# newbie question-drag and power calculation

Discussion in 'Boat Design' started by boatofhoha, Nov 18, 2013.

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### boatofhohaJunior Member

Hey thanks for the response Bandit.

I have two followup questions:

1.Do you get some efficiancy number from manufacter of the propeller, or is it something tha need to be calculated?

2.how I decide what is optimal rotation speed of the motor ?

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### keroseneSenior Member

read the human powered thread or other prop fitting threads. Really.

Props do not have a efficiency number you can just plug in - it is not fixed value it depends on speed.

3. ### Baltic BanditPrevious Member

What Kerosene said. I think you are safe with a 70% efficiency number. You can instrument it if you need more precise numbers

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### boatofhohaJunior Member

Hey,
thanks for all your help understanding the basics, I took time to learn abit too and I have some basic plan of things I need to do to make the final choice of motor and propeller, but few points I am not fully understand yet.

Her is how I understand it:
thrust needed+speed boat=w prop/eff=w motor=>choose motoer=>n speed motor=> n speed prop.
from other side choose some diameter =>use Ct/J graph to get effecianc and pitch=> choose propeller and go back to another iteration on motor.

questions:
1. is there some formulas for low spee electric motors to get needed rpm from power and prop diameter?

2. generally my process is correct? or I have some mistaked or things I missed?

3.where I can see the Ct/J graphs, some recomended site or book if you know.

thanks for anyhelp.

5. ### Baltic BanditPrevious Member

well speed of boat is intimately linked to thrust needed.

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### boatofhohaJunior Member

Yes I understand,

just stuck in the choosing process, I have some thrust, speed I need to generate, and diameter assamption, but don't know how to get all others parameters.

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### boatofhohaJunior Member

just doing some chart with your info,I know its was while ago, but two questions:
1.6.4*0.76 gives you the thrust (m/s*/kg/s=newton)?
2. its kg*m/s^2 why you deal wioth it as kg-m/s
thanks

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### rwatsonSenior Member

The chart has already been done - its included in previous posts.

Did you pick up the error in "2piR = Area so 1.9" x pi x 3" "

I cannot understand your questions myself - what is it that puzzles you ?

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### boatofhohaJunior Member

Hey I managed to get results, but now where I can choose the right motor and propeller for me?

motor power:70-100 watt / 1500 rpm
propeller: diameter=8 cm pitch:6.4 cm

thanks

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### rwatsonSenior Member

Get the motor first, as the props will have to fit the shaft
This site seemed useful, as it has a lot of info on a wide range of electric motors and some great advice on their forums - especially about Electronic speed controllers and brushless motors. The site is worth signing up for

http://www.hobbyking.com/hobbyking/store/__438__59__Electric_Motors-Boat_Motors.html

These seemed the best range and price - and they turned up in a mail as promised

http://www.ebay.com.au/itm/45X-MODEL...item2c5f8142d9

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### boatofhohaJunior Member

hey,

last stage of choosing motor and prop, still question left.

short explanation of what I did:
relying on graphs of Wageningen Series I got these results:
1.
diameter rpm 2 lades rpm 3 blades
0.04 5832.853283 6192.921179
0.05 3733.026101 3963.469554
0.06 2592.379237 2752.409413
0.07 1904.605154 2022.178344
0.08 1458.213321 1548.230295
0.09 1152.16855 1223.293072

2.
0.04 264.8517977 320.968217
0.05 211.8814382 256.7745736
0.06 176.5678652 213.9788113
0.07 151.3438844 183.4104097
0.08 132.4258989 160.4841085
0.09 117.7119101 142.6525409

now I looked in site hobbeyking and hacker motors and some others, and I could find some 200 watt motors , but the problem is kv it start from 1000 kv so it means 7000-10000 rpm minimum but we need something like 1500, we can use gear but the gaps seems too big for us, we not sure,
-can you give us indication?
-If in motor specs written 7 volts, what happen if it works on less? what are the recomended avarge voltage and current?
-what is the best battery life time we can get before charging by your asamption? we wanted something like more then 10, but in hobbey the best battery by capacety is 5Ah

thanks ,

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### rwatsonSenior Member

You are going to have to use gears. Most hobby shops sell Tamiya gearsets, so that shouldnt be a problem.

Found a good explanation of batteries that may be usefull too.

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### boatofhohaJunior Member

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