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Author Topic: Prop size question  (Read 4458 times)

Norman Castle

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Prop size question
« on: November 22, 2015, 08:10:11 pm »
I'm building a small model for which I've used a Como Drills 457-RE380 motor driving a 30mm 3-blade Raboesch brass prop.

Today's first trial in the bath shows that this thing is considerably overpowered, in that its maximum speed is going to be 2-3 times faster than needed but more importantly it's not as controllable as I'd like it to be at lower throttle settings.

Given than I want to keep the motor and the battery voltage, am I correct in assuming that the obvious thing to do is try a smaller prop, and if so, what would you guys suggest I go for?
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Netleyned

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Re: Prop size question
« Reply #1 on: November 22, 2015, 08:24:06 pm »
Change the motor to a 385.


Ned
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derekwarner

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Re: Prop size question
« Reply #2 on: November 22, 2015, 08:32:14 pm »
Norman.....2 or 3  times faster in the length of a bath must be an observation [based upon %)] rather than the actual speed of the vessel

Purely from a physics point, a number of elements come into play

1. what is the maximum motor current draw
2. does the vessel have a speed controller?
3. reducing the propeller diameter will reduce the total load on the smaller 3 blades, however this will also increase the motor speed via the reduced load....you can make a good estimate here by the reduction in total blade surface area....between the current an optionally smaller propeller
4. the function of controllability/manoeuvrability is relative to the flow of water over the rudder
5. you could even trial a DC power supply with those multi voltage tapping's....6.0v, 7.2v, 9.0v & 12v

There is also someone's rule of thumb that a propeller diameter should not exceed the diameter of the motor casing  :D

Derek
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Shipmate60

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Re: Prop size question
« Reply #3 on: November 22, 2015, 09:46:54 pm »
Keep the original set up but fit a 2 or 3:1 gearbox to the motor.
This will reduce the current consumption and increase the torque.


Bob
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inertia

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Re: Prop size question
« Reply #4 on: November 22, 2015, 11:21:57 pm »
I'm with Ned. A 385 will cost you a fiver and drop straight in with the same mountings and shaft dimensions.
DM
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Norman Castle

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Re: Prop size question
« Reply #5 on: November 23, 2015, 07:53:47 am »
Thank you, gentlemen.  385 motor now ordered as it's the cheapest way forward.

So what's the difference between a 380 and a 385 that accounts for the (hopefully) reduced performance?
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Netleyned

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Re: Prop size question
« Reply #6 on: November 23, 2015, 08:01:49 am »
A 385 is a 5pole motor which gives better slow speed control and it revs at half the speed of a 380.


Ned
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Norman Castle

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Re: Prop size question
« Reply #7 on: November 23, 2015, 08:03:30 am »
Much obliged to you Ned for that very useful information!
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malcolmfrary

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Re: Prop size question
« Reply #8 on: November 23, 2015, 08:28:22 am »
Quote
but more importantly it's not as controllable as I'd like it to be at lower throttle settings.
It might also need a better ESC.  Some (e.g. the "blue brick") make sure of a certain start by starting at about 1/4 throttle, some let you count revolutions by eye.
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Norman Castle

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Re: Prop size question
« Reply #9 on: November 23, 2015, 09:07:22 am »
Quote from: malcolmfrary on November 23, 2015, 08:28:22 am
It might also need a better ESC.  Some (e.g. the "blue brick") make sure of a certain start by starting at about 1/4 throttle, some let you count revolutions by eye.

Mtroniks tio Marine 15 should be OK shouldn't it?
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Netleyned

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Re: Prop size question
« Reply #10 on: November 23, 2015, 09:13:29 am »
All my mTroniks have very good low speed control
Rev counting by eye  :-))


Ned
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inertia

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Re: Prop size question
« Reply #11 on: November 23, 2015, 09:25:07 am »
MTroniks all make the motor squeal at low revs. This can be annoying. A 15A ESC is way OTT as well. This one is plenty big enough - and it growls instead! http://www.action-electronics.co.uk/pdfs/P81.pdf
DM
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malcolmfrary

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Re: Prop size question
« Reply #12 on: November 23, 2015, 11:58:03 am »
The squeal level need not be particularly high depending on the motor mounting but is the result of the state of electronics when the ESC was designed.  Very modern ones running at much higher frequencies might be inaudible to humans but might be the cause of upset dogs at the pondside. Growling types use the transmitter timebase as their reference.  All work equally well, the preferred, or least unwanted, sound being a matter of choice.  If the chosen ESC doesn't have its own BEC built in, there will be a need for a separate one or a receiver battery.
http://www.modelboatmayhem.co.uk/forum/index.php/board,151.0.html has a good set of drawings.
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Norman Castle

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Re: Prop size question
« Reply #13 on: November 23, 2015, 01:12:50 pm »
Quote from: Inertia on November 23, 2015, 09:25:07 am
MTroniks all make the motor squeal at low revs. This can be annoying. A 15A ESC is way OTT as well. This one is plenty big enough - and it growls instead! http://www.action-electronics.co.uk/pdfs/P81.pdf

Oh no doubt the squeal can be annoying, but actually it doesn't bother me.  Yep, a 15A ESC is way OTT, but ...

Mtroniks tio Marine 15 is £25.49 delivered, it's safe to 15A and it's compatible with LiPos.

Action Electronics P81 is £29 plus delivery (if there is a price for that on their site, I couldn't see it), it's safe to 5amp, and it may or may not be compatible with LiPos (the site doesn't say).

How might I be better off using the dearer unit, particularly when the I can use the cheaper one with far bigger motors in future?  And wouldn't I need to buy a BEC to use with it?

Sorry if I'm missing something obvious here ...
 
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inertia

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Re: Prop size question
« Reply #14 on: November 23, 2015, 01:45:32 pm »
Sorry if I'm missing something obvious here ...

Just personal prejudice preference, Norman. Some folk aren't prepared for the high-frequency noise while others, like you, aren't concerned. BTW all ESCs are compatible with LiPo batteries, but there's a few that don't have automatic voltage cut-off.
DM
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