# Crouch's Formula for high speed ferry catamarans

Discussion in 'Hydrodynamics and Aerodynamics' started by redviking, Mar 30, 2012.

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### redvikingNew Member

I am interested if anyone can give me some direction in using Crouch's formula for speed predictions on high speed passenger ferry catamarans. I am looking at re-powering of 20-40 m (65 ft to 135ft) alumininum ferries capable of up to 30 knots. I think Crouch's formula would be helpful in the initial stages to identify engine makes and models. I have read Dave Gerr's book on propellers. Whilst he talks about planing hulls in depth, his only reference to catamarans is high speed racing hull. I am not a Naval Architect, only a Marine Engineer, so my interest is what goes on inside rather than out. Can anyone give me some further detailed direction.

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Basically it is all driven by the length displacement ratio. In other words weight!

Take you cat, what is the waterline length (L)...and then what is the displacement (D).

If you work this out for each L/(D^1/3) or in words, the length in metre, dived by the displacement in tonnes to the power of (1/3), what value do you get?

If the value is around 4~5, that is low (poor)....if the value is around 7~8 (good) that is high.

The performance of your cat will be dictated by the L/D ratio.

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### Eric SponbergSenior Member

Redviking,

Just for background, you can also use other speed formulas for preliminary design. I am attaching the booklet I wrote two years ago on this forum called The Design Ratios, and the last two chapters, 11 and 12, cover various speed prediction formulae. This includes Crouch's Formula, Keith's Formula, and Wyman's Formula, as well as Dave Gerr's Displacement-speed Formula.

I hope that helps.

Eric

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### AlikSenior Member

Add Muler-Graf paper to this list; that one deals with sharp-chine cats.

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### AlikSenior Member

Crouch formula is reliable ONLY if coefficient C is defined by tests of similar vessel, and only in full planing range (FnV>4.0). Same refers to other formulas of this type - Reyes, Levi, Krivonosov, etc.

I have made the comparison of prediction methods taking parameters of L=5.1m boat, some results are on graph. Of course this is not 100% fair approach because methods chosen should be applicable to hull shape of given boat, but we assume that they are, for every speed range. The most reliable methods are classic and physically based - NPL, DeGroot, Jin, Savitsky, Robinson... Such methods as Crouch and Romanenko-Sherbakov fully depend on constant chosen. Such methods as Gerr and Wyman... well, I would not use them.

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You mean the one from Fast '93?
Its an interesting paper. Some conclusions differ from our own.

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### AlikSenior Member

There were number of papers, but they all cover their series on sharp-chine cats. I use for design/calculations often; the only disadvantages are that hull spacing is constant and L/B is related to DLR.

MÃ¼ller-Graf B. Widerstand und hydrodynamische Eigenschaften schneller Knickspant-Katamarane der VWS Series '89 (Resistance and Hydrodynamic Characteristics of the VWS Fast Hard Chine Catamaran Series '89)// 20th Symp.Yachtentwurf und Yachtbau, Hamburg 1999, Germany.

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The ones I know of are:

"SUS-A The scope of the VWS Hard Chine Catamaran Hull Series '89"
&
"SUS-A The effect of section symmetry on resistance performance and seakeeping qualities of fast hard chine catamarans".

Both in Fast '93.

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### daiquiriEngineering and Design

Did any of you guys use Radojcic method and how do it's results compare with those mentioned in Alik's post #6?

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No I haven't, maybe Alik has??

I'm lucky enough to have many many hull forms as a database. Thus just compare the L/D ratio of the 'design' to the endless variants and interpolate as required

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### AlikSenior Member

If You mean Radojcic method for planing craft, this is Series 62/62DUT. No, we have no experience using it.

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### redvikingNew Member

One of the vessel's we are considering for a repower is a 30 m alumimium cat, 104 metric tonnes lightship (including 50% fuel, 12,000 ltrs total;50% water 4,000 ltrs total;50% sewage waste 5000 ltrs total, 2 metric tonnes of stores) vessel has a survey for 405 persons. Current the vessel has twin Deutz TBD620V12 engines, 1500 kw per side. Looking to re-power with new 1340 Kw Yanmar 12AYM-WGT engines. Due to the strength of the Australian dollar, we are looking to limit the passenger number to total persons to 280 including crew. THe 620 Deutz engines are no longer manufactured with no support in this part of the world.
Thanks for all the advice. I am very heartened to see so many passionate devotees of boat design.

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### sottorfmember

The method of Werenskiold should give you pretty good answers if your hull is a typical semi-displacment catamaran. My own experience is that the method is accurate as long as you have a sectional area curve close to optimal for the design speed.

Also suggest you get the following thesis:

Zouridakis, F. (2005). A Preliminary Design Tool for Resistance and Powering Prediction of Catamaran Vessels. Ocean Engineering. Massachusetts Institute of Technology. Retrieved from http://mit.dspace.org/handle/1721.1/33595

http://mit.dspace.org/handle/1721.1/33595

It gives some good comparisons between the results of the different methods metioned by Ad Hoc and Alik.

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