Catamaran science.^

[BurnitBlue]

Why will a motorboat or other monohull sailing vessel or otherwise capsize actually easier than a catamaran of the same beam. Or will it. A bit baffled by the science. Google has diagrams of the various types but no discussion with comparison using words instead of numbers.

This leads to optimum beam/length ratio against capsize or pitch-pole. Some cats are not far off having close to square shape where beam is not far short of the LOA.

I have just returned to catamaran sailing after my last monohull.

 

[davidej]

I would like to see a mono hull with a beam equal to its length. Then we could see whether it would capsize easier than a catamaran .

I would like to add that most monohulls have a beam about a third of their length so it’s not surprisingly that they heel more than a catamaran.

 

[Chiara’s slave]

Plenty of cats around of similar dimensions to monos. Motor cats mostly, it’s true. Capsize resistance is down to the very rapid movement of the centre of buoyancy as they heel. In monos, it certainly moves, but not as suddenly. Hence that rather brisk multihull motion.

 

[BurnitBlue]

I would like to see a mono hull with a beam equal to its length. Then we could see whether it would capsize easier than a catamaran .

I would like to add that most monohulls have a beam about a third of their length so it’s not surprisingly that they heel more than a catamaran.

One fact that interferes with my perception of the big picture is that I started to consider my first catamaran as a single hull but with a masssive outrigger giving tremendoud leverage. Of course downwind she was a catamaran but when the wind moved to either beam she became a polinesian canoe.

This was aboard a fairly simple Prout Snowgoose

 

[DownWest]

One fact that interferes with my perception of the big picture is that I started to consider my first catamaran as a single hull but with a masssive outrigger giving tremendoud leverage. Of course downwind she was a catamaran but when the wind moved to either beam she became a polinesian canoe.

This was aboard a fairly simple Prout Snowgoose

Another big factor is ballast and position below the centre of buoyancy. Cats usually don't carry extra ballast.

 

[BurnitBlue]

Plenty of cats around of similar dimensions to monos. Motor cats mostly, it’s true. Capsize resistance is down to the very rapid movement of the centre of buoyancy as they heel. In monos, it certainlyi moves, but not as suddenly. Hence that rather brisk multihull motion.

Sounds logical but why would the centre of buoyancy move more rapid on a catamaran then on a monohull. Again my perception swings to leverage and the length of the arm. That Aristotle feller from Greece with his long plank moving the earth could be the first multihull designer. Oh, well I have quickly stepped outside my competance. I will shut up for a bit.

 

[mjcoon]

Sounds logical but why would the centre of buoyancy move more rapid on a catamaran then on a monohull. Again my perception swings to leverage and the length of the arm. That Aristotle feller from Greece with his long plank moving the earth could be the first multihull designer. Oh, well I have quickly stepped outside my competance. I will shut up for a bit.

You are exactly right about the length of arm. The secondary matter is the angle of that arm. If it goes vertical (mast horizontal) the length, so far as righting effect is concerned, is now zero...

 

[Chiara’s slave]

Sounds logical but why would the centre of buoyancy move more rapid on a catamaran then on a monohull. Again my perception swings to leverage and the length of the arm. That Aristotle feller from Greece with his long plank moving the earth could be the first multihull designer. Oh, well I have quickly stepped outside my competance. I will shut up for a bit.

MJ is talking about angle of vanishing stability there of course, and it’s why cats do capsize if you push them hard enough. Fortunately most of us have more sense. Centre of buoyancy… on both monos and cats, when upright the centre of lateral buoyancy is in the middle. Both the cat hulls are equally immersed. However, at, say, 15 degrees of heel (variable according to design) the mono has moved itks centre of buoyancy a couple of feet maybe, it it’s a big fat bottomed thing. Less if it’sca plank on edge cutter, etc etc. the cat, though, has shifted it’s COLB to the centre line of itks downhill hull, which is gojng to be more than a couple of feet. It’s resistance to further heeling is at maximum, and hopefully there it will stay. Time to start easing sheets, and if it is happening in every gust, well past time to reef! The mono will heel much further, picking up righting moment as it goes.

 

[DownWest]

MJ is talking about angle of vanishing stability there of course, and it’s why cats do capsize if you push them hard enough. Fortunately most of us have more sense. Centre of buoyancy… on both monos and cats, when upright the centre of lateral buoyancy is in the middle. Both the cat hulls are equally immersed. However, at, say, 15 degrees of heel (variable according to design) the mono has moved itks centre of buoyancy a couple of feet maybe, it it’s a big fat bottomed thing. Less if it’sca plank on edge cutter, etc etc. the cat, though, has shifted it’s COLB to the centre line of itks downhill hull, which is gojng to be more than a couple of feet. It’s resistance to further heeling is at maximum, and hopefully there it will stay. Time to start easing sheets, and if it is happening in every gust, well past time to reef! The mono will heel much further, picking up righting moment as it goes.

Then, one gets to extreme conditions, when a mono can reduce sail and heave too. A cat may not have that ability..

Edit: My experience in cats is with an extreme design with heavily raked mast and unusual hull design. Much later, an ex racer with a reduced rig, very fast, but in benigh conditions.

 

[Chiara’s slave]

Then, one gets to extreme conditions, when a mono can reduce sail and heave too. A cat may not have that ability..

Edit: My experience in cats is with an extreme design with heavily raked mast and unusual hull design. Much later, an ex racer with a reduced rig, very fast, but in benigh conditions.

We are multihull sailors for the last 35 years. Multis have different strategies in extreme conditions. Obviously that involves shortening sail, we start that at anything over 15kn. Our Dragonfly is probably the most extreme design we have owned though, things have been heading that way, as they have with monos. The newer Dragonflys are another small step in the same direction. The onus is on the skipper and crew to make the safe calls. A mono can look after even those that evolution has passed by. Though clearly it helps if you can think in a mono too

 

[cpedw]

Is it important that, as it heels, a cat's Centre of Gravity moves significantly upwards? Whereas a mono's CoG moves comparatively little with heeling.

 

[BurnitBlue]

I am getting a feel for the forces involved. A monohull has the leverage in the centre underneath the CoB. This will be the ballast which is free to move short distances with the heel, like a pendulum, hence the rolling motion. To move that effective leverage more outboard it needs the crew lining the rail.

As a catamaran heels the leverage moves outboard as the windward hull loses some of its buoyancy. The problem can get worse if not corrected when the windward hull lifts in an arc which vectorially will start to move the leverage inboard. Early sail reduction to keep the windward hull down Need to be watched closely.

I think this where the Wharram Cats have accidentally found a solution. To lift the windward hull the slack and stretch of the beam lashings must be taken up first. This makes a Wharram a Swinging "ama" similar effect of the swinging keel.

If I am right will we see "Swinging hulls" on future generations of cats. The cat can be made uncapsizable and faster.

 

[Chiara’s slave]

Is it important that, as it heels, a cat's Centre of Gravity moves significantly upwards? Whereas a mono's CoG moves comparatively little with heeling.

The CoG is fixed by the design, construction and loading of any boat

 

[DownWest]

I am getting a feel for the forces involved. A monohull has the leverage in the centre underneath the CoB. This will be the ballast which is free to move short distances with the heel, like a pendulum, hence the rolling motion. To move that effective leverage more outboard it needs the crew lining the rail.

As a catamaran heels the leverage moves outboard as the windward hull loses some of its buoyancy. The problem can get worse if not corrected when the windward hull lifts in an arc which vectorially will start to move the leverage inboard. Early sail reduction to keep the windward hull down Need to be watched closely.

I think this where the Wharram Cats have accidentally found a solution. To lift the windward hull the slack and stretch of the beam lashings must be taken up first. This makes a Wharram a Swinging "ama" similar effect of the swinging keel.

If I am right will we see "Swinging hulls" on future generations of cats. The cat can be made uncapsizable and faster.

Ah, no. The lashings on a Warram are to put a slight bit of flexability in the joints. An entirely rigid joint has very high stresses. The joints have no influence on the roll stability. The short rig and low sail area are much more influential and most of them have no 'top hamper' in the way of cabins.
Monos stiffen up with some heel, as the ballast keel CoG moves from below the CoB to windward, exerting more righting moment. Bit like putting crew on the rail..

 

[TLouth7]

So your hypothetical cat and mono have the same beam. Do they have the same waterline beam or does the mono have a narrower waterline? Do they both carry the full waterline beam all the way to the ends or does the mono taper to bow and stern?

As a vessel heels slightly a wedge of waterplane is submerged on one side and exposed on the other. Area further from the roll axis contributes more righting moment - initial stability is determined by the second moment of area of the waterplane. A catamaran will have its waterplane concentrated quite far from the roll axis, whereas a mono has most of its waterplane close to the centreline.

 

[AntarcticPilot]

I'm following this with interest but isn't another factor that the centre of lateral resistance can move sideways on a cat, as one hull lifts and the opposite one digs in? I don't mean when flying a hull but just normal heeling, which of course is slight but not negligible in a cat. So rather than being more or less on the centreline of the vessel, the reaction to lateral forces imposed on the centreline by the rig moves to leeward as the wind increases, thus increasing the lever arm? Ultimately, of course, that can result in a cat "tripping" over the leeward hull.

I shoudl say that I am not a catamaran sailor, and this is thinking out loud!

 

[jamie N]

I would like to see a mono hull with a beam equal to its length. Then we could see whether it would capsize easier than a catamaran .

Ah, a coracle

 

[BurnitBlue]

I'm following this with interest but isn't another factor that the centre of lateral resistance can move sideways on a cat, as one hull lifts and the opposite one digs in? I don't mean when flying a hull but just normal heeling, which of course is slight but not negligible in a cat. So rather than being more or less on the centreline of the vessel, the reaction to lateral forces imposed on the centreline by the rig moves to leeward as the wind increases, thus increasing the lever arm? Ultimately, of course, that can result in a cat "tripping" over the leeward hull.

I shoudl say that I am not a catamaran sailor, and this is thinking out loud!

That opens up another anomoly regarding dagger boards. Should the windward dagger board be down to act as drag and therefore stability leaving the leward dagger but up. Certainly in my opinion if the leeward dagger board was down with the windward dagger board up, then the cat will trip over its dagger board.

I am not sure about sliding. Apart from decreasing the apparant force. Sliding and tripping are dynamic and only experience will answer the question, good or bad.

 

[boomerangben]

The resistance to capsize is essentially the area beneath the portion of the heeling moment curve above the x axis. A quick look on google shows some graphs of both cats and monos, with the cat being a narrow pointy looking peak compared with the long undulating curve of a monohull. It seems that the angle of vanquishing stability of cats might well be less than 90 degrees, especially if the CoG is above the CoB when upright.

So I’m not convinced that a cat has more resistance to capsize than a monohull. Without real figures it’s hard to be definitive but I think I’d rather be on a monohull than a cat if the C word becomes imminent.

 

[capnsensible]

They only turn upside down once.