Angle of Vanishing Stability

[Wayward Wind]

I am currently doing my RYA theory course, and came across this statement on a catamaran's AVS, which I quote below:

"Catamarans and trimarans have a very high initial stability which is different from monohulls and have an AVS usually less than 90°. They can be easily capsized by gusts and they sail so upright that heeling by as little as 15° may cause a capsize. Thus one of the biggest risks to a multihull is being overcanvassed. Because they often have a beam almost equal to length a catamaran is more likely to pitchpole (turn end over end) than an equivalent monohull. A catamaran can cartwheel when sailing at high speed if the lee hull digs in. She stops suddenly and turns around the immersed hull."

I do not wish to start a "cat fight" but as a discussion for the sake of safety (I am relatively new to sailing), I understand that a mono heels when hit by a gust, thus depowers by spilling air. A cat sailor told me once that the cat merely goes faster, though I have read that it is better to reef early. As a newbie, how early is early, as I don't wish to sail in areas notorious for "bullets" with a reef or 2 always in.

Your advice will be appreciated.

John

[2Hulls]

Hi John - I suggest we start by addressing what I consider to be multiple errors in that quote.

"They can be easily capsized by gusts..."

It would have to be quite a gust to do so "easily."

And exactly what types of cats and tris are we talking about here? Day sailors or large cruisers?

"...heeling by as little as 15° may cause a capsize."

Huh? My flabber is completely gasted. That's a ludicrous statement. But again, what kind of boats are we talking about? Ever seen a picture of a beach cat flying a hull? That would seem to be way over 15 degrees. My crusing cat heels maybe about 5 - 10 at most. But I bet she tilts up to 15 when a freighter wake comes by.

"Thus one of the biggest risks to a multihull is being overcanvassed."

I'd say a bigger risk comes from high seas. But I don't plan to do any experiments.

"Because they often have a beam almost equal to length a catamaran is more likely to pitchpole (turn end over end) than an equivalent monohull."

First - boloney on the length to beam ratio. 50% +/- is more typical. That said, a cat is more likely to pitchpole than a mono - but because they are faster and lighter. Stuff a bow in a Hobie 16 and you're very likely to pitch it. Stuff both bows on a big cat surfing into the back of the next wave and this is when it could happen - but it's very rare.

"A catamaran can cartwheel when sailing at high speed if the lee hull digs in."

I've done this myself with beach cats. But cruisers? Nope. They won't go that fast.

"A cat sailor told me once that the cat merely goes faster, though I have read that it is better to reef early. As a newbie, how early is early...."

Here's my upwind reefing schedule:
First reef in the main at 20kts apparent.
First reef in the genoa at 25 kts apparent.
2nd reef in the main at 30 kts apparent.
2nd reef in the genoa at 35 kts apparent.
and so on...

By the way, it's better to reef early in any vessel, IMHO.

Hope this helps.

Dave

[Intentional Drifter]

I was once told or read this story, though I can't cite it to a source, but it goes like this: One of the principles at Catana got fed up with the stories of cats being capsize prone, so he took one of their boats (I don't know which, but they are all on the larger, more performance minded persuasion) out into the Bay of Biscay on a windy day with steep seas and tried to make it capsize. He couldn't. It dismasted, first.

Now, this isn't to say that it doesn't happen, as there have been examples (illustrated, even -- search for a thread on an Atlantic 42 on Lake Superior or Michigan) on this very site. Probably even more examples cited of monohulls being broken, holed, and sunk. Nothing is 100% and if God/Mother Nature/Neptune wants you, then he/she/its going to have you. But, in terms of the risks you cited, we've all seen beach cats and those incredible high performance racers do all of those things. But, a cruising cat? Not bloodly likely.

Dave's reefing schedule is pretty typical for cruising cats, but isn't that different for monos, either. I think the biggest difference is that with cats, you want to reef when you first suspect you might need to, without delay. Rarely do you actually lose much speed in such conditions, but you sure increase your safety margin and usually balance the boat a bit better, too.

If you think you're going to sail cats, I suggest you check out Chris White's book. Better yet, of course, is going out on some.

Good luck in your RYA course! That's a great accomplishment.

ID

[Tropic Cat]

They really put that in books? It sounds to me that the closest the author was to a cruising cat was a Hobie 18.

I sail what is probably one of the smallest cruising cats made. An old heavy British made cat which the builders put a reward up to anyone who could fly a hull. It has never been paid. In a gust my boat slips sideways to unload the sails. Easily done as I have no boards down.

As previously mentioned, we don't heel very much and come to think about this, Dave pretty much already covered your points.

What about what you didn't ask? Comfort, cats win hands down, we don't roll at anchor and you can leave your drink where you like when sailing.

OH..one last point. A few years ago my girlfriend and I were looking at boats and she commented that monohulls were 'caves' down below. When she saw her first cruising cat it was love at first sight. Unfortunately we are working our way up to that size boat (sigh). But she does love our boat and isn't too bad at the helm these days. The moral of the story should come as no surprise

Women love Cats..

[Sailormann]

Let me start by stating that I am a confirmed monohull sailor. I derive a certain amount of juvenile amusement from teasing multihull folk about their choice of craft, and I am often disparaging of them but, I think that the passage you have quoted is inaccurate...

If you can imagine a catamaran on it's side at an angle of 90 degrees, then you'll be able to visualise that the 90 degree figure is probably fairly accurate.

The thing that I don't like about catamarans or trimarans, is that once they have capsized they are almost impossible to right, and it is unlikely that they will do so by themselves. In contrast, a monohull will almost invariably right itself eventually.

This is not correct. For the multihull, or any boat for that matter, to capsize it must heel more than 90 degrees. The smaller daysailing catamarans are more sensitive to gusts of wind when they are heeled than monohulls are, and this, combined with the lower degree of buoyancy in their individual hulls makes it easier to get them to the point of capsize, but it does not happen at 15 degrees. You (usually) need to get up to about 30 degrees before the lee hull will start to submerge appreciably. Heeling does place a certain amount of stress on the crossbeams that hold the two hulls together, as does general wave motion. In the larger multihulls, this can be a source of concern, and sometimes a point of failure.

The same is true for monohulls.

I am unable to see any logic here. I believe the pitchpole motion would be lessened if anything in a broader boat. This is a hazard that has traditionally been asssoicated wth long, narrow, lightweight boats. I am wondering if the author was perhaps confusing pitchpoling with broaching ?

I think that this is something that is limited to small, daysailing catamarans with skipper who are maybe being a little careless or are filming YouTube clips. I have never heard of a larger multihull cartwheeling, and I am a voracious consumer of negative stories about multihulled boats.

A multihull does the same thing. It spills air. It might sped up a bit, but more likely it starts making increased leeway as theree is little lateral resistance compared to a monohull.

It is better to reef early on any boat, but maybe it would be a bit more important on a multihull, if there were gusts occurring. The one negative I am happy to share is that multihulls are not as easy to tack (turn into the wind) as monohulls. WHile they are undeniably speedier on a reach or even downwind, I harbour the belief that this speed advantage is negated by the ever-so-slow process of trying to convince the multihull to go in a different direction.

I would not recommend tht a "newbie" learn to sail on a multihull. I think that it is easier to learn the basics on a monohull, and then if you are absolutely determined to leave your wits behind you and set off to sea in one of those two or three-souled aberrations, at least you are somewhat prepared to deal with their vagaries.

However, the most important thing is for you to get out there and sail, so it is far preferable that you set off to sea on anything with a mast and sail, than stay ashore because the only boat available has more than one hull...

Cheers !

[Wayward Wind]

Dave, ID, Rick & Sailormann: thank you for the reassuring words. I am sold on the comforts of a cat, and already have an FP Mahe 36 on order. Though the text is presumably outdated, it does cause some alarm, as I have never sailed in rough weather on a cat. I will be crewing on a Mahe 36 delivery between Brisbane and Sydney in January, and so hope to be a better cat sailor, especially if sailing through "interesting" conditions while being on a schedule.

I have done a few sails on cats, but in benign conditions.

There are heaps of published and oral advice on sailing on monos, tips, tricks etc but not a lot on how to sail a catamaran better. The purpose of my post is to try and get a lowdown on better catamaran sailing techniques, so those of us new to sailing or cats, can be the better for it. Or is it that there isn't much difference in technique between mono and cat sailing? You know, something like bicycle riding as opposed to motorcycle riding (sorry Sailormann), ie same 2 wheels, same need for balance but different riding techniques.

Another book I read, The Yachting Handbook, by Dave Cox, also added to the perception that cats capsize, by starting a sentence with, "Despite their tendency to capsize . . ." before going on to ". . . they are generally seaworthy and, provided they are treated with respect, will look after their crew in bad conditions."

It is this "treated with respect" that I am trying to draw upon your vast experience for, such as when to reef. I know mono vessels have also got to be treated with respect, but in this case, anything specific to cats?

John

[2Hulls]

Originally Posted by seadog3315

It is this "treated with respect" that I am trying to draw upon your vast experience for, such as when to reef. I know mono vessels have also got to be treated with respect, but in this case, anything specific to cats?

As opposed to monos, you reef multis based primarily on apparent wind speed. On a good multi you can also feel it in the helm. Monos have the heel angle to assist them in determining when and how much to reef. Everyone should use the weather forecast and their observations before raising any sail.

Dave

[GordMay]

Excerpted from:

More Righting Moment Means Less Heel ~ By Jeremy McGeary
(Cruising World 12-Jun-2006)

Righting moment, on a cat and on a monohull

Wind force on the sails causes a sailboat to heel. Resistance to heeling, called righting moment (RM), results from the lateral movement of the boat's center of buoyancy away from the center of gravity (CG).

To compare the heeling behavior of a catamaran versus a monohull, consider a 45-foot mono as having similar accommodations to those of a 40-foot cruising cat. The mono displaces 30,000 pounds, and the cat displaces 18,500 pounds. Each has about 950 square feet of sail area.

The moment of force required to heel a 45 Ft. monohull degree is about 1,900 pounds-feet. The force to heel the cat the same amount is 9,200 pounds-feet, almost five times greater, because the newly immersed part of the catamaran is much farther from the CG than that part of a monohull

In general, the righting moment of a monohull increases close to linearly up to about 25 degrees of heel. The maximum RM for this boat would be about 80,000 pounds-feet at about 65 degrees of heel.

For the catamaran, the RM increases linearly for small angles of heel. Its maximum, 142,000 pounds-feet, occurs at the point the windward hull flies, in this case at about 16 degrees of heel.

[44'cruisingcat]

It's very dissapointing that the RYA is teaching people this garbage. Having read that,I wouldn't waste my time doing one of their courses. They clearly have no idea about multihulls, and are just passing on the usual prejudiced, outdated, ********.

AVS - depends on the hull shape. Contrary to what the monohull crowd might believe, it is possible for a cat to have an AVS beyond 90' If the hull is significantly wider at the sheerline than it is at the waterline, the boat will still be generating a righting moment at 90'. To say that a catamran will capsize at 15 degrees is pure ignorant drivel.

Cruising boats do not have a beam anything near equal to length. As was said before 50% of length would be closer to the mark. This means that it is far less likely to pitchpole. If hit by a gust, bear away and the boat will accelerate, reducing apparent wind.

BTW Sailormann, a well designed cat will tack just fine. I was on a 38 foot cruising catamaran that would happily tack on one sail, either main or jib, would tack with no daggerboard down, and would complete a tack that was started at just 2 knots boatspeed. It tacked far more quickly than a full keeled monohull. It also sailed at better than 30 degrees apparent, and showed VMG's of 6-8 knots upwind in 10 - 15 knot TWS breezes. No doubt all that would be impossible according to the RYA.

Another questionable "fact" is the self righting ability of monohulls. Even those few that are actually tested, are tested without the rig on the boat. I have seen about as many pictures of ocean-going monohulls floating upside-down as I have ocean going cats. Still, floating upside down, even in a monohull, is preferable to sinking. Just ask Elisabeth Autissier, or Tony Bullimore, or the survivors of "Rising Farrster". Monohulls invert too, and sometimes they stay that way.

[Wayward Wind]

9:31 AM Sat 7 Apr 2007

The 2007 Brisbane to Gladstone race multihull fleet suffered in heavy weather this morning, with two catamarans capsizing, both crews were rescued by other race competitors.

Just north of Indian Head, towards the northern tip of Fraser Island at about 3:15am this morning Richard Jenkin’s 11 metre Dancing Emu capsized . . .

Then about 7:00 am this morning an EPIRB was deployed, in the Lady Elliott, Lady Musgrave area . . . New Caledonian Philippe Coste and his five crew were found sitting on the hulls of the capsized Rogntudjuuu . . .

more on

http://www.sail-world.com/index.cfm?...=0&tickerCID=0

[Wayward Wind]

"At about 0215 the stern was picked up by a very steep wave and I presume a sudden wind gust (out of clear air – not a squall) our speed increased but the bows did not rise. The spinnaker sheet was released to no avail and the vessel pitch poled . . .

Makes compelling reading.

http://www.multihull.com.au/www/pdf/...ancing_emu.pdf

[Tropic Cat]

Seadog, I'm a bit confused by the point of your post about a race which occurred 7 months ago. Is it to confirm that racing catamarans in brisk winds can be risky? I don't believe anyone would argue that point.

How many monohulls would have experienced problems during a race had they been in those conditions?

Or, have I missed your purpose entirely?

[Auspicious]

Originally Posted by 2Hulls

As opposed to monos, you reef multis based primarily on apparent wind speed. On a good multi you can also feel it in the helm. Monos have the heel angle to assist them in determining when and how much to reef. Everyone should use the weather forecast and their observations before raising any sail.

That seems quite reasonable. I don't think all monohulls are properly reefed based on heel angle. I recognize 2Hulls did say "assist" so he wasn't over-simplifying. My reef points (40 Frers-designed monohull, working jib, full-batten main with a good bit of roach) are actually weather helm. I've found that when I sustain 10+ degrees of weather helm, the boat will go faster with another reef in the main.

My point is that it is worth looking at other indicators of "time-to-reef" than the easy ones. While safety is of course an issue, based on boat speed and controllability something else may be a better indicator.

[Joli]

Push any boat hard enough and bad things can happen.

[Sailormann]

BTW Sailormann, a well designed cat will tack just fine.

Well you have to say that - you own one.

I was on a 38 foot cruising catamaran that would happily tack on one sail, either main or jib, would tack with no daggerboard down, and would complete a tack that was started at just 2 knots boatspeed. It tacked far more quickly than a full keeled monohull.

Hmm - a mutlihull that changed course regardless of what you did - that sounds like a wonderful craft indeed. Ordinarily, a boat will wait until the rudder turns before it changes direction and will not going hieing off in every direction regardless of whether or not you have dropped the sails or tried to slow it down.

...I think it's great that some people sail multihulls, it would be a very boring world if we were all the same. But I enjoy teasing those folk as I find they take the bait easily...