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Old 07-05-2007, 17:30   #46
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Quote:
Originally Posted by schoonerdog
44 cruisingcat, do you remember how the divers did it in multihull world? Did they attach the lift bags to the end of the mast as well? Seems like that would get you a good portion of the way there and then you would need a good engine on a different boat to push you the rest of the way over.
Sorry about the delay in replying. I had loaned the magazine out to a friend.

OK, to start with the boat was about 26ft in length, with some accomodation in the hulls, but no bridgedeck.

They attached an airlift bag to the masthead, with the intent of righting the boat sideways, hull over hull. But the bows lifted and the stern sank so they went with that.

When the masthead bag was on the surface, they attached another at the gooseneck, and moved the first one to the back beam. They also removed the sails and supported them with a small bag. Inflating the bag at the back beam saw the boat fully upright, but obviously full of water. They then attached a bag to each daggerboard and used them to float the hulls higher in the water.
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Old 07-05-2007, 19:39   #47
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Originally Posted by Lodesman
Dennis,

OK - I'll comment. It's an interesting design, that's certain - and notwithstanding its ugliness, I generally don't judge a book by its cover and can see any boat's inner beauty. Unfortunately, the design would be impractical for a cruising cat. It looks as though it was designed to be very narrow to allow it to be trailered. With such a narrow overall beam, it lacks stability so it is inevitable that it should capsize routinely - that goes against the whole purpose of having a cat, which is to stay upright. Translate the design to a typical cat of 6m (20 ft) plus beam, and the dome would create an unacceptable amount of windage - not to mention creating a massive tipping moment by having the centre of sail effort some 12m (40 ft) from the centre of buoyancy. Not only would it have a greater tendency to capsize, it would have a greater tendency to pitchpole, too.
From an ergonomic perspective, it's certainly less roomy than a normal cat, and I would assume less roomy than a mono of similar size. If he needed a trailerable, he should have gone with a mono, or a folding cat/tri. Some people just like to be different.


Kevin
Its not a boat I would like to be in rough weather in, thats for sure. I would rather stay up right, then capsize and right every half hour.
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Old 08-05-2007, 13:50   #48
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Quote:
Originally Posted by Jeff H
Most multihulls have limit of positive stability that is roughly 80 degrees with a rare exception of a heavily balasted cat with deep keels perhaps hitting 90 degrees.
Jeff
Not strictly true. It depends a lot on the hull shape. If the sheer panels reach maximum width at deck level, there will still be a righting moment at 90' and beyond. http://www.kelsall.com/images/articl...sAndSafety.pdf
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Old 19-02-2008, 03:55   #49
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As an experienced, land bound engineer. The liferaft has a good bit of boyancy. If you saty with the cat/tri its worth hooking the main hailyard to the inflateable tender and pulling. At best you'll raise the mast to near horizontal. That's a dry hull to live in and a stable boat position. At worst it will tip a bit or sink the tender.
Flooding one of the four corners will help while the waves are still worth it, otherwise it looks like a tow is by far the best bet, especially from anything passing by. Once righted it's situation normal, lots of mess, lots of work to do, lots of damage. If it's still ON the water it's worth taking home.
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Old 07-06-2008, 12:12   #50
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10 cu ft roughly equal to 30 gallons = 300 lb trying to lift 4 ton boat , or at least trying to lift half of it. Aint going to happen.
thats the problem with "this imperial system", to easy to have an error, metric is better , look :
10 cubic feets : 283 liters .
283 liters of water --> 283KG
and 283kg are 623 pounds .
1 gallons is aprox 3.6 liters, aprox 3.6 kg , aprox : 7 or 8 pounds .

besides that, im planin my self to cicrcumnavigate sailing, i didnt decide yet, of monohull or multihull, but if a choose a multihull, will think some ideas to self rigth the vessel after a capsize, also i will have 1 or 2 sea anchors as strategic for bad weather, and in teh case of a capsize, I will wait inside, and after the storm, i will try to use, AIR ballons.
also i was thinking to have a HUGE ballon to inflate WITH WATER, and use as huge Ballast deep in the water, during a storm .

eze
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Old 07-06-2008, 14:25   #51
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So, For starters I am a mono-hull sailor. That said I am also a professional sailor and used to drive a multi in the charter trade. The boat was certified to carry 49 passengers 47’ X 24’. I sailed the boat in at least 50 knots of wind and in seas to 16 - 20 feet. I have sailed monos in over 60 knots and 30+ foot seas.
So much for background…

I have personal experience with attempting to right a multi and with flipping it to start out with. So a few comments…

First off, all the ideas about righting a flipped multi are good up until the vessel flips then they go to ****. Why? Well there is no way to know what kind of damage the boat will suffer during the capsize, but from my experience it is not trivial most likely the mast will be destroyed if you have any sail up, so using it to right the boat is out. Even if it does survive you will have a lot of water in the hulls adding tons of weight that will either need to be removed from one hull or factored into your righting plan. Removing this extra water will likely not be possible as when the boat capsizes there is a very good chance that hatches, and ports will fail and / or there will be holes on the cabin / deck due to the boat falling on itself.
So how about righting the boat at sea with another vessel? We tried this using a large power boat (salvage boat). It may be possible, but in the attempt I was involved with, we were unable to get the boat to flip back either end over end or sideways. We finally had to tow the stricken vessel upside down to a harbor where we could get a crane to do the job. Had we been in shallow enough water to set an anchor I believe we could have flipped the boat back over with a second vessel but at sea I doubt it could be done esp with a cruising multi and all its extra weight. (the boat I was involved with was only 12 thousand punds)

The event that caused my capsize, while disastrous for the cat I was sailing, would not have been any real problem for my monohull. This is in my opinion the real danger with a cruising cat or tri.
Here is what happened. We were sailing along wind about 25 – 35 knots, triple reef, reefed jib (yes that is a reefed jib) only making about 8 knots upwind seas were about 12 feet but on a short period say 20 seconds so they were steep but not particularly dangerous. The boat was very comfortable not at all stressed and in fact the sailing was good fun although very wet. I had sailed this boat in much worse conditions with no problems of any kind. Then… a bad combination a rouge wave and a microburst of wind.(60+ knots) The wave was perhaps 16 feet and as we crested it broke under the boat exposing the underside of the vessel at that instant the microburst hit us and the extra sail area (bridge deck) was enough to flip us. We were upside down in less than a second.
My mono would have perhaps been knocked down briefly but would have other wise not been unduly troubled.

About living in a capsized multi I have never had to do that as I, my crew and passengers were rescued in only hours.

Anyway, to me if part of a boats design is for the ability to live in it after it’s flipped there is a basic problem with that idea. Keep the water on the outside and a mono is the boat for me.
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Old 07-06-2008, 17:35   #52
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Catamaran stability and righting a capsized catamaran

Righting moment:

The theoretical stability of a catamaran is given in the formula, displacement in pounds x the distance between the centerline of the two hulls in feet x .5 = righting moment in foot pounds.

To use my BigCat 65 design as an example, 45,000 x 28 x .5 =630,000 foot pounds (that is, a lever of an amount of feet times an amount of force in pounds.)

Capsizing moment:

If considering a capsize from wind force, the theoretical heeling force is the wind pressure x the sail area x the CE (the height from the waterline to the center of your sail area.)

Wind pressure can be calculated from Martin's formula, which is windspeed in miles per hour squared x .004= pounds per square foot.

So, the heeling (capsize) force for BigCat 65 that generates a theoretical force sufficient to capsize it is: 9.21 pounds x 2400 sq. ft. of sail area x 41 feet (center of sail area above waterline) =630,000 foot pounds. This wind pressure is found at 48 miles per hour, which = 41.7 knots.

(One knot equals 1.15155 miles per hour, so divide the mph by 1.15155 to get the knot equivalent to the result of Martin's formula.)

So, in flat water, theory predicts the BigCat 65 will capsize at 41.7 knots in calm water if the sails are up and unreefed. This theory applies rather poorly to a biplane rig, because you can't get the full effect of the wind abeam if both sails are up, but it works pretty well for a typical catamaran with a single mast.

Re-righting moment:

If you were trying to right a capsized catamaran, you would be working against similar righting moment. If we assume an equal righting moment upside down as right side up, you can see that we need a huge amount of force to right a capsized cruising catamaran.

BigCat 65 has a mast height of 70 feet above its right-side-up waterline. If we assume that the catamaran is floating at the same depth upside down as it did right-side up, (unlikely, but not extremely far off, either,) than we would need a horizontal force of 9000 pounds at the (a) masthead (assuming the masts weren't broken off,) to right it. (630,000 divided by 70 = 9,000 foot pounds.)

This force would be the equivalent of 140 cubic feet of net buoyancy, which, in the shape of a cube, equals 5.2 feet on a side. (Buoyancy = 64 pounds per cubic foot.)

Obviously, this is all very theoretical, because a capsize at sea won't occur in calm water, using buoyancy to float an upside down catamaran probably wouldn't have a righting arm at ninety degrees to the masthead, the mast(s) might very well be broken, etc.-but it does give a way of bringing some quantification to the problem to give us an appreciation for the forces involved.
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Old 07-06-2008, 17:54   #53
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Originally Posted by cptnandy View Post
[FONT=Times New Roman][SIZE=3]
[SIZE=3]Here is what happened. We were sailing along wind about 25 35 knots, triple reef, reefed jib (yes that is a reefed jib) only making about 8 knots upwind seas were about 12 feet but on a short period say 20 seconds so they were steep but not particularly dangerous.
Can I ask the design of the boat and the location of the event please?
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Old 07-06-2008, 18:05   #54
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Ian Farrier did some work with self-righting systems for his trimarans in the early years, and came to the conclusion that it was a dead end; too heavy, too expensive, and toook up to much room on a vessel that he did not design for off-shore sailing. AYRS has published several articles on the topic, but a summary article decided that the likelihood of needing such a system was too low to justify it. If you can find my article reviewing the available statistics, you will see that multihull inversions are survivable, while monohull sinkings are not always. And they both happen at about the same RATE (percentage of losses per number of registered vessels.) They are statistically equal because the absolute majority of all fatal accidents are attributed to bad decisions in the cockpit, not on the drawing board, Despite my pretensions to the contrary, multihull people are no less capable of big mistakes (or compounded little mistakes) than those attempting to sail with an insufficient number of hulls!

But I do wonder at why a beligerantly defensive monohuller would want to spend time in a multihull topic; is it possible he wants to save us from ourselves, or is he secretly *multicurious*?

Why don't we all go over to the monohull thread and discuss balloons, bags and other self refloating systems for keelers in the same seas?
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Old 07-06-2008, 20:29   #55
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this thread is getting very good, thanks . I like to see the opinion and experiences of everyone .

so looks like if you want to sail blue water with a multihull, be prepared and have ready good storm tactics, know and DO IT early : heave-to, deploy sea anchor .
i have see this life rafts, that have water ballast to stabilize it look:
Zodiac liferaft advantages in more detail
if i use something similar and deploy it on bad weather, while heaving to, or with a sea anchor, the chances to capsize are reduced a lot . what do you think ? (even if everyone says is imposible, i will have on my storage some dive equipment, and the ballons )
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Old 07-06-2008, 20:47   #56
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Floatation bags for monohulls

"Why don't we all go over to the monohull thread and discuss balloons, bags and other self refloating systems for keelers in the same seas?"

I'll do it here, since this is where the thread is, -but- feel free to quote me if you want post elsewhere in order to harass monohull sailors.

A 42' boat displacing 20,000 pounds could be kept afloat by a bag 2' wide x 6' high x 26' long, roughly equal to the center aisle in such a boat, but it would float awash. If the boat had two double berths, bags that went over the2 double berths would have the same effect.

Awash beats sunk, but it isn't a shelter. However, it might give you the opportunity to pump out your boat if it weren't badly holed.

So, if you had bags for the aisle way and for the berths at say, 3' high, you could keep the boat floating with say, 2 feet or a bit better of freeboard. Obviously, they would have to be attached pretty well to support 20,000 pounds without breaking free and floating to the underside of the deck.

This still isn't much of a shelter, two feet of air above you in a boat full of sloshing water sounds more like a torture device than a happy home.

A typical scuba bottle contains about 25 cubic feet of air at sea level pressure, so the 325 cubic feet needed to float 20,000 pounds would take a lot (13) of scuba bottles. A 31.5 pound tank holding 6.5 pounds of air weighs 37 pounds, which times 13 weighs 481 pounds and takes up about 3.5 cubic feet. At $300 each, you are looking at $3,900 for tanks, not counting valves. Another $600 for valves, and we haven't paid for any hoses or the inflatable bag(s.)

No prices shown online, but for bag sources, see: TurtlePac - Fuel and water bladder tanks for boats and aircraft - underwater salvage lift bags for bags, or Enclosed Floatation Bags on Rockwater

It seems like a lot of money to me, but considering what people are paying for fully equipped boats these days, it's chump change.

Tim Dunn, aka BigCat
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Old 07-06-2008, 20:49   #57
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Would a small bottle, kept at the mast head, not much bigger than a small fire extinguisher filled with compressed air attached to a float bag triggered by a hydrostatic switch work? I would guess it would probably work for a cat that is turtled to put it on its side. It would then take a power boat to pull sideways on the vessel to get the cat upright or some heavy seas.
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Old 07-06-2008, 21:32   #58
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Inflatable masthead floatation

David, the answer can be calculated from information given in my posts, above, on floatation for monohulls and on righting moment for catamarans. The 9000 pounds needed to right BigCat 65 at the masthead equals 140 cubic feet, and that requires 6 standard scuba bottles to fill at sea level.

However, as the masthead is well below sea level, it would take a bit more than 3 times as much air to fill it at 70 feet of depth as it would at the surface--and the bag would explode from the expanding air as it rose, as the air would need to triple in volume as it rose to sea level.

The air pressure / air volume issue is scuba diver knowledge, if you want to get into it--66 feet = 3 atmospheres, with 1 atmosphere being the air pressure at sea level.

So, it would be very tricky to valve such a bag so that it didn't explode before you righted your catamaran. Too much venting, and you lose your righting moment. Too little, and your bag breaks. Not a simple problem. Alternatively, your bag would have to be made from a very stretchy material indeed, if you didn't vent the expanding air from it. (The air would need to expand as the float bag rose.)

So you can add these problems to the somewhat iffy issue of whether the mast would still be there after the catamaran capsized.

So, such a system on BigCat 65 would involve 750 pounds of scuba tanks, and a hose from them to the masthead via the inside of the mast, as well as a solution to the expanding gas problem to prevent breakage of the bag.

Divers use parachute-shaped bags open on the bottom to prevent this explosion when using air bags to salvage relatively small things, but they are down there to control the bag and keep it from spilling air out the bottom.
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Old 07-06-2008, 21:44   #59
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Which Catamaran and where? I think I know-

Quote:
Originally Posted by Factor View Post
Can I ask the design of the boat and the location of the event please?
I would guess--a Conser 47 which capsized near Lanai. See Honolulu Star-Bulletin News

If so, it is / was the Paragon I, and you can see it on Youtube under sail in a stiff breeze, at:


See this link for a description of the Conser 47:
1995 Conser Catamaran Conser 47: Racer/Cruisers - Boats.com
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Old 07-06-2008, 22:36   #60
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Flipping a Conser 47- doing the math

If I'm right about the flipped catamaran discussed above being a Conser 47, it is very light, at 12,000 pounds. If you add 1600 pounds for 10 people on board, that gives you a displacement of 13,600. It looks to have narrow hulls, so the beam from center of hull to center of hull would be about 20 feet. The righting moment would thus be 136,000 foot pounds.

The sail area would be 1065 square feet unreefed, and the mast would be 65' tall. I don't know the triple reefed area, but let's guess it's 425 sq. ft. Likewise, I don't know the CE reefed, but let's guess 21 feet above the waterline.

That gives you a pressure of 19.04 pounds per square foot at 60 knots x the CE of 21 x 425 =169,932. This is well over 136,000 foot pounds, and so, over it would indeed go, with no consideration given to the wave height and action at all.

Even if my guess about the catamaran above is wrong, there was a Conser 47 flipped over in very similar circumstances, so the analysis is of interest in that it confirms the math in the calculations given in my posts above on this thread.
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