# Very small hulls

Discussion in 'Boat Design' started by Saqa, Jul 8, 2021.

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### Doug HalseySenior Member

I'm sure you know this, but some others might not: The main components of resistance (skin friction & wavemaking) scale differently. Tank tests are generally done with Froude numbers matching the full size values to get the wavemaking portion accurately enough. But that means the Reynolds numbers do not match, and the skin friction (or other viscous contributions) need to be corrected or estimated in some other way.

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Hmmmm...that's not wholly correct.

Residuary resistance is merely a function of Resistance per weight at the scaled speed.

Im not sure what you're implying here?

Re of model and Re of ship are the same, except for the respective scale, that's all; thus, yielding different values of Re, but the means of calculating is identical, at its own rspective scale

Re = v.l/u

v and l = at the scale you're investigating, model or ship.

Thus the v = speed, for Re is the same for residuary, at that scale.

Thus, im not sure what it is you're trying to suggest?

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### Doug HalseySenior Member

Let me try to phrase this differently:
If you had two designs, and decided to race scale models of them to determine which one would be faster, you could easily reach the wrong conclusion because it isn't possible for the models and full-scale boats to operate at the same Froude numbers & Reynolds numbers simultaneously.

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### bajansailorMarine Surveyor

@Saqa , could you elaborate a bit more please as to what a 'wetta' is?

Last edited: Jul 12, 2021
5. Joined: Oct 2008
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If they are the same relative scale, that's not correct.
Since you're measuring the output - the result. It is what scaling is for ...

Of course not, that is what scaling is for.

But, that's like saying the model that has a motor of 5kW is slower than the ship....because you're saying, it is not a fair comparison with the full scale ship because it will be faster as it has 5000kW engine in it?!

Scaling - is just that - scaling!
It seems you're getting confused at the purpose and methodology of using scaling laws of geosims!

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### Doug HalseySenior Member

You cannot just take the model data and scale it all at once. You have to first break it into components which scale differently. And the processes to do that involve using empirical data or theoretical calculations - and must be considered approximations at best.

Sometimes they can be be seriously misleading. As an example, please consider the case of Brit Chance's infamous 12-Meter Mariner.

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### SaqaSenior Member

I regularly see a bunch of nice looking tris racing each other, with the word wetta on the side. Just assumed some sort of one make and that most here would know a lot about them than I

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That's exactly what you do.

Who said you didn't?

Of course they do... WSA scaled to the power 2... displacement scales to the power 3, and so on ... so, your point being?
But it is all done at once, and easy in excel as a spread sheet.

What are you referring to as "empirical" and "theoretical"..?

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### JRDSenior Member

It's a New Zealand built production trimaran. Has class racing and in a few countries now.
Named after the Weta, an NZ native insect rather like a large cricket that lives in the Forrest undergrowth

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