Catamaran or monohull for efficient high speed craft

[pietermariof]

Hello,
I'm joining a competition where students will build boats with the same engine given by the organizer. It will be a time trial race and a long distance race. The only limitation of the design is that the boat can fit inside a 2.5 m x 2.5 m x 2 m box and the average Froude number of the previous participants is about 0.9 to 1.2. For addition, none of the boats is in the planing mode due to the 20 kg payload.

Now I have a dilemma of choosing between catamaran and single hull. I've read some books or papers but none of them really give enlightment whether a catamaran/monohull is faster or more efficient (many recommend catamaran but how much more efficient and at what speed range?).
For now I'm more into designing a displacement catamaran, but if you have any reference or experience that monohull is faster/more efficient, please let me know

 

[Boy Griffioen]

Is it sailing??

 

[HJS]

Check if a trimaran can be a better option.

 

[pietermariof]

Is it sailing??

No, not sailing. We will use engine and propeller.

 

[pietermariof]

Check if a trimaran can be a better option.

It sounds good for sailng only I don't see trimaran powerboat quite often

 

[TANSL]

"Not quite often" but there are some, very high speed, and even pentamarans. I think it's worth investigating.
Anyway, the efficiency of your boat should be measured by being the fastest or the one that develops the maximum speed with the minimum consumption ?. Define, please, what is "effectiveness" for you.

 

[gonzo]

At those low speeds (way below planing) skin friction will dominate. A monohull will be more efficient.

 

[TANSL]

At those low speeds (way below planing) skin friction will dominate

I would not be very sure about that, I would have to check it; I would not give it as an axiom. At low speed the resistance by wave formation can be, in relation to the total resistance, high and of course greater than in planing mode in which the friction is, relatively to the total, greater.
I guess this, without having done any number, is debatable.

 

[Boy Griffioen]

At those low speeds (way below planing) skin friction will dominate. A monohull will be more efficient.

That would be a question? What's the speed you want to design for? A narrow hull has a more gradual transition from below hull speed to above hull speed. So you could maximise length and put the boat in the box diagonally and maybe push hull speed to 4 knots. Then you will be good until 3 knots before increase of resistance. If you want to go above 3 knots then a hull with a length to beam ratio of 8+ will probably have advantages. But depending on available power even hydrofoiling is possible.

Just saying Video http://www.jetfoiler.com/gallery-1/

 

[pietermariof]

"Not quite often" but there are some, very high speed, and even pentamarans. I think it's worth investigating.
Anyway, the efficiency of your boat should be measured by being the fastest or the one that develops the maximum speed with the minimum consumption ?. Define, please, what is "effectiveness" for you.

There will be 2 categories of the competition and we will have to use the same boat. The first one is time trial so maximum speed is more important, the 2nd one is long distance race where we have to finish as many laps as possible in the given time until the battery run out (usually in this category the participant will keep their speed around Fn = 0.5 - 0.7)

I know that there is no type boat that can be the best on both categories, but what is the type of boat that will perform good enough in both categories? (in this case monohull vs multihull). Thanks

 

[pietermariof]

That would be a question? What's the speed you want to design for? A narrow hull has a more gradual transition from below hull speed to above hull speed. So you could maximise length and put the boat in the box diagonally and maybe push hull speed to 4 knots. Then you will be good until 3 knots before increase of resistance. If you want to go above 3 knots then a hull with a length to beam ratio of 8+ will probably have advantages. But depending on available power even hydrofoiling is possible.

Just saying Video http://www.jetfoiler.com/gallery-1/

The design speed is 10 knots and yes some participants use foils (they performed well in the time trial category). However, some catamaran and monohull are also have decent speed compared to the foilers.

"So you could maximise length and put the boat in the box diagonally and maybe push hull speed to 4 knots."

Ah, this has been my question for a while. With the same length and displacement, will a catamaran be faster than a monohull since the demihulls of the catamaran have higher length-displacement ratio?

 

[Boy Griffioen]

If you can make the length to beam ratio more than 8 wave drag becomes less and less obvious and wetted surface is king. So a narrow long monohull could potentially beat a catamaran.
Also because there is interference between the hulls. A well thought out skiff could be efficient.

 

[Mr Efficiency]

I think you need to publish the exact rules of this competition, to get any considered replies.

 

[Ad Hoc]

... the average Froude number of the previous participants is about 0.9 to 1.2.

Displacement cat will be the most efficient for you...given those Fn's.

 

[tdem]

Sounds like the Hydros hydrocontest. Some videos can be found on Youtube, unfortunately they are very highly edited and don't show how well any of the boats performed.