Why Do Catamarans Capsize?

[aqfishing]

Originally Posted by fxykty

Ummm, no in short, your reserve stability theory is wrong.

The righting moment (what you’re calling reserve stability) of a typical cruising catamaran increases as the windward hull lifts up from its resting position. This is because less of the hull’s displacement is supported by water. The maximum righting moment is at the moment when the entire windward hull is just out of the water. It is absolutely not zero! It truly is the maximum righting moment.

Righting moment decreases to zero as the cat heels more - the point of zero righting moment is at about 80-85 degrees of heel. The fact that the righting moment is now decreasing means that a crew has to be very vigilant - I would think only racing crews ever get to this point.

It is highly unlikely that a cruising cat, including performance cruising cats like our one, could actually lift the windward hull out of the water through wind action alone. That would take a huge amount of force and likely the rig and/or structural elements supporting the rig would fail first.

Wave action certainly can add to wind force to lift the windward hull, but wave actions are usually intermittent and the windward hull crashes back down between waves. There’s demonstrably plenty of reserve stability at that point.

Correct. That was my thought exactly regarding the righting arm. Would be fun to see a stability curve of an average cat.

[Kaimana]

Originally Posted by fxykty

Ummm, no in short, your reserve stability theory is wrong. The maximum righting moment is at the moment when the entire windward hull is just out of the water.

I didn't even mention maximum righting moment in my post, so I have no idea why you're talking about it and telling me I'm wrong. I was talking about reserve stability, which is completely different.

I didn't say "Your righting moment is gone", I said "your reserve stability is gone". That's because there's NO MORE LEFT!

Once your windward hull is out of the water, you have no additional hull weight in reserve to hold that side of the boat down. That's what reserve stability means; there's some left. But once that hull is out, the reserve stability is gone, and you're sailing on a very thin edge.

It doesn't matter that you are at your maximum righting moment with the hull a foot out of the water! If you get a long enough gust that's 15 to 20 knots higher than the wind that put your hull out of the water, you're going over! Yes, I'm sure you will let go of sheets in plenty of time to fend off a capsize.

And I'm sure that many of the people who capsized big cats thought they had it under control, right up to the moment they went over.

It's just a risky attitude to take when your entire boat might be at stake, not to mention people's lives. And that's often one of the major causes of accidents at sea: someone thinking they're in control.

You want to read an almost-capsize story that happened on a 56-foot trimaran with 450,000 foot-pounds of righting moment? Go here: https://oceanpeople.org/why-a-proa/. I had years of sailing multis and tens of thousands of sea miles under my belt when that happened to me, and it scared the crap out of me.

Anyone else out there know any cats with 450,000 foot-pounds of righting moment, or close? Just out of curiosity, I'd like to know how big they are.

With Warm Aloha, Tim

[parachute]

ONe capsizw: https://www.sailingscuttlebutt.com/2...aran-capsizes/

[fxykty]

Originally Posted by Kaimana

I didn't even mention maximum righting moment in my post, so I have no idea why you're talking about it and telling me I'm wrong. I was talking about reserve stability, which is completely different.

I didn't say "Your righting moment is gone", I said "your reserve stability is gone". That's because there's NO MORE LEFT!

Once your windward hull is out of the water, you have no additional hull weight in reserve to hold that side of the boat down. That's what reserve stability means; there's some left. But once that hull is out, the reserve stability is gone, and you're sailing on a very thin edge.

It doesn't matter that you are at your maximum righting moment with the hull a foot out of the water! If you get a long enough gust that's 15 to 20 knots higher than the wind that put your hull out of the water, you're going over! Yes, I'm sure you will let go of sheets in plenty of time to fend off a capsize.

And I'm sure that many of the people who capsized big cats thought they had it under control, right up to the moment they went over.

It's just a risky attitude to take when your entire boat might be at stake, not to mention people's lives. And that's often one of the major causes of accidents at sea: someone thinking they're in control.

You want to read an almost-capsize story that happened on a 56-foot trimaran with 450,000 foot-pounds of righting moment? Go here: https://oceanpeople.org/why-a-proa/. I had years of sailing multis and tens of thousands of sea miles under my belt when that happened to me, and it scared the crap out of me.

Anyone else out there know any cats with 450,000 foot-pounds of righting moment, or close? Just out of curiosity, I'd like to know how big they are.

With Warm Aloha, Tim

I wasn’t familiar with the concept of reserve stability, so apologies for misconstruing your comments.

Even so, there is diminishing righting moment, but not absent righting moment, when going past the point of maximum righting moment. There is still plenty of energy addition required to go further and get to the point of capsizing and I don’t agree with you that it is a foregone conclusion that a cat will necessarily go on to capsize.

In a cruising situation, offshore, most cruising cats are typically under canvassed for the conditions to keep speed down for comfort. With full white sails most have gust resistance of 30-35 knots. That’s from a position at regular waterlines - 15-20 knots of gust is not going to capsize them. What cruiser sails around with their windward hull out or nearly out of the water in the first place?

The 50 knot gust that you describe with Tropic Bird was almost 3x the regular wind strength, wow! And even so, you didn’t capsize, despite going past zero reserve stability as you describe it.

There are very few reports of cruising cats, while cruising, of capsizing due to wind and/or waves. That’s why we know well of the few that have taken place - it’s rare and remarkable.

Unlike those cats that capsize while racing - those are often examples of the diminished reserve stability that you cite, with the windward hull flying for more speed. It takes a good helmsperson with knowledge of multihulls, as well as a fast and experienced crew, to race a multihull and understand that they’re close to the edge and what to do. Mistakes happen. But when cruising you should be a long, long way away from that edge.

Note that when sailing a cat or tri, if the wind is anything more than 50 degrees apparent then the correct gust response is to bear away so that the turning force pulls the windward hull down. Let fly the jib sheet as well, but not the main sheet as the added twist and depth would add more power up high.

If close hauled then letting go both sheets is correct, as they’ll lose their shape before more power can develop. But don’t turn to windward, as the turning force would reinforce the capsize force.

Systems like Up Side Up use load cells and heading information to determine the correct response - bear away, ease one or both sheets, etc.

[Icarus]

Never heard that the rigging will fail befor the cat turns turtle, it's nonsense.
I guess the form stability of a cat is good up to 14° of heel, then it diminishes rather quickly.

[aqfishing]

Originally Posted by fxykty

I wasn’t familiar with the concept of reserve stability, so apologies for misconstruing your comments.

Even so, there is diminishing righting moment, but not absent righting moment, when going past the point of maximum righting moment. There is still plenty of energy addition required to go further and get to the point of capsizing and I don’t agree with you that it is a foregone conclusion that a cat will necessarily go on to capsize.

In a cruising situation, offshore, most cruising cats are typically under canvassed for the conditions to keep speed down for comfort. With full white sails most have gust resistance of 30-35 knots. That’s from a position at regular waterlines - 15-20 knots of gust is not going to capsize them. What cruiser sails around with their windward hull out or nearly out of the water in the first place?

The 50 knot gust that you describe with Tropic Bird was almost 3x the regular wind strength, wow! And even so, you didn’t capsize, despite going past zero reserve stability as you describe it.

There are very few reports of cruising cats, while cruising, of capsizing due to wind and/or waves. That’s why we know well of the few that have taken place - it’s rare and remarkable.

Unlike those cats that capsize while racing - those are often examples of the diminished reserve stability that you cite, with the windward hull flying for more speed. It takes a good helmsperson with knowledge of multihulls, as well as a fast and experienced crew, to race a multihull and understand that they’re close to the edge and what to do. Mistakes happen. But when cruising you should be a long, long way away from that edge.

Note that when sailing a cat or tri, if the wind is anything more than 50 degrees apparent then the correct gust response is to bear away so that the turning force pulls the windward hull down. Let fly the jib sheet as well, but not the main sheet as the added twist and depth would add more power up high.

If close hauled then letting go both sheets is correct, as they’ll lose their shape before more power can develop. But don’t turn to windward, as the turning force would reinforce the capsize force.

Systems like Up Side Up use load cells and heading information to determine the correct response - bear away, ease one or both sheets, etc.

“ Note that when sailing a cat or tri, if the wind is anything more than 50 degrees apparent then the correct gust response is to bear away so that the turning force pulls the windward hull down. Let fly the jib sheet as well, but not the main sheet as the added twist and depth would add more power up high.

If close hauled then letting go both sheets is correct, as they’ll lose their shape before more power can develop. But don’t turn to windward, as the turning force would reinforce the capsize force.”

Hi,

I have often pondered this concept. Lets say you in a cruising cat chugging along and a massive gust hits you. As you are a lone helmsman I would say the only way you can respond with speed is with a helm adjustment. The thought of suddenly dumping sheets while under pressure and having them whipping through cleats at high speed scares the crap out of me. ( my thought here is that there would not be time to gently ease the sheets in a controlled manner.) There is then also the risk of a tangle and jamming the cleat, and this could be disastrous.

So lets say plan A is an immediate helm adjustment. At what point would one turn into the the wind, your comment above seems to suggest never, but lets say you are at 30 to 50 deg apparent (yes I know most cats don't do 30, but humour me!) would it not be a quicker way to get wind out your sails? Would that benefit not out weigh the turning force increasing the capsize force issue??
Then at what point is it definitely better to bear away? Obviously this would increase the risk of pitchpoling. But hopefully then your sails will be over-sheeted, thereby de powering them. This as all about getting you hull back down and allowing you time to adjust your sheets.
Keen to hear thoughts on the above. I for one do not know the answer.


Then on another note. It would be interesting to know how many cats that went over were on auto pilot at the time with no one behind the helm? (Helms person gone to make coffee or check on an issue).

[aqfishing]

Originally Posted by Kaimana

I didn't even mention maximum righting moment in my post, so I have no idea why you're talking about it and telling me I'm wrong. I was talking about reserve stability, which is completely different.

I didn't say "Your righting moment is gone", I said "your reserve stability is gone". That's because there's NO MORE LEFT!

Once your windward hull is out of the water, you have no additional hull weight in reserve to hold that side of the boat down. That's what reserve stability means; there's some left. But once that hull is out, the reserve stability is gone, and you're sailing on a very thin edge.

It doesn't matter that you are at your maximum righting moment with the hull a foot out of the water! If you get a long enough gust that's 15 to 20 knots higher than the wind that put your hull out of the water, you're going over! Yes, I'm sure you will let go of sheets in plenty of time to fend off a capsize.

And I'm sure that many of the people who capsized big cats thought they had it under control, right up to the moment they went over.

It's just a risky attitude to take when your entire boat might be at stake, not to mention people's lives. And that's often one of the major causes of accidents at sea: someone thinking they're in control.

You want to read an almost-capsize story that happened on a 56-foot trimaran with 450,000 foot-pounds of righting moment? Go here: https://oceanpeople.org/why-a-proa/. I had years of sailing multis and tens of thousands of sea miles under my belt when that happened to me, and it scared the crap out of me.

Anyone else out there know any cats with 450,000 foot-pounds of righting moment, or close? Just out of curiosity, I'd like to know how big they are.

With Warm Aloha, Tim

https://yacht-express.net/the-russians-came-to-help/

[chrisr]

Originally Posted by Icarus

Never heard that the rigging will fail befor the cat turns turtle, it's nonsense.
I guess the form stability of a cat is good up to 14° of heel, then it diminishes rather quickly.

sorry : on the typical cruising catamaran that we are talking about it is not nonsense

the heeling moment required to overturn a 10mt 6m beam cruising cat is more than a typical rig & sails could withstand

add wave action and it's a different scenario of course, but here we are talking about wind alone

cheers,

[Kaimana]

Originally Posted by chrisr

sorry : on the typical cruising catamaran that we are talking about it is not nonsense. the heeling moment required to overturn a 10mt 6m beam cruising cat is more than a typical rig & sails could withstand. add wave action and it's a different scenario of course, but here we are talking about wind alone.cheers,

1. The heeling moment required to capsize your catamaran is only a little more than that which is required to get your windward hull out of the water. If you can do that, without collapsing your rig, then your rig is strong enough to capsize your boat.

2. Provide some numbers. What IS that heeling moment on that 10mt X 6mt cat you refer to? How much does the windward hull weigh? What's the staying base? What's the rig? What size are the stays and shrouds? How much sail area does it have? What size is the spar section, and what's the longest unsupported section of mast?

Your "conclusions" are meaningless without knowing the strength of the rig, the righting moment of the boat, the sail area, and so on. I think your contention might be true, on an extremely under-rigged boat. If you have a "normal boat", spar and boat designers usually design the rig to handle shock loads of two to three times the maximum expected sailing load, and not collapse. This is because any normal boat can be expected to have to handle those loads; you get surprised with gusty wind conditions all the time if you're cruising.

3. There's ALWAYS waves! Where can you possibly sail where there is "only wind alone"? SF Bay? Lake Michigan? Chesapeake Bay? I've been to all those places and seen 10-foot wind-driven waves. But.

If you take all the sails down and remove the mast from a catamaran, it becomes extremely stable, just like a barge, and it is very unlikely to capsize unless you have hurricane-force winds and waves.

If you're sailing a cat hard and fast in big waves, and you poke a bow into a wave, your cat can "trip" and go a**-over-teakettle into a capsize (pitchpole). In this instance, the wave contributed to, but didn't cause the capsize. It was the "sailing too hard and fast" that caused the capsize.

Now, when you hit the water in a 10-15 knot pitchpole after sailing too hard and fast, THAT could easily snap your mast! This is because the mast, sails, and rig are designed for the forces of WIND, not the force of WATER, which is a lot more solid and unyielding.

With Warm Aloha, Tim

[fxykty]

Originally Posted by aqfishing

“ Note that when sailing a cat or tri, if the wind is anything more than 50 degrees apparent then the correct gust response is to bear away so that the turning force pulls the windward hull down. Let fly the jib sheet as well, but not the main sheet as the added twist and depth would add more power up high.



If close hauled then letting go both sheets is correct, as they’ll lose their shape before more power can develop. But don’t turn to windward, as the turning force would reinforce the capsize force.”



Hi,



I have often pondered this concept. Lets say you in a cruising cat chugging along and a massive gust hits you. As you are a lone helmsman I would say the only way you can respond with speed is with a helm adjustment. The thought of suddenly dumping sheets while under pressure and having them whipping through cleats at high speed scares the crap out of me. ( my thought here is that there would not be time to gently ease the sheets in a controlled manner.) There is then also the risk of a tangle and jamming the cleat, and this could be disastrous.



So lets say plan A is an immediate helm adjustment. At what point would one turn into the the wind, your comment above seems to suggest never, but lets say you are at 30 to 50 deg apparent (yes I know most cats don't do 30, but humour me!) would it not be a quicker way to get wind out your sails? Would that benefit not out weigh the turning force increasing the capsize force issue??

Then at what point is it definitely better to bear away? Obviously this would increase the risk of pitchpoling. But hopefully then your sails will be over-sheeted, thereby de powering them. This as all about getting you hull back down and allowing you time to adjust your sheets.

Keen to hear thoughts on the above. I for one do not know the answer.





Then on another note. It would be interesting to know how many cats that went over were on auto pilot at the time with no one behind the helm? (Helms person gone to make coffee or check on an issue).

This describes gust response where the wind momentarily increases 50%+.

Going to windward 30-50* AWA, it will be faster to dump the sheets rather than to turn the boat to depower the rig. Try both in moderate conditions. And if you have any speed at all, the lifting force of the fast turn is very big and exactly what you don’t want if the windward hull is lifting.

There won’t be all that much sheet running - on our boat 2:1 jib sheet and 4:1 mainsheet would only pull sheets out about 2m for the jib and 5m for the main, when going upwind. Working sheets should always be flaked and ready to run on a multi. This is how we keep our sheets ready to release on our self tailing winches (clutches are always open on working sheets).

Anything more than 50* AWA, or very close reaching to beam reaching, turn to a broad reach. You can blow the jib sheet but it will likely start to be blanketed by the main anyway. If there is more than one person then the second person can wind in the main to reduce its exposed area.

Remember, this is a gust, so wave conditions will be relatively low - attentive helming at an angle to the waves will avoid pitch poling. At night with zero visibility just keep the AWA at 150-160*.

Unlike Hobie Cats (I raced H16s for many years in my youth and can attest to many pitch poles and trips around the forestay), modern cruising cats have plenty of volume in the bows (not sure about reverse bows though) and it would need to be a really big wave to pitch pole - not something that you would encounter in a gust from 10 or 15 or 20 knots of steady wind.

We’ve been caught by squalls on passages a few times, usually nighttime but once in a fast moving front, where we’ve done one or the other. It’s pretty exciting powering on a broad reach during the gust and really hoping it finishes soon.

Regarding autopilot, for cruising cats most likely. Though hand steering could be a good response to squally conditions, in addition to radar, flaked sheets, prepared quick releases, and over reefing.

[DHLyman]

I ran this question by my son, whose in Solaent U, studying marine engineering and yacht design. He said, cats are designed to be most stable, upside down. Keel boats will take a knock-down and right themselves. A keel mono-hull can roll 360 degrees, turn-turtle, and right itself. A cat can’t do that. As some one has written, when a cat reaches a particular angle of heel, it quickly becomes unstable, and flips upside-down, and stays that’s way. Maybe they can fix an auxiliary mast and rig to the bottom of an in-turned cat, and sail it that way.

[qwerme]

Originally Posted by DHLyman

He said, cats are designed to be most stable, upside down.

That's a BS way of putting it. By the same logic keelboats are designed to be most stable at the bottom of the ocean.

Cats have stability of form contrary to stability of mass. You can look this up, a lot of modern fast monohulls are also designed around stability of form, which means they also have a larger area of stability upside-down like cats.

an inverted IMOCA 60 will not right itself more than a Cat if it's canting keel isn't moving.

[chrisr]

Originally Posted by DHLyman

I ran this question by my son, whose in Solaent U, studying marine engineering and yacht design. He said, cats are designed to be most stable, upside down. Keel boats will take a knock-down and right themselves. A keel mono-hull can roll 360 degrees, turn-turtle, and right itself. A cat can’t do that. As some one has written, when a cat reaches a particular angle of heel, it quickly becomes unstable, and flips upside-down, and stays that’s way. Maybe they can fix an auxiliary mast and rig to the bottom of an in-turned cat, and sail it that way.

firstly : yes, the mono will right itself...assuming the keel has not fallen 0ff

secondly : ask your son how well a mono full of water floats. the honest answer will be "like a rock"

thirdly : "designed to be most stable, upside down" implies this is the intention of the designer, which clearly it is not

what is true is that a mono has zero righting moment when vertical whereas a cat has nearly it's maximum righting moment in that condition.

cheers,

[barryglewis]

[QUOTE=aqfishing;3375052]

Originally Posted by barryglewis

CG is centre of gravity, CB is centre of buoyancy[/QUOTE

Ok. Thought as much. I know them as G and B.
Maybe I should have been a bit clearer in my view. Lets say you take a cat and move the hulls further apart therefore increasing the beam. For sake of simplicity we will ignore the added weight. It will in any way it will only have a very slight effect on B. So basically B will stay in the same place. We agree that increasing the beam will improve stability, so then seeing as B still the same, G will have to move down vertically in order to increase your GM which is a measure of your stability and stiff/tenderness.

Now as far as I understand, the distance between G and B has no relation to the vessels stability, but GZ will indicate the vessels ability to right itself at various angles.

Hence the saying that G will move vertically up or down as the beam increases or decreases.

CB only stays in same place while vessel is upright. When heeled the CB moves toward the low side , giving a lever arm opposing the CG.
Increasing the beam increases the righting arm, the horizontal distance between CG and CB.
In the case described above, where the assumption is made that increasing the beam has a negligible adding weight the CG will remain in the same place.
Here is a definition of CG from the introduction to "The Principles of Ship Stability" , by Capt. Taylor:
"The weight forces of a ship, which are made up of the weight of the ship itself and the added effect of stores, water and cargo, act vertically downward through the centre of the mass. This point, through which these forces act is the Centre of Gravity and is entirely dependent for its position upon the distribution of the various weights. It is in no way affected by any other factor. "

[aqfishing]

[QUOTE=barryglewis;3378554]

Originally Posted by aqfishing

CB only stays in same place while vessel is upright. When heeled the CB moves toward the low side , giving a lever arm opposing the CG.
Increasing the beam increases the righting arm, the horizontal distance between CG and CB.
In the case described above, where the assumption is made that increasing the beam has a negligible adding weight the CG will remain in the same place.
Here is a definition of CG from the introduction to "The Principles of Ship Stability" , by Capt. Taylor:
"The weight forces of a ship, which are made up of the weight of the ship itself and the added effect of stores, water and cargo, act vertically downward through the centre of the mass. This point, through which these forces act is the Centre of Gravity and is entirely dependent for its position upon the distribution of the various weights. It is in no way affected by any other factor. "

“” CB only stays in same place while vessel is upright. When heeled the CB moves toward the low side , giving a lever arm opposing the CG”

We are not talking about the vessel healing, we are talking about what happens when you increase/decrease the beam. Then G will move in a vertical line as per the OP of that discussion. I think you missing the point of the discussion.

Nice definition of G. However as per your definition increasing or decreasing the beam will have an effect on how the items mentioned effect G.