# Frictional & Wave making Resistance

Discussion in 'Multihulls' started by Bala5278, Feb 16, 2018.

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### Bala5278Junior Member

Hi, I'm calculating the frictional and wave making resistance for 3 different hulls with same beam and different length. I already have the coefficient of these two resistance but is there a way to calculate the frictional resistance and wave making resistance(I also have the total resistance value) instead of coefficients?

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### TANSLSenior Member

Could you explain a little better what you want to do? What coefficient do you mean? In all these formulas there are usually several coefficients. What method are you using?

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### Bala5278Junior Member

Hi , I'm calculating the total resistance, frictional resistance and wave making resistance for a CAT. I used three different lengths 16.76m, 21.336m, 24.286m with same beam 1m. I used maxsurf to calculate the resistance but it gave only total resistance(Rt) and Coefficient of frictional and wave making resistance ( Cf, Cw). But I would like to now if there is a way to calculate the frictional resistance and wave making resistance with their coefficients since maxsurf didn't give those values.

Thank you

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### TANSLSenior Member

I am sorry but I do not understand. In Maxsurf the method applicable to catamarans is the Slender body, by Mollan, and in it, in Maxsurf, no coefficient appears, only the form factor.
Probably to find out those coefficients that you need you would have to go to articles thate describe the method you are trying to use. I'm sorry I could not help.

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Well, it is not as simple as Frictional + Residuary = Total resistance. There are many more sub elements of the total resistance.

However, to answer your simple question is a simple answer. The hulls that you use must have a displacement and thus must be floating on assumed draft. From the hydrostatics, you can obtain the wetted surface area. Once you have the displacement and thus WSA you can calculate the frictional resistance using the ITTC standard method.

Simple.

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### jehardimanSenior Member

As Ad Hoc and TANSL said, there is a lot more going on here, but to put a simple spin on it...
Total resistance (T) = Friction resistance (F) + Residual resistance (R)
These are non-dimensionalized into the coefficients Ct, Cf, and Cr by dividing by (1/2 rho V^2 L^2) where L^2 for Cf may be S, the wetted surface, so check your documentation. (IIRC ITTC now suggests only using powers of L and favors dropping S.)
The problem you are going to have is that Cr is a lot more things than just Cw. As TANSL points out Cw is normally calculated using non-viscous slender body theory and does not include all the things that make up Cr. Because this is a catamaran Cr will be more influenced by hull spacing because slender body theory may not apply in this case. The pressure effect from one demi-hull will effect the other so all Cw values will be wrong by some unknown amount. Additionally, Cf may be calculated by several methods, most based upon length only, and this will tend, for a constant speed, the make the longer demi-hulls have a lower Cf but for geosims a larger overall drag.

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### Bala5278Junior Member

Hi everybody thanks for the reply. Yes i understand what you are trying to say.
Yes I have the displacement, wetted surface area, draft, coefficient of frictional , wave making, total resistance. I'm trying to find the resistance for different lengths of a CAT with same displacement and same beam. Can you tell me the ITTC formula for the frictional resistance and wave making resistance. I'm particularly asking this because that's exactly what I was asked to do that's why. Also I'm just looking for simple figures not very accurate results. This is just an research. Thank you for your support.

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### TANSLSenior Member

In the Holtrop-Mennen method the following formula is used for viscous resistance (see attached file)

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### John PerrySenior Member

Have you come accross the Michlett software covered at this thread here:

A couple of years ago I also used Michlett to try to find out which is 'best' - a catamaran or a trimaran (sailing boats) - a big question without a clear answer anywhere I have looked! Also a strange way to want to spend your free time during retirement! Michlet does allow for interference drag when hulls are close together and it also enables you to input hull sinkage and pitch - for multihull sailing boats these both vary with course and sailing conditions. It became basically a VPP with my custom software writtain in the Python language linking up with the results from Michlett. I never properly managed to get my software to tell Michlet what to do and to 'pipe' the Michlet output automatically into my programe - I realise Leo does not really want you to do that. Instead I used Michlet to make big lookup table for total drag for a 'typical' shape cat hull, a tri main hull and a trimran float all at various sinkages and pitch angles. I did get as far as producing a few polar performance plots including one for a trimaran with 'foil assist'. As they say, a work in progress, except that at the moment it is not in progress, too many other boat projects to distract.

Last edited: Feb 21, 2018
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Friction:

Thus you then have wavemaking resistance= Total - Friction. That's it....very very simply!

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### Richard WoodsWoods Designs

John - try towing from the side with a long horizontal spar so the test boat is out of the wake turbulence. But variations in drag resistance between different designs are very small especially at low speeds so you have to be very precise in your measurements and keep repeating runs. Better to comparison test a known performer against the test boat rather than just the new design

a 10% performance increase between two similar boats is extremely unlikely. Even a 2% improvement is seen as a design break through

Sailing Catamarans - Why Predicting Performance is an Imprecise Art http://sailingcatamarans.com/index.php/faqs/19-sailing-and-performance-questions/107-performance-predictions

Richard Woods

Last edited: Feb 22, 2018

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### TANSLSenior Member

Some researchers of the viscous resistance, of international recognition, have considered it was necessary to affect this resistance by a factor "(1+ k1)", form factor for bare hull, which was obtained statistically. So, according to these researchers:

Last edited: Feb 23, 2018
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