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Author Topic: Electric power and drive options?  (Read 708 times)

Gazzalene

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Electric power and drive options?
« on: April 11, 2019, 11:14:02 PM »

I am new to the forum and the hobby and have asked a few questions about first wooden rc boat to get etc.
Having looked at the kits available I have decided to build a basic "speed boat" from a plan.
I was lucky to find a free plan online that looked easy and nice looking.
The boat is " Cobra 21".  28" long by 8.5 broad and says for engines .09 - .21. Its seems a very old plan because the elec example on the plan shows a wiring diagram instead of a esc!

I have no idea of "drive train" options in general. I have seen the prop tube style and rudder just aft AND the style where the prop is behind the transom and the rudder is there also. I do not know the reason to have one rather than the other?

I would like to know what "drive train" I will use before I start the build. The plan shows a old none can style motor with very long prop tube with the prop about 3.5 inchs in front of the transom,with this old low power motor.

How to I start finding out what I need and where its placed. I don would like a fast model,i don't care if brushed or brush less. will be 3 cell lipo. Plan says to buy prop of correct dia and pitch!

The boat is ply skinned so what is the best motor mount style.

Sorry for all the questions but I am that new to the hobby,i have probably posted this in the wrong section!

I am about to post in suitable section about using balsa instead of ply.
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canabus

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Re: Electric power and drive options?
« Reply #1 on: April 12, 2019, 09:57:44 AM »

Hi Gazzalene

I recommend a 28mm brushless motor and depends on how fast you wish to go.
Starting with a 3S 2650mah Lipo battery and a 45Amp ESC.
Prop a 2 blade 32mm 1.4 pitch.
These can be used on a number of motors.
As I use Hobbyking and Banggood I will list a few motors for you.

T2836-1200kv 340Watts 13320 rpm on 3S.
Prop Drive 2836-1400kv 15540 rpm on 3s.
Prop Drive 2836 1800kv 438Watts 19980 rpm on 3S.
Turnigy X2845- 1980kv 792 Watts 21978 rpm on 3S.
The last motor is a beast and will get you into 45 KPH plus speed !!!
As all the motors have the same bolt pattern and shaft changing up in motors is a very easy way to go.

Canabus
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Gazzalene

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Re: Electric power and drive options?
« Reply #2 on: April 12, 2019, 10:43:15 AM »

Canabus, thanks for the info,that certainly helps with the power side.

2 questions please?  I am not going to go crazy fast but I realise the componants will get hot. Aircraft have some airflow but this boat is all inclosed.
what about cooling,i have seen water cooling for motor and esc. When do you need to water cool? I presume the esc is the main issue?

lastly,if you were building this "basic" desisn speedboat of 28" length and 8.5 broad.  how would you choose type of prop? length,dia placement?
is there a calculation of where the prop should sit?

I have been looking for articles with no luck
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canabus

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Re: Electric power and drive options?
« Reply #3 on: April 12, 2019, 01:21:02 PM »

Hi Gazzalene

The prop on my Sea Hornet(25 and 1/4") is 50mm from the stern to the trailing edge of the prop with 10mm clearance from the prop tip to the top of the hull.
I use the Car 45Amp ESC's(part No.HK-45A) because they have an electric fan on top.
You will require the program card(part No.HK PROG-CARD) and I can post you a setup for it.
Motor cooling is not required, but, you can get a bolt on one( Part No. BP22-MMWC or WTCMT01).
Over size props for the  motor and ESC generates over loading and heat.
The brushless motors like to rev freely.
The X props are for under the hull and the S props are surface ones(half the prop out of the water).

The 1800kv will run on 2S at 12960 rpm with the same setup.
Basically the kv times the voltage is your unloaded rpms.
As for a calculation where the prop sits, I have not seen one, but the lower the prop shaft angle the better.
Canabus
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Gazzalene

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Re: Electric power and drive options?
« Reply #4 on: April 12, 2019, 01:38:08 PM »

Canabus, I have inc some pictures so you can see what the plan shows!
The prop to transom in 4inch AND distance from prop to hull is touching!
Is this boat design similar to yours? wondering if I should use your positions?
The plan is old, look at the motor .
Also rudder looks very small.
Do you think I have posted this in the correct section?

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Andyn

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Re: Electric power and drive options?
« Reply #5 on: April 13, 2019, 01:05:00 AM »

The prop to transom in 4inch AND distance from prop to hull is touching!


That's exactly how you want your prop to be. Too often I see models with the propshafts looking like they're about to fall off the bottom of the boat. The more shaft angle you have, the more energy you are wasting trying to lift the boat upwards, which then creates all sorts of funky handling issues.


Obviously its a completely different beast, but race boats often have cutouts dremelled into the bottom of the hull for the prop tips to sit in. Some hulls even have this pre moulded...
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Gazzalene

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Re: Electric power and drive options?
« Reply #6 on: April 13, 2019, 08:05:36 AM »

Hi Andyn, thanks for the info.
is the prop distance from the trans ok?  And would you change the rudder at all?
It says to make from brass sheet . can buy ready made item now,so would you go bigger/longer/wider?
thank you
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canabus

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Re: Electric power and drive options?
« Reply #7 on: April 13, 2019, 08:58:20 AM »

Hi Gazzalene
I would lengthen the prop shaft so as the prop has a 5 to10mm clearance even if you have to move the rudder back a bit.
You do require clearance to remove the prop(between the end of the prop shaft and rudder).
The water pickup can be moved or deleted.
The  rudder looks about the right size, but rack it forward 4 degrees this will keep the bow up in the turns.
The old motor in the plans looks like a brush 540 one(about 90 Watts).

Cool looking boat, I have my eye on a similar style one with an old IC motor.
A solid looking boat with a more pointed bow.
The paint work looks like he used a wire brush to paint with !!!

Canabus
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Gazzalene

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Re: Electric power and drive options?
« Reply #8 on: April 13, 2019, 11:48:32 AM »

Sorry guys, I am not sure on one thing.
is the prop placement on the plans ok regards distance to rear?
its apx 4.5 inch frpm prop to transom
sorry if I have missed the info
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canabus

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Re: Electric power and drive options?
« Reply #9 on: April 13, 2019, 12:22:17 PM »

Hi Gazzalene
The prop placement is totally wrong !!!
Redraft the plans to get the prop 2" from the transom and remove the rudder to suit.

Canabus
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Gazzalene

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Re: Electric power and drive options?
« Reply #10 on: April 13, 2019, 02:51:09 PM »

Hi Canabus, now I am getting there!
2" from the transom, is the angle of the prop on the plan ok?
I realise I will have to find the correct length of propshaft so the motor will be located suitably .
So I will end up with trailing edge of rudder pretty much level with trans?
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Andyn

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Re: Electric power and drive options?
« Reply #11 on: April 13, 2019, 02:51:27 PM »

Prop placement look absolutely correct, as does the rudder. 10mm clearance for a prop is absurd, especially on a hull of this type. 5mm would be convenient if trying different props, but if you're using the motor and prop combo recommended to you then there's no real need for it.


See photo below for a correctly proportioned  setup. Having the prop 2" from the transom would not give you enough room to remove the prop with the rudder anywhere other than hanging off the back of the boat, which would not only give you poor handling but make it very susceptible to being damaged while not in the water.

The thing to remember  is that plan was drawn after being g tested, and is designed  to work as it is

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