# Scale down weight to test a model

Discussion in 'Boat Design' started by Hgraham, Jun 27, 2015.

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### HgrahamNew Member

I have built a number of smaller boats but this is my first time designing one. I have drawn up a 16ft river boat on autocad. To check my numbers I have build a 1/4 scale model. The boat should weight 1200 lbs with gas and outboard motor. The surface area of the bottom goes from 49 sqft on the 16ft boat to 3.08 sqft on the 1/4 size model. I am presently trying to set up the weight distribution for the boat. Scaling down the weight by 1/4 doesn't work also scaling down 1/16, the area of bottom doesn't work. What factor should I be scaling the weight down to test the model.

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### Jamie KennedySenior Member

Weight is proportional to volume, so 1/4 x 1/4 x 1/4 = 1/64

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### rwatsonSenior Member

For weight - if you have Excel you can do it like this

1:4 scale

Real Weight = 1200
Scale Weight = (Real Weight) / (4 x 4 x 4)
Excel formulae = (Real Weight) /( 4*4*4)

which is the cubed value.

1:5 would work like this

Real Weight = 1200
Scale weight = (Real Weight) / (5x5x5)
Excel formulae = (Real Weight) /( 5*5*5)

Its only speed that can be divided by the ratio eg 25 knots/4

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### Mr EfficiencySenior Member

It is a can of worms, because the dynamic lift does not scale in concert with your displacement.

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### PARYacht Designer/Builder

More like eating a can of mixed nuts, with your eyes closed . . .

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### TANSLSenior Member

What a strange statement!

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### rwatsonSenior Member

Please feel free to explain the comment

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### rwatsonSenior Member

I started to ask for more info about this - when I fortuitously found a reference to the topic. Larger commercial projects have a lot more to take into account than home-brewed designs, thats for sure

"Boundary layer
The boundary layer is relatively thinner in full-scale flows than in model test conditions. The wake fraction is therefore larger in model tests than in full scale. Propulsion improving devices operate (at least partially) in the boundary layer, which results in different behaviour of such devices between model scale and full scale.

Flow separation and vortex formation Flow separation is generally delayed in full scale and vortices encounter higher damping.
Thus ship wakes in the propeller plane are significantly changed. Vortices from bilge or struts are much weaker and vanish sometimes altogether in full-scale simulations, e.g.
Visonneau et
al. (2006)
.

Wave breaking "

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### TANSLSenior Member

No thanks, I do not want you to say that I speak "ex cathedra" from my pulpit.
Just do not understand your statement. Explain it, if you want, if not, as friends as before.
Cheers.

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### Mr EfficiencySenior Member

What exactly is a "river boat" ? Something Kenny Rogers sung about ? But I am assuming it is a planing contrivance in the modern context, or maybe not ?

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### rwatsonSenior Member

Velocity is a purely linear calculation - Scaled velocity also.

If the speed of a 1:10 size object is 15 klms per hour, then the equivalent speed of the full size object would be 150 klms per hour. Just a pure mathematical extrapolation.

However, what you were "hinting" at may have been the best method to realistically calculate a real boats performance from a model boats performance - which is a totally different problem.

Best explained here

"... the speed the model skipper should shoot for should be, to be realistic, the actual ship speed/scale, not /sqrt(scale). I have read the Froude analysis published by a British boater S.J. Booty (he has a string of engineering degrees behind his name). Booty is often cited for the model speed calc you presented above, which is why I mention him specifically. He misapplies Froude's work wrt model boat speeds.

Froude investigated why shipbuilders could not use model results to predict actual boat speeds. The models always sailed faster than the real ship once it was built (that is, the equation real ship estimate=modelspeed * scale greatly overestimated the performance of the real ship). Froude calculated that if the ship builders did as you say (real speedestimate=modelspeed*sqrt(modelscale), then the equation would more closely predict the actual speed once the real ship was constructed. "

It may be worth reading the whole discussion here

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### PARYacht Designer/Builder

I have a series of "riverboat" designs, ranging from 24' to 50' and there's a distinct difference between them and other craft similarly configured. Most would consider them a houseboat, but these tend to do nothing but stay parked, occasionally moving from one slip to another, but rarely head out into a current, contrary wind or leisurely cruise in protected and semi protected waters. This is the difference between a riverboat and a floating Winnebago. A riverboat can cruise, possibly efficiently like some of my models, much more so than a houseboat, can take on semi protected water, the ICW and even the "great loop" passage if properly equipped and handled. Typical houseboats wouldn't dream of these types of adventures, preferring to stay at their slip, a radio blasting, the BBQ blazing and the teenagers slithering down a water slide.

His (the OP) idea of a riverboat may be different, likely a simple dayboat, basic accommodations and propulsion. Without a pretty good idea of his SOR, we'll have to guess at what he's looking for, but a 48" model will offer very limited information. A 16' puddle jumper isn't especially difficult to get fairly close to right, given all the available offerings in this size, so a look at the SOR would be most helpful at this point.

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### TANSLSenior Member

I would never have suspected. Probably I not quite understand the nuances of this assertion.
My knowledge in this field are not the same as yours, that is very clear, so I am surprised about such categorical statements. When I have some time, I delve into all these studies you indicate me. Thank you my friend.

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### Jamie KennedySenior Member

There are similar problems scaling down sailboats, as they become less stable as they get smaller if you do everything proportional.

Never thought too much about speedboats, but if I did the math right the dynamic lift of a small speedboat should be proportional to the weight as long as the speed is proportional to the square root of the length.

So all else being proportional, a speedboat 1/4 the length and going 1/2 the speed should have 1/64 the dynamic lift and 1/64 the weight. It should only require 1/256 the power however, since it is going slower as well as being lighter. I think this is why model boats can be relatively fast with just an electric motor.

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### rwatsonSenior Member

Good points, but the aim seemed to be to get the weight allocation right. I think a mocked up scale motor, passengers etc in a large model would work fine - and may be easier and just as accurate as doing the weight table in cad.

Its true that performance extrapolation is a bit of a problem.

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