Water cooling brushless motor

[Chris ConnettParticipant @chrisconnett17898]

Is it required

[Chris ConnettParticipant @chrisconnett17898]

Can you please give some advice if it is necessary to watercool an Inrunner 3500kv brushless motor going into an 18 inch speedboat (kaon). Thank you for your help.

[ashley needhamParticipant @ashleyneedham69188]

Hi Chris and welcome to the forum. 3500 Kv is very high for marine use. Is the motor geared? What battery voltage are/will you be using and is this a surface piercing setup? Very high revs can give rampant cavitation leading to severe loss of propulsive force. Ashley

[Richard SimpsonModerator @richardsimpson88330]

I'm afraid there cannot be a definitive answer to will it need cooling. You will have to try it carefully over time, slowly increasing the load and duration to see what sort of temperature it reaches. The common sense route would dictate that you fit cooling anyway as an insurance against any inadvertent overheating. If you don't need it you haven't lost anything but better than than having to try to retro fit it.

As Ashley suggests however you might find it difficult to get the prop to grip at those revs although, on the plus side that will keep the load and therefore the heat down!!

[Charles OatesParticipant @charlesoates31738]

Unless you are going to load the motor to near its limits, no you won't need cooling. However, pay heed to what Ashley said, that's a lot of revs and will need some careful setting up of the boat to work properly.

[harry smith 1Participant @harrysmith1]

Hi Chris

3500kv is too powerful for a 18" boat.

More like 1800-2000kv on 3S Lipo with a 30-32mm 2 blade prop.

My 25" Sea Hornet has a 28mm B28-57-15L 1900kv (750Watt) on 3S with a 32mm 2 blade prop.

It's been GPS at 45KPH which is dam fast.

21,950 rpm (1900kv times 11,1 volts(3S).

Harry Smith

[Malcolm FraryParticipant @malcolmfrary95515]

The KV number only tells you what the motor would like to do at a given voltage when unloaded. It says nothing about its ability to handle power. There can be many sizes of 3500Kv motors, with a wide range of power handling abilities.

[Chris ConnettParticipant @chrisconnett17898]

Thanks everyone for your help. In summary I think I’ll look for a slower motor and may for safety go for water cooling although I achnowledge it may not be necessary depending on power drawn and run time. I understand that I will need to experiment with the prop. I’m hoping to build a surface piercing prop. I’ll probably fit a 3s lipo. I’m not sure if I need to water cool the esc. Thanks again for your help, Chris

[Dave MilbournParticipant @davemilbourn48782]

Chris

Is the model being built from a plan? If so, what motor/prop combination did the designer use? That's usually a good place to start.
And build a surface-piercing prop?? Really?? Sorry to say it, m'duck, but this all sounds like an awful lot to aim for unless you really know what' you're doing and have some remarkable workshop skills and equipment.

Dave M

[Chris ConnettParticipant @chrisconnett17898]

Question for Harry Smith, did you need to water cool your motor.

[Ray Wood 3Participant @raywood3]

Hi All,

Yes KAON is a Glynn Guest free plan with a surface piercing prop set up, so tried and tested engineering strut & rudder, looks like it was a brushed 540 motor 21" LOA, can't see a date published but has WASP an ic design on the other side. Good luck with the build.

Regards Ray

[ashley needhamParticipant @ashleyneedham69188]

I assume that Chris means build a surface piercing type boat, not an actual propeller?

[Ray Wood 3Participant @raywood3]

Chris is building a G G published design, no rocket science involved.

Regards Ray

[Dave MilbournParticipant @davemilbourn48782]

MFA 540/1 3-pole brushed motor on 7.2v = 9,000 RPM

XYZ 3500kv brushless on 3S LiPo = 38,850 RPM

QED?

DM

[harry smith 1Participant @harrysmith1]

Hi Chris

Have never used watercooling on my brushless motors both inrunner and outrunner .

Ran my Perkasa 37" with a 3639-1100kv at the weekend on a 27 degree day for 3/4 hour, no problems.

Harry

[ashley needhamParticipant @ashleyneedham69188]

As this is a tried and trusted design, perhaps it would be a good stratagem to try a 28mm outrunner giving approx the same, or slightly higher revs than a “normal” 540 , 9000 rpm being a starter then, and of 80 or so watts so as not to overpower the design and have torque difficulties.

If 80 watts does not seem to be much, consider the output of a cooking 540, anything from 30 to 50W I would suggest.

My cat, Hornet, achieved a very respectable speed on a 540 or Graupner eco600.

Ashley

[harry smith 1Participant @harrysmith1]

Hi Chris

I prefer the Prop Drive V2 2836-1800kv 28mm inrunner ( less torque).

Amps =30

unloaded 2S= 12960rpm

3S=19980rpm

4S=26640rpm

Shaft=3.175mm(1/8&#39

Max power 438Watts@4S, lower on 2-3S.

ESC 45Amp (HK-45A) car one, small and only 69 grams.

A number of other motors would bolt up to the same setup which can increase performance.

Two blade props cause less torque problems and keep the prop max to 32mm.

Larger will over load the motor and ESC.

These motors like to rev !!!!

Harry

[Chris ConnettParticipant @chrisconnett17898]

Hi, and thanks for the encouragement. Yes the plan is from the December 1999 issue. Unfortunately it appears that the prop setup as published is no longer available. As I said I am new to boating and I do enjoy a challenge and are researching my options before I settle on a way forward….I agree, the surface piercing prop does seem to be complex given I can’t buy a pre-built unit. All Comments provided here have been very helpful thanks.

[Malcolm FraryParticipant @malcolmfrary95515]

The difference between a surface prop and a submerged on is placement rather than design. Because they are mostly used on fast boats, they tend to use fast boat props.

The big difference is driveline design, needing to put the prop boss at the running waterline while having a vibration free run to wherever the motor needs to be. If reverse is not required, a flex drive can solve a lot of problems. I am sure that there are vendors of the appropriate hardware out there, given the popularity of fast electrics using it, there is probably a lively market for replacement parts.

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