# length/beam ratio of around 20

Discussion in 'Multihulls' started by PetterM, Apr 21, 2015.

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### PetterMSenior Member

I am looking for resistance data/prediction methods for very slender hulls, with length/beam ratio of around 20 and Froude number of up to 0.8-1.0. Anyone?

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### daiquiriEngineering and Design

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### hump101Senior Member

For such slender hulls you wouldn't be far out with just using ITTC curve and allowances, wavemaking will be minimal provided you aren't dragging a transom.

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### daiquiriEngineering and Design

That is true. The resistance will be almost entirely frictional. So the easiest method is to just calculate the wetted surface of the hull and the friction coefficient, and get the resistance. It will be given by the formula:

R = 0.5 rho V^2 Awet Cf.

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### Mr EfficiencySenior Member

I assume, (maybe shouldn't !) these could be hulls of a catamaran ? If so, the issue of wave interference comes into play.

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### Ad HocNaval Architect

Indeed. Since an L/B of 20 is not very stable!!

We use L/B ratios of 15-20 for our cats.

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### oldsailor7Senior Member

The late Edmond Bruce extensively researched L/B ratios in non heeling situations. He showed that form drag only increases as the displacement increases. Skin friction drag increases as both displacement and waterline length increase. Wave induced drag is bad At low L/B ratios , but improves very greatly from about 7/1, to 12/1 where, it practically disappears. Improvement continues until it reaches it's limit of diminishing returns at about 20/1 providing the displacement, (weight) is kept light. The super light Tornado Cat, at 20/1 is a case in point. A compromise is reached by designers of cruising Cats and Tris, usually between 8/1 to 12/1, and performance multis from 13/1 to 20/1.
Interference drag between the hulls of multis is not usually a problem these days as modern cats and tris have much wider overall beams than in the early years of multihull design.

Last edited: Apr 28, 2015
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### Ad HocNaval Architect

Sorry, it is an issue no matter the "age" of the deisgn.

It is greatly influenced by the hull spacing and is not a constant either. In other words it also varies with speed too. There is no one size fits all.....one must take interfere into account bad ensure your target speed is achievable for your hull spacing and L/D ratio.

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### Gary BaigentSenior Member

On square and oversquare sailing multihulls (modern and also advanced for their time "old" designs) of fine hull beam/length ratios. like above 12/1, wave interferance is a non issue - as OS7 writes - and with lifting foils, you can say it doesn't exist.

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### Leo LazauskasSenior Member

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### Ad HocNaval Architect

Oh dear Gary, you're at it again. Subverting your opinion as fact.

Here is a simple fact for you. Below is residuary resistance...that's wave making resistance to you,... of a hull that has an L/B of 15.1.

The monohull is plotted versus various multihull versions with increasing spacing..oh, what do we see..the opposite of what you are saying. Same old..same old.

I do admire your persistence in ignore facts for the sake of a good story and to sound knowledgeable though.

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### Mr EfficiencySenior Member

Now Ad Hoc, kindly explain that graph a little, are the 4 dotted lines the resistance of each hull of a catamaran, and S/L is what ?

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### Ad HocNaval Architect

The thick dark line near the base, is the residuary resistance of a monohull with an L/B ratio of 15.7.

S = hull centreline to hull centreline distance of a catamaran using the same hull. So 2 monohulls of L/B ratio of 15.7 forms a catamaran.

The S/L is the separation (S) divided by the length (L).

So S/L = 0.2 means the hull CL to CL divided by L = 0.2
S/L = 0.3 means the hulls are a tad wider, S/L = 0.4 wider still and so on.

So you can see 2 hulls have more resistance than 1 single monohull of an L/B greater than the 12, contrary to Gary's "beliefs".

With increasing separation the residuary resistance of the catamaran version reduces. Only becomes more consistent at higher Fn. But as can be seen, it is not constant either nor does it get anywhere near that of a single hull, unlike Gary's assertions.

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### Mr EfficiencySenior Member

Alright, but Gary Baigent specified "square or oversquare" catamarans, so the S/L could be 1.0, how does that look on the graph ? Seeing the others are very convergent at the higher Fn, is it similar there too ?

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### Ad HocNaval Architect

It is all about trends, not absolutes.
If you look at the curves from Fn 0.7+, what does it tell you?...and there is your answer.

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