Trawler Motor advice

[Terry PlumridgeParticipant @terryplumridge66794]

I am building a Billing “Boulogne Etaples” stern trawler. It is 56cm long with a 20cm beam, and has a 40mm dia 3 bladed prop. I will be using a 6 volt SLA battery. I am not sure what motor (brushed) I should be using. Any help greatly appreciated.

Thanks, Terry.

[Terry PlumridgeParticipant @terryplumridge66794]

[Dave MilbournParticipant @davemilbourn48782]

Terry

This is quite a short fat boat and will need a fair bit of thrust. I would suggest a geared motor for a prop that size on 6 volts. Perhaps the MFA 919D 2.5:1? **LINK**
If the sound of meshing gears isn't to your liking then this one might do the same job with direct drive **LINK** I do have one of these but I haven't tried it in a model yet.. The speed is approx the same @ 6v as the geared 540 motor although the torque will be less – but adequate for the prop.

Dave M

BTW there is a full review of this model on this website at http://www.modelboats.co.uk/news/article/boulogne-etaples/528/  The reviewer uses a 380 motor with reduction pulley drive.

DM

 

Edited By Dave Milbourn on 30/09/2013 11:14:41

[Terry PlumridgeParticipant @terryplumridge66794]

Hi Dave,

Thank you for your reply. The model review was very helpful. I have been given a Multiplex Permax 400 7.2 volt motor, would this be adequate running at 6 volts?

Terry.

[Dave MilbournParticipant @davemilbourn48782]

No. Quoted RPM is 16000 on 7.2v which makes it about 13500 on 6v. That's about three times as fast as you need, so you'd have to reduce it down by at least 3:1 using gears or pulleys and belt.
DM

[Terry PlumridgeParticipant @terryplumridge66794]

Hi Dave.

Just one more, possibly stupid question. In order to reduce the motor speed, could that not be achieved using the Speed Controller?

[Dave MilbournParticipant @davemilbourn48782]

Would you put a Formula One engine into a Fiat 500 and then advise the driver to "go easy on the throttle"? No. Same thing here. A work-boat needs a motor with high torque and low speed to get the available power from that prop (which is designed to run at low speed). You have there a racing motor which has the opposite qualities. The only way to increase the torque and lower the speed at the prop is to gear it down – just like your car. It will not run efficiently at low speeds and you'll end up overheating it and/or flattening the battery very quickly. I'd advise you to save the Permax motor for a smaller and much faster model.

BTW – and this applies to any of my replies to anyone about anything – I am aware that I can come across as a bit of a know-all and sometimes – heaven forfend – even pompous! Forgive my enthusiasm. It's all just my opinion, albeit based on experience with a bit of science chucked in there occasionally. Please feel free to disagree and/or just ignore it if you wish. I really won't mind.

Dave M

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