# scaling up (down) planing hull

Discussion in 'Boat Design' started by tonipu, Mar 30, 2021.

1. Joined: Dec 2014
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### tonipuSenior Member

I made a model of my planing hull 1:20

when i scaled down engine it is 720hp=544kw : 20:20:20 gives 68 wats

thats that i mesured on dc motor about 7.3volt x 12Amp 87 wats eficency on electric brushed motor maybe 80% so it sceled down very well

model speed froud number 1.72 at 4m/s

maxsurf prediction for real boat
11meter waterline
740hp gives maybe about 38kts so froud number 1.83

it matches pretty close?
is it my math good, i know that the similarity theory is not that simple i would like to hear opinion and learn something

thank you

also the weight of the model i scaled and match

7000kg :20:20:20 0.875kg and the waterline matched

2. Joined: Sep 2011
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### TANSLSenior Member

I think the Froud numbers should be the same for model and original, not very similar but the same.

3. Joined: Aug 2020
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### mc_rashJunior Member

You should take a look on model laws!

Edit: give us the dimensions, speeds, power etc. from both - model and real scale - and we can take a look.

4. Joined: Aug 2020
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### mc_rashJunior Member

Since Fn ~ 1.882 is equal for both 'models' you can calculate the model speed v = 4,372 m/s
Now you could calculate en measure the resistance etc. but you also can scale the power on modelscale by dividing the real power with scaling factor to the power 7/2.
=> P_model = P_real/(20^(7/2)). The power on model scale is now = 530 / (20^(7/2)) = 0,0148 kW

5. Joined: Aug 2020
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### mc_rashJunior Member

You also could determine the force F (since P = F * v)
-> F = P/v = 530/19,548 = 27,113 kN
you'll get the force on model scale by dividing the force with the scalefactor^3
-> F_model = 27,113/(20^3) = 0,003389 kN = 3,389 N

now you can calculate the power required for you model
P = F*v = 0,003389 * 4,372 = 0,0148 kW

6. Joined: Aug 2020
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### mc_rashJunior Member

You should keep in mind that there is no resicentance calculation at this point. This will be another step to determine the needed power for your scale model. The calculated power above may be too low to reach the Froude number 1.882 (I saw today someone with a selfbuild planing hull model and it did'nt reached the planing mode.. )
I'll try to help you if you have more questions on this topic!

7. Joined: Dec 2014
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### tonipuSenior Member

tomorow i will mesure model speed with accurately with gps logger mounted on model.
real boat will be 11meters on waterline, 3 meter beam on waterline including chine, 10cm chine wide, and 15 degres constant deadrise till half of the boat(monohedron)

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### tonipuSenior Member

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### tonipuSenior Member

The model planes very well; I posted a video

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### tonipuSenior Member

I say froud number is similar because I didn't measure the model speed accurately, Tomorrow I will do with gps logger and I will try to log the trim data

11. Joined: Aug 2020
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### mc_rashJunior Member

Nice project!
What do want to do with the towing test results? On the "open sea" you don't have control over wind, waves, flowdirection of the water etc.. And also the speed is not that accurate you want to achieve and would achieve with a towing tank test

12. Joined: Dec 2014
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### tonipuSenior Member

I look more to sea keeping capability than resistence, but i mount a rc motor mesure the electric power and i noticed that pretty good match the maxsurf prediction scaled town to a model, but i am not expert in similarity theory. So i am curios to know more

13. Joined: Dec 2014
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### tonipuSenior Member

I know the raynolds number is issue when scaling and is different in model, than to a real boat

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