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Author Topic: Prop size on Pilot boat  (Read 904 times)

Hurric123

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Prop size on Pilot boat
« on: October 27, 2018, 10:43:20 am »

Hi, I am putting a bigger brushed motor in my 33" pilot boat to get a bit more speed ( dont fancy brushless in this one).
Currently it has a 500 motor with a tired 6v lead acid batter on a plastic 2 blade 45mm prop.
I want to change the motor for an 800 brushed and fit a 12v lead acid battery.
My question is, am I generally better to put in a larger 2 blade which will just fit. Or am I better to fit a 3 blade plastic prop? I guess they are cheap enough to try both options but an opinion will help.
Also when looking at available plastic props the 3 bladers have a choice of LH or RH, whereas the 2 blade ones seem seem to be multi directional, is this for when using a twin setup?
Thanks and regards
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JimG

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Re: Prop size on Pilot boat
« Reply #1 on: October 27, 2018, 11:31:37 am »

Don't bother with a 3 blade plastic prop. They generally have a poor performance especially in reverse. If you must have a 3 blade prop go metal. If looking for better performance go NiMh or LiPo for your batteries not Lead acid, a 7.2 or 7.4V pack will give more power than a 6V. If you need 12V then a 10 cell NiMh pack wii do this.
Jim
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Hurric123

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Re: Prop size on Pilot boat
« Reply #2 on: October 27, 2018, 02:05:53 pm »

Thanks Jim
Why is 12v lead acid battery not the best choice. I dont mind having the weight and size on board, and I find charging them no problem. Plus they are fairly cheap new. Will it not deliver the amperage for as long as the NIMI or LiPO. I do like to potter about at scale like speed most of the time, with surges of high speed when it suits me.
Brian
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Netleyned

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Re: Prop size on Pilot boat
« Reply #3 on: October 27, 2018, 03:02:18 pm »

Looks like you are upping the motor for more speed then slowing it down by a heavier battery???
Ned
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Hurric123

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Re: Prop size on Pilot boat
« Reply #4 on: October 27, 2018, 06:10:09 pm »

Hi,
The old 6v 10ah lead acid battery weighs 2.05kg. A 12v 7ah lead acid battery weighs 2.54kg so there is a weight penalty.
But I think I can neutralize the extra weight by moving it forward a couple of inches and taking a bit of lead from the bow area.
It is  a 34" hull so the extra battery weight is not an issue and these 12v batteries are less than 12 .
Brian
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JimG

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Re: Prop size on Pilot boat
« Reply #5 on: October 27, 2018, 07:03:55 pm »

Brian if you are mostly running slow (scale speed) then lead acid will do. Your original post spoke about increasing speed and the lead acid is not so much use for this. Increased speed generally means higher current and lead acid cells have a higher internal resistance so will drop voltage at higher currents. Lead acid chemistry means that the voltage will constantly drop as capacity is used (this allows you to get an idea of the amount of charge left by measuring voltage.) NiMhs have a very different voltage profile, they have a small but rapid drop from fully charged but will maintain their running voltage until they drop rapidly when near fully discharged.For the type of battery useage I normally have my preference is therefore for NIMh or nowadays LiPo as I generally try to keep weight down.
Jim
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g4yvm

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Re: Prop size on Pilot boat
« Reply #6 on: October 28, 2018, 07:46:03 am »

There seem to be so many advantages to NOT using Pb I cant see why you wiuld.  Ok so the weight is only an extra half kilo, but bent over retreiving the model, a pound's a pound!


With NiMH cells you have the option of adding cells cell by cell...by go from 6v to 12v when you could go via 8, 9, 10 or even 14 or so and tune your set up to your tastes. 


I can think of more advantages, but Im not here to persuade you, just offer ideas.


David
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roycv

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Re: Prop size on Pilot boat
« Reply #7 on: October 28, 2018, 08:58:11 am »

Hi all, there is a difference in how the amp hour capacity is measured between Lead Acid batteries (except cylindrical) and NiMh etc. 
The SLA is measured at the disharge rate of 20 hours where as the others are measured at the 5 hour rate.

The effect of this is that in theory an SLA at 10 Ahrs wil be able to deiiver 0.5 amps for 20 hours, ( 0,5 x 20) higher current and it heats up and you lose stored energy.

A NIMh battery at 10 Ahrs capacity will deliver 2 amps for 5 hours.  ( 2 x 5 ). As has been said the internal resistance of the batteries is different and this determines the maximum power that can be used.  N.B. not maximum efficiency.

If your batteries run hot that is energy not going to your motor but coming out as heat.  This is a case where you are not comparing like with like.

Hope this helps, theory can be boring but there is usually a reason why things happen.
regards

Roy
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Hurric123

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Re: Prop size on Pilot boat
« Reply #8 on: October 28, 2018, 09:40:45 am »

I am certainly willing to look  into NIMh. Maybe I need to move on?
What is the number of cells I will need and the current capacity? Where can I buy these packs made up to suit the 800 brushed motor.
I have various chargers that will work but I have never had large NIMh packs.
Regards
Brian

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roycv

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Re: Prop size on Pilot boat
« Reply #9 on: October 28, 2018, 10:52:23 am »

Hi, NiMh batteries are 1.2 volts each so you get packs of 6 to give 7.2 volts. 

If you buy LiPo then these cells are 3.7 volts so 2 of them come together as a 7.4 volt, so very similar but the Lipo battery weighs less.  Cells are individual units when you put 2 or more together they become a battery.

Each needs its own charger type, you want to buy an intelligent charger this will charge the battery and switch down to trickle when charging is complete.
If you check on the motor there will be a recommended voltage and current for the motor.  The current used will be decided by the propellor and how big it is.  Always check when the prop is in the water, i.e. when under load.

Specification for the MFA 800.

Approx motor diameter 50mm by 70mm long.
Operating Voltage 12 volts.
 Current approx. 5.28A at max efficiency.
RPM at 12.0v - 4289 at max efficiency.
Weight 595g (approx) Shaft Diameter - 6.35mm (1/4")

If you fit a prop that on load is taking about 5 amps on 12 volts then it is running efficiently.  However this is 60 watts quite a lot of power, so how much do you need?

You can buy batteries made up for 12 volts or LiPo 11.1 volts. without problem.
Going back over your previous threads you will go much faster and for longer at a cost!

An esc that can deliver 20 amps (gives you some leeway) and make sure it will work at the voltage you want, many just run on 7.2 volts.

I suggest you could have a good turn of speed from 40 watts power  that is 12 volts at 3.5 amps or 7.2 volts at just over 5 amps.

The prop size will determine the current taken, so a bit of a balancing act.  If you used 12 volts at 3.5 amps and use 5000Mah NiMi battery then you will get an hour plus run time.

Good luck

Roy




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g4yvm

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Re: Prop size on Pilot boat
« Reply #10 on: October 28, 2018, 11:07:17 am »

If you can solder make your own packs.  Buy tagged cells.  Id go for 3000mah cells maybe?  Like these...


https://www.ebay.co.uk/itm/1-2V-NiMH-SubC-SC-Single-Cells-2200-5000mAh-for-DIY-custom-battery-packs/361612590574?hash=item5431ca3dee:m:myE_OHdShVcY-bMx0Jl6jJw



David
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