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19-05-2020, 20:27
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#31
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Registered User

Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43 and OPBs
Posts: 12,891
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Re: Beam vs length on catamaran
Quote:
Originally Posted by Chotu
How can you NOT tell from the Morelli and Melvin page with a picture of the boat on it?
Hulls don’t look too beamy to me. (Screenshot grabbed right off the same page I linked to).
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Again, you're the one with the faulty logic. The hulls being "not too beamy" negates your arguent.
The " less beamy" the hulls are, the closer the LWL/BOA and LWL/BOC figures are to each other and the LOWER the LWL/BOC ratio for the same BOA.
If the hulls were " more beamy", then the LWL/BOC ratio would be comparatively higher than the LWL/BOA.
i.e. if the hulls were just vertical planks, the LWL/BOA and LWL/BOC would both be the same 1.797:1 on the quoted figures. As the hulls get beamier, if the BOA remains the same, their centrelines must be closer together and the LWL/BOC ratio increases.
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19-05-2020, 20:35
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#32
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Registered User
Join Date: Jun 2012
Location: Port Aransas, Texas
Boat: 2019 Seawind 1160 Lite
Posts: 2,126
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Re: Beam vs length on catamaran
Seawind 1000, LOA 33 ft, BOA 19.4 ft (1.70 ratio). Mast height was 52.5 ft above water. Sailed very well, and was roomy on deck and in saloon, and even maneuvering at dock was good - actually excellent! Put those dual outboard motors pretty wide, and it'll turn in its own length. It's exceptionally wide for its length, but I found no issues whatsoever.
My Seawind 1160 Lite is 38 ft long, 21.3 ft beam (1.78 ratio). Mast height is 61 ft above water level - I think. We've never hit VHF antenna on any 65 ft bridges!
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19-05-2020, 21:04
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#33
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Registered User

Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43 and OPBs
Posts: 12,891
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Re: Beam vs length on catamaran
Quote:
Originally Posted by sailjumanji
Seawind 1000, LOA 33 ft, BOA 19.4 ft (1.70 ratio). Mast height was 52.5 ft above water. Sailed very well, and was roomy on deck and in saloon, and even maneuvering at dock was good - actually excellent! Put those dual outboard motors pretty wide, and it'll turn in its own length. It's exceptionally wide for its length, but I found no issues whatsoever.
My Seawind 1160 Lite is 38 ft long, 21.3 ft beam (1.78 ratio). Mast height is 61 ft above water level - I think. We've never hit VHF antenna on any 65 ft bridges!
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LWL?
BOC?
LWL/BOC ratio?
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19-05-2020, 21:07
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#34
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Registered User
Join Date: Sep 2007
Location: Queensland, Australia
Boat: None at present--between vessels. Ex Piver Loadstar 12.5 metres
Posts: 1,476
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Re: Beam vs length on catamaran
Yes--the wider the beam the less it wants to stay on course.
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19-05-2020, 21:15
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#35
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Registered User

Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43 and OPBs
Posts: 12,891
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Re: Beam vs length on catamaran
Quote:
Originally Posted by Cliffhanger
A couple of points. First, overall beam can be misleading, as can overall length. Waterline length and hull centreline width (centre of bouancy) are the important measurements. A cat with narrow hulls will be very different to a cat with wide hulls even if the beam to length ratio is the same. Modern (cruising) cats tend to have wider hulls to allow more accomodation. So. the overall beam has to increase, thus ratios around 1:1.7 are becoming common. Add in plumb or even reverse bows on modern boats and overall dimensions become meaningless. Once you remove those variations, I think you will find that the hull centerline width to waterline length ratio is typically closer to 1:2.5 for almost all boats
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2.15:1 LWL/BOC for my "condomaran" Belize.
(Getting a published BOC figure is far from easy - I ended up establishing it myself)
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19-05-2020, 22:20
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#36
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Registered User
Join Date: Jun 2012
Location: Port Aransas, Texas
Boat: 2019 Seawind 1160 Lite
Posts: 2,126
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Re: Beam vs length on catamaran
Quote:
Originally Posted by StuM
LWL?
BOC?
LWL/BOC ratio? 
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Seawind 1160, 38 ft LOA, 37 ft LWL
Seawind 1000, 33 ft LOA, 32.3 ft LWL
Either way - LWL or LOA - it's still a wide boat. If you want to look at LWL/BOA, its 1.74 and 1.66, respectively. I've never seen a BOC published, but the hulls are not wide like a Lagoon or Leopard.
My comment was that these two boats are below the 2.0 threshold that someone said was necessary and ideal, and they are both very good sailing boats.
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19-05-2020, 22:32
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#37
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Registered User
Join Date: Jun 2012
Location: Port Aransas, Texas
Boat: 2019 Seawind 1160 Lite
Posts: 2,126
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Re: Beam vs length on catamaran
The Formula 60 "Earth Voyager" is 60.00 LOA, 58.00 LWL and 44.00 BOA (all in ft). It sails pretty decent. ;-)
Gougeon 35 is 35.50 LOA, 33.60 LWL and 22.20 BOA (all in ft).
Maine Cat 30 is 30.00 LOA, 29.25 LWL and 18.00 BOA, all in feet.
TRT 1200 is 39.33 LOA, 38.00 LWL and 24.92 BOA, all in feet. A very good sailing boat.
I'll stop. But I don't see where the limit should be LOA/BOA or even LWL/BOA at 2.0 or greater.
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19-05-2020, 22:34
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#38
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Registered User
Join Date: Jun 2012
Location: Port Aransas, Texas
Boat: 2019 Seawind 1160 Lite
Posts: 2,126
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Re: Beam vs length on catamaran
Quote:
Originally Posted by Mike Banks
Yes--the wider the beam the less it wants to stay on course.
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What?
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19-05-2020, 23:17
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#39
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Registered User

Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43 and OPBs
Posts: 12,891
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Re: Beam vs length on catamaran
Quote:
Originally Posted by sailjumanji
What? 
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It's theoretically feasible.
The wider the beam, the greater the turning moment if there is an imbalance in drag between the two hulls.
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20-05-2020, 02:59
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#40
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Moderator
Join Date: May 2014
Boat: Shuttleworth Advantage
Posts: 2,806
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Re: Beam vs length on catamaran
In a sideways capsize event the shape of the hull must also be considered. Keels or a deployed daggerboard will increase the tripping moment whereas a retracted daggerboard will not.
A wide beam with daggerboards up and sails down provides the safest platform in survival mode.
Capsize is now determined by relationship between beam and wave height and wave period.
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21-05-2020, 10:02
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#41
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Moderator
Join Date: May 2014
Boat: Shuttleworth Advantage
Posts: 2,806
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Re: Beam vs length on catamaran
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21-05-2020, 10:06
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#42
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Registered User
Join Date: Feb 2006
Posts: 2,816
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Re: Beam vs length on catamaran
Better title for this would be -"When does a boat become a raft?"
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21-05-2020, 11:22
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#43
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Moderator
Join Date: May 2014
Boat: Shuttleworth Advantage
Posts: 2,806
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Re: Beam vs length on catamaran
Rafts are very stable and very slow. How to build a fast raft.
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21-05-2020, 22:50
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#44
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Registered User
Join Date: Apr 2012
Location: Florida
Boat: Seawind 1000xl
Posts: 2,592
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Re: Beam vs length on catamaran
Quote:
Originally Posted by sailjumanji
Seawind 1160, 38 ft LOA, 37 ft LWL
Seawind 1000, 33 ft LOA, 32.3 ft LWL
Either way - LWL or LOA - it's still a wide boat. If you want to look at LWL/BOA, its 1.74 and 1.66, respectively. I've never seen a BOC published, but the hulls are not wide like a Lagoon or Leopard.
My comment was that these two boats are below the 2.0 threshold that someone said was necessary and ideal, and they are both very good sailing boats.
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If the truth be told while the Seawind 1000 was an OK boat the Seawind 1000xl was far superior. This was due to a one meter sugar scoop added to each hull. It made a huge difference in how sea kindly the boat was and probably added speed as well.
I had an interesting encounter with the USCG when switching the documentation on my boat. Since the sugar scoops don't add to the inter volume of the boat they are not counted in the length according to the USCG. I also have a six foot bow sprit (that folds against the forward cross member). So when I go into a marina they wanna charge me for a 42 foot boat but the USCG says my boat is 32'8" according to their rules. Really like to show the USCG documentation checking into the Bahamas for cheap entry.
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21-05-2020, 23:49
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#45
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Registered User
Join Date: Jan 2018
Boat: 50ft Custom Fast Catamaran
Posts: 12,231
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Re: Beam vs length on catamaran
Quote:
Originally Posted by StuM
Again, you're the one with the faulty logic. The hulls being "not too beamy" negates your arguent.
The "less beamy" the hulls are, the closer the LWL/BOA and LWL/BOC figures are to each other and the LOWER the LWL/BOC ratio for the same BOA.
If the hulls were "more beamy", then the LWL/BOC ratio would be comparatively higher than the LWL/BOA.
i.e. if the hulls were just vertical planks, the LWL/BOA and LWL/BOC would both be the same 1.797:1 on the quoted figures. As the hulls get beamier, if the BOA remains the same, their centrelines must be closer together and the LWL/BOC ratio increases.
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Ridiculous.
There is no faulty logic here.
I didn’t say anything opposite to what you just did. The less hull beam there is, the closer the overall beam is to the individual hull centerline beam is. How could anyone think otherwise?
What are you even talking about??
The overall beam and centerline beam are very close BECAUSE the hulls are narrow. SO AS I SAID, the Fusion 40 on the Morelli and Melvin site, as shown and as listed HAS A 2:1 beam. And performs properly.
Length Overall
40' 0"
Beam 22' 0"
2:1 is appropriate. There are deviations, but by and large, good performing cats are 2:1. If you skew that too far, you end up with less righting moment (easier to roll by wave action or unable to carry a decent rig to get her moving) or way too much righting moment, which could encourage a large rig and pitch pole/cartwheel behavior as you start burying bows.
2:1 is the magic ratio to hit the sweet spot between these 2 extremes.
Nothing I’m saying is ground breaking in the catamaran design world, so I’m kind of surprised there is a weird reaction here. It’s an accepted truth. You’re way off if you don’t understand this simple, fundamental truth to performance cats.
PS: your last paragraph is absurd. No one would ever constrict overall beam and widen hulls inside that, pushing the hull beams closer and closer together. The hull centerline is where the catamaran “grows” from as you add beam to individual hulls. So overall beam AND individual hull beam both rise at the same time as you fatten up an individual hull (and destroy the performance of the boat by adding all that hull beam).
In fact, overall beam grows 2x as fast as hull centerline beam as you fatten hulls.
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