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Author Topic: What brushless?  (Read 1813 times)

stobby256

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What brushless?
« on: October 29, 2008, 07:36:44 PM »

Hi All!

Could anyone advise on a brushless, not costing an arm and a leg?

Boat - Fibreglass hull, 32" long, deep 'V', single prop.  6 Cell Lipo, I'll start with 40mm prop.  With I/C (Im an aircraft fanatic!) you get 25, 36, 40, 46 etc sizes across all makes...

Brushless's are different...  I understand the Kv rating...

Thoughts anyone?

Steve
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Garabaldy

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Re: What brushless?
« Reply #1 on: October 29, 2008, 07:58:27 PM »

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stobby256

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Re: What brushless?
« Reply #2 on: October 29, 2008, 08:42:24 PM »

How could I have forgotten overlander... stalwarts of many a model show!!!

All their motors rated in Watts too!

Cheers   :-))
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Martin [Admin]

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stobby256

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Re: What brushless?
« Reply #4 on: October 30, 2008, 09:34:31 AM »

Many thanks to all...

but...

Exactly what motor...!!??  <*< Wattagge? Kv?
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andrewh

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Re: What brushless?
« Reply #5 on: October 30, 2008, 10:01:51 AM »

Stobby

You have asked the question which we are trying to get a comprehensive answer to!

As an aircraft man you can translate motor sizes 46, 61 etc back into power expected - say 1HP (750 watts) from a good 61.  (THE MANUFACTURERS EXXAGERATE TERRIBLY)  This is a PEAK power, at one point in the rev band - you don't need to copy it to get blinding performance.

So you are looking for a brushless giving say 600Watts
You have a 6-cell LiPo (I look on you with increased respect) that's a nominal 22 Volts* if they are in series and the current for 600W would be 27Amps

*PLEASE don't anyone take exception to this - the voltage quoted for LIPO cells is ENTIRELY arbitrary - corresponding to about the half-discharged condition

Now you havn't said what velocity you are aiming for - is it an out and out speedboat?  a scale model of an offshore racer? or a deep Vee tug :}

To sort out WHICH motor now requires some decisions - are you geared?  Direct drive?  What Prop?  How much room do you have for a prop diameter etc.
Conferssion - I know nothing about boat props so what follows is logical but may be completely wrong

Say you are going direct drive, with a 40mm dia x 62.5mm pitch prop, suitable for the speed you expect
Thats 75mm advanced per rev (but slipping a bit - say 25% (guess) so the actual advance is  62.75 x 75%= 50mm
So every 1000 revs advances 1000 x 50mm , or 50 metres

if the motor did 1000revs/min the boat will cover 50 metres/min or 50/60 metres per sec - nearly 1 metre per second

If you expect to reach say 10 metres/sec boat speed you need the motor to turn the prop at 10,000 revs/min
You are feeding the motor with 22V so you need the version of a motor with KV of 20000/22 rpm/volt or Kv=454

There are a lot of assumptions  in here, and some can only be decided by experience . 
Nonetheless I hope its a little helpful - if another mayhemmer can suggest a starting point for prop and speed, we can select a motor

andrew
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Garabaldy

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Re: What brushless?
« Reply #6 on: October 30, 2008, 10:10:02 AM »

andrewh,

Im not disputing nothing here but wanting to understand the numbers more.  How did you calculate a 40mm dia prop with 62.5mm pitch will advance 75mm?  Sorry if this sounds dumb but i would love to understand the maths behind all this.

Cheers!


G
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andrewh

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Re: What brushless?
« Reply #7 on: October 30, 2008, 10:22:46 AM »

G,

Sorry, brain fade on my part; should be the pitch distance = 62.5mm

Apart from silly errors like that the maths is not hard - the challenging part is finding somewhere to start (for me anyway)
andrew
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stobby256

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Re: What brushless?
« Reply #8 on: October 30, 2008, 12:02:24 PM »

Great - I feel as though Im getting somewhere...

Boat details - deep V mono hull, speedboat built for speed, but not about to break any records. Direct drive, I have as much room for a prop as necassary as the strut will protrude rom the back of the boat. Boat length 32" beam approx 15".  YOur maths checks out OK with me (apart from the 75mm !)  so the Kv rating looks good.

You guessed at a 600W motor - this is the question will that be enough or oo much?  OK - 48" trainer aircraft = 40 size motor, or a 60 if you want to liven it up... 50" 3D aircraft = 120 size engine and that should do anything... and does...

Remaining questions...

1) Lipo rating - go easy on me here, my wallets twitching already!
2) Motor wattage
3) ESC rating - obviously based on (2) with of course an overhead

BAsed upon your answers I'll put my mortgage on hold  {-)

Many thanks to all!
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