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Old 07-12-2021, 21:04   #1
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Downflooding in Catamarans

The term "downflooding" has been used to describe the event where massive amounts of seawater enter the hull from the cockpit or an open or broken hatch. Being a monohuller who wants to become a multihuller, I am always puzzled by the fact that none of the multis I've seen made any provision to prevent a large wave from astern to flood down into the hulls through the open stairways.

Of course this could only happen if the aft bulkhead in the salon was breached by the wave. But that seems quite possible to me, when I look at the large sliding glass/plastic doors and windows of most designs. Be that as it may, is the idea that any large sea will be stopped at the aft bulkhead and never enter the salon?

I've heard the argument that "multihulls can't sink". This may be true in the inverted position for some length of time, but certainly not in the upright position if both hulls were nearly full of water one would expect the salon floor to end up at or too close to water level to prevent complete flooding of the hulls.

So what is the reason for not having closeable and strong hatches, such as those necessary in monohulls, at the entry points to the hulls of catamarans?
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Old 07-12-2021, 21:44   #2
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Re: Downflooding in Catamarans

It's the same reason that catamaran fanboys ignore the possibility of capsize. It can't happen to me because...(fill in the blanks).

Not only sliding windows and doors, but large vertical windows, forward cockpits, and wedding cake profiles. It's not a problem at the boat show docks.
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Old 07-12-2021, 22:09   #3
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Re: Downflooding in Catamarans

A mystery?

Pessimistic enough to require two hulls but optimistic enough to not require keeping the water out of them?
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Old 07-12-2021, 22:23   #4
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Re: Downflooding in Catamarans

Hi Waterman, here is a good video that you may not have seen of a Leopard 39 on a delivery passage in the Southern Ocean. The passage was 65 days straight in winds up to 65 knots & as we all know the conditions always look smaller on film . In this clip they are running under bare poles & no sign of any of the very large following.waves entering the cockpit. This must have been a much scarier situation at night.

I thought this real life video may go further to providing information on your question as opposed to comments from those who have never experienced more extreme conditions anywhere near this.



Here is another good video which is a pitchpoling risk as well as a following sea.

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Old 07-12-2021, 22:32   #5
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Re: Downflooding in Catamarans

The most interesting response would be one containing actual experience.

Cats have flaws, and I don't like those doors, but I've just never heard of this actually happening. Heck, even in pretty snotty weather, the swim platforms just barely get wet, and never above the first step. They just rise.

Perhaps if riding to drogues was common enough. You would have to slow the boat and hold the transom down. Then a big breaking wave... but again, has it happened?
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Old 07-12-2021, 22:33   #6
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Re: Downflooding in Catamarans

Can you point to one instance of a catamarn ever being swamped in this way?
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Old 07-12-2021, 22:46   #7
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Re: Downflooding in Catamarans

Is it possible? yes but never have heard of it happening.

But it's far less likely.
- Unlike a monohull which has several thousand pounds of lead holding it down, on your typical cat, the stern simply rises with a wave coming up on the stern because it's lighter.
- There usually is not a stairway down in the cockpit. As long as the door to the main cabin is closed, it's not a big issue. This is different from monohulls where it's a hassle to close up the cabin and put in the wash boards. On ours, the door is typically closed when under way. If we need air inside, the window in the upper half of the door is opened. A wave that could get that high up would be absolutely massive and steep.
- Many cats have wide openings at the rear of the cockpit so any water that makes it into the cockpit simply runs out the back as the wave passes limiting the time any water that makes it into the cockpit would have to find it's way in to down flood.
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Old 08-12-2021, 00:27   #8
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Re: Downflooding in Catamarans

The OP asked for actual experience.

Skippering a West to East trans Atlantic delivery of a Lagoon 400 one early May I got stuck in a very nasty four day storm with wind aft at well over 50 kts, seas to match, and even larger seas abeam from even worse weather to the north of us.

Needless to say those seas were huge, confused, and breaking. Running off with only a tiny bit of foersail unfurled, we had two waves break on us from abeam on separate days. The best way to appreciate the experience is to imagine sitting on the boat on stands on the hard and being hit broadside by a cement truck going 35 miles per hour. The violence of the event was incredible. I believe that our heel angle exceeded 60° on both occasions.

We were also pooped once, with the tender and cockpit (really an outdoor saloon) filled. About twenty gallons of water found their way under and around the closed aft sliding door and down the companionways to the bilges before the cockpit drained aft and through the deck grates.

By the time we made the Azores the dinghy chaps were in tatters, there was damage to the stack pack and other outside canvas, and I had to replace some chafed reefing lines from earlier in the storm. Just about every boat that arrived in Terceiera in the days before and after us had received major damage. By comparison we came off pretty well indeed.

These were by far the worst conditions I have ever experienced or expect to see again. With the exception of the passages between Madagascar and South Africa and that to New Zealand I see no need for a prudent cruiser sailing typical cruising grounds who is not bound by an unreasonable schedule to experience the like. If you are heading to Tierra del Fuego or the Antarctic I can give you a lot of reasons to choose a boat other than a production cruising cat.

I relate this long winded tale by way of saying that a well made, carefully sailed catamaran will get you home as well as a well made, carefully sailed monohull. I will grant you that the Lagoon 400 is a reasonably proportioned boat and I'm not so sure we'd have done as well on some of the more freakishly tall models on the market today. I will say that the vast majority of the, "A following sea will carry off the tender, smash the saloon slider, and sink the boat," crowd haven't got out of their armchairs to sail a cruising cat in challenging conditions.

I
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Old 08-12-2021, 01:30   #9
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Re: Downflooding in Catamarans

Quote:
Originally Posted by sv Grateful View Post
The OP asked for actual experience.

Did he? Where?
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Old 08-12-2021, 01:34   #10
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Re: Downflooding in Catamarans

I have been buried in the back of a wave up to the mast with large quantities of green water flooding into the cockpit over the coach roof. This drained instantly through a 200cm by 30 cm scupper with 17 x 7cm drainage holes traversing the door threshold. No water entered the cabin. Never have we had water enter the cockpit from astern. I also have 10 sealed bilge compartments covering the majority of the of the hulls. During our build on a tidal mud berth we forgot to turn off an unconnected sea cock, the area above the sealed floor flooded to a depth of 10 cm and we pumped it out the next day. So there is a remote possibility of down flooding but the consequences will be minor and easily dealt with. But remember not all cats are equal. I personally do not like or consider it safe to have open transoms on multis or monos.
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Old 08-12-2021, 03:06   #11
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Re: Downflooding in Catamarans

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Originally Posted by StuM View Post
Did he? Where?
Right you are, it was "Thinwater" who cited the need for actual experience.

Thank you for your insightful contribution to the conversation.
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Old 08-12-2021, 03:48   #12
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Re: Downflooding in Catamarans

IIRC that is what happened to the GUNBOAT “Rainmaker,” but it had issues before that particular part of the event. In any case, despite the downflooding, the RM did not sink and has been rebuilt albeit as a power cat

Suggest the OP, if not actually trolling, read up:
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Old 08-12-2021, 04:17   #13
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Re: Downflooding in Catamarans

Quote:
Originally Posted by tamicatana View Post
IIRC that is what happened to the GUNBOAT “Rainmaker,” but it had issues before that particular part of the event. In any case, despite the downflooding, the RM did not sink and has been rebuilt albeit as a power cat ...
Story & Photos of Gunnboat catamaran “Rainmaker”
https://www.superyachttimes.com/yach...t-55-rainmaker



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Old 08-12-2021, 04:22   #14
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Re: Downflooding in Catamarans

Not really related to downflooding but here's a cool vid of a Crowther tri surfing some huge waves at a bar crossing. The fun starts al about the 11 minute mark.
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Old 08-12-2021, 05:02   #15
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Re: Downflooding in Catamarans

Valhalla nailed it.

It’s because it’s less of an issue. I have said this many times on the forum, but a good catamaran floats on the water like a piece of cork. Like a piece of styrofoam. A monohull floats in the water like a message in a bottle. It’s a simple way to picture the difference.

That’s why large waves are less of an issue. The catamaran pops up over the wave, the monohull goes through it.

However, some of the newest dogs of catamarans with huge displacement for their waterlines may no longer follow this line of thinking. There sure are some crap cats at the boat shows lately.
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