# Towing resistance – surface friction

Discussion in 'Boat Design' started by kolloff@get2net, Mar 1, 2011.

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### fastwaveSenior Member

Some might even manage to shot their hand that is holding the gun.

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### Leo LazauskasSenior Member

The Table gives the value of Fn^4/CFM in column 10 so a qualified person (obviously, not a mathematician ) could probably use that in a Prohaska analysis to find the form factor.

Unless,of course, the CFD code already uses the ITTC line to estimate skin-friction.

I get a lot of laughs out of CFD papers that describe in great detail the enormous computing power and sophisticated mathematics used on some ship hydro problems, except for skin-friction, where they resort to the ITTC line for skin-friction.

Leo.

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### Leo LazauskasSenior Member

Maybe the little umbrella in his holiday drink obscured that part of the screen.

Cheers, indeed!
Leo.

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### Leo LazauskasSenior Member

Give it to a mathematician and he will look down the barrel to see where the noise came from.

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### AlikSenior Member

+100

Question: does mouse have a trunk?
Answer, engineer: No, it does not!
Answer, mathematician: Yes, but of zero length!

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### MikeJohnsSenior Member

The reality of CFD in a situation like this is that it's next to useless. CFD must be tied to real world tests and then it's useful for SMALL changes in hullform.

Gun analogy: take the expensive laser sights off a well sighted gun and fix them to another then declare that the target is 5 feet to the left of where it actually is because that's where the bullets go.

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It was all the sunblock causing the persperation from a hard work out on the tennis courts blocking my vision. Well, that's my excuse...and D...yup..i miss read the WSA for Lwl.....that's what happens when you're chilled and not paying attention.

But i'll be back home in a few dyas....i'll go through this properly with my charts/tables etc.

I have my own little program for converting model to ship data from tank testing too....as we do our own too

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### Leo LazauskasSenior Member

You'll lose that chill pretty soon
When does the academic year start in Japan?

Happy holidays, John!
Leo.

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### Leo LazauskasSenior Member

"All models are wrong. Some are useful." - George E. P. Box.

My wife has a corollary concerning husbands, but with a less flattering conclusion.

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### MikeJohnsSenior Member

What's her sample population for this statistical observation Leo?

But seriously; a model can be wrong and still be very useful, something Prof Box was well aware of as a statistician. It's all a matter of how wrong the model is and when the discrepancies make the model useless.

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### daiquiriEngineering and Design

Pretty much true. All you have to do is to find a suitable theory which the model will then correctly confirm.

Example: tobacco or oil industry-sponsored scientific research.

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### MikeJohnsSenior Member

When CFD came along I got quite excited, FEM in structures made life so much easier and I naively assumed CFD would follow suit . I though people like Leo laboring away would make all those wave cuts obsolete and we could even add real world ocean surface to the models for accurate prediction methods......ITTC would be disbanded...test tanks around the globe would become swimming pools..... HaHa !

A numerical approximation of a mathematical model can become quite abstracted from reality with something as complex as fluid flow, it's just a matter of whether it's too abstracted.

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The values posted by MARIN are incorrect.

Looking at what they have given (and assuming their nomenclature for model and resistance)

Their Rn are correct, the CF is correct. Their “as calculated” values for the "form factor" are correct.

(One doesn’t need to do a form factor calculation when doing tank testing. The form factor is only for hydrodynamicists/mathematicians to correlate what is observed and what is measured. All you need are the dimensions and of course the total drag).

However, where it all goes wrong is on two fronts.

Firstly, the form factor, r, (RM/RFM) shows values of 5.308, 5.635 ...6.002. When using this method (with a form factor), with increasing Rn the form factor should become constant.

But the main error, as you correctly highlight, is the residuary resistance.

With a vessel at such low Fn it is impossible to have a resultant Cr (residuary coeff) so high, leading to the form factors of 5.308 etc..when in reality this should be around 1.1~1.30.

Therefore the figures of RMN, I assume is residuary resistance model, are 100% incorrect. Perhaps it is something as simple as a miscalculation from kg to N. Maybe the data was already converted from kg to N by one person, and the next person didn’t know and did the conversion thinking it was kg not N. Whatever the reasons, their values of RMN are wrong.

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### Leo LazauskasSenior Member

You must be thinking of another Leo .
I have never been a fan of CFD because it is used in situations where far simpler classical methods work, and because results depend on the mesh used.
I do concede that they are very good interpolation methods, once the answer is known from experiments ;P

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