Catamarans in 50kt winds

[donradcliffe]

Acoording to ISO 12217-2

3.2.2
knockdown
event when a boat reaches a heel angle sufficient to immerse the masthead, and from which it may or may not recover without intervention

Table 5 — Required minimum angle of vanishing stability
Design category Required minimum angle of vanishing stability, φV(R)
A (offshore) φV(R) = (130 - 0,002m) but always ≥ 100°
B (nearshore) φV(R) = (130 - 0,005m) but always ≥ 95°
m is mass of boat in kg

Now, lets hit a mono with a big gust of wind in sheltered water. The boat heels over, and several things happen:

1. The righting moment increases, typically up to a maximum at 75 degrees or so, and does not go to zero until the angle of vanishing stability--say 120 degrees, when the mast is well into the water.
2. The forces on the sails decrease, going to zero at about 90 degrees, or before the mast hits the water.

3. The boat develops severe weather helm, which rounds it into the wind, further decreasing the pressure on the sails.


All of this happens without human intervention, and the boat finds another stable point (with a bunch of flapping from the sails and an increased heel level. The reality is that a mono can't get into a whole bunch of trouble without wave action (unless you are a gung-ho racer with a spinnaker up, which is another chapter).

Now lets look at a catamaran in a similar situation. When the gust hits the boat starts to heel, and several things happen:

1. Instead of increasing like the mono, the cat righting moment starts to drop after one hull lifts out of the water, or at about 10 degrees of heel.

2. The pressure on the sails drops like the mono, except that once the hull starts to lift, the wind pressure on the wide structure on the boat starts to make an increasing contribution to tipping it over.

3. The cat hull shape does not produce much weather helm as heel increases, which means that the boat will not round up as severely as the mono, and an autopilot may be able to keep course.

Once you start to fly a hull, the catamaran is in an unstable situation. Unless there is intervention in the form of releasing at least the mainsheet (a sharp turn downwind may help, but is not as effective), the next stable position for the cat will be inverted. Even releasing the sheets does not work once the heel angle exceeds 50-60 degrees, because the wind pressure on the hull and bridgedeck is enough to push the boat over.

The catamaran is blessed with significantly greater initial stability than the mono, which provides for the possibility of improved performance by carrying more sail. However, that increased performance comes with a downside risk. If you increase the sail area to displacement on a cat, it lowers the gust required to capsize it.

Here is what ISO-12217-2 says about cats :

7.5 Stability information
Since sailing multihull boats and form-stable monohulls may capsize, information as listed in a) to d) below shall be:
— displayed in a durable manner inside the boat close to the main companionway or navigation station, and
— provided in the owner’s manual (see Annex F).
NOTE “A durable manner” means a rigid plate or flexible label affixed to the craft in such a way that it can only be
removed by the use of tools.
a) The stability hazards to which these boats are vulnerable, including the risk of capsize in roll and/or pitch,
particularly in breaking seas.
b) The apparent (i.e. relative) wind speed (expressed in knots or metres per second) at which the area of
any practical combination of sails should be reduced when sailing in calm water in the minimum operating
condition, taking account of the hazardous effects of gusts. Additional information relevant to the maximum
load condition can also be provided if desired.
This information can be calculated using Annex G (which includes a margin for gusts), or alternatively be
derived from sailing trials. The method of determination shall be stated.
NOTE For form-stable monohulls, refer to 6.5.2 e).
If derived from sailing trials, the wind strength quoted in the owner’s manual shall correspond to a wind
speed of not greater than 70 % of that required to
1) lift the windward hull of catamarans out of the water, or

[softgoodsint]

Re Dulcesuenos comments. My question: while I understand hull speed limitation physics in mono's, but I'm unfamiliar with multi's, but it sounds like it's a planing hull (?) to hit 12kt on a 30' cat. I completely understand that when you're skimming on the water vs. in the water, that limitation disappears (e.g. windsurfing or foil designs, etc). But clearly, speed when it's not blowing F5+ is a major difference favoring multi's. I also know a larger factor than speed or even safety, is "wallet depth" eg. emptiness. In otherwords, the big difference multi vs mono discussion is the price tag differential. Especially in todays almost 'give away' market place for used, but still serviceable (another debate topic) craft. (what other era could you pick up a boat for less than a seasons' docking fees?)

[Souther Wombat]

Originally Posted by tp12

And equally it's not easy to blow a catamaran over. It takes a lot of wind and a lot of sail up. Put a mono with equally large sail area up. and in this case a performance mono per the cat in question, in the same wind it has the potential to be knocked down and even roll over. If your argument is that a knockdown will never result in rig loss, potential flooding etc then I think we'll have to agree to disagree.

We get that you're not a fan of cats in storm conditions, and that's fair enough. But those arguments don't hold water. Pun intended

It's not even sensible to suggest any normal ballasted monohull sailboat could be rolled in sheltered water from wind heeling moment alone. It takes a breaking wave to do that.

[arsenelupiga]

Originally Posted by barnakiel

Yes. A cat only half a roll over actually. A mono, depends on how much rig left after the first roll over. Nearly always there is no rig left after the first try.

No production boats designed with rig parameters to allow for roll overs. maybe some purpose built ones, but I have never heard of.

Some time back a sailing boat purpose built for rough weather got wiped out off Bergen. You can google up her images. The crew abandoned (airlifted).

http://bi.gazeta.pl/im/dc/e1/10/z177...e-w-Bergen.jpg

b.

nothing better than cat-mono to boost site visits.

anyway, it appears to me mono people live in la-la land.

[Snowpetrel]

Originally Posted by Souther Wombat

Ben My bad Apparently the mainsail was still covered and lashed to the boom when she was re righted and the folk we spoke to had seen her motoring before so they were apparently confused. I'd like to know just what they were flying. Also weren't the conditions were fairly benign before hand except the sudden squall and no time to throw the sheet ? This is a seaworthy boat from her past exploits. How could they get it so wrong ? That's where any monohull would have coped with the sudden squall with no more than a split sail.

Sounds like you have some good info I haven't heard. Both cases I have only heard about second hand as well, so I might be wrong about the details. What did someone once say? Believe nothing you hear and only half what you see... From what I gather she was certainly tearing around pretty fast all day, then got caught as a bigger than normal squall came though. My freind was on a neaby square rigger and saw it happen, they helped with the rescue but he isnt an experienced sailer so may have missed some important details. Id love to know what really happened from the owners perspective.

This is the wind info from two nearby weather stations. Note the bigger 40 knot gust around 1615-1630 ish. I wonder what time she flipped?

[tp12]

Originally Posted by Dockhead

Another cat vs mono buying decision thread . . .


In my opinion, most of these statements are false. The key thing to making this comparison intelligently is to compare like for like. A 45' catamaran is not "like" a 45' monohull. It has two hulls, so it's more like a 55' monohull in terms of interior volume.

Once you compare like for like, everything looks different:

1. Cost is about the same.

2. Cat is not faster, and is probably slower if it's a loaded-down condo-cat. Cat has better fineness ratio, but a comparable mono will have a longer waterline.

3. Cats do NOT point worse than comparable monos. Condo cats don't point, but loaded down cruising monos ALSO don't point. High performance cats point very well.

4. Monos are NOT safer. Cats flip but monos sink. Statistically there is no difference between cats and monos in terms of safety.

5. What is more comfortable is a matter of taste. Cat has wonderful high and roomy salon but prison-cell like cabins. Mono worse view and light in salon but better cabins (as a gross generalization of course).

6. Lack of heeling a very profound advantage of cats, for long ocean passages. Less important for more typical cruising regime of short passages between long stays in port or at anchor, but still pleasant. Living on a heel sucks.


The biggest advantages of cats you did not mention:

1. Redundant propulsion, greatly enhancing reliability and maneuverability.

2. No ballast, so much more efficient motoring. Cats are totally superior motorboats, and this is important considering most cruisers motor more than 50% of the time.


Advantages of monos:

1. For some people, more pleasant motion at sea.

2. Easier to find berthing.

3. Better load carrying capacity.

A very reasonable post. Thank you Dockhead


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[UpOnStands]

do believe that beaches are far more common than 50 k winds.

[tp12]

Originally Posted by Souther Wombat

It's not even sensible to suggest any normal ballasted monohull sailboat could be rolled in sheltered water from wind heeling moment alone. It takes a breaking wave to do that.

If a cat can be flipped by the wind that same wind can knock down a mono. I'm honestly not sure why we're even discussing this. If you don't agree, fair enough. I can't agree with you either.


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[Factor]

A stupid discussion that flowed from a silly question. Why am I not surprised.

[805gregg]

Watch "SV Honeymoon" on youtube, they were heading down wind with bare poles in 50 knots of wind, surfing along at 19 knots in a Lagoon 38

[tomfl]

Originally Posted by softgoodsint

Re Dulcesuenos comments. My question: while I understand hull speed limitation physics in mono's, but I'm unfamiliar with multi's, but it sounds like it's a planing hull (?) to hit 12kt on a 30' cat.

SNIP

Not sure your understanding of the speed limitation physics is the same as mine. A displacement hull creates a bow wave and the faster the boat goes the bigger the wave. At some point the wave gets so large that the displacement hull is basically going up hill on the wave and this is the max speed.

But there is another consideration, wider hulls produce bigger waves faster and narrower hulls produce smaller waves more slowly. At some point a hull is so narrow that it has to be going really fast to produce a big enough bow wave to limit its speed. This point is normally some where around a ten to one ratio between a hull's length on the water line and the hulls beam. Performance cats are normally defined as having a hull ten times longer than the hull is wide. If you look at a generic condomaran the beam of its hulls is often two or three times the beam of a high performance catamaran. It is also common for some catamarans like a Shuttleworth to have hulls that are very narrow below the water line and then quickly expand above the waterline.

There are also boats designed to plane, both monohulls and multihulls. These boats normally have a flat bottom or a very shallow V bottom. They are also either very light or have very large engines or lots of sail area. As a rule they are not really suitable for cruising.

There is also a consideration with some multihulls with the bow waves created by two hulls interacting between the hulls and slowing the boat down. This is somewhat related to deck clearance and probably too far out in the weeds for this thread.

[stillbuilding]

One of the posts above touched on something important to me, which is the constrained staterooms on a cat.

I am 6'2" and like some "room to move". The tight spaces "down" in smaller cats gives me a claustrophobic feeling which I don't get in a bigger monohull.

My 60' project boat is very comfortable for two couples and a mouse. Many 50' monos I have been on are very open and spacious in the shared spaces and in staterooms.

I find cats are mostly great "upstairs" but horrid in the hulls. I hope that would improve in 50' cats but have very limited experience in that size.

Anyone with experience care to comment?


Sent from my iPhone

[REsCat]

Originally Posted by stillbuilding

One of the posts above touched on something important to me, which is the constrained staterooms on a cat.

I am 6'2" and like some "room to move". The tight spaces "down" in smaller cats gives me a claustrophobic feeling which I don't get in a bigger monohull.

My 60' project boat is very comfortable for two couples and a mouse. Many 50' monos I have been on are very open and spacious in the shared spaces and in staterooms.

I find cats are mostly great "upstairs" but horrid in the hulls. I hope that would improve in 50' cats but have very limited experience in that size.

Anyone with experience care to comment?


Sent from my iPhone

If you would like to start a new thread on that subject I would be glad to comment on what our cabins in the hulls are like with a picture or two,
as I'm sure other owners would as well.
Like every boat ever talked about on this forum there are positives and negatives of that particular design and some features are better executed on some than others.
There are many factors which come into play regarding cabins/staterooms in Cats, these being influenced by hull shape, performance objectives and materials used for the fitout etc.

Bob

[barnakiel]

Originally Posted by stillbuilding

One of the posts above touched on something important to me, which is the constrained staterooms on a cat.

I am 6'2" and like some "room to move". The tight spaces "down" in smaller cats gives me a claustrophobic feeling which I don't get in a bigger monohull.

My 60' project boat is very comfortable for two couples and a mouse. Many 50' monos I have been on are very open and spacious in the shared spaces and in staterooms.

I find cats are mostly great "upstairs" but horrid in the hulls. I hope that would improve in 50' cats but have very limited experience in that size.

Anyone with experience care to comment?


Sent from my iPhone

Yep.

I have been aboard this:

Xquisite Yachts - Xquisite Yachts

The difference between a typical older design 50' cat and the new designs is beyond words. Browse their interior images to get an idea.

As the poster above noted - you may want to move this part of the discussion to a separate thread.

Cheers,
b.

[Cadence]

Originally Posted by tp12

If a cat can be flipped by the wind that same wind can knock down a mono. I'm honestly not sure why we're even discussing this. If you don't agree, fair enough. I can't agree with you either.


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I have no dog in the fight but I disagree. It would be possible for wind to get under the bridge deck on a light cat.. Likelihood small. Knocking down a mono at anchor with 50kns isn't going to happen unless someone removed the ballast