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Method for choosing the right prop and motor

Is there an analytical method for calculating prop, gear, motor and voltage for a scale model?

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Mark Beard03/10/2010 23:20:48
164 forum posts
84 photos
I'm relatively inexperienced in model boats, but I know quite a bit about motors.
It seems to me that when building a scale model, the correct choice of prop, gear ratio, motor and battery voltage is absolutely crucial from the get go.  Because the motor and prop shaft need to be accurately aligned, they need to be matched and set up when installing into the hull.  Once the shaft is bonded in place, any change to the motor mount, or heaven forbid installing a gearbox, can be catastrophic to this critical alignment.  So, without the experience of all you experts out there, we beginners need a way to be sure we have the right motor for our model at the start of our build.
While I understand the advice to join your local club and ask someone, advice from 'experts' does seem to be inconsistent at times.  And the beginner may not be able to tell who's got real experience, and who's wanting to sound as though they are an authority.
As a professional engineer, I can't get away with popping any old motor into a client's application and expecting him to pay for the re-design if it doesn't work out right.  Neither can I always ask someone else for advice.  I need to calculate the performance up front and do the sizing calculations.  Then I have a good reason to be confident that  the selected motor will perform as required.
What I'd dearly like to do is to determine a methodology for taking a given model, (starting with scale displacement hulls only), assigning the correct prop and shaft speed, then determining the correct gear and motor combination to drive the prop at the required speed and torque, and finally give the applied motor voltage and current draw to select the battery pack.
This all sounds wonderfully straightforward, but since there are so many forum questions about motor and prop choice, I'm sure it is far from simple in practice.  I have heard all sorts of 'rules of thumb' like "a prop diameter should not exceed the motor diameter" or "the model power should be the prototype power divided by the scale cubed".  It must be possible to bring a 21st Century engineering method to this problem.
Does anyone have any experience of attempting such a task?  Does anyone have any tried and tested rules of thumb relevant to this?  Has anyone solved part of this problem, such as finding the right prop size, which can be incorporated into the full solution?  Would anyone be interested in collaborating in such a project, perhaps leading to publication?  Or would anyone have reason to caution me against jumping in where angels fear to tread?

Tony Burton04/10/2010 09:37:34
133 forum posts
49 photos
Hi mark
      I honestly dont really know where to start on the subject. A lot depends on whether you are building a kit or scratch building from plans. In both cases  either the kit manufacturer
will usually recomment the motor and batteries or there will be some sort of clue on the plans
Colin Bishop04/10/2010 10:54:35
5011 forum posts
6115 photos
414 articles

Hi Mark,

Thank you for an interesting post. It’s quite a coincidence actually as there is a possibility that there will be another Special Issue of Model Boats next year and I was only discussing yesterday the possibility of putting in something about choosing motors and props!

You talk about ‘starting with scale displacement hulls’ but that is in fact the most difficult area. With competition models there are class rules which generally determine the size and shape of the hulls and power options. There is usually only one prop and it is intended to push the boat as fast as possible in one direction only. As the models are competing against each other, the best power drive configurations sort themselves out and there is plenty of evidence as to what setup works best which beginners can tap into.

When it comes to scale hulls you are in an entirely different ball game as the number of variables needed to establish a satisfactory power configuration increases exponentially.

Firstly there is the overall size of the model expressed in both terms of dimensions and displacement. Then there is the shape of the hull itself, a model tug will need more power to push it along than a destroyer of similar weight. But then within the range of scales we build to the destroyer would be expected to actually go faster than the tug to give a realistic appearance on the pond. As far as props are concerned, these are available in a wide range of diameters, blade configurations and pitch, all of which affect the power generated to propel the model. On top of this, your choice of prop may be constrained by what is scale in appearance rather than being able to opt for the most ‘efficient’ one which can require more or less than optimum power to be provided. Some props give good forward thrust but are pretty useless in reverse and a scale boat needs to be able to stop and manoeuvre effectively. Many models have two props and motors (some three or four!) which obviously need a different power input to that of a single screw vessel – but you can bet it won’t be half the power of the single for each of the twin motors!

Electric motors usually benefit from being geared down but gearboxes at our model sizes tend to be both inefficient and noisy. A satisfactory alternative, which I use, is to fit a belt drive. Others prefer not to bother with this extra complication and plump for a larger motor/smaller prop combination which usually works fine in practice if possibly at the expense of higher power consumption – but if you are using lead acid batteries as ballast this doesn’t matter.

Trying to calculate all these parameters for a particular boat would be way beyond the majority of us modellers so, being practical types, we cast about for empirical solutions and usually the most effective is to find another boat of around the same size, shape and dimensions which performs well and use that as a guide. Sometimes this doesn’t always work though and two apparently similar models can perform very differently. This is probably due to the detailed hull form, particularly at the stern, and the way in which the prop interacts with the water flowing around the hull.

So, in practical terms I rather feel that the best we could hope for is to produce a table of commonly built models together with the motor/prop/battery combinations that work for them and people building other models could use these as a base reference for fitting out their own boats. As Tony says above, the kit manufacturers usually give practical guidelines for their models based on experience and these are also a good starting point. However, these are my own personal views and I’m sure that other members will be happy to pitch in with their own which may be different.


Mark Beard04/10/2010 12:11:26
164 forum posts
84 photos
Thanks for such rapid and comprehensive replies.  I appreciate that scale is the hardest area, as this seems to be where many forum posts raise these problems.  It is this area I think is in most need of some help.
If the prop sizing can be established, either by analytical means, by manufacturer's recommendation or by use of tables derived empirically, then there is certainly enough known about motors to calculate how well a given motor will perform in the installation. 
My experience many years ago of scratch building a hard chine 19" cabin cruiser, fitted with a 35mm high pitch prop, an RS540 motor and 7.2V NiCd pack, as suggested in the plans, was that it went like absolute stink for 5 minutes.  Then the battery died and the motor was smoking!  So much for recommendations!
My current build is a 760mm Conserver survey vessel from Model Slipway, and has a roughly tug-shaped hull.  It has a 50mm brass 4-blade prop and when I asked the seller about using a Decaperm motor I have lying around, he said "oh, that'll do very nicely".  So I wanted to do some calculations before installing it to see if it was anyway suitable.
So this is what I did:
I found a graph of prop sizes and suitable speeds posted by nick_au75 at  I got a reasonable fit to his curves by calculating shaft speed based on a fixed tip speed of 10m/s (chosen to fit Nick's curves and limited to avoid cavitation).  For a 50mm prop, this works out to be 3800rpm.
Then I developed an Excel model of motor performance and checked it out against the excellent motor specifications given at  I did struggle to find comprehensive specs for the Decaperm and had to measure the speed constant Kv, no-load current and motor resistance Rm.  At 12V this gave a peak efficiency of 63% and a working speed range of 8,200 to 9,700rpm (at 30-70% of peak motor power).  With the Decaperm's integral (and rather noisy) 2.75:1 gearing, this gives a shaft speed range of 3,000 to 3,500rpm.  Remember this is top speed and will be throttled by the ESC during sailing.
Conclusion: on paper, the Decaperm is a reasonable choice in terms of shaft speed for the 50mm prop in the Conserver kit.
This is the kind of methodology I'm proposing, albeit with simplified tools to aid calculation.  Of course it would need to be validated against performance in the water!  And this is where a collection of vessel models with prop, motor, battery and performance data would be invaluable.  Some may find the answer to their problems directly from the table.  Others with not-listed models may interpolate.  Others still, perhaps with a bent for design calculations, will appreciate such a methodology being published.
Further questions, comments, (even "you must be out of your mind") and suggestions gratefully received.
Colin Bishop04/10/2010 13:16:35
5011 forum posts
6115 photos
414 articles
Mark, you obviously have the tools and the knowledge to approach this in a scientific manner and I think your result with the Decaperm demonstrates that it works as I would have said that was a good motor (if a bit pricy!) for Conserver simply from my own modelling experience.
However, many, if not most 'average' modellers would not necessarily have the tools or expertise to do the measurements and calculations you refer to above. This means that the whole thing needs to be reduced to some sort of simple  'look up' basis which is what I was getting at in my own post. i.e . 'If I've got a boat like this then my motor/prop options fall into these ranges' sort of thing. Some notes on how to do more precise calculations for those with higher skill levels could be provided as well.
Whether this is possible I'm not sure, but you could have some fun trying and we might all learn a bit more along the way.
Mark Beard04/10/2010 13:34:00
164 forum posts
84 photos
Agreed, the plan is to reduce this work to something easy for modellers to use.  I'm a bit odd in finding 'fun' in this sort of thing I guess!  If anyone is interested in collaborating on this, particularly with prop selection and info on which setup works with which model, I'd love to hear.
Gareth Jones04/10/2010 16:33:06
799 forum posts
1078 photos
Maybe I am a bit odd, too as I have been experimenting over the past few months to try and achieve the same aim as you.  I began building my first model boat about 5 years ago (a TID tug) and I was surprised there were no recommendations on matching propellers and motors.  In the electric model aircraft world there seems to be much more information with advice on thrust available from various battery and propeller sizes for a given motor.  My second boat was a three motored PT boat (Snogg as described in the Sept issue of Model Boats) and that turned out to be a bit of a disaster initially.  Even the second attempt has not turned out to be very good after about a years running with ever higher motor temperatures.  I have done some testing and research and now I am much more confident that I have a good working match of propeller and motor, at least for Snogg.  I have already written to Paul Freshney and suggested a follow up article on the work I have done, Snogg the Sequel perhaps.  I am also supposed to be giving a presentation on my experience to the club that I sail at, (Goole MBC), probably in November.
Essentially I have gone down two routes.  I have compiled a spreadsheet for our limited fleet of models (I say our because my wife built some of them).  This documents the build standard details, size, weight, type, battery, motor, propeller size etc.  I have also measured performance using a wattmeter to measure the power into the speed controller, a tacho to measure prop speed and a digital set of fishing scales to measure thrust from the propeller.  I have put this data into the spreadsheet and rated all the combinations I have covered, ranging from a 1:12 narrow boat through Springer tugs, Steam drifter, TID and PT boat. I have come to the conclusion that the best performance is with a propeller that lets the motor run at around 60-70% of the free running speed,  i.e. somwhere between the max efficiency and max power points on the motor performance chart.  I have also done some experiments with different propellers and found some surprising variations as you can see if you look on the forum under R/C and accessories of a few weeks ago.  I am not an expert in electric motors but I spent nearly 40 years as  a Flight Systems engineer with BAE Systems so I have a lot of experience with collections of equipment connected together and being forced to work together.  
I would be interested in collaborating in some further work as you suggest.  I thought it would be relatively easy to come up with some analytical tool to predict performance but as Colin Bishop has said its not as easy as it looks and there are more variables than is first apparent.  If you would like to get in touch I could send you the spreadsheet as a starting point.
ashley needham04/10/2010 19:04:08
7433 forum posts
159 photos
  Would you also be contemplating brushless motors? 
Mark Beard04/10/2010 20:22:05
164 forum posts
84 photos
Gareth: Glad to hear from a fellow odd bod!  Your focus on prop testing and my focus on motors sounds like a great fit.  Will contact you separately.
Ashley: No reason why brushless motors cannot be modelled using the same techniques, the principles are precisely the same.  From reading your many post replies on beginner's questions about motor sizing, I'm sure you will be able to tell us where we can improve our model (this time of course I refer to mathematical modelling of props and motors, not model boats - English has its limitations).
ashley needham05/10/2010 19:28:32
7433 forum posts
159 photos
Nope, least not as a mathematical modelling goes.
Although I have been free with the "knowledge" it is gained either from personal experience or having chatted to other chaps to see what they have done, and is of a "this should be satisfactory" standpoint rather than a "best/most efficient" view. Compounded by my thoughts that model boats in a pond should be overpowered so as to have "something in reserve" when an errant boat comes along, or dog or goose or "oh dear I have taken my eye off the boat as i was looking at that young lady and it is just about to smash into the bank and I need to reverse (fast).
As for being clever.. i will retell the tale of a man at the poolside with an e-boat, May have been a plastic kit conversion perhaps. He had three teeny weeny long thin brushless motors, and three props about...oo... 15mm diameter or something even smaller. I thought....well its not going to go very fast with that lot is it...just thrash the water to death. (but kept quiet)
He set it off, and ye gods..did it fly ! almost literally.. the motors screamed at such a pitch that all the dogs started barking!  wow. Keep your mouth shut at the pondside is my motto now.

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