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Author Topic: Propulsion advice please  (Read 895 times)

colh107

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Propulsion advice please
« on: August 02, 2019, 06:11:27 PM »

I'm looking into resurrecting an old craft (35yrs+) - a Modav 'Sea Vixen' same as this
and hoping for helpful suggestions as to a suitable electric motor.
It is, in theory, a planing hull - hard chine with a chine rail on each side. LOA 64cm, beam 22cm, weight (without motor/batteries/RC) 1200g.
Any thoughts please for giving it a reasonable turn of speed? (I'd also be asking for appropriate prop ideas too).
Thanks.
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RST

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Re: Propulsion advice please
« Reply #1 on: August 02, 2019, 09:34:18 PM »

Hi,

Are you missing a picture on the post?  Does it already have a shaft and propeller (diameter and type, e.g no of blades)?
Do you want to stay brushed motor because I guess at some point soon you'll get allot af answers and some will say brushless and Li-Po.  Some folk's definition of a "reasonable" turn of speed is different from others.  I'm old school brushed and Ni-Mh so with the pic and propeller details it might help put something together.


Rich
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colh107

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Re: Propulsion advice please
« Reply #2 on: August 02, 2019, 09:51:50 PM »

Hi Rich, I think 'this' in my msg is a link to a pic? (If I've posted it right). Or https://www.modelboats.co.uk/sites/2/images/member_albums/121166/old%20boat.jpg
The shaft is in there  - 8BA or 6BA thread, iirc. Yes, it's from that far back! I have a two-blade prop on there at the moment, 40mm but I guess that's easily changed as necessary.
Brushed/Brushless is more modern than my antique knowledge, but I would expect brushed would do me. I would probably be looking towards LiPo for the better energy vs. weight (and I have a charger for all types). I can dig into ESC etc once I know what motor/voltage/current I looking to.
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canabus

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Re: Propulsion advice please
« Reply #3 on: August 03, 2019, 05:47:23 AM »

Hi colh107

Rock Crawler Brush motor(540(35mm) specs @ 7.2 volts(2S Lipo).

20turn 18400 rpm

27turn 15000 rpm

35turn 12500 rpm

45turn 9500 rpm

55turn 7700 rpm

65turn 7000 rpm

80turn 5500 rpm

The Raboesch prop chart recommends a maximum rpm for their props.

eg Two blade C Type 40mm is 11000rpm.

Other companies may have other ideas.

Plastic and CNC prop handle more rpm.

So a 35 turn brush motor should be OK.

On Lipo batteries a Turnigy 2650 mah 2S 30C is 136 x 43 x 12mm, connectors 4mm weight 139 grams.

Brush ESC I use the Hobbywing Quicrun 60 Amp 2-3S on all my brush motors.

Setup is by changing links for battery type and forward/reverse with NO BRAKE !!
Just straight forward into reverse.

It's simple and it works !!!

If you wish to change to brushless divide the voltage into the rpm require to get the KV rating.
eg 7.2 divide my is rounded up 1800kv.

Canabus
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colh107

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Re: Propulsion advice please
« Reply #4 on: August 03, 2019, 02:34:23 PM »

Thanks for that Canabus.
Do you have views on what the Quicrun's BEC is like? I see from the manual that it's a switcher not linear. More efficient sure, but they can have their own problems (speaking from many experiences getting switcher-powered electronics through EMC regs!).
Would you advise an external BEC instead?

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colh107

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Re: Propulsion advice please
« Reply #5 on: August 03, 2019, 03:34:04 PM »

You'd recommend I get rid of the existing speed controller then...??  {-) {-)
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colh107

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Re: Propulsion advice please
« Reply #6 on: August 03, 2019, 05:00:20 PM »

I've taken a look at props. I have two, both 2-blade 45mm. One of them is a 'Radio Active' X type - I still have the bag it came in (50p, back then, from the Newcastle Model Shop!)[/size]One is coarser pitch than the other, so I guess I could try both? (I'll post pics if it would help anyone advise). They would also have the advantage that I wouldn't need to upgrade the shaft to M4 thread.
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canabus

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Re: Propulsion advice please
« Reply #7 on: August 04, 2019, 07:35:20 AM »

Hi colh107

I have had no problems with the BEC's on the Quicrun ESC.
I got four and ordered another one last night.

Ran the Hellen fishing boat (14 pounds weight) for 2 hours and the 4000mah 2S Lipo only drop to 7.1 volts on a 3 blade 50mm prop.

The general pitch is 1.4, but you can get a finer 1.2 and a courser 1.6.

Canabus
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