Old Motor Torpedo Boat

[John O’CParticipant @johnoc]

Hi Guys

Thinking about this motor: Turnigy AquaStar 3974-2200KV water cooled brushless motor.

Any comments welcome and/or advice on controller etc'

Cheers

J

[Dave MilbournParticipant @davemilbourn48782]

That's an inrunner, John. These are generally too fast for anything except a genuine fast-electric hull and a surface drive. If you think about it, 18.5v x 2200kv = a lot more RPM than an ED Otter – by a factor of at least five! As I said, I've no experience of large brushless motors but I think an outrunner with a kv of less than 1000 would be a much better bet for this model with its fully-submerged prop.

Dave M

[John O’CParticipant @johnoc]

Hi Dave

How do you cool an out runner motor or does the spinning cool it sufficiently?

John

[Dodgy Geezer 1Participant @dodgygeezer1]

I think I mentioned earlier that outrunners can have cooled mounts.

Allowing some air movement past the motor is always a good idea, and simply running an engine below its maximum voltage will mean that it doesn’t get too hot in the first place…

Edited By Dodgy Geezer on 15/11/2016 07:51:40

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by Dave Milbourn on 14/11/2016 16:03:57:

That's an inrunner, John. These are generally too fast for anything except a genuine fast-electric hull and a surface drive. If you think about it, 18.5v x 2200kv = a lot more RPM than an ED Otter – by a factor of at least five! As I said, I've no experience of large brushless motors but I think an outrunner with a kv of less than 1000 would be a much better bet for this model with its fully-submerged prop.

Dave M

Well, lets stick my neck out and try to provide some actual data, even though I have no experience in the field! I also have not used a large brushless motor, but we should be able to get some ball-park figures by considering power requirements.

The big Aerokits used to be suitable for motors of around 10cc. I think the 3.5cc motor installed would have made it plane, but would not have had an 'exciting' performance. I used to use a Merco 35 in a Sea Commander, and I could have done with a bit more poke…

Now the 10ccs used to be able to put out a bit more than 1Hp – perhaps about 1Kw? And if we guess that you will be using 12-15V, that equates to about 100Amp (in round figures). That would probably be able to run quite fast, so running it less fast would minimise the cooling requirements.

Now I think that above 50A or so you tend to hit a 'price point'. Below this there are a lot of motors at quite cheap prices – above this there are fewer motors, and many become very expensive. 50A-60A is meant to be the equivalent of the old 10cc (0.60cuin) motors – See, for instance, the Turnigy G60 range **LINK** and these are 60A. Of course, these are expected to run in a cooling draught in front of an aeroplane.

So I would start off by looking for a brushless motor perhaps 50mm in diameter – between 50-100 Amps. Down at the lower end of the Amp range I would be using higher voltage batteries and adding cooling – by about 70Amps I would not be worrying about this so much.

I'm happy to be corrected if anyone knows better – but those figures seem to make sense to me…

How does that sound, DM?

[Malcolm FraryParticipant @malcolmfrary95515]

With any motor, the bit that gets heated is the bit with the wire wrapped round it.

In a brushed motor, its the rotating bit which also heats the outer case which is what gets cooled. This works quite well when needed.

In a brushless inrunner motor, the wound wire is part of the case which can be cooled in the same way as a brushed motor but is probably more effective. An outrunner has the heating elements coils attached to the backplate, so cooling the backplate is just as effective.

[Dodgy Geezer 1Participant @dodgygeezer1]

Brushed motors also have a 'hot-spot' at the brushes. I think resistance here is the limiting factor for these motors, and I can recall that the big brushed high-performance marine motors of the 60s (not the Taycols!) had water-cooled brushgear…

[John O’CParticipant @johnoc]

Dodgy

Firstly thanks for all the info' and I have bought the motor. The Turnigy G60 Brushless Outrunner 300kv.

Saved £24 by going with Hobby King and not Amazon!

Just about to buy the TURNIGY TRUST 70A SBEC Brushless Speed Controller unless somebody says "Don't do that"

Thanks again

J

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by John O'C on 15/11/2016 12:17:37:

Dodgy

Firstly thanks for all the info' and I have bought the motor. The Turnigy G60 Brushless Outrunner 300kv.

Saved £24 by going with Hobby King and not Amazon!

Just about to buy the TURNIGY TRUST 70A SBEC Brushless Speed Controller unless somebody says "Don't do that"

Thanks again

J

 

That motor is the 'equivalent' of a 10cc motor, so it should have the 'oomph' to push the boat well.

If the prices are not too great I would go for a bit more amps on the esc – perhaps their 90A or 120A. Overspeccing the esc is safer, and means it will run cooler.

What voltage are you thinking of? The G60 seems to like around 20v to get all the power out…

 

Edited By Dodgy Geezer on 15/11/2016 12:47:21

[John O’CParticipant @johnoc]

How about this one? Turnigy AquaStar 90A Water Cooled ESC

J

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by John O'C on 15/11/2016 13:52:50:

How about this one? Turnigy AquaStar 90A Water Cooled ESC

J

Yes – I would prefer that level of overspecification. Picking an ESC that is close to the maximum drain on the motor is not ideal. But DM is really the person to advise.

The actual amp draw will depend on the voltage and the prop you use (assuming that your battery can deliver as many amps as the motor asks for!).. If you use batteries which cannot output the amps you want, you will not get the right performance. Lipos are the obvious choice here – but they don't come cheap! I wonder if Component Shop has anything suitable – LiFe seems to be possible, but NiMH would need parallel connections…

And of course you will need to have good connectors and thick wires to get the power into the motor without obstruction… most people use Deans connectors or those gold-plated torpedo fittings.

It's probably worth investing in a cheap WattMeter – something like this will let you know what is going on.. **LINK**

[Dave MilbournParticipant @davemilbourn48782]

As I said earlier, I've no experience with these larger brushless motors. My various models all run on 28mm dia motors on 3S LiPos with a 30A ESC. Even after ten minutes of thrashing around at full chat, nothing gets remotely hot. The no load current is less than 500ma. Because of the lack of a heat problem I've not bothered testing them with the on-board wattmeter yet.

I can't fault DG's maths and I would agree 100% about the watt-meter, gold-plated bullet connectors and using LiPo's. NiMH are also in the frame but avoid SLA batteries. They are wrong for this type of on many different levels! As regards the rating of the speed controller, if price isn't a consideration then buy the higher-rated one. You may need it for something bigger later, whereas if you buy the lower-rated one it will get warmer and will be of less use later e.g. for a more powerful motor and/or larger boat.

Dave M

[ashley needhamParticipant @ashleyneedham69188]

It is a minefield this. The hobbyking website gives a typical 17A current draw on the suggested air-prop sizes suited for the motor. It does say 60A max for 15 secs only…..

​Have not re-read all of thread but on 18.5V (which was mentioned recently) this only gives 5.500 revs, probably sufficient for a quite big prop….but

Could you reiterate the battery voltage you intend to use, and the likely prop size?

​Ashley

Edited By ashley needham on 15/11/2016 15:50:09

[Dave MilbournParticipant @davemilbourn48782]

Yes – I would have gone for the 600kv version, too.

DM

[John O’CParticipant @johnoc]

Glad I cancelled the order now!

Where's the rubber bands?

J

[Dodgy Geezer 1Participant @dodgygeezer1]

I wouldn't dignify that post with the term 'maths' – it's just a ball-park estimation of orders of magnitude…

You can do proper engineering if you have some proper data to build on – and the problem for model boats is that we don't have that. We have rules of thumb, like don't prop a brushless motor with a prop bigger than it's own diameter, but in reality the props we use have so many different pitches and different working conditions that anything can only be a guess.

My small brushless experience agrees with DMs – the motors/ESCs rarely get warm, let alone hot. When you get to tuning the boat for 'best power' the following points may help:

1 – can your batteries pump enough current out? The spec may be ok, but if you have a dodgy one performance will drop

2 – does your wiring harness carry the power adequately? A bit of thin wire or a poor contact stops the power effectively. Don't include a switch on the powerline…

3 – have you got the best prop? Different props make an amazing difference. Pick up a few cheap ones and swap them around. You should be able to see the difference, and certainly measure it on an on-board watt meter. Cheap Chinese wattmeters are readily available: **LINK**

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by Dave Milbourn on 15/11/2016 15:49:39:

Yes – I would have gone for the 600kv version, too.

DM

Agreed. Most of my brushless are KV900 to KV1400, running on 7.2v. That means the lower voltage batteries are cheaper, and if you want to vary power it's easy to up the voltage. Going up to 35v or so gets expensive…

[ashley needhamParticipant @ashleyneedham69188]

I have some 1200-1400Kv 28mm outrunners rated between 50-100W depending on battery voltage, and they turn props up to 40mm ok on 2s and perhaps s40 or 35mm props on 3s (more voltage, smaller the prop is the rule here).

​I have a 1300Kv 35mm motor of poss 100+w and this gets fairly hot on an x50 prop, but mainly as this boat has a small hull volume and no airflow through. With airflow I would expect it to get quite warm, but that is ok.

​If you have no battery I would say a 3s or 12V Nimh pack is an economic choice and for a prop 40-50mm diameter (the likely sort of size for a single shaft on this boat) then something like 8000+rpm or a bit more would be needed.

The g60 and others similar make 400…500 + watts……and on 4s or 5s so you can see the difference.

​Ashley

Edited By ashley needham on 15/11/2016 17:17:43

[John O’CParticipant @johnoc]

Wow

I'm not sure if the motor is being delivered or not as my cancellation my have crossed the robotic despatch!!!

If I am getting the gist of all of this there is no need to go for water cooling providing the motor and esc are oversized and run at a lower voltage, say the 11.1 volt as per Dave's earlier post.

If they have despatched the motor is it worth keeping or should I definitely return it for a 600kV version.

Sorry for all the questions but you guys lose me with electrics.

Plumbing and heating I understand so may fit a bathroom and a condensing boiler instead!!!!!

Don't mention the controls for that!!!!!!!!

J

[Dave MilbournParticipant @davemilbourn48782]

It depends on whether or not you can afford to buy/justify buying another motor. A 300kv motor would suit a single-screw workboat such as a tug, fishing boat or coaster, so if you think you might build something like that in future then hang onto it. I have drawers full of stuff which I've bought and kept "just in case".

The final answer must be yours – some decisions are beyond simple advice.

Dave M

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by John O'C on 15/11/2016 18:07:40:

Wow

I'm not sure if the motor is being delivered or not as my cancellation my have crossed the robotic despatch!!!

If I am getting the gist of all of this there is no need to go for water cooling providing the motor and esc are oversized and run at a lower voltage, say the 11.1 volt as per Dave's earlier post.

If they have despatched the motor is it worth keeping or should I definitely return it for a 600kV version.

Sorry for all the questions but you guys lose me with electrics.

Plumbing and heating I understand so may fit a bathroom and a condensing boiler instead!!!!!

Don't mention the controls for that!!!!!!!!

J

 

Don't mention a boiler! I have been trying to persuade DM to take up amateur plumbing recently…

 

The first point to make is that we are talking 'probably'. Without real experience to go on we can't guarantee performance.

DM is looking to run the motor at a lower voltage level – which would certainly cut the cooling requirement – and the battery cost. High voltage Lipos can go for £100 or more! You could certainly run your boat fast on a 300-odd KV motor. Here is a Sea Queen – similar in size to your boat, using a 50mm brushless outrunner.

This one is 380Kv, running on 25v, giving around 10krevs. It's spinning a 65mm 4-blade prop. I'm guessing it's drawing around 50A – it uses an 80A ESC – and I suspect that it needs cooling. With a 600KV motor you could get the same effect at 15v.

**LINK**

 

With that sized prop under that power I would expect some torque reaction, and I think I can see the boat tilting over to the right under power. The Sea Queen is quite beamy – you might be well advised to  go for a slightly smaller prop… 

 

Edited By Dodgy Geezer on 15/11/2016 23:34:20

[ashley needhamParticipant @ashleyneedham69188]

DG that is a wonderful clip . I wonder how many people play with their boats on such waters? It is what those hulls were made for!

The point about higher voltage batteries is well made, but you could simply run two 3s in series to give 24v.

The boat in the video is certainly going fast enough.

Ashley

[Dave MilbournParticipant @davemilbourn48782]

The point about higher voltage batteries is well made, but you could simply run two 3s in series to give 24v.

The point about using two batteries is that first you have to BUY two batteries, Ashley!

I'm sorry to say it but the model in the video appears grossly overpowered. To my mind Sea Queen is a river cruiser and was never intended to scream across a choppy lake like that, with just the rear six inches of hull in the water. There are much better-shaped boats around for that sort of thing.

Dave M

[Dodgy Geezer 1Participant @dodgygeezer1]

Posted by Dave Milbourn on 16/11/2016 09:30:34:

The point about higher voltage batteries is well made, but you could simply run two 3s in series to give 24v.

The point about using two batteries is that first you have to BUY two batteries, Ashley!

I'm sorry to say it but the model in the video appears grossly overpowered. To my mind Sea Queen is a river cruiser and was never intended to scream across a choppy lake like that, with just the rear six inches of hull in the water. There are much better-shaped boats around for that sort of thing.

Dave M

Quite right – that is well overpowered. You can see the slamming in particular. When I first saw it I thought that the hull would perform better with underbody strakes, and, of course, as DM says, it's the wrong hull for that job anyway. A deep Vee like a Swordsman or Surfury would look a lot better – and would keep the prop in the water. On the fast runs the flat underbody aft simply lets air through to destroy the drive. I wonder if it does the same in flat conditions…?

But it shows what you can do with a 380KV motor if you're prepared to buy 25v worth of high-discharge batteries…

[Gareth JonesParticipant @garethjones79649]

I dont know whether this will help or just confuse everyone but I have extensively measured the performance of my PT boat so I have some numbers to give guidance.

My model is a 1:24 scale ELCO boat, 40 inches long and weighs about 13 lb. It is powered by a pair of Graupner Speed 7307 motors driving scale sized 3 bladed brass propellers, 30 mm diameter. The battery is a 3S LIPO of 6000 mah capacity.

Performance of the boat is excellent, its as fast as I can cope with and planes beautifully, although its probably a bit on the heavy side.

At max power each motor is drawing 24 amps at 11.9 volts (285 watts) and the prop speed is 11400 rpm with the boat static in the water. If I were to start again I would fit a pair of brushless motors and based on the measured data I would suggest the following size. For good efficiency, the motor needs to run under load at around 80% of the free running speed. This suggests the free running speed will be 11400 divided by 0.8 which is 14250. Taking the battery voltage as 11.9 suggests the motor KV will be 14250 divided by 12.9 which is 1100KV.

I have found that the power rating for most brushless motors is biased towards an installation behind a big cooling fan, i.e. an aircraft propeller. You need to be fairly conservative in specifying the required power and I usually look for something capable of giving about 3 times what you need in the boat. For this application I think my starting point would be something like a Turnigy D3542/5 which is 1250 KV and capable of 665 watts.

If you are prepared to sacrifice the attention to scale detail a bigger, slower prop would probably be more efficient. I have found 11400 rpm in the PT boat to be prone to vibration problems unless you are very careful with alignment and use a good universal joint.

Gareth

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