Brushed or brushless?

[GrumpsParticipant @grumps]

I'm bracing myself for my next project, which is a scratch build of a 1/24th scale Thornycroft MTB, presented as a post war private motor boat conversion. It's an old Vic Smeed design, which was originally for 1.5cc IC, running a single, non-scale prop. I'm going to build it with the twin props that this boat would have had – 35mm would be approximately to scale, I think. The boat will be 933mm long with a beam of 216mm when built.

I bought a couple of 750 brushed motors some time back for this project, intending to run them from a 12v lead acid battery. The motors are rated up to 12v and produce 16k rpm at 8.4v, with a 21A stall current. I'm now thinking this is a rather Luddite approach, given that I'd like the boat to be able to plane. My thoughts are now turning to brushless motors – for example the Mtroniks Hydra 30 packages, running from 11.1V lipo battery/batteries. Problem is, I know nothing about brushless and I'm a bit windy about going Lipo without knowing a lot more. I realise they're sensitive to charging arrangements and to maximum load. I know the old MTBs were explosion hazards, but I don't want go too scale in that direction!

I'd be hugely grateful for any advice and suggestions – particularly for pointers towards useful reading. Thanks!

[GrumpsParticipant @grumps]

[Richard SimpsonParticipant @richardsimpson88330]

Hi Grumps, I have a RAF Fire Tender of around the same size and hull shape as yours which was fitted with a single large brushed motor when it was refurbished a few years ago. I estimate the motor to be around an 800 size. It is powered from two 7.2 Nimh packs, connected in series to give 14.4V. This gives a suitable performance with the model planing at around half way down the hull. The problem is that it only lasts around 15-20 minutes. I have thought frequently about changing the motor out for a brushless equivalent but, like you, have been put off by the complexities of a new technology, buying new batteries, chargers, learning new procedures etc.. I am also put off by the frequency of failures of Lipo batteries that seem to invariably go in a spectacular fashion.

If I was starting from scratch and knowing the limitations of my set up I might go for brushless but, I'm also quite happy with limited endurance and a technology I am familiar with.

[GrumpsParticipant @grumps]

Thanks for this, Richard. Nice looking tender! Are speed controllers generally ok for 14V? I thought most were rated for up to 12V. I'd heard that Lipo batteries were temperamental – maybe something to avoid. But that does leave the possibility of brushless with NiMH, I suppose. I've assumed (maybe wrongly) that brushless are more efficient than brushed. Like you, I'm not too worried about endurance. I was mostly concerned that it would be difficult to get the boat to plane with a motorbike battery in the stern, or, even worse, further forward! Thanks again.

[Colin BishopModerator @colinbishop34627]

Grumps,

I'm sure you will get some more advice in due course from our members. In the meantime you might like to take a look at this article on the website. It is a few years old now but the principles are stil the same.

**LINK**

Brushless motors are more efficient than brushed ones but they do need individual speed controllers which can up the cost a bit. They don't mind where the power comes from, Lead Acid, NiMH or LiPo but I would rule out lead acid for a boat like this as the power to weight ratio is not good if you want planing performance.

NiMh batteries are cheaper and easier to maintain. LiPos do need more care and don't last as long. Richard doesn't say what the capacity of his NiMH batteries are or the current draw, but NiMH cells are available in quite high capacities now.

My personal take on you siituation would be to fit brushless motors with hassle free NiMH batteries but I'm sure our members will be able to come up with specific solutions.

I have a couple of Fairey powerboat models powered by LiPos but store the batteries out ot the models in fireproof pouches in a tin box! My other models use a mixture of NiMH and lead acid batteries which are built into the models which is more convenient.

Colin

 

Edited By Colin Bishop on 27/01/2023 18:40:47

[Richard SimpsonParticipant @richardsimpson88330]

I'll check the capacity tomorrow. Yes you can get speed controllers capable of handling 14V, I'll check what mine is but I know it's water cooled!

[Dave Cooper 6Participant @davecooper6]

Hi Grumps,

Most of my experience with LiPo's comes from the world of powered RC gliders.

Here, they get a real hammering (especially in a vertical dive situation !). Yes, there are some safety rules that you should obey – like charging them correctly and inside a fire-proof envelope…

If you see any signs of 'bulging' then, effectively, they are 'time-expired'. I usually plug in a fresh battery after about 4 flights (say, 20 minutes each). I often tire before the batteries do !

Well worth considering for a fast launch application I would have thought. The up-front cost, charger, ESC(s), motors etc. is a bit of a swallow, but, once done they should last for years…..I think some of my batteries are now coming up to 5+ years old….my motorcycle lead-acid died after just 2 years !

Happy Modelling,

Dave

[GrumpsParticipant @grumps]

Thanks for the advice, Richard, Colin and Dave. And thanks for the link Colin – very helpful. Looks like brushless is the way to go – and not with lead acid. I'll need to think of another project for my redundant 750s. Suggestions on a postcard please….

[GrumpsParticipant @grumps]

Just looking at brushless ESCs and most are rated at 12v max. I suppose I could wire 7.2 and 4.8v NiMHs in series?

[Colin BishopModerator @colinbishop34627]

I'll leave others to comment but I doubt if you will need to exceed 12v.

Some NiMH battery packs here:

**LINK**

Colin

[GrumpsParticipant @grumps]

Thanks Colin. I wasn’t thinking of exceeding 12v – just didn’t realise there were 12v NiMH packs. Thought I’d have to wire up in series to get to 12! The large capacity 12v packs in the link you sent look rather useful. I think this is probably the way I’ll go. Wondering now if the move to brushless from the 750s will be worth the considerable expense.

[Colin BishopModerator @colinbishop34627]

I would tjhink that the 750s would be likely to get the model up on the plane, the question is at what voltage and at what current draw? A lot will be dependant on the propeller size and type and others on here will be able to advise you better than I can.

If you stick with the 750s you will need two speed controllers if you want independent control, each of which will need to be able to handle the maximum motor current draw. You could use one speed controller but that would need to be rated at the total current draw. The 5000MAh NiMH packs have a maximum discharge rate of 30 amps so would be drained after about 5 minutes at full power.

I think the the reality is that you won't need all that power to get the boat onto the plane. Richard's figures quoted above demonstrate what a practical brushed motor setup would deliver.

If brushless motors are substituted then they will be a lot more efficient and can still use the the NiMH packs so you will get more running time. Ultimately you will just have to look at the costs of the options.

Switching to LiPo batteries offers more power but the boat might not be able to use it and it will cost more.

Decisions!

Colin

[GrumpsParticipant @grumps]

Thanks Colin. Yes – LOTS of decisions!

[Richard SimpsonParticipant @richardsimpson88330]

Grumps, if you go to Component Shop they will make you a battery pack in whatever configuration you want to suit your model. For instance a single strip, cells side by side, two sets of cells side by side with a link etc..etc..

For 12V you would want 10 cells in your battery so perhaps five either side and a cable link between the two, or even two separate five cell battery packs, joined together with plug in links to connect the cells in series.

Not been out to the workshop yet, I'll have a look later.

[Ray Wood 3Participant @raywood3]

Hi Folks,

Time for me to chime in, this is my size of boat and have built a couple of the same size, so here goes …………… The MTB will plane easily on one 750 brushed motor and a 40mm prop on a 9.6 volts NiHM battery.

But as you want twin props here goes, White Marlin built on a Vosper MTB hull AKA HMS Fervant is the same size as your proposed Vic Smeed MTB and runs well on 2 x 480 brushed motors driving 35mm handed L/R 3 blade brass props on 7.2 Volt NiMH

The second is a scratch build of MTB 71 to 1/24th scale also by Vosper Thorneycroft runs well on 2 x 400 brushed motors driving handed L/R 35mm props on 7.2 volts single 20amp Mtronics speed controller. I have run this on 11volt Lipo 2200mah, not a massive difference in speed.

Both models plane at scale like speeds, most of the brushless boats at my club will go at a scale 200mph and are a hazard to all other craft in the vicinity !!!

I've been using Lipo's for 15 years with no problems, but they have shorter life span, once they start to vent (Puff up) you need to throw them away. You will hear horror stories about them but same for mobile phones ! we all still have one in our pockets

The choice is yours, but 2 750's are a bit hefty in my view, but as you have them stick them in .

More photos of each boat in my album Ray's Boats for reference.

Regards Ray

Edited By Ray Wood 2 on 29/01/2023 15:21:44

[Richard SimpsonParticipant @richardsimpson88330]

 

OK, hands up, my apologies, It's got two five cell packs in not two six cell packs in so it is running at 12V. As they are 5000mah cells the motor must be pulling a significant current for it to only last around 20 minutes, (ball park, 15a?). The two batteries are separate but connected in series by a plug in lead. The motor and speed controller are cooled by another battery supplying a small water pump. The batteries are Component Shop batteries.

Edited By Richard Simpson on 29/01/2023 15:54:28

[Charles OatesParticipant @charlesoates31738]

A bit more practical experience for you to consider.

I've an RTTL, it's about the same size as the mtb you're planning

It's powered by a little brushless motor, a Turnigy 28mm 1100kv. I.e. It turns at 1100 revs per volt.

If I use my 11.1 volt lipo, it is massively over powered and looks ridiculous, but great fun, and I've a throttle to slow it down.

If I use a 9.6 volt nimh battery it's nicely over scale speed and runs for about 30 mins plus depending on how I drive. The nimh is a 4ah and the boat has a standard pitch 35 mm prop to keep the amps down.

I chose to use one shaft to keep things simple, cheaper and lighter and consume less power than two would have, but that's down to personal choice.

I've built a few quick scale models over the years, and I think one very important rule is to keep the weight down, it makes a huge difference. I build light anyway, but the advent of lightweight motors and batteries has made everything a lot easier. Don't even consider lead acid batteries, and seriously consider lipo ones, they are remarkable.

If you do go for lipo, get a good charger, they make everything easier and safer, then just follow the safety rules.

I hope that helps a bit.

Charles

[Charles OatesParticipant @charlesoates31738]

I forgot the pictures.

[ashley needhamParticipant @ashleyneedham69188]

Size is not everything. Those little 28mm motors are powerhouses, and even a cheap one puts out 160 watts, and its that which you want to compare to the output of the 750.

A 1000Kv 28mm motor will power a 40mm prop on a 3s battery (12v) easily. If you were not worried about reverse, a 30A esc and brushless 28mm vombo can be had quite cheaply. Get a Y lead, job done.

ALSO….its a third the weight of a say 600 can brushed motor

Ashley

[Colin BishopModerator @colinbishop34627]

Quote from a post on the Mayhem Forum:

Further to the original post….we’ve had a bit of a traumatic time of late.
Whilst charging s lipo battery for my club 500, the battery exploded and gutted my garage with damage to the attached house, so we’ve been in hotels and rented accomm for the past 3 or 4 months.
All modelling gear and tools gone….

Just a reminder that you need to treat LiPos carefully.

Colin

[Charles OatesParticipant @charlesoates31738]

I saw that post too. I noticed that there was no mention of how it was being monitored or charged.

The simple fact is that any of the re chargeable batteries we use can explode/ catch fire etc if they are abused.

If you put 'over charging batteries' in to you tube there are some spectacular results, including a 6 volt sla resembling bonfire night.

The whole point with lipo batteries is that they can hold and deliver more power than other types, so they need more care. If there was a real problem I doubt they would be popular in thousands, probably hundreds of thousands of model aeroplanes, care and boats.

Does anyone remember a news story from years ago about a chap who put some batteries in his pocket next to his keys? He went to hospital with burns to his ……. Well, you can imagine.

Charging safely and correctly isn't difficult, I'm getting very ancient and I can cope ok.

 

Edited By Charles Oates on 29/01/2023 23:45:21

[Richard SimpsonParticipant @richardsimpson88330]

We once had a Club 500 mysteriously explode on the pond while in operation. Totally destroyed the boat. No-one knows what happened or why it should have happened then but that was a Nimh so, as Charles states, they all have potential. (Quite literally!)

When I get into the conversations at the pond side with someone relating to the likelihood of a boiler 'blowing up' I frequently mention the fact that there is considerably more stored energy in a lead acid battery than there is in a small boiler. Either way, if you release that energy instantaneously, it's going to get interesting.

[GrumpsParticipant @grumps]

Thanks everyone for your hugely helpful – and entertaining – advice. I realise now that my original question was rather uniformed, so thanks everyone for your time and patience. It's much appreciated. I'm pretty sure I don't have a club very near me (I'm near Clare, in Suffolk), so forum members' help is really valuable. In view of the last couple of posts, I'm now wondering what cross section of rubber band would get my proposed project to plane! At least it would rekindle memories of building Keil Kraft Hurricanes in my youth…

[Author]

Posts