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  #1  
Old 03-23-2009, 07:58 PM
ZOA33 ZOA33 is offline
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Holtrop versus ITTC-78

Hello!,

I would like to compare Holtrop's Method versus ITTC-78's to get the resistance of a vessel. I compared Rv and the results are similar, but Rw is not.

I compared Rv in Holtrop with the follow term of the ITTC:
RV=(S+Sbk)/S*((1+k)*Cf+IncremetCf)

and Holtrop's Rw with ITTC's CR.

I would like to get some conclusions comparing both methods. Advantages and disadvantages of each one.

Thank you!.
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  #2  
Old 03-23-2009, 09:44 PM
Guest625101138 Guest625101138 is offline
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The only method I have found reliable for looking at wave resistance is Michlet. It applies to slender hulls but I have found it works well with L/B of 4 or higher. It uses an analytical method that no empirical approach will match across a range of hulls.

The viscous resistance seems to be better nailed empirically. I have good test data against the ITTC1957 combined with Michlet wave drag data.

If you post details of the vessel you are looking at and your results against the other methods I will do a run against Michlet for comparison.

Rick W
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  #3  
Old 03-24-2009, 08:25 AM
ZOA33 ZOA33 is offline
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Hello Rick!,

thank you for your answer.

I just really want to know the conceptual differences between Holtrop and ITTC-78, I did the power prediction by both methods and now I have to give some conclusions. I represented Rv-V by both methods and Holtrop's Rw-V versus ITTC's Cr passed to Rr-V, but now I would like to write the reasons why are similar or different depending on the speed, kind of reistance, etc...
In other words justify the results I have obtained.

Best regards!.
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  #4  
Old 03-24-2009, 08:39 AM
Guest625101138 Guest625101138 is offline
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The value of Michlet is that it will give the correct answer so you can see which of the other methods gives the nearest correct answer.

Rick W
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  #5  
Old 03-24-2009, 09:47 AM
Ad Hoc Ad Hoc is offline
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The ITTC 78 is a modification of the Hughes–Prohaska method. It is generally
more accurate and also considers the air resistance. Such as, the frictional resistance component is taken as the ITTC ’57, (which already includes the global form effect), however, for the full scale ship there is the added correlation coefficient of the air resistance. This is calculated as 0.001At/S, where At is the frontal area of the ship above the waterline and S wetted surface area.

I’m not overly familiar with the Holthrop, since I do not use software tools for resistance calculations. I use actual model tests. But what little I do know is that is it based upon a prescriptive set of hulls. As such, like any parameter, if your data lies outside, then the results will be affected. Just as any CFD software program, check the limits.

But a search suggests that you should try these papers for further reading, I don’t have them so can’t offer any more than that, sorry.

Holtrop J. & Mennen G.G.J.
"An approximate power prediction method"
International Shipbuilding Progress, 1982

Holtrop, J.
"A Statistical re-analysis of resistance and propulsion data"
International Shipbuilding Progress, 1984, pp. 272-276
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  #6  
Old 03-24-2009, 02:59 PM
ZOA33 ZOA33 is offline
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Thank you for your answers.

Regards!.
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  #7  
Old 03-24-2009, 11:45 PM
Ad Hoc Ad Hoc is offline
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ZOA33
Since I’m not too familiar with the Holthrop, i did a bit if digging, interesting!

Holthrop has the total resistance as:

Rf(1+k1) + Rapp + Rw + Rb + Rtr + Ra

Rf is as ITTC’57 but the form factor (1+k1)is somewhat more complex.

ITTC’78 has resistance (in coeff’s nomenclature) as:

Cf(1+k) + Cw + Ca + Caa

So isolating each will help in determining where your errors/differences are coming from.

You can see that Holthrop has ‘additional’ coef’s in his formula. As such comparing Cw and Cf is probably your best bet to establish where the differences are, as these account for the lions share of the resistance.

So it comes down to the differences between theCf and Cw, but more importantly the source of your Cw and then the differences in form factor between the ITTC’78 and Holthrop, which is rather complex. Also since his method is based upon a small set of data, the algorithms used to create his method/predication are obviously suspect to deeper statistical analysis. Consequently it is anything but ‘correct’. Since the Cw’s in Holthrop are ‘simple’ interpolations between his small sets of data. So you really need to see the source data to see whether your hull form fits into the parameters. If it does not, then clearly your results will differ between the two methods.

So bottom line is that Holthrop is great for inputting into a computer and creating a resistance prediction. Just save slots of number crunching. As such used by many software programs. It does not replace the standard model testing for more accurate results (owing to its caveats) for establishing Cw’s. It is just cheaper and quicker at estimation when time and funds do not permit a ‘proper’ tank test.

Not sure if this helps?
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Old 03-25-2009, 06:53 AM
ZOA33 ZOA33 is offline
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Thank you very much Ad Hoc!,

your post help me to have a actuation mode and re-enforce "line work" I had.

Thank you again and take care.

Best regards!.
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  #9  
Old 04-28-2009, 07:42 PM
Marvic Marvic is offline
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Holtrop

ZOA33

This spreadsheet may help you.
I will make it available in the forums spreadsheet area as well.
Attached Files
File Type: xls Holtrop - 1984-1.xls (45.0 KB, 1178 views)
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