Why Do Catamarans Capsize?

[edexter]

Great article on multihull stability in the January 2021 Practical Sailor magazine, see https://www.practical-sailor.com/sai...ize-risk-check. Discusses reasons why they go over, provides examples, and how to prevent it.

[barryglewis]

How does increasing the beam move the CG down?
Increasing beam increases stability by increasing the distance between CG and CB when the vessel heels.

[aqfishing]

Originally Posted by barryglewis

How does increasing the beam move the CG down?
Increasing beam increases stability by increasing the distance between CG and CB when the vessel heels.

Hi,
What is CG and CB???

[barryglewis]

CG is centre of gravity, CB is centre of buoyancy

[Manateeman]

Catamarans capsize because of improper construction material.
Much too rigid. Reed catamarans have a proven ocean crossing record. 8,000 km.
All catamarans should be made from reeds stitched together.
If they were made out of reeds, the mast would gently fold over in extreme winds. Reed boats are difficult to pitch pole as well. The flex nicely in huge waves.
Since there is no hull interior, there is no way to become trapped.
Reeds, logs and coconut fibers are all natural materials and require no petroleum to produce. Derelict catamarans make nice beach cookout fires. No need to use toxic materials from which boatbuilders become ill or allergic.
Is all the research and proof of concept done by Thor Heyerdahl to be ignored ?
Again, “the need for speed “is the culprit.
My manatee crew moves at a speed which is barely discernible except when they smell pizza or hear a beer being opened. Cruising...not racing to paradise.
Having answered the question, I find it time to nap.
Happy trails to you.
Mark and his sleepy manatee crew.

[Kaimana]

Why Catamarans Capsize So Easily

Why catamarans capsize so easily is simple: because the closer the windward hull gets to lifting out of the water (that is, the harder you drive the boat), the less "reserve stability" you have. And reserve stability is what keeps you from capsizing. The amount of reserve stability you have at any given moment is your "buffer zone" between you and a capsize. In fact, here’s a joke about it:

When is a catamaran least likely to capsize?

When anchored!

Lemme explain that in English:

When a catamaran is sailing in light winds, the entire windward hull's weight is holding that side of the boat down; and there’s no way you're going to flip her without a huge and badly mishandled spinnaker.

At this point, you have lots of reserve stability; the entire weight of the windward hull, times the lever arm of the distance to the fore-n-aft centerline of the boat, is your reserve stability in foot-pounds.

Whether you know it or not, reserve stability translates directly into “margin of error for capsizing your catamaran”. As your reserve stability decreases, the margin of error that keeps you from capsizing also gets smaller and smaller.

As you get out in the bay, and get into more wind, you'll notice the windward hull has lifted a little bit more out of the water. You’ve lost a little of your reserve stability, but you’ve still got lots left.

You get into a bit more wind still, say 15-18 knots closehauled, and now the windward hull, which is normally submerged 30” or so on a 45-foot cat, only has 15” in the water. This is pretty exciting sailing; ripping along at 12 to 15 knots closehauled! Racing cats can go even faster than this; we’re talking about a family-style Caribbean cruising catamaran here.

Soon you find yourself in 22-25 knots with the 150% genoa still up. You were shorthanded today and taking it down and putting the yankee up is so much work. And you thought the wind was going to decrease as the sun came up more. But it didn’t.

And now your windward hull is skipping entirely out of the water every so often, as it lifts off the crests of the 5-foot wind chop that has developed. When this occurs, it stays airborne for three or four seconds before coming back down with a slap. This is REALLY exciting!

Still in control? Barely .

Because as soon as that hull lifts entirely out of the water, even if just for a few seconds, your reserve stability is GONE. The heeling effect of the sails has already lifted ALL the weight of that hull out of the water, and there’s no more weight “in reserve” to hold that hull down.

What isn’t obvious, unless you’ve capsized your catamaran before, is that there is a very fine margin between lifting your windward hull out of the water “a little bit” for a few seconds, or lifting it out of the water a lot, and going all the way over.

And something else happens as soon as the windward hull of a cat lifts out of the water “a little bit”: now the wind can come in UNDER the hull and exert an upwards lifting force on the wing, that will add to the forces trying to capsize the boat.

If you watch catamaran capsize videos, it becomes immediately obvious that at a certain degree of heel (and this is surprisingly low, often when the windward hull is only a few feet out of the water) the boat is simply going to go over, regardless of what steering or letting go of sheets the crew is doing.

I’m not just talking about Hobie and Prindle cats; just search “catamaran capsizes”, “images” on Google, filter out all the Oracle racing cats and the Prindles, and you’ll find that about half the photos are of large cruising cats that went over. A fair number of the photos are of cats with large accumulations of fouling from being upside down for weeks or months.

Now, to address the "catamarans don't capsize, they pitchpole" undercurrent in this thread: it don't matter which happens to you if you end up upside down!

The ONLY upside to capsizing is that's it's less final than sinking! My good friend Jim Brown wrote a whole treatise on how to set up your cruising trimaran to survive a capsize offshore; everything in that is applicable to catamarans too (You'll find it in this book: https://outrigmedia.com/outrig/case-...sing-trimaran/, along with a lot more stuff that's applicable to cruising catamarans).

A capsize can take all day to recover from, if you’re lucky and it happens in the bay, and you may get your boat back without too much damage. It can take your whole life to recover from, if it happens offshore out of range of assistance; and you’re lucky to escape with your life. Forget about the boat.

And it isn't just cats that can capsize: see here (https://outrigmedia.com/outrig/case-...sing-trimaran/) towards the bottom of the page, for my description of how I almost capsized my 56-foot long, 34-foot wide trimaran Tropic Bird. That was a boat I thought I could never capsize; I affectionately referred to her as my "ten-ton sailing dump truck".

I once did a 2-1/2-day, 450 mile delivery of a 40-foot cat our boatshop built, in light winds, with an inexperienced crew and no windvane; we had to hand-steer the whole way. I slept three hours out of that time when we were motoring through a calm, otherwise I didn’t sleep at all, I steered and watched the weather like a hawk. (See photo of 40-foot cat below)

To contrast to that, I did a 3,000-mile singlehanded passage in a good 37-foot trimaran with a good windvane, and I slept well the entire trip, with no worries. Except that was a stupid thing to do, for all the reasons you can think of, and I don’t do singlehanded passages now. (See photo of 37-foot tri below)

A Professional Boatbuilder Dude: Been designing and building my own cats, tris, and proas from 24' up to 56' since 1978.

With Warm Aloha, Tim

[refuge]

If you’ve sailed Hobie Cats then you know.

[Kaimana]

Originally Posted by refuge

If you’ve sailed Hobie Cats then you know.

I was the trapeze guy on a Hobie 18 that was doing 20 knots, when the skipper stuck the bow in. The boat pivoted a**-over-teakettle around that bow.

The trapeze wire slung me around and I hit the water at full speed 20 feet in front of the boat. Then the boat fell on top of me.

I remember swimming around a lot, unshackling things and tying loose stuff to the boat, and even in my wetsuit getting pretty cold.

[aqfishing]

[QUOTE=barryglewis;3374775]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.

[smj]

[quote]You get into a bit more wind still, say 15-18 knots closehauled, and now the windward hull, which is normally submerged 30” or so on a 45-foot cat, only has 15” in the water. This is pretty exciting sailing; ripping along at 12 to 15 knots closehauled! Racing cats can go even faster than this; we’re talking about a family-style Caribbean cruising catamaran here.[end quote]
.................................................. ..........,,,............

Nah.

[Cat_sailor]

Originally Posted by chrisr

one of the few sensible comments in this thread...

cheers,

I very much agree! Great input!

[Cat_sailor]

Originally Posted by Seabeau

I suggest you review the excellence treatise , "Heavy Weather Sailing" by Adlard Coles who concludes that the shallower a vessels hull and appendages are, the safer that vessels will be from knockdown by breaking waves. Shallower drafted vessels simply slide sideways when stuck.
Subsequently, the number of hulls is immaterial.

Thank you very much for a constructive and useful reply!

[Buzzman]

Originally Posted by sailingabe41ds

Is there such thing as a variable tension winch? My idea is that if the main sheet is attached to it and there is a gust of wind that the winch allows that main sheet to roll out like the spool in a fishing line thus allowing wind to spill and decreasing the chances of flipping.

Jim Brown designed a DIY version for his SeaRunner, and it's a common enough installation. Essentially the cam cleat retaining the mainsheet is on a hinge, with the hinge attached to bungy cord.

Adjust the tension of the bungy cord for the prevailing wind, and anything higher than that, like a sudden gust, overcomes the bungy and the hinge flips up, and the sheet pops out of the cam cleat (as the pull is then in the wrong direction, so it no longer 'holds' the line).



Installing such a system to me seems a no-brainer for anyone on a multi, especially if you are anywhere a gust or bullet might be found.

Jim calls it a 'fuse'. The fuse pops and gives you a chance to regain control.

[chrisr]

Originally Posted by Buzzman

Jim Brown designed a DIY version for his SeaRunner, and it's a common enough installation. Essentially the cam cleat retaining the mainsheet is on a hinge, with the hinge attached to bungy cord.

Adjust the tension of the bungy cord for the prevailing wind, and anything higher than that, like a sudden gust, overcomes the bungy and the hinge flips up, and the sheet pops out of the cam cleat (as the pull is then in the wrong direction, so it no longer 'holds' the line).



Installing such a system to me seems a no-brainer for anyone on a multi, especially if you are anywhere a gust or bullet might be found.

Jim calls it a 'fuse'. The fuse pops and gives you a chance to regain control.

i think you've missed the point, which is that the cruising cats we are talking about here do not capsize by wind alone.

something like this might be useful on a powered up race boat or beach cat but academic interest only for most of us...

(btw, cats our size don't use a cam cleat for the mainsheet...it's either via a rope clutch or in a self-tailer)

cheers,

[fxykty]

Originally Posted by Kaimana

...
When a catamaran is sailing in light winds, the entire windward hull's weight is holding that side of the boat down; and there’s no way you're going to flip her without a huge and badly mishandled spinnaker.

At this point, you have lots of reserve stability; the entire weight of the windward hull, times the lever arm of the distance to the fore-n-aft centerline of the boat, is your reserve stability in foot-pounds.

Whether you know it or not, reserve stability translates directly into “margin of error for capsizing your catamaran”. As your reserve stability decreases, the margin of error that keeps you from capsizing also gets smaller and smaller.

As you get out in the bay, and get into more wind, you'll notice the windward hull has lifted a little bit more out of the water. You’ve lost a little of your reserve stability, but you’ve still got lots left.

You get into a bit more wind still, say 15-18 knots closehauled, and now the windward hull, which is normally submerged 30” or so on a 45-foot cat, only has 15” in the water. This is pretty exciting sailing; ripping along at 12 to 15 knots closehauled! Racing cats can go even faster than this; we’re talking about a family-style Caribbean cruising catamaran here.

Soon you find yourself in 22-25 knots with the 150% genoa still up. You were shorthanded today and taking it down and putting the yankee up is so much work. And you thought the wind was going to decrease as the sun came up more. But it didn’t.

And now your windward hull is skipping entirely out of the water every so often, as it lifts off the crests of the 5-foot wind chop that has developed. When this occurs, it stays airborne for three or four seconds before coming back down with a slap. This is REALLY exciting!

Still in control? Barely .

Because as soon as that hull lifts entirely out of the water, even if just for a few seconds, your reserve stability is GONE. The heeling effect of the sails has already lifted ALL the weight of that hull out of the water, and there’s no more weight “in reserve” to hold that hull down.

What isn’t obvious, unless you’ve capsized your catamaran before, is that there is a very fine margin between lifting your windward hull out of the water “a little bit” for a few seconds, or lifting it out of the water a lot, and going all the way over.

And something else happens as soon as the windward hull of a cat lifts out of the water “a little bit”: now the wind can come in UNDER the hull and exert an upwards lifting force on the wing, that will add to the forces trying to capsize the boat.

[FONT=&quot]If you watch catamaran capsize videos, it becomes immediately obvious that at a certain degree of heel (and this is surprisingly low, often when the windward hull is only a few feet out of the water) the boat is simply going to go over, regardless of what steering or letting go of sheets the crew is doing.
...

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.