Beam vs length on catamaran

[belizesailor]

Originally Posted by tomfl

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.

Or find a yard with an articulated trailer and the beam becomes irrelevent. Also easier on the boat.

[belizesailor]

Originally Posted by Old fella

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.

Not an issue/rule for cats. Its common to have beams well over 50% of LOA. Mine is about 63%.

[dustman]

Originally Posted by Happ

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

I hope to gain stability, I don't want to worry about being knocked over taking a wave on the beam, I hope to be able to carry a decent amount of sail without the risk of being blown over by a gust, I hope to build it where I am and trailer it to the ocean. With my design the hulls and crossmembers would be removable so beam is not a limitation, as much as length. The hulls will have no accommodations.

[Buzzman]

In the multihull design threads over on BoatDesign.net, it is generally accepted that beams are wider now becasue it's desirabe for speed, as providing a greater righting moment and therefore resistance to heeling, thus enabling carriage of taller mast and larger sail area.

As someone pointed out, tris are getting up towards 1:1 in the extreme racing categories, but 75-85% beam/length ratio is considered 'normal' whereas in the Seventies and even Eighties tris for example were more like 60-70% beam/length.

There is also lots od discussion on volume of floats. Smaller beach cats (Hobie et al) used to have tiny volume floats, and so would pitchpole easily. This has increaswe dover the easrs such that performance floats are often 100% displacement up to 200%, but 150% considered more usual.

Again, this is to resist heeling so as to enable taller mast and larger sails and thus more speed.

Note the above is more about tris, but similar 'push' is evident in cats.

Production cats different story as haul out ease has to be considered.
Racing cats and tris hire a crane to lft them straight into the water, as very few ramps or slips wide enough, never mind travel lifts.

[Chotu]

Finally, people who know what they’re talking about here. It’s been an extremely painful thread with the utter lack of knowledge regarding modern catamaran design so I just stopped. Life’s too short to argue with people who don’t read and just want to argue what they “believe “ without actually understanding the current trends in performance SALING catamaran design. Not talking about those power cats they put a little mast on top of the layer cake and call it a sailing cat.


To the OP:

You’re on the right track.

[catsketcher]

Peter Wormwood is right. You have to balance all the different types of stability. I sailed an almost square trimaran for about 7 years - it was a bit strange as it was much more stable sideways than fore and aft. There were three capsizes of the boat design, all of them over the nose, by pitchpole.

Sideways stability is easy to calculate and easy to increase, make the boat wider centreline to centreline and/or increase weight. But increasing fore and aft stability is trickier.

You could try to increase bow volume. You can do this by increasing beam of the sections forward. If you do this too much then when the hull is stuck into a wave at speed then the deck will provide a downward acting force, counter to the reason for adding volume. This is why racing cats have reverse bows - to increase volume without increasing and even reducing immersed drag at speed. It is also the reason I shake my head at reverse bows on cruising cats. Reverse bows work because immersed drag is low, and yet the reverse bow cruising cats have forebeams, nets, stanchions, cleats and other paraphenalia that ensure there is no reduction on drag when immersed anyway.

Calculating the fore and aft (longitudinal) stability of a cat at speed is very tricky and so usually not done well. Our boats, especially cruising cats can undergo a reversal of stability increase which confounds easy design. As a bow is driven lower (say on a broad reach) stability increases but as soon as the bow goes under stability decreases because the moment (or couple) reduces as the bow cannot push up as hard as it did with its deck just above the surface. So you pitchpole. Go play on a beach cat and observe.

It is also really dumb to increase sideways stability more than longitudinal. That is because you will be sailing upwind, using sideways stability and then bear away and need longitudinal stability. You can ease mainsheet to get the boat back on its feet if sideways stability is low, but when square running or broad reaching, easing mainsheet does not help bring the boat back when the bows go down. Or you don't want a boat to have heaps of siadeways stability and then bear away and pitchpole because longitudinal is low. My square tri was like this, great on a beam reach but a little dangerous on a square. It was a vice I lived with but I did not want again.

So in the end about 50% beam to length (centrelines and LWL) is nice. This gets you to Peter Wormwood's clever tip about both modes of stability being equal and the boat being without surprises in terms of stability.

[Tupaia]

There is an excellent article here using real life designs that analyse in detail the Beam vs Length issues for multihulls.

Multihull Dynamics, Inc. - News Article

The summary is here
Summary.jpg

[sailjumanji]

Originally Posted by Chotu

Finally, people who know what they’re talking about here. It’s been an extremely painful thread with the utter lack of knowledge regarding modern catamaran design so I just stopped. Life’s too short to argue with people who don’t read and just want to argue what they “believe “ without actually understanding the current trends in performance SALING catamaran design.
.

Geezus Chotu, you have a lot of nerve with that one. "Utter lack of knowledge"? Lets see, it was only a few posts back that you said:

Originally Posted by Chotu

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.

And then you quoted an example of the perfect Fusion 40 (which by the way is a Formula 40) design by Morelli and Melvin, which was 40 ft LOA and 22 ft BOA and was your perfect 2:1. Which, by the way, is only 1.8 to 1. So in your argument would be too wide for your perfect sweet spot, that everyone should just accept, because otherwise we are too dumb to understand your simple, fundamental truth.

Oh, and this gem, when you redid the math and proudly proclaimed that numbers direct from Morelli and Melvin website yielded - incorrectly BTW, because it was still 40 ft LOA and 22 ft BOA - a ratio greater than 2:1.

Originally Posted by Chotu

The REAL formula 40 specs. Straight off the Morelli and Melvin website... MORE than 2:1!

You cannot even get the math right. Frankly, I thought you quit posting on this subject because you finally realized it wasn't OUR "utter lack of knowledge". Even after several people provided a list of good-sailing cats that were lower than your 2:1 ratio.

And finally, how many modern catamarans are at risk of pitchpole/cartwheel behavior? Since this is YOUR reason that a cat has to be at least 2:1 LOA to BOA ratio?

[sailjumanji]

For those wondering about cruising cat LOA/BOA ratios. Seems there are a lot of designers that don't feel a minimum 2:1 ratio is necessary.

[smj]

Originally Posted by sailjumanji

For those wondering about cruising cat LOA/BOA ratios. Seems there are a lot of designers that don't feel a minimum 2:1 ratio is necessary.

Pretty sure the Norseman 43 was the old 40 with stern extensions which puts the beam at about 25’ not 29’.
Of course I’m saying this because I want to see the TRT 1200 at the top of the list!
Also if you look at the Shuttleworths I think they would be at the top of the list.

[NevilleCat]

I don't understand why this discussion even continues using BOA and LOA when surely the main things that matter as far as stability goes are the BWL and LWL.
Sure Shuttleworth designs have wide beam overall but that's mostly because of massive flare above the waterline.
As far as the OP's intentions, he has already stated there will be no accommodation in the hulls, so, depending on what size vessel he is designing and it's load carrying ability then he could conceivably make the hulls quite narrow so BWL is what he needs to judge. The interesting thing would be the engineering involved in making it all demountable and still have the widest beam practical. Or perhaps I am misunderstanding and it is only demountable for the purpose of moving it from his place to complete the assembly closer to a launch place?

[smj]

Originally Posted by NevilleCat

I don't understand why this discussion even continues using BOA and LOA when surely the main things that matter as far as stability goes are the BWL and LWL.
Sure Shuttleworth designs have wide beam overall but that's mostly because of massive flare above the waterline.
As far as the OP's intentions, he has already stated there will be no accommodation in the hulls, so, depending on what size vessel he is designing and it's load carrying ability then he could conceivably make the hulls quite narrow so BWL is what he needs to judge. The interesting thing would be the engineering involved in making it all demountable and still have the widest beam practical. Or perhaps I am misunderstanding and it is only demountable for the purpose of moving it from his place to complete the assembly closer to a launch place?

Wouldn’t the flare above the waterline ad a lot of buoyancy to give more stability on the beam as well stop the bow from diving which would give more stability fore and aft?

[StuM]

Originally Posted by NevilleCat

I don't understand why this discussion even continues using BOA and LOA when surely the main things that matter as far as stability goes are the BWL and LWL.

No, it's LWL and BOC


BWL = Beam Water Line i.e. the width of a single hull at the waterline
BOC = Beam On Centreline (sometimes also called BCB)

[NevilleCat]

Originally Posted by smj

Wouldn’t the flare above the waterline ad a lot of buoyancy to give more stability on the beam as well stop the bow from diving which would give more stability fore and aft?

Yes you are right; I was referring to the OP's original question.
Although with Shuttleworth's flare being more like a knuckle quite a way above the waterline I assume increased buoyancy doesn't really come into play until the hull is so depressed that the captain needs clean underwear.

[NevilleCat]

Originally Posted by StuM

No, it's LWL and BOC


BWL = Beam Water Line i.e. the width of a single hull at the waterline
BOC = Beam On Centreline (sometimes also called BCB)

Great, thanks for that; beam on centreline is what I meant. BOC then.
Not usually quoted but that's what matters in this case rather than the overall beam yes?