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Old 12-05-2014, 09:15 PM   #41
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Originally Posted by BruceK View Post
the former can have water tanks either side which can be filled on the weather side to counteract heeling
Icebreakers have used similar systems to rock the vessel back and forth to help break up the ice.
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Old 12-06-2014, 02:07 PM   #42
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I've always wondered about how initial stability is actually measured.
The most I've ever seen was less than 40 degrees, and that was plenty scary. I can't imagine 70 or 80. You'd be walking on the bulkheads.
Transverse stability is the righting action of any floating object. It's the result of the buoyancy force acting upwards and the displacement force acting downwards. The displacement force is equal to the weight of the vessel and operates through the center of gravity (CG) of the boat. According to Archimedes, "An immersed body is buoyed up by a force equal to the weight of the fluid it displaces", this is the buoyancy force, acting through the center of buoyancy (CB).

As the boat heels the center of buoyancy moves to one side (the lowest one) while the center of gravity remains (hopefully pretty much) stationary. This sets up a rotational force to right the vessel. The horizontal distance between the downward displacement force (on the centerline) and the upward buoyancy force is the righting arm. Multiplying the length of the righting arm by the buoyancy force (equal to displacement) gives you the righting moment.

Rolling big boats around in real life is awkward and expensive. It's rarely done, except with some Pilot boats, Coast Guard, some ocean racing sailboats, and the occasional stunt as mentioned in the note above about the Elling. I'll come back to that....

Today we have cheap personal computers and inexpensive software that can do stability calculations all day long with no sweat. From the above we can see that, if you have an accurate hull model floating at the correct level, and an accurate center of gravity, and know the density of the fluid our ship floats in, we can accurately calculate her stability (righting force) at any heel angle.

So large angle stability is not usually measured, it is modeled in a computer program, very carefully. Taking into account things like tanks and their contents, downflooding points, and the flooding of deck spaces (cockpits, etc).

What is physically measured is small angle (up to 3 degrees heel) stability. By heeling the boat with a known weight a known distance from centerline, and measuring (very accurately) the heel angle, and using geometry, we can precisely locate the center of gravity of the boat. This is known as the Inclining Experiment. The results of inclining experiments are often surprising, the center of gravity is always higher than anyone expects. And it's always higher than the NA calculated. Thus it's a vital part of any realistic stability study.

To return to rolling boats over over with cranes and watching them pop upright, it's spectacular but not really useful IMO. Some classification rules require this proof-positive of self-righting for certain vessels, almost always specialized commercial boats. A few years back a new 60'ish Pilot Boat was being tested like this in the PNW. During the inversion with a crane, a fire extinguisher fell out of it's mount and cracked a window. The boat started flooding, the water forming ballast inside the roof, the Pilot Boat failed the test and underwent expensive repairs. Just think about the 100's of pounds of stuff that would land on the roof should your average pleasure boat roll over. It makes these sort of "tests" seem pretty silly.
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Old 12-06-2014, 02:52 PM   #43
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What is physically measured is small angle (up to 3 degrees heel) stability. By heeling the boat with a known weight a known distance from centerline, and measuring (very accurately) the heel angle, and using geometry, we can precisely locate the center of gravity of the boat. This is known as the Inclining Experiment. The results of inclining experiments are often surprising, the center of gravity is always higher than anyone expects. And it's always higher than the NA calculated. Thus it's a vital part of any realistic stability study.
That's the part I was curious about. That's really interesting. I wonder why the CG is always higher than expected? Variation in the density of the hull materials or something?
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Old 12-06-2014, 03:19 PM   #44
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That's the part I was curious about. That's really interesting. I wonder why the CG is always higher than expected? Variation in the density of the hull materials or something?
Actually the hull material is the easiest thing to predict. What designers can't predict is what the owner will add to the boat.

I might imagine an owner will buy a lightweight dinghy and store it on the swimstep. Instead he'll buy a heavy hard-bottom inflatable with a monster outboard and center console, add 3 extra gas tanks, and a crane to lift the thing, and install it all on the roof! Then he'll add four solar panels with ss mounts, a giant radar and two sat phone antennas, and a new aluminum mast to hold it all......and so it goes.

Two bigger anchors, more chain, a hardtop on the flying bridge, a barbecue and fridge on the flying bridge....all additions above waterline, raising the CG and reducing stability......

Most additions by owner's are equipment on deck or upstairs inside, usually such additions are not included in stock boat stability studies.
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Old 12-06-2014, 06:26 PM   #45
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Originally Posted by Tad Roberts View Post
Actually the hull material is the easiest thing to predict. What designers can't predict is what the owner will add to the boat.

I might imagine an owner will buy a lightweight dinghy and store it on the swimstep. Instead he'll buy a heavy hard-bottom inflatable with a monster outboard and center console, add 3 extra gas tanks, and a crane to lift the thing, and install it all on the roof! Then he'll add four solar panels with ss mounts, a giant radar and two sat phone antennas, and a new aluminum mast to hold it all......and so it goes.

Two bigger anchors, more chain, a hardtop on the flying bridge, a barbecue and fridge on the flying bridge....all additions above waterline, raising the CG and reducing stability......

Most additions by owner's are equipment on deck or upstairs inside, usually such additions are not included in stock boat stability studies.
Many of the things you point our are the reason we greatly prefer a "stock" boat or a semi-custom that is consistent equipment just slight variations to custom. We can actually use one first. Even more, the builder has experience with very nearly the exact boat. The more custom and unique the boat, the greater the chances of surprises. Now if your custom builder is Feadship or someone of that ilk, it's not likely to have surprises but we'll all seen results like New World/Northern Marine. They were doing modifications far beyond their skills and knowledge.

I've been on a lot of production, but very much built by hand and will little computerization or technology, high performance boats where no two of them rode or handled the same. I also have seen a boat purchased and refitted by the owner with the waterline at the stern six inches or more lower than it was before. It rode so bow up and the stern just dug in deeper as they accelerated.

The biggest place I've seen people mess up the stability and other characteristics has been with batteries and tanks.
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Old 12-06-2014, 08:05 PM   #46
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Originally Posted by Tad Roberts View Post
Transverse stability is the righting action of any floating object. It's the result of the buoyancy force acting upwards and the displacement force acting downwards. The displacement force is equal to the weight of the vessel and operates through the center of gravity (CG) of the boat. According to Archimedes, "An immersed body is buoyed up by a force equal to the weight of the fluid it displaces", this is the buoyancy force, acting through the center of buoyancy (CB).

As the boat heels the center of buoyancy moves to one side (the lowest one) while the center of gravity remains (hopefully pretty much) stationary. This sets up a rotational force to right the vessel. The horizontal distance between the downward displacement force (on the centerline) and the upward buoyancy force is the righting arm. Multiplying the length of the righting arm by the buoyancy force (equal to displacement) gives you the righting moment.
Beautifully explained, Tad.
I can appreciate your comment about (hopefully stationary) CoG.

I see so many boats that have heavy objects which are not secured well enough for when the SHTF. Furniture, appliances, hot water tanks, tool boxes, storage lockers. And also things that are very difficult or impossible to secure - Extra large fuel and water tanks which are half full, heavy chain in a deep locker, human bodies - which all move and affect the CoG.

As you mentioned about the fire extinguisher; Besides affecting your Center of Gravity, shifting ballast can damage and sink your boat. If heard more than one story of batteries coming loose and snapping off seacocks.
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Old 12-12-2014, 07:40 AM   #47
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If heard more than one story of batteries coming loose and snapping off seacocks.

Thru hulls just installed with a valve and no installed seacock are a huge danger.

A bronze seacock installed with at least 3/8 bolts , can take a heavy hit!
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Old 06-11-2017, 01:42 PM   #48
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Hi, I hope you can help me , I have a Eurobanker 28 made in Denmark in 1978 . five years ago a previous owner had a bow thrust installed at the bow and the bottom of that forward locker had to be cut out for the installation and has not been sealed up , it looks as though a tank runs from the bow to the engine room bulkhead under the forward cabin floor and the width of about two feet ,at the point where you step down into the forward cabin there is a hole in the floor under the steps and it has a white plastic bung in it . I thought it was the shower sump but it contained 22gal water , does anybody know if this is a water ballast tank . Thanks if you can help . Dave
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Old 10-19-2017, 04:50 PM   #49
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Halsey's Typhoon

While on vacation this summer I was given a book to read by an old friend.
The book:
Halsey's Typhoon
What a GREAT book!!

some excerpts

synopses
"Describes how, at the height of World War II in the Pacific Theater, Admiral William "Bull" Halsey unwittingly sailed his undefeated Third Fleet into the heart of a powerful typhoon while supporting General Douglas MacArthur's invasion of the Philippines, a destructive act of nature that devastated the fleet and inflicted untold damage and loss of life."

One item that becomes apparent is how a number of the destroyers and destroyer escorts succumbed to the seas due to extremely low fuel loads, and the addition of lots of new hardware (radars, etc) to their upper superstructures.....lost of some of the stability they had been designed to originally.

GREAT book and story


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Old 10-21-2017, 03:02 PM   #50
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Anti-Rolling Tanks

A few days ago I got this private message update from the owner of this 58' Roughwater vessel Swan Song

Dave Cooper said:
Quote:
message: G'day Brian,just ran into an old thread on the boatdesign.net.
Anti-Rolling Tank https://www.boatdesign.net/threads/anti-rolling-tank.32925/page-3
You had quoted a right up I did on Swan Song's roll tank back then.

Just wanted to update you re the ongoing experiences since I wrote that.

110% better than described in that write up years ago and many 10's of thousand's of offshore miles since.
Never have even thought of using the 'dump valves' in any ocean state.
Just made a run 0f 2800 nm from Honolulu to Los Angeles. Upwind, upwave and up current for the most part.
3 gales, seas always from the beam forward. Days over 10' a few days over 15' two close to 20'.
We also took 2 waves in a 24 hr period broadside from nowhere that broke over the boat. 25'+
Nothing in all that was busted or even any stress crack from the roll tank over the pilothouse right down to the bottom of the hull.
I did a complete inspection once in LA as I was expecting some stress from twisting loads etc...None!
Our 15' tender on top of the boatdeck did try to come free even though I keep it totally deflated on a passage just so it wouldn't have much lift for waves over the deck which is 11 ft over the waterline.

One more time,... all I can say is that Dr Bass took all the data that I'd given him back in 2003 and worked his magic to design ART for Swan Song's specific parameters in all his modeling sea states.

My crew, unpaid as a friend, was a seasoned ocean delivery captain with a YachtMaster UK license. Many ocean deliveries under his command over the years in everything type of boat. Sail/Power/Monohull/Cat/Tri configurations. In gales and clam seas.
He said he'd never ever had a boat take the variety of seas/conditions that we had and do so well. zero faults in his experience. Just one more of the many many who share his opinion.
They have all been aboard if varying sea sates vs sitting at a computer running 'stabilization software'. Gotta be a disconnect there somewhere ;-)

It's funny how I read threads and see how folks, yacht designers in particular, continue to downplay ART's. Somehow the sea states in real life and Dr Gray's specific design for Swan Song and our implementation of it has well proven that there must be some missing data/algorithms in all those 'computer' runs on stabilization methods ;-)

Just thought, that as you had posted that long ago, an update might be interesting to you.

BTW we're leaving LA and heading down to the Canal and back into the Caribbean after 8 years in the Pacific. More than Likely we'll bring Swan Song into Cape Coral/Ft Myers, Fl around Christmas.
I plan on hauling her for a bottom job and need to replace the Wesmar bow thruster. More than likely in Glades boat storage yard in Moore Haven.
Could be a good time to have a look at this boat that seems to perform outside of the world of boat/yacht designers computers ;-)

Cheers
Dave
This is the original posting I had found on another forum back in 2010,...
Quote:
We had looked into these units when Swan Song was earlier in the stage of re-construction. At the time, according to the factory man in Japan, we'd need two of the mid-sized units to keep Swan Song from rolling less than 25 degree each side. If a sea state was at the level that we were rolling this much the power required to keep them both spinning was just over 8 KW. This all according to them. In a really nasty sea state they would need to be turned off as if they hit the stops often and with too much force they might self destruct. Suffering and surviving a knockdown wasn't in their vocabulary! All in all we opted not to go in that direction as the predictability of sea state isn't within our ability in making a passage.

As most of you know we went the route that few have taken and for the life of me I don't know why. An Anti-roll tank, ART, is our only roll reducing system on Swan Song. A totally passive device with no maintenance to date, 5 years, and only one moving part, water.

I wish I could say exactly what the numbers are for roll reduction but I can't. What I can say is that in 15,000 odd nm we have never rolled more than 30 degree and perhaps on 20-25. This includes a tough patch coming into Hawaii last year in waves that were occasionally in the 30' range. We can sit with no way on in 8-10 ft. seas with very little movement. So little that you can work on things and not be chasing tools all over the place. This is a very comforting fact especially when you have to shut down the engine for daily checks at sea.

For some reason Naval Architects hate them. They are afraid that they will somehow encourage a capsize or something similar. We have found all of this is bunk. Swan Song is stiff and has a high righting moment but it is round bilged and likes to roll and once started would go on for 12-15 rolls as a minimum. We tested all of this with full inclination tests, etc. Our boat parameters were fed into the computers that Dr Bass up in Newfoundland runs with his software and came up with the design tuned to our boat. $10,000 later it was constructed and put in place on top of Swan Song's Pilothouse. 1500 lbs. of water over our heads ;-)

Everyone who has ever been aboard either on the hook or underway is astounded by how Swan Song behaves in the water. At first they are puzzled and then as they watch other boats nearby whether they are sailboats with the masts waving in the sky or power boats showing lots of bottom paint they realized that Swan Song doesn't hear the music that the other boats are dancing to. As Seahorse John (now departed) once said in Bequia after an evening on board with the ferryboats passing 50' away without spilling the wine, "I thought you must be aground as you aren't rolling like the others". Then as he watched us round the point heading north out of the harbor the next day waiting for the first African tradewind swell to pin us down Swan Song just went on her merry way. "**** I got to get that system".

Bob Phillips, Another Asylum, has the same system done by Dr Bass. He is the one that sold me on it. Check with Bob as I think he'll compliment just about everything I've said.

I can't compare if paravanes would be as good but I don't think so especially at slow speeds, stopped or in shallow water plus they do require effort. Active fins are nothing but trouble long term and because of our size, propensity to roll, and slow speed the size of a system for Swan Song is large and in the order of $75K with all the attendant maintenance over the years. We would already be due for an overhaul. So for us the option was the paravanes or ART. Glad we made the choice we did and have never looked back.

Also as to significant wave height measurement, significant wave height is the average of the highest 1/3 of the wave. The actual wave height can be as much as 3 times that on occasion and from my experience usually is. Our height of eye is 12' off the water sitting in the pilothouse and beam waves up to this height aren't comfortable but offer no problem on the beam. Even an occasional crest coming aboard just tosses you sideways. Bigger waves that this and we are either taking them further forward or further aft. Dead down we are fine even in that 30' stuff near Hawaii. 20-30 degrees off our stern is our weak point. We get both the pitch and a roll so the corkscrew motion gets uncomfortable pretty quickly

Dave & Nancy
Swan Song
Roughwater 58
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Old 10-21-2017, 09:16 PM   #51
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I work occasionally on a research vessel (RV Savannah R/V Savannah - Skidaway Institute of Oceanography ) which has an ART. Truely incredible - the stability it provides, especially in a beam sea. It's the opinion of the crew that it is significantly better than using birds on outriggers while under way. This was how the previous ship was stabilized.

I can't speak to its ability in much above~10'. It's pretty hard to look under a microscope/bang on a computer in those conditions!
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Old 10-21-2017, 09:27 PM   #52
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Art?
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Old 10-21-2017, 09:35 PM   #53
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Anti Roll Tank. It has another name but I'm not firing on all cylinders right now. Someone will chine in I'm sure.
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Old 10-22-2017, 01:07 AM   #54
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Maybe U-Tube Tanks or Frahm Tanks?

The use of these tanks was pioneered by Frahm in Germany at the start of the 20th century and they are often referred to as Frahm tanks. These partially filled tanks consists of two wing tanks connected at the bottom by a substantial crossover duct. The air columns above the liquid in the two tanks are also connected by a duct. As in the free surface tanks, as the ship begins to roll the fluid flows from wing tank to wing tank causing a time varying roll moment to the ship and with careful design this roll moment is of correct phasing to reduce the roll motion of the ship. They do not restrict fore and aft passage as space above and below the water-crossover duct is available for other purposes. - Wikipedia
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Old 10-22-2017, 07:23 AM   #55
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Quote:
Originally Posted by brian eiland View Post
While on vacation this summer I was given a book to read by an old friend.
The book:
Halsey's Typhoon
What a GREAT book!!

some excerpts

synopses
"Describes how, at the height of World War II in the Pacific Theater, Admiral William "Bull" Halsey unwittingly sailed his undefeated Third Fleet into the heart of a powerful typhoon while supporting General Douglas MacArthur's invasion of the Philippines, a destructive act of nature that devastated the fleet and inflicted untold damage and loss of life."

One item that becomes apparent is how a number of the destroyers and destroyer escorts succumbed to the seas due to extremely low fuel loads, and the addition of lots of new hardware (radars, etc) to their upper superstructures.....lost of some of the stability they had been designed to originally.

GREAT book and story


Yes, terrific book. Incredible heroism. The modifications to the older destroyers included adding several 5-inch gun mounts, a lot of weight high up.
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