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Old 18-05-2020, 17:27   #1
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Beam vs length on catamaran

Is there any potential problem with having a really wide beam in relation to length on a catamaran? For example a beam as wide as LWL.



I'm not talking about structural concerns, I understand that a wide beam requires significantly greater strength in the crossmembers.
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Old 18-05-2020, 17:32   #2
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Re: Beam vs length on catamaran

Bridge deck slamming
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Old 18-05-2020, 17:43   #3
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Re: Beam vs length on catamaran

Difficulty in finding a marina which can accommodate a wide vessel and likely will charge you much more than a standard catamaran dimension.

The greater weight of the wide body bridge will require wider and deeper hulls displacement.

But as mentioned above the impacting of waves on the underside of the deck when the waves traverse athwartship could be catastrophic as the boat literally could get high centered and the hulls dewatered, suspended in the air. That is why they invented Trimarans.
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Old 18-05-2020, 17:51   #4
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Re: Beam vs length on catamaran

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Is there any potential problem with having a really wide beam in relation to length on a catamaran? For example a beam as wide as LWL.
Charles Kanter writes about this in Cruising in Catamarans. If you increase the beam, there's a temptation to increase the power of the rig. This means not only more pressure on the leeward hull, but more pressure on the lee bow when going downwind. This can lead to pitchpoling in a worst-case scenario. There's also typically more weight for each hull to support, and cats don't like to be overloaded.

I used to sail a Woods Sagitta, which was wider than average for its length. It was a nice boat, but I never pushed it hard.

And then there is the issue of finding marina slots, and somewhere to haul out. The wider your boat, the harder it is to find a yard that can accommodate you.
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Old 18-05-2020, 18:07   #5
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Re: Beam vs length on catamaran

I believe catamaran designers are looking for transverse stability? Not sure if thatís the correct term, but if the beam isnít wide enough you stand the chance of flipping sideways. To wide and you tend to pitchpole. The transverse (?) stability is a compromise in between.
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Old 19-05-2020, 02:49   #6
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Re: Beam vs length on catamaran

What is the reason behind your question?
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Old 19-05-2020, 03:45   #7
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Re: Beam vs length on catamaran

2:1 is about right.

A beam as wide as the length gets weird for all the reasons people stated above. Burying a bow and cartwheeling is more a factor with something that wide assuming you’re basing the rig on the beam rather than the length.
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Old 19-05-2020, 04:15   #8
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Re: Beam vs length on catamaran

i think manoeuvring - esp under sail - would become more difficult. a super beamy cat would be an absolute bu@#$%er to turn...

agree with chotu...2:1 seems about right. certainly our boat is 12m loa x 6m bm

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Old 19-05-2020, 04:39   #9
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Re: Beam vs length on catamaran

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2:1 is about right.

A beam as wide as the length gets weird for all the reasons people stated above. Burying a bow and cartwheeling is more a factor with something that wide assuming you’re basing the rig on the beam rather than the length.
2:1 is at the end of the range and typical of early cat designs due mainly to structural limitations. Very early catamaran designs were often over 2:1.

A better guide would be between 2:1 and 1.5:1

Ignoring stability the main benefit of wide beam means that the bow wake(s) meet astern not under the bridgedeck, negating the need for a wave breaker of nacelle.
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Old 19-05-2020, 04:49   #10
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Re: Beam vs length on catamaran

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2:1 is at the end of the range and typical of early cat designs due mainly to structural limitations. Very early catamaran designs were often over 2:1.

A better guide would be between 2:1 and 1.5:1

Ignoring stability the main benefit of wide beam means that the bow wake(s) meet astern not under the bridgedeck, negating the need for a wave breaker of nacelle.
2:1 is what catamarans that sail well are. It’s the sweet spot of weight, bow wakes, stability, righting moment, etc.

It’s basically the gold standard in modern performance catamaran design. Not sure where you’re getting old design from?

Look at the best performing catamarans out today. They’re all 2:1. Racing takes it even higher.
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Old 19-05-2020, 05:32   #11
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Re: Beam vs length on catamaran

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2:1 is what catamarans that sail well are. It’s the sweet spot of weight, bow wakes, stability, righting moment, etc.

It’s basically the gold standard in modern performance catamaran design. Not sure where you’re getting old design from?

Look at the best performing catamarans out today. They’re all 2:1. Racing takes it even higher.
Think you might mean lower.

Catana 43 1.8:1
TS42 1.75:1
Prout 37 2.45:1
Lagoon 500 1.85:1
KH56 1.7:1
Iroquos 2.24:1
Formula 40 1.63:1

The hull fatness vs centre line beam will also have an effect on the measurements.

Racing trimarans are typically 1.5:1

When you get very big Playstation, Explorer 2:1 is more common but the difference between CL beam and OA beam is very small.
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Old 19-05-2020, 05:41   #12
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pirate Re: Beam vs length on catamaran

Actually 'old design' were narrower than 2.1..
Iroquois, Prout, Catalac, Hirondelle etc were all below that.. the Catalac for example 8.92m x 4.19m, Iroquois was 30ft x 13ft..
The Iroquois had an emergency float at the top of the mast to stop them inverting when they flipped.
It was James Wharrams Designs success with wider beams I believe that started designers on the 2.1 track and more stable platforms.
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Old 19-05-2020, 05:48   #13
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Re: Beam vs length on catamaran

Don't forget beam is just one part of the equation in terms of stability. Weight plays a major role as does the rig height and COE. It's not linear either.

The lighter the boat the wider the beam or lower the rig needs to be. Increase the rig hight and you need to increase the beam or weight in the hulls. Decrease the beam and you need a shorter rig.

And so on.

My snowgoose 35 is roughly 2.35:1 of course that doesn't take into the account that amidships is wider than the bow or stern.
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Old 19-05-2020, 06:01   #14
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Re: Beam vs length on catamaran

The older British cats were usually beam equals half of waterline length
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Old 19-05-2020, 09:51   #15
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Re: Beam vs length on catamaran

Take a look around Sailing Catamarans - Home
the Richard Woods web site. Or just try this google search
: site:Sailing Catamarans - Home beam ".
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