MFA385 – A Story of Confusion

[Chris EParticipant @chrise]

The humble 385 motor is the basis of many models and is often recommended to beginners but when one asked me which 385 motor to get I was surprised as I had assumed there was one and that it would be obvious.

MFA actually list 3 different 385 motors but we can probably forget the 24v version for now.

The RE385 is a 66gm motor spec 4.5-15v (although it says 6-15 on the motor) that has maximum efficiency at 12v 9869rpm 0.99A

The RE385LN is a 110gm motor spec 6-12v with max efficiency at 12v 5950rpm 0.9A

Now lets look at the retailers:

Component Shop list both and show the same specs as MFA !!!! Congratulations Component Shop.

Model Boat Bits list both the RE385 and the RE385LN but then give them both the same spec, which is different to either MFA spec, 6-15v max efficiency 12v 9280rpm 0.85A. They do give you the opportunity to pay £2 more for it as the LN version however. I guess that the LN spec might be a mistake but the RE385…..?.

Cornwall and Howes list the RE385 using approximately the same spec as Model Boat bits. Has MFA changed the spec of the RE385 motor?

My local model shop, Pegasus Models, list the 385 motor but when you decode the model number you find it is the LN version with its very different spec, rather than the base RE385.

As an experienced modeller I can see my way through this but a beginner cannot. I do not understand how something so simple can be made so confusing. I wonder how many beginners have not got the motor that was recommended to them.

[Chris EParticipant @chrise]

[Colin BishopModerator @colinbishop34627]

Interesting points Chris. the LN (Low Noise) version obviously has a lot more torque at much lower revs than the standard 385.

I wonder how this would translate into actual boat performance. Presumably the LN version could swing a bigger prop?

Mine are the standard 385s on Raboesch 35mm 3 blade brass props. bought from Component Shop.

Colin

[Chris EParticipant @chrise]

Colin

I am not sure of the real world difference but I would be surprised if it wasn't significant. I don't build planing hulls so I always prefer to run my motors lightly loaded. The maximum efficiency spec means something to me.

I note that at maximum efficiency the base motor is turning at 9280rpm or 9869rpm (depending on where you look) whilst the LN motor is turning at 5950rpm. Both are just under 1A which suggests a considerable difference in the size of a suitable prop. With the 6v/7.2v favoured by many beginners around 3000-3250rpm for the LN doesn't sound a lot when connected to a small prop. Fit a larger prop and you could end up with around 2000rpm

I suspect that the 2 different specs for the base 385 are due to a change in spec, or at least a change in what is stated, by MFA.. I doubt that 9280rpm / 0.85A is very different to 9869rpm / 0.99A.

I suggested the beginner buy from Component Shop.

[ashley needhamParticipant @ashleyneedham69188]

Good point. Mine are all "standard " ones(I think?). They all run without noise capacitors, mostly, and I dont have any issues with this.

At this sort of consumption I would not worry about the different current draws for a few hours boating on a 3700mAhr stick, but the seemingly much lower revs of the LN version would be a worry, and seriously underpower almost anything i have with a 385 fitted without using massive props or bigger voltage batteries.

Efficiency in the model boating world, for anything other than racing craft or items with very high current draws is something to think about…..or is it??? What benefit would a slightly more efficient drive make to one of my landing craft, for 2 hours water time???? Would it be worth the worry?

As an aside, a static wattmeter test on a "standard" 385 on 9.6V and an s35 two blade plastic prop was 2.5A. Not that this means much on its own, but The motor takes it all in its stride and doesnt even get warm with this abuse.

Ashley

[Richard SimpsonModerator @richardsimpson88330]

Very interesting post Chris and certainly something I had not considered. My choice of motor, to date, has simply been based on size, when I'm not putting a steam engine in of course!

It makes you wonder whether the two motors have been designed specifically for different uses, i.e. the LN for high pitch propellers and high torque use in say tugs and scale steam models and the base motor for the speed required by higher performance props, or whether it is more accidental that the two motors should be so different.

It also begs the questions, 1) Are all other sized motors similarly available in more than one configuration and 2) Why are vendors not more familiar with their products and more careful with their descriptions and recommendations?

Maybe you could develop this into a very useful article for Model Boat Magazine?

[Chris EParticipant @chrise]

Richard Simpson

I don't think that the MFA motors were designed for specific model boat use at all. They are general small dc motors that can be used for pretty well anything that requires a small dc motor. I agree that you could put a larger prop on the LN motor.

If we look at the next range up – the 540 – the situation becomes even more confused. These are just 5 examples but there are many more out there with wildly varying specifications.

Mtronics 500 which gives at maximum efficiency 12v /4700rpm /2.7A

MFA list the 540.1 and the 540LN. The 540LN is a 5 pole motor which gives 12v /5258rpm/1.42A. The 540.1 is a 3 pole motor giving 12v/ 6180rpm/2.1A.

Component Shop used to list a 540 giving 12v /13360rpm / 2.85A

Hobby King list the Mabushi 540 – 6527 which gives 9.6v/ 20040rpm(!!!) /6.8A

 

Add to this the tendency of beginners to look for the “free” motor that came out of a power tool and about which they know nothing.

 

Edited By Chris E on 28/09/2020 11:36:06

[Ray Wood 3Participant @raywood3]

Hi Richard,

I doubt if any of the motors are designed with model boats of any type in mind. Lots of mine come from the grandchildren broken toys, they must be sold by the million ?

Regards Ray

[ashley needhamParticipant @ashleyneedham69188]

Ah, the 540. I am not even going to start to comment on this beast. It is just too confusing.

Suffice it to say that virtually all the 540`s I use are the MFA 540/1 types of which I have experience of and buy new.

I regard any unmarked 540 with great suspicion. Motors from drills….do we all have some in the "come in handy box"??? it always seems a shame to chuck a nice shiny motor away, but that`s what we should do. I have probably half a dozen motors I should chuck (but wont, just in case….).

Ashley

[Richard SimpsonModerator @richardsimpson88330]

I completely agree gents. To be honest I'm not really sure that any motor is specifically designed for model boat use. I have always considered motors produced by the motor manufacturers to be designed to a set of parameters and it is up to us to decide as to which one matches our requirements.

Some may be supplied by model boat vendors, Dean's Marine Kingfishers spring to mind, but even they were almost certainly not specifically designed as a model boat motor. That is simply Ron Dean doing the thinking for us and selecting a motor that matches the model's requirements from a motor manufacturer's catalogue.

The responsibility still lies with us to ensure that the motor we put in the model is fit for the purpose. If the motor has no means of identification or if we cannot identify the motor's characteristics then I'm sure it is best not to use it.

By coincidence I have just read a post on Mayhem from a modeller who has declared he is leaving the hobby because his model is making a lot of noise. When you then read he is using an 800 motor through a double universal joint you can't help but think he is going to struggle to get his head around the most basic of motor characteristics.

As for 20,000 rpm, I hope its well balanced!

Edited By Richard Simpson on 29/09/2020 09:52:53

[Chris EParticipant @chrise]

Posted by Richard Simpson on 28/09/2020 10:38:23:

……………………… My choice of motor, to date, has simply been based on size………………….

Um……………… You have been very lucky if you have been successful using this method of selection.

I have also read the thread from the gent with the barge. It is the sort of thing that an experienced modeller would sort out very easily (probably by avoiding the problem in the first place) but I guess that there is nobody to help. Unfortunately forums are not ideal for sorting it out.

[Richard SimpsonModerator @richardsimpson88330]

Posted by Chris E on 29/09/2020 11:56:40:

Posted by Richard Simpson on 28/09/2020 10:38:23:

……………………… My choice of motor, to date, has simply been based on size………………….

Um……………… You have been very lucky if you have been successful using this method of selection.

I have also read the thread from the gent with the barge. It is the sort of thing that an experienced modeller would sort out very easily (probably by avoiding the problem in the first place) but I guess that there is nobody to help. Unfortunately forums are not ideal for sorting it out.

Obviously that was probably over simplifying things a little but odd for you to consider me lucky as you started the thread of with:

"The humble 385 motor is the basis of many models and is often recommended to beginners but when one asked me which 385 motor to get I was surprised as I had assumed there was one and that it would be obvious."

I am sure you would not rely on luck any more than I would as a degree of experimentation with a meter balanced with an evaluation of performance would head us in the right direction.

[Chris EParticipant @chrise]

Yes knowledge & testing are a great help.

[redpmgParticipant @redpmg]

Here is a simple guide to Mabuchi motor designations – may help somewhat

[ashley needhamParticipant @ashleyneedham69188]

Hmmmmm….

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