Discussion on Motors and Props

[Colin BishopModerator @colinbishop34627]

This topic is intended to support my request for firther data on selecting motors and prop to revisit an earlier magazine article on the subject.. See:

**LINK**

The above topic should be used for data, this topic can be used for general discussions and points that might inform the updated article for the magazine.

Feel free!

Colin

Edited By Colin Bishop on 18/09/2020 11:19:25

[Colin BishopModerator @colinbishop34627]

Updating data for a magazine article

[Chris EParticipant @chrise]

Colin

Firstly let me say that I regard a wattmeter as essential so that you know what is going on as soon as you deviate from a few well trusted combinations.

Propellers

To be useful you need rather more propeller data that you have asked for. The make and model of propeller makes a large difference to its performance and loading. With model aircraft props the data is available to compare any 2 propellers and be very precise about what they will do.. With model boats it is alchemy.

Take the simple example of a 40mm 3 blade propeller. The Raboesch list above lists pitches of 41mm, Proteus Design (Propshop) are 45mm, Graupner Plastic are 21mm, etc. These differences are enormous and haven't started to include the effects of the different blade designs that are out there.

Even with basic 2Bl plastic props there are large differences between P and X types.

Without good propeller identification I would find the results of very little value.

Edited By Chris E on 18/09/2020 11:08:47

Moved from data topic. Colin

Edited By Colin Bishop on 18/09/2020 11:18:26

[Chris EParticipant @chrise]

Colin

Brushless Motors

My experience of brushless motors in aircraft is that specifying them is simple. There is very little difference in the performance of different brands of motor that have the same specification although the more you pay the better the marginal performance. Indeed the same motor can often be purchased in many colours with many different labels and for many different prices. Better quality expensive motors do have advantages but they are beyond the scope of what I am saying here.

What matters is the kv of the motor and its ability to dissipate heat. Test data is available for brushless motors that tells you exactly how many watts of power they use if loaded to any particular rpm ,With model aircraft I know for any propeller exactly how many watts is required to turn it at any given rpm and how much thrust will be achieved. It is thus easy to match motor and prop.

With model boat props it is guesswork but see my previous post above. For model aircraft I work on around 3W per gramme of motor weight but that is with a nice big cooling fan. For model boat use I would go less – particularly if I didn't use a cooled motor mount.

Moved from Data topic. Colin

Edited By Colin Bishop on 18/09/2020 11:18:05

[Colin BishopModerator @colinbishop34627]

Chris – thanks for your comments. I have moved them from the other topic which, as I said, should be used for data only otherwise things get confusing.

I don't know if you read the original article but people did find it useful I think. It was mainly aimed at the less to medium experienced modeller rather than someone like yourself willing to do the maths.

I have amended the data for props to include more detail as you suggest but I'm afraid that will lilkely only apply to new information unless previous contributors update their original data.

I agree 100% that a wattmeter is invaluable. These were not so readily available when the article was originally published which relied on ammeters and voltmeters for measuring purposes. Obviously this will all be updated.

Again, thanks for your useful input.

Colin

[Mark Beard 1Participant @markbeard1]

I fully agree with Chris E’s comments (which I received as email notifications and for some reason are yet to appear here), that specific data are needed for motor and propeller combinations to enable good mathematical modelling of boat performance. The kv parameter and motor count are key for motors, and drive voltage for the battery. Gear or belt speed reduction ratio is key for the drive system. Diameter, blade count, blade pitch and prop count are key for the prop(s). The most difficult to enumerate is the hull shape, as there are enormous performance differences between, say a full displacement hull, such as a tug, and a fast planing hull as in an MTB, with everything in between. The aim should be a mathematical model which accurately predicts scale performance for a model boat, as easily as is achieved for model aircraft. Notwithstanding the fluid dynamic difficulties of hull characteristics, our field should not be inferior!

Edited By Colin Bishop on 18/09/2020 14:03:56

[Chris EParticipant @chrise]

Mark

Nice as it would be I wasn't really proposing developing an all inclusive modelling programme for model boats but rather observing that some real data for props would make drive examples that were posted much more useful in a wider sense. The motor data is already available.

Edited By Colin Bishop on 18/09/2020 14:07:36

[Colin BishopModerator @colinbishop34627]

Mark,

As you will see from my earlier post on this topic, we are wanting to keep things really simple for the magazine article using an empirical approach. Chris' suggestion of more precise information on props being used ties in with this but I'm afraid that mathematical modelling techniques will fly right over the head of the average kit builder, for example, who simply wants a proven setup to install in their boat.

It is unfortunately the case that many people struggle with even basic concepts like watts, volts and amps so fluid dynamics characteristics would not be something they would readily engage with!

Colin

[Gareth JonesParticipant @garethjones79649]

Hi Mark,

Glad to see you are still around and still got the anorak.

Colin,

I will post some data when I work out the best way of displaying a table. I have used a Springer tug as a test bed for a number of brushless motor/prop/battery combinations but it would probably be easiest to use as a comparison if they were all listed side by side.

Gareth

[Colin BishopModerator @colinbishop34627]

Gareth, thanks, info will be welcome but it might not be printed that way as it will depend on how the designer needs to display it so don't spend a lot of time trying to format it specially.

Did you come up with one or two combinations which are the best ones to use? If so use those as space in the article will be limited to some extent.

If you were using the same model all the time I assume you have a favourite?

Colin

[Chris EParticipant @chrise]

Lots of pictures and very litte information seems to be the rule these days.

What I would like to see is a simple comparison between different props but all the same size. Take any size of prop (lets say 40mm) and connect it to a motor with plenty of power then give us the watts consumed and the thrust delivered for a range of props – 2 blade plastic "S" & "X" and 3 blade plastic and, 2, 3, 4 & 5 blade brass (all 4 being one make & model).

I have never seen that done but it would be so useful and increase the value of any examples enormously as we could see that if the example worked well with say a 40mm 3 blade brass it would/wouldn't work well with a much cheaper "X" plastic .

I realize that this makes a whole load of assumptions but I am convinced it would be useful and might debunk some of the odd statements that are made here & elsewhere.

 

 

Edited By Chris E on 18/09/2020 17:53:34

[ashley needhamParticipant @ashleyneedham69188]

Before this gets too detailed, I think the gist is….you have a model, it works well enough with the motor and prop/battery combo you have chosen….write it down.

Endless theorising is NOT what`s required for the purposes of the article as I understand it, I believe it is aimed at knowing what works with various types of hull and that`s that. (plse correct if wrong)

Are we including airboat prop or EDF combinations to bring the list fully up to date??

Ashley

Very few of my boats are of any use for such an article as most of them are specials and thus the combinations I used are of no practical use for anyone.

[Colin BishopModerator @colinbishop34627]

Interesting Chris, but rather outside the scope of the intended article which is aimed at scale models.

I am presently building a 1:150 scale of the Aberdeen line passenger liner ss Miltiades of 1903 to 1:150 scale. I have some 1:1200 waterline drawings, some photos of the orignal plans taken in the National Maritime Museum last year plus a lot of photos taken of the builder's model which used to be displayed in the London Science Museum.

From the builder's model, the draught marks on the hull indicate that the three blade merchant style props are 15 feet in diameter. At 1:150 scale that would be pretty much 30mm on the model. Raboesch sell this type of propellor in this size so that is the back end of the model taken care of. (it's a scale model so it has to look right, no black plastic two bladers)

I am using a conventional brushed motor/NiMH battery setup as it is convenient, relatively inexpensive and will do the job. I know from experience that 280 size motors would struggle a bit with this size of prop and an M4 13 inch shaft. A 540/545 type would be overkill and too heavy so the sweet spot is a 385 low drain type which is fitted in many other models of this overall size and weight. I also know that a 7.2v battery would probably be OK but an 8.4v pack would give a bit of power in hand. After that the speed will be dependent on how far I push the throttle stick.So, for me, job done using empirical practical experience withput any need to enter into tests or calculations although I will check the current draw to confirm it is within reasonable limits.

Obviously the less experienced modeller, or one coming into the hobby (who is the target of the article) won't have that experience to draw on but he may be building one of the Deans Marine traditional merchant ships which have siimilar dimensions and could therefore fit a similar setup to mine in the reasonably sure knowledge that his model will perform similarly – and that is really all he wants to know.

A lot of the points made above are in fact made in the original article which explains pitch, blade area etc. etc. so that readers have a basic idea of the factors involved without having to reach for a calculator.

Colin

Edited By Colin Bishop on 18/09/2020 18:36:44

[Chris EParticipant @chrise]

Posted by ashley needham on 18/09/2020 18:05:30:

…………………………………Ashley

Very few of my boats are of any use for such an article as most of them are specials and thus the combinations I used are of no practical use for anyone.

Ashley

I would have thought that you Landers, Mintanic & Sans Pareil (+possibly others) were good candidates. No example will ever be identical to all models of the same size.

Edited By Chris E on 18/09/2020 19:53:23

[Colin BishopModerator @colinbishop34627]

One other thing not mentioned above but which was covered in the original article is the beneficial effects of gearing (gears or pulleys) n helping to reduce the disparity between optimum motor and prop speeds. Something that is frequently overlooked. Several of my models feature pulley drive which considerably improves efficiency.

My big 48 inch Fishery Cruiser has pulley drive and cruises at 2.5 amps.

Colin

[ashley needhamParticipant @ashleyneedham69188]

I used the wattmeter on my medium lander, twin 385 can motors! S35 two blade plastic props and on 9.6V on the day, and it gave me a 5A reading.

Looking in the shed, I have measured up a few boats propelled by the humble385 motor,

largest first. Props for all mostly s35 or brass 30-40mm. All dimension in mm

LST. 1600 x 300. 3x 385 Flat bottomed so not as heavy as it looks

Titanic. 1200 x 130. 3x 385

Argus a/c carrier 900x 140 2x 385

LCT 750×130 2×385

LCT 500×130 1x 385

In addition, the Devastation (not measured),

The motor copes with 40mm props, and the motors barely get warm. Landers normally run on 6 cell Nimh, the others on 7 or 8 cell for a bit more oomph. Neither the LST or Titanic are underpowered on 9.6v. Hasto be a go-to motor if ever there was one!

 

Ashley

 

Edited By ashley needham on 24/09/2020 09:02:29

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