Beam vs length on catamaran

[StuM]

Originally Posted by Chotu

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.

Fusion 40 or Formula 40? Two different cats.


Once again we see a totally illogical argument from you.

The figures you listed from the website give a LWL/BOA ratio of 1.797:1.

Previously you used the fact they the boat has very narrow hulls to supposedly explain how that figure changes from 1.797:1 to an un-referenced 2:1.

You now agree that the very narrow hulls means that the LWL/BOC will be close to the LWL/BOA, negating your previous reasoning.

Yet somehow in your mind that very narrowness explains how 1.797:1 becomes 2:1.

[StuM]

Originally Posted by Chotu

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).

.
No, your interpretation of it is absurd.
Who mentioned construction?

I was simply illustrating the fact that for a listed BOA (which EVERY builder advertises widely), the wider the hull (rarely stated), the smaller the BOC (even more rarely stated).

[StuM]

Still waiting to see a reference to the alleged 2:1 for the Formula 40. I can't find that number anywhere on the linked M&M site.


Incidentally, here's another Formula 40 with very different dimensions. So which one was Tupaia talking about in post #11 before Chotu chimed in with his numbers?


http://www.multihulldesigns.com/desi...ck/f40cat.html

[StuM]

And another Formula 40 built to the same rule, which is more like 1:1 ! (But it's tri )



Length overall: 39'-11" (12.16 m)
Length at waterline: 39'-9" (12.16 m)
Beam: 39'-4" 12.0 m
Draft: 1'-3"/8'-10" (0.38/2.7 m)
Weight: 3,950 lb (1,791.7 kg)
Sails - Main: 787 sqft. (73.1 sqm.)
Blade: 323 sqft. (30.0 sqm.)
Spinnaker: 925 sqft. (85.9 sqm.)
Displacement: 4,466 lb (2,025.74 kg)

[StuM]

And another Gino Morelli Formula 40, but this time 40ft x 24ft as opposed to the 40ft x 22ft on the M&M site.


Boat for sale - Morrelli Formula 40 Catamaran RD Boatworks Soma - 40'


IOW, debating the L/B ratios (plural) of a "Formula 40" is a totally pointless exercise

[Tupaia]

Originally Posted by Chotu

Length Overall
40' 0"
Beam 22' 0"

This is 1.818:1 not 2:1

[NevilleCat]

Originally Posted by StuM

.
No, your interpretation of it is absurd.
Who mentioned construction?

I was simply illustrating the fact that for a listed BOA (which EVERY builder advertises widely), the wider the hull (rarely stated), the smaller the BOC (even more rarely stated).

Well that's true. I had an old Seawind 31' open bridgedeck cruising cat which was 23' wide, but the hulls had a lot of flare so were over 6' wide at deck level.
So I guess the centre-line beam was probably only 17' on maybe 29'

[Happ]

Originally Posted by dustman

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.

Why would want to? What do you think you would gain?

[Night_Sailor]

Originally Posted by dustman

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.

Well for one, only a few shipyards on the east coast that can haul-out a really wide catamaran (30' +). Most marina's can't haul-out these wide cats.

Wider is more stable, but some of the wider cat's needed structural reinforcement on the cross beams long after constructed. I forget whether it was the Priviledge 65 or Lagoon 67.

But to answer your question. NO! IT MAKES NO SENSE TO HAVE SOMETHING AS WIDE AS IT IS LONG.

[Calculator]

I have an Excel file with data for 1110 cruising catamarans. Graphing waterline length vs. beam centerline and seeking the trendline shows a ratio of about 0.52 at 20 feet LWL to 0.35 at 150 feet. There is a cloud of points around 30-50 feet at 0.45. It's important to note that a boat sails on waterline length and beam centerline, not length and beam overall.

Wharram's boats range from 0.37-0.57.

The database is mainly from boats from the 1960s through 2000. Today's designers and manufacturers are very reluctant to provide valid design or cruising displacement values.

[davenrino]

Originally Posted by barnestim

A good observation, to which it might be worth adding that pitchpoling risk can be mitigated to some degree by bow volume, among other things. As I understand it, pitchpoling is usually the result of digging in the lee bow, leading to sudden deceleration, rapid increase in apparent wind, and the concomitant increase in overturning moment.

On a much smaller scale, our Hobie 16 loved to pitchpole when flying a hull. I had to keep a close eye on the lee bow and still occasionally took a ride in the trapeze around the front of the boat. The Hobie 18 and the Prindles had much deeper bows and weren't so prone to cartwheeling. Weight was also a factor as it wasn't as much of a problem solo in the trapeze as when two big guys trapped out.

[Svan]

Our PDQ is 36 x 18. 18’3” including the rails. This barely fits in the widest of travel lifts.

On the water performance considerations are academic at best for practical matters as manoeuvring close to dock and taking it out every year.

[motu42]

I find these kinds of problems are easy to visualise if you take them to extremes.


1) make the beam 1/10th of the waterline (10:1)- it will fall over sideways in 5knts
2) make the beam 10 x the waterline (10:1) - it will fall over forwards in 5 knts.


Around 2:1 is typical, but plenty of variation exists. Boats are all compromise

[tomfl]

Originally Posted by Svan

Our PDQ is 36 x 18. 18’3” including the rails. This barely fits in the widest of travel lifts.

On the water performance considerations are academic at best for practical matters as manoeuvring close to dock and taking it out every year.

Fairly easy to find travel lifts that are 50 feet wide if you are willing to travel, and you can get them special order even wider. I have never had an issue finding a travel lift for my Seawind with a 19' beam, Marathon, Key West, Tampa, Pensacola were all used at one time or another.

[Old fella]

Originally Posted by dustman

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.

I read somewhere years ago that the max beam of a boat can no more than 45 per cent of the length, if I remember correctly it had something to do with it's handling. But this might be only for mono's, not sure.