# Multihull power requirements

Discussion in 'Multihulls' started by Brian Alsum, May 27, 2021.

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### Brian AlsumJunior Member

I've been working on a way to calculate (not generalize) the power needed for a catamaran of a given size. I want to do it with the intent of replacing the diesel with electrics and be able to accurately calculate the range. I've seen lots of charts that generalize such as 4kW per tonne. this is all well and good except it doesn't do anything for estimating consumption at different speeds. I recently did this with Monohulls and managed to get my calculations to within about 5%-10% of the real world. I installed 5.4kW on a 22' Catalina. I revised the gerr formula to take into account the efficiency losses of an electric motor and spit out the actual kw required for a given speed. I'm now looking to do the same with a larger cat (~15m , 15-20 Metric Tonnes) I've seen people talk about (in general ways) that cat hull speed and power requirements depend on one of these ways
1. Wetted area and Displacement only
2. LWL and Prismatic Cof Only
3. Displacement and LWL (much like Mono's)

So does anyone have a somewhat concise formula for calculating speed and power?

Here is the formula I used to build the sheet in the next image.

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### AlikSenior Member

You can't use simplified formulas for catamarans in near hump range. It never works. The resistance near hump can be +/-40% depending on spacing between hulls only...

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### Brian AlsumJunior Member

Doesn't necessarily need to be simplified, the mono worked out that way but really that formula I showed has a few generalizations and contains about 4 other formulas that I simplified down.
also, can you expand on what you mean by hump? I'm an eng just not a naval arch/eng

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### Mr EfficiencySenior Member

By "hump" he means the speed point where the transverse wave system generated by the boat reaches a wavelength where the second crest is behind the stern, and the boat trims nose up. In some catamarans of the slender hulled variety, it is not a big deal, but frictional drag is, and I would think that for modest electric power, that is what will be the chief enemy.

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### AlikSenior Member

Looking at specs above 5.9m at WL and 1.5tons of weight, I am sure there is a hump, and it affects a lot!

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### DejaySenior Newbie

Have you tried prelimina.com?

From my novice understanding it's pretty perfect for resistance / power prediction for a monohull or catamaran. There is a thread here. You need to explot the hull as obj and figure out the waterline offset to get the correct displacement.

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There is no concise formula.
This is not a James Bond villain with a white fluffy cat and a piece of paper with a secret formula written down that solves the world, type of scenario!

You need a range and series of hull with varying length-displacement v residuary resistance.
Noting the subtle differences in L/B ratio too. Since low L/B ratio are more influenced by 'B' since this increases resistance to the power of 2.
Then you will need the WSA of your target hull.

In other words - a trend.

That's it.

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### Brian AlsumJunior Member

The spreadsheet is for a mono hull (a 1983 Catalina 22') not a cat.

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### Brian AlsumJunior Member

So im actually trying to do the math on an existing hull (a leopard 48) so with some generally known specs. do you know of some basic formulas that i can use to calculate speed and powe

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### AlikSenior Member

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### Brian AlsumJunior Member

That's interesting, It defiantly throws out a lot of info. sadly the hull or even once close to it isn't in the pull-down and I don't have an obj of the actual hull. It hard because I don't need extremely granular data for a very specific hull. I need a (somewhat) simple formula that will cover most sailing catamaran hulls that will allow me to derive the power required to push a boat through the water at a given speed (<20kn). I've found very general guidelines for engine sizing in a boat but nothing that will give me a kW output for a kn input

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### DejaySenior Newbie

Sorry I got slightly confused and thought you were looking for a specific case.

I don't think you can get around something like what michlet or prelimina do. They are between simple formula and CFD. If you just wish to do get the aproximate power requirements for your own boat I'd just remodel your own boat approximately and export as obj.

I wish prelimina would add a kind of generative hull design for standard multihulls (e.g. displacement / semi with length, beam and displacement) and a way to automatically process the output. To find for example a sweet spot for a given displacement or length.

Theoretically you could write a javascript to generate a hull, export to obj, load to prelimina and then read the output. But you should ask Josip Basic for permission first.

Other than these methods (you can implement from papers but I assume the math gets kinda advanced) maybe some machine learning could work with enough data points. I'm sure you'd find papers on that too. There is a paper about doing this for a trimaran hull configuration to minimize resistance.

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### DogCavalrySenior Member

I think it's been mentioned, but particularly confounding is the wave interaction in the space between the hulls. Low L/B ratios will generate enough waves that they can reinforce each other, such that the underside of the the bridge structure is contacted, for such an increase in drag that the curve is almost discontinuous.

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### Mr EfficiencySenior Member

It seems to me that if you have an existing boat powered by diesel(s), and you have the speed vs RPM and better still the fuel flow at the RPM points, you should be able to calculate the resistance of that actual boat, and use that to calculate the electric motor and battery etc needs

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### DogCavalrySenior Member

And see the catastrophic loss of range.

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