help with motor selection?

[Krys JoyParticipant @krysjoy]

First of all hello to everyone on here, I have just joined this forum

I am wondering if anyone can help me, I am currently in the middle of building a 46" model of the Vosper RTTL, the project is well on its way and I have got to the point now where I need to start thinking about purchasing some suitable motors. I believe these boats had two engines so I am wanting to use two motors to make it as realistic as possible. Problem is I am unsure of the setup on twin motored boats as I only have boats with single motors. i.e what size motors should I be looking for to get a fast yet scale speed for the size of my boat? Also do I need to have 2 speed controllers or can they both run off one? and lastly would I be best with a brushed motor or brushless?

Hope someone can advise me.

Krystian

[Krys JoyParticipant @krysjoy]

[Gareth JonesParticipant @garethjones79649]

Based on my experience with a slightly smaller Elco PT boat model (1:24 scale, 40 inches long) I would suggest a pair of Graupner Speed 700 motors part number 7307, driving 30 mm 3 bladed props. I used a pair of 9.6 volt NiMh cells but a better solution, if you could stretch to the expense. would be a pair of 11.1 volt LiPo cells. On the 9.6 volt cells in my model, each motor will take about 22 amps. Personally I would use independent speed controllers, one for each motor. Whichever type of battery you chose, get the largest capacity you can afford (unless you are space limited in the model)

If you go down the NiMh route I suggest you use a separate battery to supply the receiver, rather than a BEC supply from one of the speed controllers. I found that at high powers the main/motor battery voltage can be pulled down below the receiver operating voltage and you may lose control.

If you go down the LiPo route you either need LiPo compatible speed controllers or low voltage warning systems to ensure you dont take the battery voltage below 3 volts per cell and write off a pair of pretty expensive batteries.

A brushless set up would probably be more efficient but unfortunately I dont have enough experience to make a recommendation what to use.

Edited By Gareth Jones on 12/07/2012 22:05:33

[Dave MilbournParticipant @davemilbourn48782]

I would recommend this set-up BUT with Graupner Speed 700BB Turbo 12v motors (Part # G6317) and a 12v NiMH or 11.1v 3S LiPo pack. Avoid Sealed Lead-Acid batteries – they are far too heavy for a planing model. 40-45mm brass props (3 blade) would be a good size to start with. **LINK**

Dave M

Edited By Dave Milbourn on 13/07/2012 08:06:53

Edited By Dave Milbourn on 13/07/2012 08:07:17

Edited By Dave Milbourn on 13/07/2012 08:07:50

[ashley needhamParticipant @ashleyneedham69188]

As an alternative…Pair of MFA 850 motors and X50mm props on 12v Nimh battery.

yes yes I know

Ashley

[Francisco MolinerParticipant @franciscomoliner82444]

you can use a 400 brushless motor, you dont need two motors, brushless is better than electric motors

[Dave MilbournParticipant @davemilbourn48782]

Posted by Francisco Moliner on 29/07/2012 16:49:31:

you can use a 400 brushless motor,

400 what? Watts? Revs per Volt? Amps continuous current? Please would you clarify what you mean? I hope it's not a Speed 400 equivalent………

This is a BIG model and would need a very powerful (aka current-hungry) brushless motor and equally hefty LiPo cells to make it 'sing'. They are certainly more efficient than most brushed motors but they still obey the basic principles of physics i.e. you only get out as an absolute maximum the same energy that you put in. At 18v a 1KW motor is sucking 55A out of your LiPo pack; I hope you like quick sailing sessions!

I'd also suggest that two motors is probably the easiest way of turning two props.

Dave M

[Paul TParticipant @pault84577]

Hello Krystian

Have you considered using petrol power?

Paul

[Francisco MolinerParticipant @franciscomoliner82444]

no, 400 is the size of the motor…. but…. you can need a 540 outrunner motor, only one, with gears you can move two propellers, the outrunner motors are very poweful!!

[Malcolm FraryParticipant @malcolmfrary95515]

"with gears you can move two propellers, the outrunner motors are very poweful!!"

I take it that this is a gearbox that allows for one of the shafts to reverse, and there would be some means of controlling this, to go towards the same level of control as 2 motors.

For simplicity, two direct drive motors, each with its own ESC. Again, Dave's suggestion of 700BB Turbo motors is valid for pushing that size of boat along at the speed wanted – I doubt that there is a 400 size equivalent brushless motor (incidentally, brushless motors are electric) than can do the same job as 2 of 700BBs, regardless of any theoretical increase in efficiency.

[ashley needhamParticipant @ashleyneedham69188]

Krys. Proven performance… twin 700 motors, 12v NiMh battery, twin esc (to allevate overheating) (or possibly 850 motors)…. job done. This may not provide the absolutly best performance, but you are on your own if going on the brushless route, or gearing shafts together, or using Lipo batteries.

Francisco. You are dead right. 400 is the size of A motor.. a 400 motor. it describes the physical can diameter only, and not the electrical characteristics. Similarly then a 500 motor… covers everything with a 5 at the front…. 500.. speed 500..purple power 540, standard 540…540/1….. 545…550. (as I understand it)

Brushless motors described as "400 replacements" are inevitably more powerful, and completely different in size. The only way you can even start to compare brushless with brushed is to describe the electrical characteristics.

Ashley

[Francisco MolinerParticipant @franciscomoliner82444]

for me… simply… BRUSHLESS MOTORS RULES!!

[ashley needhamParticipant @ashleyneedham69188]

Francisco. No one here would dispute the efficiency or other benefits of a brushless motor. The problem we have is with equating what we already have and know about, to what is on the brushless table.

Too many phrases like "brushless 400" are bandied about that are meaningless in themselves without a description of the capabilities of the motor.

Ashley

[shipwrightParticipant @shipwright]

Francisco – it might be worth re-looking at the thread started by Paul entitled "Brushless Motors" on 26/7/2012 at 08:11:59 (Paul was considering the feasibility of replacing i/c engines with brushless motors) The conclusion was that if you have a large model boat that needs a lot of power to propel it through the water at speed (Paul quoted 5bhp = 3.8 kWatts for his i/c engine) then there really is no practical alternative to internal combustion engine (i/c). On the other hand if less power is acceptable – say a few hundred watts – and duration on the water of a few minutes is acceptable – then brushless will meet the need. This is a situation that is familiar to people that opt for an electric car. In the latter example a compromise is reached by using hybrid technology (small i/c combined with very large capacity Li batteries). At the end of the day it is "horses for courses".

Ian

[dave garrettParticipant @davegarrett24229]

@Paul T,

most of the smaller brushless motors are just as fast as the if not faster and better for the enviroment, plus a lot of club waters here in the uk do not let you use nitro or gas engines because of the gunk that they throw out so i think that nitro and gas boats are going to become a thing of the past very soon.

the best thing to do would be to build your own lake in your garden the you can run what you like lol

[Chris EParticipant @chrise]

I rather hope that he has been running the model for the last 10 years.

[Chris FellowsParticipant @chrisfellows72943]

Yes, and I think brushless motors and batteries have moved on a touch!

[Dave Cooper 6Participant @davecooper6]

I've got a feeling the RTTL's had three petrol engines running on 100 octane aviation fuel. (Same as the Air-Sea Rescue launches my dad used to drive ?).

Perkins diesels were also used (sans goveners), again, 3 of I believe by some yards.

If you're going for realism, something to consider perhaps…

Dave

[Dave Cooper 6Participant @davecooper6]

Just remembered – my RAF launch has a stepped hull built in a shallow "V" from bow to step. Flat 'floor' aft of the step leading to a single prop /rudder. It leans in towards the centre-of-turn quite nicely. No noticeable torque lists anywhere in the throttle range – even 'flat-out' and, it tracks straight as an arrow.

I don't really know the hydrodynamic reasons for this, but, I suspect the 'v' in the step has something to do with it. Incidentally, the hull as originally built, was 'flat floor' bow to stern. The step was built on afterwards as a sort of 'double hull' section (I think there may be a photo or two in the album).

So, I think this is something that you could retro-fit as a sort of experiment…..I just love to tinker ! Others are more purist….

Dave C

ps Album is called "Dave's RAF Launch"

Edited By Dave Cooper 6 on 13/12/2022 11:19:22

[JANA FRANCESParticipant @janafrances58379]

Gentlemen, this is getting confusing.

I'm in the process of acquiring a 1:24 model of a Thorneycroft ASR launch in need of some work and engines.

Since my dad used to coxwain one of these, I'm aware of the engine options they had, and that the hull was designed to skim over the water. Hence lie the OP I would like her to growl, sit up on her tail and positively fly around the lake! Two engines minimum (and probably maximum, considering space and weight).

I appreciate the environmental issues of running petrol or diesel engines on waterways, but a) full-size boats still run on IC engines, b) I'm not too bothered what club regs say, I'm not much of a joiner, and c) it will be transported to water in a 2.5 turbodiesel Range Rover, 32 years old, so it's polluting but also paid its environmental dues.

SO, original question: growl, hull up…engines please?

[Colin BishopModerator @colinbishop34627]

Hi Jana,

I’m afraid you will find that most publically accessible model boating waters do not permit i/c engine boats on the basis of noise, pollution and disturbance of wildlife. Whilst you may not agree with this, it is still the reality. It isn’t ‘Club Rules’ as clubs normally use waters under licence from the landowner, often the local authority, who impose conditions. Publically owned free sailing waters are subject to bylaws which will usually rule out i/c craft. This has all been the case for many years now.

It is of course possible to run models on private land with the permission of the landowner. The Model Power Boat Association supports competitive i/c boating and there are venues where this can take place under controlled conditions but this is obviously not what you are looking for.

On the plus side, if you want i/c performance for your boat then brushless electric motors with LiPo batteries can match the old i/c engines and give you planing performance. Our members may have some suggestions in this respect it that appeals to you.

Colin

[Ray Wood 3Participant @raywood3]

Hi Jana,

Assuming you have somewhere to run your boat, which I'm guessing is around 36"- 38" Long at 1.24th scale ?

Westonuk in Kent still support the IC community with marine glow motors.

I think a pair of West 12 Marine motors would be suitable @ £220 each. = £440

Keeping 1 glow motor going for any period of time is interesting to say the least, keeping 2 going would be a bloomin miracle

They won't produce a growl either they will scream.@ 15,000rpm

Go electric as Colin suggests and buy a sound system that growls !!

Good Luck

Regards Ray

[Kevin BeallParticipant @kevinbeall91525]

My son raced RC Touring cars for a few years at club level, I can't drive 10 yards without crashing so was his "pit crew", and they are all running brushless/Lipo due to the power to weight ratio. You can find the quoted power output for most motors in watts for a given number of cells, example 140w for 2-4 cells, so take this as the max power quoted 140w is using 4 cells and half the output for 2 cells and so on.

I have a 24" Police launch with a 600 ECO brushed motor, using 6 Nimh cells 7.2v and that pulls about 6 amps so maybe 50 watts on a good day, perhaps 70% efficient and you are looking at 35 watts at the shaft.

I'm building a 36" 200 series Seaplane Tender, so a similar hull to the start of the post. I'm looking at a pair of Tornado Thumper V3 2836/08 1120KV motors quoted as 270 watts with 4 cell Lipo, I'll be using a 2 cell battery so 135 watts each…Plenty I hope! Perhaps it will turn from a Tender to a Seaplane itself!

The real benefit with brushless is that they are able to just add another cell to the battery pack if you want to up the performance, the limit is just how big your battery pack is.

[Kevin BeallParticipant @kevinbeall91525]

Also, as most know, one ESC per brushless motor. Non sensor motors will probably run with a shared ESC but not very well as they are basically AC motors and they will be out of phase.

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