1/6 scale model prototyping

Discussion in 'Boat Design' started by jakeeeef, Jan 7, 2015.

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jakeeeefSenior Member

I am playing with various designs for a small speedboat I hope to build. I have built a scale model 1/6th of the size, which is fitted with RC equipment as with my skills and budget this is the only way (or the only fun way) I can roughly test things like trim, will it plane etc. without building the real thing.

Scale speed: I am guessing this is simply found by multiplying by 6. Please correct me if this is wrong.

Clearly weight does not scale the same as speed. The finished vessel will be (with engine and passengers) about 750 kg. 750/ 6 = model boat sinking incident.
What is the correct formula for scaling the weight?

Also, power is also not going to scale as linearly as speed either I'm sure.

My power source for the model is an RC outboard (electric with lipo battery). It seemed a reasonable starting point as the finished boat will be outboard powered and one has to start somewhere.

The cheapest way to get the parts was to buy an existing boat. I got some numbers off this with a GPS before stripping out the RC gear. It does 25.3mph in real life. It is a 1/10 scale boat- so I am guessing this relates to 253mph. I can believe this as it's quite a handful at this speed, can only be used in glass calm water and indeed ended up several feet up a tree on the bank after one cornering incident. If the power is scaled up it must have thousands of hp. But then aero and hydro drag aren't linear either, so the above calculation overstates the speed considerably because a model boat never goes fast enough to have major aero or hydro drag.

It also is probably disproportionately light. It does not have a driver. It only carries enough battery power for running about 10 minutes hence this ridiculous scale speed.

I can find out how much power the motor produces. I can guess the loss from motor to prop. How do I scale this power?

What I want to get out of this:
I am using this 1/10 scale motor on the back of a 1/6 scale model, so I'd like to know first what the power would be if scaled up 10 times- so its 'scale power'. It clearly is not a linear scale as it won't even be producing half a horsepower, multiply that by 10 would be 5hp- which would not make a grown up powerboat go 250mph!
Once I know this relationship I would then like to find out how to work out what this power would be equivalent to in my 1/6 scale model. Then i can work out how big the engine would need to be to create the same power in my full sized prototype. (Although I'm quite aware this won't produce this scale speed).

If I know how to scale the power, and know how to scale the weight, I can at least start by giving my model the correct power to weight ratio. I can then ask more questions on what the power to weight ratio needs to be in small but full sized planing craft.

I'm not looking for precise answers- just like to have some idea of what power my model engine is putting in and therefore what power of full sized outboard I would need to achieve the same power/ weight ratio. And also how much weight to put in my model to make it more realistic.

Sorry for such basic questions. I'd like to say I'm 14 to get the sympathy vote, but in reality I'm 40- just didn't concentrate in Maths lessons.

I'm sure this is all well troden ground. Any links to resources that would explain gratefully received

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keroseneSenior Member

different things scale in different rate. In general single dimension (like length) scales in linear fashion (1/6), Area in square (1/(6*6) or 1/36) and volume or mass in cube (1/6*6*6 or 1/216).
So based on that you could conclude that you need 1/216 the mass. Which is correct for sitting in its lines. But unfortunately many things don't scale like that on boats. Exactly because different things scale at different rates. For example scale sailboats need (in relation) massive ballasts as the mass has scaled down much faster than sail area. (area in square, mass in cube or power of 3).

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RurudyneSenior Member

Half scale, 1/8th displacement, which is to say it varies by the cube root. So 1/6th scale will have 1/216 displacement.

Power is more complex because drag will be friction and wave formation based. Friction varies by the square as does surface area; however, I'm guessing wave formation, a function of the displacement of water, will vary by the cube.

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RurudyneSenior Member

Heh, faster poster.

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tom28571Senior Member

Speed is scaled at square root of size ratio or 2.45.

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TANSLSenior Member

Can you give us more details of this simple formula. Do it have something to do with the Froude number?. Thank You.

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rwatsonSenior Member

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tom28571Senior Member

Speed of a wave in open water is directly proportional to the square root of the distance between wave crests (wave length). A scale model running in open water will generate waves in that same proportionality. Therefore the model scale speed is the square root of the ratio of the model size to the full sized boat. In your case the model is 1/6 as big as the full size boat so the ratio is the square root of 6, which is 2.45.

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TANSLSenior Member

I hope you know forgive my ignorance. The fact is I do not understand anything. You make statements that seem to help demonstrate what you say but, of course, first have to prove that these claims are true.
I repeat that my skills are not great, but are not null. I think the wave speed in open waters has nothing to do with the wavelength, or its square root. The frequency of the waves created by the stem of the boat depends, I think, on the speed of the boat but the wavelength, in my opinion, no.
I hope I have not said many nonsense and that, in a simple way, you can get me out of my error. If you think I do not know enough to understand you, do not worry, forget me. Thank You.

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DCockeySenior Member

Wave speed in deep water is proportional to the square root of the wave length, and is the basis of the Froude number and Froude scaling. Two boats with geometrically similar shape but different sizes produce the same wave patterns in deep water when their relative speeds are proportional to the square root of their relative sizes. If one boat is 6 times larger than the other, the wave patterns in deep water will be similar when the larger boat is moving 2.45 times the speed of the smaller boat.

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TANSLSenior Member

It is clear that my lack of knowledge prevents me to understand what is clear and obvious but I refuse to accept things without knowing what principle or theory are based.
I suppose you talking about the length of the wave created by the boat because if not, I do not find any sense. That wave, how is it generated? because I think it can´t be the train of waves generated by the stem hitting the water.
I do not see clear that the speed of propagation of a wave, in general, has something to do its wavelength. Nor do I see clearly that the speed of propagation of the wave, or wavelength, has something to do with the boat speed. But probably I'm wrong.
On the other hand, I guess both, ship and scale model, have the same Froude number and hence the relationship between their lengths and speeds.
How many things I said are wrong ?. Thanks a lot for your time and your patience.

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tom28571Senior Member

Since the whole boating world thinks that the speed of a wave is directly proportional to the square root of the wavelength, please do not ask me to prove what is long established fact. Any basic text will help you to get some understanding of this and the other elements you are interested in.

Mid ocean tsunami waves travel at speeds of many hundreds of miles per hour but the wave height is so low that you would not even notice them in a boat.

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DCockeySenior Member

The waves caused by the boat moving through the water, frequently referred to as the "wake". These waves are caused by the entire portion of the boat hull which is submerged, not just the stem.

Wave speed, wave length and wave frequency are all connected. For a given water depth you can pick one and the other two are determined. For deep water the effects of water depth vanishes. In very shallow water (water depth much less than wave length) the wave speed approaches depending on the water depth only.

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TANSLSenior Member

Whole boating world knows that there is a principle, called Archimedes principle, which governs the floating bodies immersed in a fluid. No need demonstration, but just knowing that Archimedes demonstrated. From there, I can begin to understand some things.
tsunami : Okay, so you're giving me the reason.
With all humility, I do not want you to do any theoretical demonstration, I just want you provided me, if I may abuse your kindness, on what principles it is based stuff.
Thanks again.

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DCockeySenior Member

Water waves have been studied extensively for several hundred years. The relationship between wave length and wave speed has been observed experimentally, and then water wave theory has been developed which agrees with the experimental observations.

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