floating your boat

[ebb]

The boat being our Pearson Ariel/Commander.
Check me out here if interested.
I was wondering how much foam it would take to positively support a flooded Ariel with closed cell foam.
Foam supports about 60# per ft3 -cubic foot.

If our 5500# A/C is roughly 1/2 lead and 1/2 fiberglass, how can we find its Specific Gravity? S.G is a number value for an object immersed in (salt) water.
Lead is: 11.0
Fiberglass is: 1.5 to 1.8. I use the 1.5.
Mahogany is: .6 (wood floats)
Human is: 1.1 (naked man sink)
Sea Water is: 1

To avoid 34 pages of engineering formulas,
can't we just take 1/2 the SG of lead AND 1/2 the SG of frp and combine them to get a representative Specific Gravity number for the 5500# empty boat? There are so many other material factors (wood furniture, metal fittings), plus 1 AND minus 1, that you could argue their total SG values neutralize. So I've chosen to ignore the contents of our MORC. Not scientific, but......maybe ballpark.

1/2 of 11 = 5.5
1/2 of 1.5 = .75
A + B = 6.25
That's what I propose for the S.G. of a half lead / half fiberglass object in seawater - to find out what it weighs when immersed.

Weight of A/C totally immersed:
W = Weight in lbs of A/C in straps
SG = Specific Gravity of the boat
PB = Portion of weight in lbs with Positive Bouyancy
NB = Portion of weight with Negative Bouyancy
F = Flotation value of foam in lbs per ft3. Cubic foot.*
Since water takes away some of the weight of an immersed object: W is divided by SG.
5500/6.25 = 878.6 PB
This PB is way under the weight of the boat, therefore to find the NB of the boat: 5500 minus 878.6 = 4621 NB.

NB divided by F = 4621/60 = 77 ft3 ---required to support 5500# immersed.

Multipling in a commonly used "safety factor" = 1.33.
77 X 1.33 = 102 ft3 of foam needed. One hundred cubic feet of foam to keep the boat up - where would it ALL go?
3'X3'X3' is a cubic yard.

It'll take 3.8 cubic yards of well placed closed cell foam to float an Ariel or Commander DECKS AWASH!
How do you like dem apples???
__________________________________________________ ________________________________________
*Nearly all the figures above are borrowed from FIBERGLASS BOATBUILDING FOR AMATEURS - Chapter 25 Flotation - by Glen-L Marine Designs. Thanks!
The SG whole boat ap, I take the blame for.
__________________________________________________ __________________________________________________ _____

[Rico]

Early in my career I designed a couple aluminum work boats that HAD to float (Stay on the surface - not go to the bottom...) if holed...

No need to get too complex...

The simple way to figure out the amount of flotation is to determine the amount of water that needs to be displaced...

There are many variables - The density of seawater varies with temperature, depth, and salinity at any one specific location of the water especially if in different oceans. As temperature increases, density decreases. As salinity of the water increases, density also increases. Somethings on the boat will aid flotation, some cavities will retain some air, etc...

But all this just confuses things with little impact to the end result. You can get pretty technical with all this, but in the end all that matters is that the boat remains on the surface.

Although the density of seawater varies at different points in the ocean, a good average of its density at the ocean's surface is 1025 kilogram per cubic meter. (or 63.99 lbs/ft3 - compared to the 62.4 lbs/ft3 of fresh water) .
We will take the specific gravity of seawater which is around 1.025. (Salt water is a bit denser than fresh). And ponder the following:

How many ft3 of water do we need to displace 5,500 lbs of water?
Divide 5,500lbs by 63.99lbs/ft3 = ~86 ft3 or about 3.18 yds3

(A bit more volume than Ebb's Calculation if you take Ebb's included safety factor into account - but almost the same number for all intents & purposes!)

This volume will get you neutral buoyancy. (No safety factor) Add a bit more to keep the mast & you out of the water!

End result: It is not practical to make your ariel Unsinkable!

A better solution would be external inflatable bags that fill upon activation of the EPIRB, or a quick release mechanism for the lead ballast!

[ebb]

Hey Rico, I like that quick and dirty formula, cool and refreshing!

Wasn't focused on any other aspect of flotation. Trying to find some numbers. Gratifying it came out like yours!
It becomes a problem when you try to do something practical.
Never say never.


OK, suppose you had a bunch of, say, 10"D, 10 feet long, inflatable tubes in the main cabin with an easy way of inflating them with a CO2 bottle? The main idea still is to keep the A/C floating even if swamped. (It would be mo'betta to have as much emergency bouyancy as possible. The actual boat with all its cruising gear - and the rig - would have to be estimated.)
We know what it takes now to keep a bare boat from sinking.
100 cubic feet.
In bags of air, not all that much volume.

In designing a PFD for the A/C I'd add in all the 'safety factor' possible!

The long tubes could be stored folded along their length - breakaway tied - in position ready to be inflated. Part of the interior decor.

In theory we need 1/3 less volume with air bladders compared with foam. 10 to 12 10' bladders could do it?* Have to be sure the system floats the swamped boat on her waterlines.
Connecting tubes for inflating the bladders could be arranged in the cockpit behind plastic access plates that you'd push/pull connect the gas bottle to. Or maybe at the companionway each bladder would have its own air cannister that could be reached from the cockpit and yanked to activate.
Individual canisters would have the correct amount of air for a bladder. And the system should be able to be tested and reusable.

Enough tubes would get the boat up off its sheer. Inflating tubes low down in the aisle might even displace water out of the cabin, raising the boat.
In an original Ariel interior, you might have these bladders under the side decks on both sides running stem to stern. A third to a half of the symmetric flotation needed - right there pretty much out of the way.

Can also see that it won't be as simple and not as inexpensive as it should be to get something like this to be practical.
Inflated tubes could keep you from getting below. Partial deflating might get you in. A number of tubes might let you slither in between them to retrieve stuff, close the hole, plug the seacock, or bail.
AND when things calm down - get the boat going again.


Don't we all agree that staying with the boat is the best way?
There are records of people abandoning ship never to be seen again - yet later the ship is found drifting, or sailing by itself. Those who stay with disabled boats have a better chance to survive. Be great to find a functioning flotation apple that would keep any boat afloat in the worst of times.
I believe the technology exists - it's the application that needs to be tweeked - and the costs to be realistic.


But most of us Commander and Ariel Captains - if swamped or holed
- would be depending on our PFDs and an unflated vinyl airboat.
and trying to remember when we last toasted the gods.
__________________________________________________ _______________________________________
AHOY CARL! Post #4 below here.....
[Later Post] maybe this is the one.
Found an Aussie site but can't bring it up. Key word is turtlepac.com
http://www.turtlepac.com/collapsibleair.htm
[LATER POST] The Dashew's SetSail web site reveals a note from Nov '99 (TEN YEARS AGO) where somebody wrote them asking what happened to Yachtsavers - he was interested in their airbags. A third party writes, "Hi Steve and Linda, I was reading the Q.A section and I noticed the comment about Yachtsaver. You were right someone picked up the business, it is Turtel Pack (sic) in Australia. www.turtlepac.com Email: turtlepac@yahoo.com au - Tel: (07) 5598 1959)"
Noise on the subject from other web sites complains about their non-responsive web pages and not answering the phone........BUT, they have superior products. Wouldn't know.
Punching in YACHTSAVERS on google produces about 6 pages.

[carl291]

A few years back there was a company that made a system that Ebb describes or quite similar,air bags and CO2 cylinders, After a few years of touting their system, they later admitted it's failure and quietly went out of business if I remember correctly.

[ebb]

It would be very interesting why Yachtsavers couldn't carry on.
The denominator is always money.
But the numerators are the people and the energy it takes to succeed.
But it may have been simple math. (See Below)

To me the prime purpose of an emergency system is to save the vessel.
My philosophy is like a lot of others:
Don't depend on the military to save your life - or the crew's - or the boat.
As a recreational vessel I think we have the responsibility to take care of the boat.
I wonder what the concensus is among cruisers about this subject?
What then is the difference needed to keep the boat afloat when faced with sinking?

Most skippers don't have all their eggs in the one basket of their A/C. They have another life on shore.
I have a feeling MY eggs will be in the one basket when I leave harbor.


Imagine that all emergency operations will be from the cockpit. There are probably dozens of scenarios of what could have caused the emergency.
Huge coamber over the stern - a 360 - a hole forward from hitting an immovable object like a container off a freighter - getting run down by one of those guys - a broken seacock - and 19 other ways.

Getting the water out of the cabin and repairing underwater damage are the most important things to accomplish. If you don't have an issue with the hull then I think you are way ahead. If you are awash with an airbag system working, it's not quite a walk in the park, but close.

Interior volume, how many ft3 is it? If you sealed the whole accommodation off would that float the boat?
If the boat is floating just below the sheer because the emergency flotation is working, that is a pretty secure platform in a storm - so I've heard! Certainly less buffeting and jerking is going on. If we weather the storm in that condition (and of course with a drysuit, a gallon of drinking water, and some Marsbars) we would carry on.

A couple of differences:
We want to get the boat to float above the deck. Aside from neutral flotation we have to get more water out. Pumps will only work if the boat is no longer taking water over the side - and you have the strength and energy. Low down inflation between the settees will displace a good volume of water, don't you think? Three or four long bags in the aisle will float the boat high enough to eventually pump and bail. That's minimum.

So, can see a two part system:
A) Keep the boat from sinking. (minimum flotation system - semi or totally automatic)
Primary flotation would probably be evenly positioned throughout the length of the boat, inboard but along the hull.
B) Get enough water out of the interior to float higher. (secondary airbag inflation - semi automatic or manual)
These bags displace water as quickly as possible.

This assumes you will be able to DIVE in the interior to plug whatever is letting the water in. That would mean that emergency flotation is arranged so that some access is available to work on the hull inside Hopefully.
The secondary airbags would attempt to float the boat higher but probably would not allow access below.

The secondary airbags could be vertical - attached only at their bottom. Kind of like BOZZO the clown punch bags. Vertical bags might be more versatile, allowing egress by pushing and deflating. Their main purpose is to displace water. Once it's calm and the boat is floating they can be removed, deflated, and reloaded later.

In ten years since Yachtsavers demise, there probably have been significant advances in thin, puncture-proof airbag fabrics, valves and automatic inflation. It would very good if an outfit customed an innovative system to individual boats, rather than simply supplying stuff out of a catalog.

Time to toast the gods again. There must be a bottle around here somewhere in this mess!:D

[ebb]

Think ole blowhard here has discovered why Yachtsavers quietly disappeared.

Let's figure out what a 10" diameter 10 foot long tube means.
It's just an out-of-the-hat size that seems managable:

pi.r2 of the circle X 120 will be the cubic inches (volume) of that cylinder.
78.5" X 120" = 9420 cubic inches. To get FT3 divide by 1728 (cubic inches in a FT3)
and the answer is.......
5.45 FT3.
100 FT3 (our required neutral bouyancy factor, NBF)
divided by 5.45 gets us 18.3!!!!!
18.3 10" diameter, 10' long bloody tubes.


What is the usable volume of an Ariel or Commander taking into account the cabin furniture and gear? 500 FT3? And another bouyancy design consideration is that a flooded interior does not go any higher than the companionway threshold. Unless of course the boat is sinking.

It looks like the cabin interior will be stuffed to the gills with airbags to get to a 100.
That's another assumption, but it certainly looks that way.
Course if we set off the compressed air on 18 high tech airbags all at once down in the accommodation,
the explosion will blast most of the water right out of there, and probably the windows too.



Any suggestions???
(Helium is not an option.)
__________________________________________________ ____________________________________________

Out of the box LOW TECH AIRBAGS

Getting to 100 FT3 could be done with 6 or 7 AIR MATTRESSES. One twin size (74X39X9) campbed has about 15 FT3. $25 to $75. Often come with 12V pumps (Aerobed, LLBean, Coleman, etc.)
Larger volumes will take longer to inflate.
Pool air mats maybe other cheap source.
Can we get pvc bags to inflate underwater ie in a flooded compartment?

Might be worth looking into.....

[kendall]

There is a ton of unused space in an Ariel that could easily be dedicated to flotation.

While looking over mine I saw a lot of spaces that the first thought I had was about making the space usable, most of it was extremely difficult to get to, or shaped so strangely that the only thing you could store in it was small items that would get lost easily, or soft goods.

With factory layouts, you could build a band of foam along the shelves on each side to provide a backrest and flotation. that's an easy way to come up with at least 20 cubic feet of foam, that since it at the same height on both sides and evenly laid out could also define your 'swamped' level.

Under the berths there is a LOT of space, but most of it is useless for regular storage use. It's oddly shaped, and with factory type construction needs to have fairly small access ports.
For the side berths, build a box roughly 12x6x60 inches long in it, and fill the rest with foam. You will lose some actual storage space, but gain usable storage space.
Building usable space into those areas and commit the rest to flotation is a great trade-off. Do the same in the V-berth, if you don't use the factory water tank, remove it and fill the space with foam, with or without a storage box built into the area.
Foam can be applied to the underside of the cockpit sole on outboard versions, or pack the area with removable blocks/sheets for access.

You can have 100 cubic feet of storage space, but if it's oddly shaped, or hard to use it may as well not be there.

Also, Might as well face it, if you absolutely -NEED- every possibly available cubic inch of storage in a boat the size of the Ariel, the boat is too small for you and you should go bigger.

With custom interiors, the options really open up, one issue I have is that the side berths feel too low, I'm debating whether I want to go through the trouble of raising them a few inches or live with it. If I rebuild them, it would make them more comfortable, and with the addition of a platform, would allow for a double berth that's right around 7ft long, great for when my daughter and 6'5" son-in-law spend time on the boat.
I could go with foam/glass construction, possibly faced with 1/4 inch wood along with the storage boxes, which would add quite a bit of flotation.

The counter I'm building in my Ariel has several drawers in it, the cabinet is 24 inches deep, and since I don't need or want 24 inch deep drawers I have approximately 8 inches of 'wasted' space behind the drawers, which would provide roughly 10 cubic feet of space for foam, counter top is 26 inches deep, with the wings over the berths being roughly 24x28 inch, on mine I have bands that are about 9" tall under them to pretty it up, that's another 6 cubic ft that could be filled with foam.
Many structures inside could be built with foam sheets covered with either glass or a thin wood, which would provide a light yet strong structure that incorporates flotation. (I like the looks of nice woodwork, but prefer the laminate on the bulkheads etc to be painted.)

Think that those ideas would provide 100+ cubic feet of foam that would never be missed by the average sailor. Also, many things in the boat can be counted as flotation, cooler, propane tanks etc.

ken.

[carl291]

Hey wasn't the Titantic engineered to unsinkable???
If being rundown by a large ship, the bow wave helps to push you off to sustain less damage. A collision with a container at what 3-7 knots will probably on be a thud.
With these boats it isn't like a grounding or a whale collision will rip the keel off.
A Navy Commander once said you only abandon a ship when you have to step UP to enter a liferaft.
Many many boats are abandon today not because they are sinking, more often the crew is simply uncomfortable.In defense of the crew , it seems the SAR team always want to remove the entire crew than risk having to come back out later. Even when hatches and the companionway are left open the yacht manages on it's own to stay afloat.
Wasn't it the Fastnet 69 race where a unconscious crew member (assumed dead) was left on board after an air rescue of the crew only to regain consciousness and continue sailing and save the boat only later to find out his crew had abandon him and the boat?
I vote for easy and quick access to the damaged hull area and the necessary means to stem the water flow along with material to effect a emergency repair . And a much larger pump or two, than you think will ever be needed:D

[ebb]

Carl,
Here's an informative 2 page article on the Titanic and a mammoth sail assisted 22,500 ton, 700' long iron passenger/cargo steamship built 56 years before the ill fated Titanic. The Great Eastern (IK Brunel) had a double hull. sixteen watertight bulkheads, a watertight 'bulkhead deck', and longitudinal bulkheads isolating the boiler and engine rooms.
The ship sideswiped a rock - much like the Titanic an iceberg - sustaining an 83' X 9' gash in its side. Didn't sink.
More than a half a century later the Titanic comes along with "a more efficient work space for the engine crew (that) meant the watertight bulkheads didn't carry as high, and she had no double hull (only a double bottom), no watertight bulkhead deck, or longitudinal bulkheads in her engine and boiler rooms."
The damage caused by the ice opened a long hole across several bulkheads.

"Safety features that were POSSIBLE were eschewed in favor of what was PRACTICAL - as Walter Lord wrote, 'The appearance of safety was mistaken for safety itself.'"

snopes.com The Unsinkable Titanic
Sinking the Unsinkable - is the title of the piece.
http://www.snopes.com/history/titanic/unsinkable.asp

Imco that statement about SAFETY relates to our Commanders and Ariels. Compromising requires at least having a persistant awareness.

Our Albergs may not be unsinkable, but I agree with you, our hull form makes it more likely to ride up on a submerged immovable object. We don't have fins that can be sheered off or knocked cockeyed. Hugo says for a glass boat to get holed it needs a lot of cracking first. We are probably light enough to bounce back some when we crash into something - better a glancing blow. Our hulls have no flat areas. Engineered double-hulled 'unsinkables' probably have very thin skins that could get holed or torn open..... Producing a very uncomfortable scenario of their own.
__________________________________________________ ___________________________________________
Kendal, I haven't 'done my sums' on built-in interior positive flotation.
In Little Gull I opt for Baldwinizing lockers and bins. Shop made water-resistant access lids are a problem - I have found NO inexpensive quarter-turn cam latches (including Southco) on the market. Many lids for small spaces require dozens. Each lid is an event to seal.

* Hugo du Plessis, who wrote the bible on 'Fiberglass Boats' in 1962, writes a comprehensive paper for the Caribbean Compass November 2004 titled "Fibreglass Boats and Damage Control"
google - Damage Control
www.caribbeancompass.com/damagecont.htm

This 6 page reality check covers just about everything on the subject including "The Watertight Lockers Option" that I am attempting in Little Gull A338. This does not solve the problem of the proportionately huge living space that can still fill up with heavy surging water and sink us. As he says, we have to do our sums. But offers no solution for emergency flotation.
[Imco there is NO way an existing Ariel or Commander can build in enough rigid flotation to float it. Maybe we can get close with sealed bulkhead compartments and lockers. They would always have to be closed - which isn't realistic. We will need a full and reliable compliment of airbags to displace water in the cabin when it gets in. We need WAY MORE THAN ENOUGH NEUTRAL BUOYANCY and we need it IMMEDIATELY. 100 lbs less than we need it's DaveyJones.. Depending on pumps is not realist either in a dire emergency. imco]

REQUIRED READING
__________________________________________________ ____________________________________________

Unsinkable fiberglass boats HAVE TO BE DESIGNED. ETAP's are well known racer/cruisers.
google - Sadler 26 archive details -Yachtsnet Ltd. online UK yacht brokers -

Nice photo presentation (especially by a yacht broker) on the"unsinkable" fin or twin keel double-hull Sadler 26. 1974, with complete particulars and pictures - except interior height.
".....many examples are found to have absorbed some water into the (URETHANE) foam."
Even with a nice open accommodation much of the storage is taken up with foam, they say.
We can believe it because we know how much flotation is needed just to keep a boat in neutral buoyancy when completely swamped. Hugo du Plesis in the above paper also points out that much more than neutral is essential to rehab a crippled boat at sea.

I believe Kurt is incorporating foam panels into KatieMarie. Which see here in the Gallery.
James Baldwin has created sealable lockers into his Triton, ATOM. See his webpage.

__________________________________________________ _____________________________________

Imco any boat is ALWAYS vulnerable and that there is no way to prepare for every emergency.
Every disaster is different.
Every response is different as well. It depends on your 'edge', your 'crisis reaction factor', attitude, faith, quickness (age), money.
Preparation and rapport with the gods..;)

[carl291]

EBB,
Thanks for the link,That should be a very interesting read.

[ebb]

Many independent lockers, many manageable airbags.

LOCKERS
Watertite lockers along the cabin length on both sides is achievable.
These lockers will displace water in the space we most want water displaced.
If you keep the original layout but rebuild the settees and and lockers to be sealable, that would be easy to figure.
But the number of cube feet attained would probably be very disappointing.

Filling in under the deck everywhere with lockers to my mind will destroy any feeling of spaciousness. People short enough to sit on the settees will not be able to. Sealable lockers can be put in against the hull as seat backs and storage - with some pullout seat extensions to get berth room. Lockers on the lower hullsides can be added into emergency flotation.
Can imagine a sealed locker isolating a hole or leak.
Seacocks can have at least a partially sealable locker built to contain them.

AIRBAGS
The space directly under the interior side/sheer decks could take emergency airbags.
But the technology doesn't exist to get them to inflate on demand all at the same moment. They would have to be coated nylon fabric to fold small. They wouldn't have to be automatic like automobile crash bags. It could be a pull the toggle thing. If we had just enough time to activate. Remember, no electricity. Each bag might have a battery operated CO2 cannister on a closed dedicated circuit that most airbags are connected to.
A bag or two for under the cockpit. A couple bags each for the cockpit lockers. A bag or two for the forepeak. They could be in breakapart cannisters in unoccupied spaces. The way to do this hasn't been done. So far as I've seen there is no R & D existing either.

Many separate bags add a measure of redundancy. Bags might not open with certainty in the anchor locker for instance. But if we were holed up there one of two inflated bags might close the hole for awhile. Bags would have relief valves against over inflation - ie two bags opening in a one bag location.

Settees could have bags built in on top to bag up still attached to the seat.. Bags could be ready in the edge of the aisle, securely attached. Most important airbags would be at the bottom of the accommodation. We want them to displace much of the weight of the water. Once the water is out, the leak detected and taken care of, most if not all bags can be deflated.

Four stage inflation:
All bags in inaccessible locations.
Bags fore and aft.
Bags in the cabin. Lowers first, Uppers if needed.

An Edson 1gal a stroke manual pump in the floor of the cockpit would be a godsend.

The technology for automobile crash bags is very complicated and exact..
It's hard to imagine anything automatic working well for long on a boat.
Imco a semi-automatic demand system has to be possible. Pull the toggles, bags fill up. Sounds simple, but will take time and $$$ to get a simple system to work. Somebody' right at this moment is holding a patent and looking for venture capital.

In the meantime is there anything more we can do?

(Suppose you want to design exterior water wings running along the sheer of an Ariel/Commander?
It would take two 25 foot long 20 inch diameter tubes to get enough air to theorectically float. Add to that the airbags are really too high up to do any good. Wouldn't float the boat correctly either attached along the sheer.
Another guy suggested a couple giant air donuts that you'd inflated around the front of the the boat and around the stern. No comment.

This kind of thing means to me that the only bet is INTERIOR FLOTATION of some sort.
Has to be thousands of skippers who want this level of safety for their boat.
Yeah, for themselves, for thier crew.

[ebb]

http://dayton.en.craigslist.org/boa/1515971092.html

This is a three bag system complete with a 26"X6.75" 16LB aluminum CO2 cylinder and connecting tubes to 3 (24"X9"X4") canistered pillows that will inflate to 5'X4'X3' each. By the seller's math each bag displaces 35ft3 of water which is about three long tons. It is therectically possible that these three bags could be inflated in the aisle of an A/C and provide the flotation necessary for neutral bouyancy.
If the cabin was flooded and completely filled there would be plenty of water weight still below with those bags inflated. Would assume that.
Also imco just inflating the bags isn't good enough - the inflation to do the job has to be attached low around the cabin sole. For example if we have a bag floating at the bridgedeck level inside the cabin, what use would it be?

Maybe what is obvious about the deployment of Yachtsaver bags, since they are in breakaway boxes is that the bag is meant to expand into a locker or closed space - would guess that the bag is not attached to anything but the inflation tube.
`I think we should be attempting to quickly displace tons of water and therefore at least one side (the 5' side probably) would have to be pre-connected to a select part of the boat before inflation.
I wonder if this illustrates what the problem was with this system? Airbags could be folded into long break apart tubes, with the bag, of course, attached along one edge inside the tube which is attached along the edge of the aisle (as I see it in our case).
You know, just an idea that pops up.

Maybe there are too many technical ducks to keep in order.
Yet it all seems rather direct and relatively simple. It certainly would be excellent to have a boat saving system that would also still be ready to go after ten years in a boat.
The Yachtsaver system for sale must have gone into service some ten years ago as it "was used on a harbor patrol boat located in Charleston, SC." It can't be new, but it might be unused. How would you go about testing? Yup, pull the toggle.


But anyway there is a photo of the Yachtsaver in the craig'slist ad.
Hopefully it is still up on the net to see.
I'm certain a simple system of compressed CO2 and bags is possible to put together.
__________________________________________________ ____________________________
5'X4'X3' = 60ft3. A ft3 of air supports 62lbs.
(The seller on Craigslist says the bag is PILLOW shaped and supports only 35lft3 which indeed supports about a ton, or displaces a ton. )
The designed empty weight of an A/C (5280#) is a good number to aim for to achieve neutral buoyancy in a loaded cruiser. Or a weekender with a large crew.
6 tons may be more realistic.
' Yes sir, yes sir, three bags full.'