Topic: Article on matching motor size to hull size? (Read 10047 times)

TurboTyne · « **on:** August 30, 2014, 07:44:36 am »

Posts here and elsewhere in 2010 refererred to the preparation of an article that was to appear in "Model Boats" and that was going to discuss and summarise information relevant to matching size/power of motors to size/speed of models.

Did that article get published? If so could someone please let me know the issue in which it appeared?
I'd like to obtain a copy in the hope it may enlighten me about how much power I'll need for a planned model and how the theoretical "scale speed" relates to what would seem like visually acceptable speed for a model warship.

Thanks, Mike

J.beazley · « **Reply #1 on:** August 30, 2014, 08:25:05 am »

Out of interest what model do you plan on building?

There are some very clever people on the forum that can suggest motors for specific boats having tried and tested what works and what doesn't.

I've mostly gone with the theory that motor can shoud be around the same size as the prop your trying to swing.
Plus I don't see the point in over powering a model as like you say you want it looking scale on the water.
Having said that to give you a rough idea my 8foot Richelieu has 4x graupner 500e motors spinning 45mm props and gets upto some pace across the lake running 12v.

Jay

TurboTyne · « **Reply #2 on:** August 30, 2014, 02:40:01 pm »

Quote from: J.beazley on August 30, 2014, 08:25:05 am

Out of interest what model do you plan on building?

There are some very clever people on the forum that can suggest motors for specific boats having tried and tested what works and what doesn't.

I've mostly gone with the theory that motor can shoud be around the same size as the prop your trying to swing.
Plus I don't see the point in over powering a model as like you say you want it looking scale on the water.
Having said that to give you a rough idea my 8foot Richelieu has 4x graupner 500e motors spinning 45mm props and gets upto some pace across the lake running 12v.

Jay

Hello Jay

Thanks for the reply. The ship is HMS Rigorous, an R-class destroyer built in 1916.
Guided by advice in another thread I've decided to make it at 1:42 scale so the LWL of the model will be about 69", beam 6.7" and displacement 22 lbs. The two props, if to scale size, would be roughly 57mm diam.

From your comment and from other posts I get the impression that scale speeds are generally much faster than what is really needed - is this correct? I estimate that scale speed for the Rigorous would be 36 knots divided by sq root of 48 = about 6 mph. Also, from scaling the power of the prototype (27,000 shp) the model would need 27,000 divided by (48) cubed = 0.24 hp. This is equal to 0.24 hp x 746 = 182 Watts. This seems to me to be a lot of power (I have a 1/4 hp motor driving my metal turning lathe). So I am wondering: are these calculations incorrect and/or is scale speed more than what is needed.

I gather that the article in Model Boats was going to give examples of hull size/form along with motor sizes that gave good performance and this would be a useful guide for me. I should add that I have no experience in model boat building but have several reasons for wishing to build this model.

Regards Mike

Colin Bishop · « **Reply #3 on:** August 30, 2014, 02:58:39 pm »

I provided a general article in the 2011 Model Boats Construction Special Issue on the principles of choosing motors & props and it was accompanied by some working examples supplied by members of the Model Boats website Forum which you can see here: http://www.modelboats.co.uk/forums/postings.asp?th=45505&p=4

The article didn't cover models of the size you are contemplating building however!

People have built cruisers to that physical size which should give you some idea of a practical working setup.

Colin

inertia · « **Reply #4 on:** August 30, 2014, 03:36:21 pm »

Little boats in full-size water are always a nuisance when it comes to theory, so why bother with it? "Suck it and see" is always a reasonable maxim. I'd try two of these, direct-drive on 12v http://www.componentshop.co.uk/775-dc-motor.html 50mm dia 3- or 4-bladed props should be fine; there's plenty of torque here.
DM

morley bill 1 · « **Reply #5 on:** August 30, 2014, 04:19:16 pm »

biggest motor you can get in the wind and water are 1/1 scale our water is 7acres and 34ft deep so spare power is allways welcome....Bill..

Time Bandit · « **Reply #6 on:** August 30, 2014, 09:18:59 pm »

There is a pretty easy rule for displacing non planing boats.
3W per kg weight to reach hull speed.
Its a little bit depending on the shape of the hull, tight warships will need a little less power.

The rest is pure calculation.
Motor rpm x prop pitch x 0,7 = speed and vice versa.
You will probably need around 3000 rpm to reach scale speed.

TurboTyne · « **Reply #7 on:** August 31, 2014, 08:52:42 am »

Thanks for all the responses.

Colin, thanks for that link. Actually I had already seen that thread on the MB website and I thought the actual article might contain more words of wisdom that would be helpful for me. Now you’ve told me the issue I looked for it but found it is not listed as for sale on the back-issue pages of “Model Boats”.
Anyway, I shall now search for info on builds of model cruisers.

I fully accept that scaling calculations will not be reliable and that is why I was seeking info on actual models. But it seems a good starting point and I wanted to compare the results of scaling calculations with actual successful designs in other models so as to help me assess the limitations of theory for my planned model. I am trying to design things in advance as well as possible because I am currently involved in building a flash steam plant and it would be great to have a rough idea of the power the plant needs to output. I can measure the power of the plant but have little idea what it needs to be.

BUT, I have just spotted an ERROR in the calculations I mentioned previously. I had found the scaling calculations in various forum posts and had assumed that they have some validity since the principle has been used in reverse for over a century in designing full size ships and assessing their power requirements etc using model hulls in towing tanks. This morning I decided to seek out info on these scaling methods and found this useful page:
http://en.wikipedia.org/wiki/Similitude_of_ship_models
It seems that dividing prototype power by scale factor raised to the power of 3 (i.e. cubed) was wrong since this gives the power needed to propel a scale hull at full-scale speed (i.e 36 knots in my case). Apparently the correct scaling calculation should be to divide by scale factor raised to power of 3.5. In my case this gives a power requirement of 26 Watts – instead of 182 Watts.
It seems difficult to find examples of models of non-planing boats where the power output of the model motors can be identified and also where the power of the prototype is known. I have found a few so far and I’d be happy to post the results of the comparisons if it would be of any interest.
Thanks again, Mike

malcolmfrary · « **Reply #8 on:** August 31, 2014, 09:35:11 am »

If trying to scale exactly, when looking at power, it is as well to remember that power figures quoted for the real thing are output power. The figure for an electric motor in a model is the input figure. Its a big model of a fast ship, expect large numbers. I do suspect that a pair of 12 volt motors pulling an amp and a bit each would cause observable movement, but nothing that would look in any way right.
Daves solution is the best - look at what others with a similar size and shape boat have done, and run with their wisdom. To send a boat that size and weight along at a proper looking speed, two 12 volt motors pulling about 15A between them doesn't sound at all unreasonable.

TurboTyne · « **Reply #9 on:** August 31, 2014, 09:54:25 am »

Quote from: malcolmfrary on August 31, 2014, 09:35:11 am

If trying to scale exactly, when looking at power, it is as well to remember that power figures quoted for the real thing are output power. The figure for an electric motor in a model is the input figure. Its a big model of a fast ship, expect large numbers. I do suspect that a pair of 12 volt motors pulling an amp and a bit each would cause observable movement, but nothing that would look in any way right.
Daves solution is the best - look at what others with a similar size and shape boat have done, and run with their wisdom. To send a boat that size and weight along at a proper looking speed, two 12 volt motors pulling about 15A between them doesn't sound at all unreasonable.

Actually MFA/como do give output powers and also state the efficiency of converting electical input power into output (about 50 - 60% in the ones I've looked at) and it's these values I have been seeking. (So far I've only seen one other brand of motors which give output power).

But your advice about the sort of motor power you think should be needed is very helpful and I'll be guided by that and by comparison with other successful similar sized models rather than rely on theory.

Mike

Time Bandit · « **Reply #10 on:** August 31, 2014, 09:41:32 pm »

Quote from: TurboTyne on August 31, 2014, 08:52:42 am

It seems that dividing prototype power by scale factor raised to the power of 3 (i.e. cubed) was wrong since this gives the power needed to propel a scale hull at full-scale speed (i.e 36 knots in my case). Apparently the correct scaling calculation should be to divide by scale factor raised to power of 3.5. In my case this gives a power requirement of 26 Watts – instead of 182 Watts.

Scale^3,5 is correct, regarding power.
BUT, you assume that your propulsion system has the same efficiency like the real one.
And that´s wrong.
Efficiency is less since the shape of the props is not perfect.... so you need some more Watts to reach the scale speed.
If you calculate with 3W/kg you will be always right, if we talk about non gliding boats that reach hull speed.

I always design my motors, that the model reaches hull speed, even if the scale speed would be lower.
The reason is the wave pattern the hull creates.
Hull speed is v(km/h)= 4.5 x squarerroot(length of waterline in m). Very narrow hulls can reach higher speeds.

Half gliding boats at a maximum of double hull speed need around 20-25W/kg. At 1 and 1/2 times hull speed it´s around 15W/kg.

At higher speeds we talk about fully gliding boats, which can´t calculated that easy, too much factors kill any calculation (wetted area of the hull...)

Unfortunately I´m not so aware of the motor´s available in the UK.
Here in germany I´d recommend those motors for your boat (if you want to use 55-60mm brass props): http://www.hobby-lobby-modellbau.com/onlineshop/product_info.php/info/p5616_B-hler-Star-Max-II---12--Volt.html

That one here might help (warning it´s german)
http://www.jochen-schwarzer.net/Motorcalc_V12.xls

roycv · « **Reply #11 on:** September 01, 2014, 12:39:48 am »

Hi just had a look at the web site. It seems Clevises are called forkheads!
Rpy

TurboTyne · « **Reply #12 on:** September 01, 2014, 02:47:08 pm »

Hi Tobias
Thanks for that explanation and for the spread sheet. (I can just manage the German, having worked in Tuebingen for over 2 years, too many years ago). I found the motor you suggested but cannot find its power rating.

Prompted by your post I worked out the hull speed for an R-class destroyer. For the full-size ship the maximum speed (66.7 km/h or 36 knots) was 1.65 times the hull speed (40.4 km/h or 21.8 knots). For a 1:48 model the scale max. speed (11.0 km/h or 5.2 knots) is 1.9 times the hull speed (5.8 km/h, 3.1 knots).

Is the significance of hull speed that it indicates how the model will look relative to the waves it creates? If so, then am I right in thinking that, in theory, to look the same as the real ship, the model would need to achieve a ratio of speed to hull speed of 1.65? If so, this means it will not be necessary to attain scale max. speed.

In practice I conclude I should aim for plenty of power since power can always be turned down. I’ve been looking at the implications of the all the expert advice provided in this thread.
‘Inertia’ suggested two motors of a type which, calculated from the data for speed and torque at maximum efficiency, each generate 31 watt output. But I guess they can give more power than this and I guess this was why Malcolmfrary referred to motors drawing 7.5A each at 12V. Assuming an efficiency of 70% this means an output of 63 watts for each motor. The total of 126 watts is about 5 times greater than the theoretical scale power.

Tobias suggested the formula 3 watts per kg. For the R-type destroyer at 1:48, the displacement will be about 22 lb = 10 kg. and that gives a result of 30 watts which is just a little more than the scale power. I wonder if the 3 watts per kg formula works best for hulls of a broader shape and lower speed than the WW1 destroyers. These ships were shaped rather like a pencil. It is because of their small displacement per length that I need to build a longer model than I really wanted - so as to give more scope for the steam machinery.

Following Colin’s advice I have so far found one example of a cruiser of similar length – although obviously with much greater beam and draft. This is “bluebird’s” lovely HMS Exeter: http://www.modelboatmayhem.co.uk/forum/index.php/topic,33734.0.html

Assuming my spread sheet is set up correctly, scale power for the Exeter would be 7 watts which seems ridiculously small for a 6ft model. This low figure seems to result from the very large influence of scale on the calculations and this model is 1:96 scale versus the R-type destroyer at 1:48. Bluebird has installed what looks like 4 Graupner speed 500 motors although I don’t know which type. But whichever they are, I think they can each produce up to over 100 watts of output giving a huge excess over the scale power. So, bearing in mind this and comments such as Morley Bills, I think I shall aim for at least 120 watts.

Sorry to witter on about what must seem like a reinvention of the wheel but this shows I do listen to the advice offered. Also, it helps me get my ideas straight and open to any further comment.

Regards, Mike

Calimero · « **Reply #13 on:** September 01, 2014, 03:30:03 pm »

Isn't that power calculation just overthinking it ? We're talking about scale boats doing 10km/h not offshore powerboats doing 80km/h.

Finding the right shaft RPM is probably a bigger issue than power. Especially with brushless motors these days.

TurboTyne · « **Reply #14 on:** September 01, 2014, 04:08:21 pm »

Quote from: Calimero on September 01, 2014, 03:30:03 pm

Isn't that power calculation just overthinking it ? We're talking about scale boats doing 10km/h not offshore powerboats doing 80km/h.

Finding the right shaft RPM is probably a bigger issue than power. Especially with brushless motors these days.

I'm sure that in many ways you are correct - but I started out looking for any & all ways to estimate the power requirements for my planned model and at the beginning of this thread I had no idea whether scaled power estimates were useful or not and could not find any previous general discussions of the topic. That was why I seeking the MB article. (But, also, I must admit that I tend to get a bit carried away with exploring topics like this

).

Now since you raise the issue of shaft RPM, can you please advise me where to turn for guidance? Since I'll be building the flash steam plant I should be be able to adjust its output speed to suit the needs of the model - if only I knew what speed range to aim for.

Thanks Mike

Calimero · « **Reply #15 on:** September 01, 2014, 04:23:58 pm »

I would aim for just a little above scale speed ("vorbiltgetreue Modellgeschwindigkeit" in TimeBandit's Excel spreadsheet, that is (real speed)*sqroot(scale)). If the figure is really too low, round it up. Our boat "scale", water and wind don't.

Then find the right prop from Raboesch, Prop Shop, Rivabo, .... Once you have the final pitch you can work you way back to shaft RPM. And then get the right motor/battery.

And keep in mind it's always possible to limit speed on the TX (servo travel, or simply a sensible throttle finger) if you got a bit carried away.

My springer has a ... 600W brushless motor in it. Even with the meager 6V I feed it could probably output up to 150W. But I estimate it actually draws between 1.5 and 4A (9-24W) depending on the propeller type and type of sailing.

TurboTyne · « **Reply #16 on:** September 01, 2014, 06:35:16 pm »

Quote from: Calimero on September 01, 2014, 04:23:58 pm

I would aim for just a little above scale speed ("vorbiltgetreue Modellgeschwindigkeit" in TimeBandit's Excel spreadsheet, that is (real speed)*sqroot(scale)). If the figure is really too low, round it up. Our boat "scale", water and wind don't.

Then find the right prop from Raboesch, Prop Shop, Rivabo, .... Once you have the final pitch you can work you way back to shaft RPM. And then get the right motor/battery.

And keep in mind it's always possible to limit speed on the TX (servo travel, or simply a sensible throttle finger) if you got a bit carried away.

My springer has a ... 600W brushless motor in it. Even with the meager 6V I feed it could probably output up to 150W. But I estimate it actually draws between 1.5 and 4A (9-24W) depending on the propeller type and type of sailing.

Thanks very much for that Calimero.
I looked up Raboesch and Rivabo, but only the former seems to give pitch data.
So, picking a 55mm diam prop, this has 49mm pitch.
With scale max speed = 11 km/hr = 183 metres/min and pitch = 0.049 metres
I estimate rpm = 183 / 0.049 / 0.7 = 5,345 rpm
I included the factor 0.7 since this was mentioned by Time Bandit.
Does this all sound O.K. ?
I read on the following web page that max revs for Raboesch 55 mm props is 6000 rpm.
http://www.rcgroups.com/forums/showatt.php?attachmentid=3402635
One thing I don’t understand is that different props can have different blade areas and so, I assume, different propulsive powers and different power requirements for the same rotational speed. Presumably, if a blade area is too small to give the required thrust at a given rpm, there will be more slippage so there must be another factor that enters the process of choosing a prop? There seem to be a few choices of blade style to pick from for, in my case, 3-bladed props.
Regards MIke

Calimero · « **Reply #17 on:** September 01, 2014, 06:52:09 pm »

I don't know exactly what type of props where in use in 1916 on such ship. I'll just go with Raboesch 3-blade type #146 propellers which are listed as "Before 1940" in their catalog.

Pitch for the 55mm dia. prop is 56mm.

11km/h is indeed 183m/min.

183/0.056/0.7= 4668 rpm

That's rpm under load.

Let's say you want to power your boat with a 2S LiPo battery or a 6-cell NIMH pack. That's 7.2 or 7.4V. I'd go with a pair of 750kv brushless motors. But that's just me and my fondness for BL motors.

You should be able to find something close with brushes. See Inertia's suggestion a few posts earlier. Very close, but running on 12 volts.

Anyway with two 55mm props, you won't have any thrust issue as your ship is very light.

TurboTyne · « **Reply #18 on:** September 01, 2014, 08:19:25 pm »

Thanks very much for that further guidance. That is a great help!

I was looking at type 162 describeed as: Low-cavitations propeller screws with high output at a low RPM for high-speed 1920-1960 warships. But apart from type 146 all are 49mm pitch.

By the way - I am building STEAM propulsion - that's what I'm fond of

!!! - no batteries or electric motors except to power the fuel and water pumps

.

Regards Mike.

Calimero · « **Reply #19 on:** September 01, 2014, 09:34:02 pm »

Ha !
The Prop Shop's web site is currently down but I remember they have a range of props for steam propulsion, with high pitch, as steam engines tend to be on the low side.

TurboTyne · « **Reply #20 on:** September 02, 2014, 12:04:09 pm »

Thanks - I was wondering why search results for Propshop lead to a strange web page.
I'll keep trying.
Mike

Time Bandit · « **Reply #21 on:** September 02, 2014, 06:11:25 pm »

To be honest, your destroyer is a bad example

Normal (non gliding) boats can only reach hull speed. No matter how fast your motor turns the boat won´t get faster than hull speed. This count´s for all no gliding boats with a length/width ratio up to around 7:1.
If you increase motor rpm you just make the waves bigger (sometimes wanted e.g. for tug boats to create bigger waves). A good value is up to 30% bigger calculated speed than the hull speed. Everything above will increase the power consumption very much.
Mainly depending on the motor specification (it gets choked down to lower rpm and higher amps since hull won´t run faster) but it also needs a lot of energy to create the waves.

In the early 20´s somebody found out that if you increase the ratio of length and width to like 10:1 you can minimize the creation of waves by the hull.
That´s why destroyers run faster than hull speed but are not real "half gliders".
Same theory is still used today for fast competition rudder boats which can reach speeds of nearly 3 times their hull speed with just 8 HP (while H stands for human

)

I personally would use a setup which also makes it possible to reach the same 1,65 times hullspeed with your model. It probably wont run faster, since the hull shape will make it impossible to get the bow out of the water without increasing the power to 300W+.
The "slip" factor of 0.7 is also variable (depending on hull shape and especially prop diameter and blade area, in case of that hull shape it will be more or less probably only 0.8 if you use props with 60mm diameter.
You will probably also need a little more power than 3W/kg. I´d calculate with around 5-6W/kg.

The different type´s of prop don´t have such a big factor on a free running non gliding ship if you don´t overdo it.
30mm props on your boat would create a slip factor of at least 0.5.
Generally you need "energy" in the water, no matter on what kind of prop it is supplied.
Biggest influence is on acceleration but the time is so short you won´t even notice the different shape of the prop.
There are differences on the props (Raboesch C-type for example have a progressive pitch which creates a little more thrust while A- and D-type pitch is linear) that´s why I calcuate with diameter/pitch ratio of 1.0 for C-type and 0.9 for the rest.

If you are after bollard pull the prop shape has a big influence, but I think we can forget this here.

BTW:
You can´t calculate our models perfectly since you have to many unknown factors.
Don´t try it

I stopped it years ago to even try and work with my general factors gained by experience and a data logger.

TurboTyne · « **Reply #22 on:** September 02, 2014, 10:57:10 pm »

The R-class destroyers had a length to width ratio at the waterline of almost exactly 10:1. This photo shows HMS Marmion at speed. She was an M-class destroyer, but these had exactly the same hull size and form as the R-class.

6W / Kg means an output of 60 watts = just under 0.1 HP (horses in this case not human). I'll be very sad if my steam plant cannot achieve more than that!!

Thanks for the further advice. Where do you find all the detail info about the propellor forms? - I missed it if it is on the propellor company web pages.

I am realising that designing a model ship is an excellent example of Murphy's first and second laws.

(The third and fourth laws are certain to also come into effect during and after construction).

Are Murphy's laws well known in Germany?

My main focus is making the PIC-controlled steam plant but I need a boat to put it in and, for me, that has to be a model of HMS Rigorous.

Regards Mike

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Author Topic: Article on matching motor size to hull size? (Read 10047 times)

inertia