steam, triple expansion. Still a bit of a pipe dream, especially with this coronavirus. Will this affect the scale speed though? "

There should not ne a problem. Your engine would not be susceptible to catching coronavirus and the steam should kill it if it did. LOL.

Sorry for that, Paul

[William BeckParticipant @williambeck]

Thank you for all your replies, a great help.

Interesting it should be the same, useful as I've just found a portable tacho!

Plan is to attempt a working model of the Titanic's engine room, although still just thinking about it. It will have scale shafting and condensers so hopefully runs smoothly.

[Sam SpoonsParticipant @samspoons83065]

Just a thought WRT concealed flywheels, could you build an additional compartment aft of the engine room to hide a flywheel?

[Dave HalfordParticipant @davehalford22513]

Don't forget to include the coal bunker fire

[Clive FosterParticipant @clivefoster55965]

Ultimately its a question of aesthetics. The engine needs to look right when turning over. Which generally means running slow enough to see what is going on. Basically running at an idling speed so you can see the parts moving not rushing round in a blur.

Most likely something in the region of 30 to 40 rpm will look right. Small engines seem to need to run more slowly than larger ones to look right. Presumably some weird combined perspective and distance effect.

Going to be a bit of trial and error to get it looking right. Expect to need some form of concealed load as well as a smoothing flywheel.

Michaels point about a model needing to run at the same speed as the real thing because its supposed to look just like the real thing would from further away is well taken.

But in practice that is pretty much impossible to achieve when viewing a model in the real, 12" to the foot scale, world. We have all seen photographs and films where its impossible to tell if the subject is a model or full size but when we are looking at it we know the real world is full scale and that we are looking at a model which upsets our perception. Odds are that if viewing the real thing running at normal speed from equivalent distance we'd not be able to see the interesting moving part details properly anyway.

In many ways the idea of scaling speed lives down the same rabbit hole as scaling mass because "you can't scale nature". At some point you have to accept "representationally correct" over "mathematical exactitude".

Clive

Edited By Clive Foster on 11/04/2020 11:22:42

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