Multihulls vs. Monohulls

Multihulls are faster - it's a statistical fact.
Any decisive explanation? Ideas?

 

Well...

You can prove anything with stsistics. That's what 's so good (BAD) about it

 

Sail area is generally proportional to initial stability.
Light air performance is generally a function of sail area to whetted surface ratio.
Heavy air performance is generally a function of righting lever over (the square root of) the midsection (i.e. frontal) area. (Most people would state it in terms of sail area/displacement, but I contend the way I've stated it has more merit. Length and length to beam ratio, beam taken of the load-bearing hull individually, are also important).

 

You can prove many things...

but not that real-world monohulls are faster than multihulls.
Have a look at WSSRC website:
http://www.sailspeedrecords.com/

 

If we are comparing multihulls to monohulls, we should establish some parmeters. For example, equal waterline length and sail area. In certain conditions multihulls carry more sail area for the displacement, therefore getting more speed. In very light winds, monohulls have less wetted surface and more speed.

 

I agree about the need to establish parameters

Total displacement is the first parameter that comes to my mind since it's more of a primary requirement than part of a specific design solution. Another natural but less "neutral" parameter may be length at waterline since it has to do with "Hull Speed"- a parameter common to anything that moves in water. In any case the results will be the same for sailing boats: monohulls are a category apart - a slower one.
I wonder what statistics are in motorized boats. Are there any?

 

i learned there are more than i ever thought yipster

for speed and multi hull tunnel boats have a look at: http://boatdesign.net/forums/showthread.php?s=&threadid=1315
but powerboat worldrecords double that "easely"

 

Thanks! One link didn't open but the other

offered a display of magnificent twinhulls but I couldn't find monohulls - Is it because this competition / championship is for multihulls only or because monohulls didn't make it?
If the multihull advantage goes across means of propulsion maybe it will be possible to find a general explanation that's independent from propulsion factors

 

Are you talking about sail- or motorboats?

I think, for sailboats it comes down to "what is the real size of a boat".
An example: one 38' catamaran has an all up weight of 7,5t. If you deduct the weight of the keel, a 45' mono from the same yard weights also 7,5 t. You can say, the 2 boats are of same size (the keel is deducted, because it is very cheap in relation to weight).
Both boats offer the same accomodations, the mono better usable space below, the cat more deck area.

And, surprise?, the 45' mono is slightly cheaper as the 38' cat.

Florian

 

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So:
what boat will be faster, the 38' cat or the 45' mono?

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I have no idea but

there seem to be a rule of thumb about this: multihulls are 20% faster than comparable (in volume) monohulls, or in other words a multihull can be 20% shorter that a comparable (in volume) monohull and be as fast.
Take a look at http://www.wavewalk.com/COMPARISON.html
-there are some links there to articles on precisely questions like this.

 

Faster by what parameter?

Knots/Length
Knots/Pound
Knots/Rating
Knots/$

The question doesn't mean much without clarifing comparable measures of effectiveness.

 

Regarding comparison of the 38 ft catamaran to a 45 ft monohull.
Assuming that both lengths are the length on their design waterlines. This is best way to rate. Do not use the sales length on overall.

The monohull length is somewhat like 18 to 20% longer. The cost over a similar 38 ft mono hull by stretching is likely 30% to 40% depending on whether you will add depth or beam.

A shorter catamaran still provide you with the necessary deck area. But you may not have enough displacement to support the extra weight of multihull. Multihulls are heavier and will have more wetted surface in drag. Multihulls can reduce wave resistance, a primary element in speed hulls.

For all small crafts, the running speed desired usually end up in the high speed zone in Speed/sqrt(L) function. If you want to trim the powering requirement use the planing hull form for speed = 3+ x sqrt(L (ft)).

Catamarans are not usually planing hulls. But they can trim the wave resistance by optimising the wave resistance parameter B/d. You need displacement, so your primatic coefficient cannot be optimised to low values, else you will end up to increase the size of the craft.

The High beam draft ratio in mono hulls indicated the cause of the relatively high wave formation and powering requirement in the high speed range.

Building multihulls can be justified only if speed is a primary target. The extra cost can be pay off.

Best comparison on mono/multihull case, is to base on the same length, total added up beam on water and the hull depth. Then you will see the performance index on common baseline.


Peter

 

Looks like Peter got the practical

side of the issue fully covered.
Doesn't everybody want to go faster?
I assume everybody does, so monohulls are still very popular mainly because of tradition and $ factors.
What about human powered boats?