[NA]Ship/sub buoyancy [NA]

Let'sgoflying!

Touchdown! Greaser!
Joined
Feb 23, 2005
Messages
20,264
Location
west Texas
Display Name

Display name:
Dave Taylor
So, diesel is lighter than water. But it has mass.

As a ship takes on diesel, will the diesel add buoyancy or will it weigh the ship down, increasing its draft?

Child-like analogies welcome, I can't seem to figure this out.
 
What's the fuel replacing inside the ship? Is the fuel denser than what it's replacing?

Bingo. The tanks are inside the hull, which is air displacement. When you put in fuel, you are replacing air, so the total mass increases.
Theoretically, if the tank was full of water, then displacing the water with fuel would reduce the ship's draft.
 
Yep. Whenever someone says they aren't getting something, I find it's helpful to ask questions that may force them to think about the fundamentals of the problem and make the light bulb go on.
 
Aye-Aye.

<and yes, in the CG I said that ALL THE TIME>

For non-sailors:

"Aye-Aye" means "I have heard, understood and will comply with your order".

"Yes, Sir" oftent means "I heard you, and think about it, and will at least appear to be in compliance."
 
Last edited:
Bingo. The tanks are inside the hull, which is air displacement. When you put in fuel, you are replacing air, so the total mass increases.
Theoretically, if the tank was full of water, then displacing the water with fuel would reduce the ship's draft.
Correct. If it has water compensated fuel tanks like our DDGs, then the ship will rise as the fuel is taken onboard and the water is pushed out. If you add fuel to empty tanks with no water, the draft will increase.
 
Correct. If it has water compensated fuel tanks like our DDGs

Interesting, I didn't know the FFGs did that. Is the idea to keep the draft and handling as consistent as possible throughout the range of fuel loads?
 
So, diesel is lighter than water. But it has mass.

As a ship takes on diesel, will the diesel add buoyancy or will it weigh the ship down, increasing its draft?

Child-like analogies welcome, I can't seem to figure this out.

The fuel replaces air, not water. Fuel is heavier than air and the ships draft increases as she bunkers.
 
Correct. If it has water compensated fuel tanks like our DDGs, then the ship will rise as the fuel is taken onboard and the water is pushed out. If you add fuel to empty tanks with no water, the draft will increase.

Are you sure the compensation isn't done using separate ballast tanks rather than putting water in the fuel tank (which has to be the dumbest idea ever)?
 
FFGs (at least OHP class) do not use sw compensated fuel tanks like the DDG's do. The result is the FFGs bob like a cork when their fuel level is low.
The seawater is in contact with the fuel(on the sw compensated vessels) but since the fuel floats on the sea water the suction is taken from the top of the tank and a series of clever cross tank connections (in use/ holding) helps prevent any transfer of sw. The levels are monitored carefully.
 
Are you sure the compensation isn't done using separate ballast tanks rather than putting water in the fuel tank (which has to be the dumbest idea ever)?
Why so dumb? Aside from stability and space (no need for a trim tank), there's also little or no air space above the fuel- harder to get a fire started without air. Fuel vapor tends to be explosive. I'm pretty sure these are advantages for a war ship.
 
Why so dumb?

The dumb part is fuel in the water going to the boiler or IC engine. Of course the navy probably runs fuel centrifuges and day tanks to help catch any water. These arrangements require extra maintenance and personnel.

As with any design question, there are trade-offs. Inert gas can be used to prevent explosions so where do you want the money to be spent? fixing the inert gas system? or fixing the centrifuge? or paying the sailor to fill the day tank and sample the fuel?
 
The dumb part is fuel in the water going to the boiler or IC engine. Of course the navy probably runs fuel centrifuges and day tanks to help catch any water. These arrangements require extra maintenance and personnel.

As with any design question, there are trade-offs. Inert gas can be used to prevent explosions so where do you want the money to be spent? fixing the inert gas system? or fixing the centrifuge? or paying the sailor to fill the day tank and sample the fuel?
I was never in the Navy, I'm hoping those who actually were in the Navy chime in and teach us something.

As R&W said, it's not really a problem as they've been doing it for years and years. Others have indicated in the thread it reduces trim problems and keeps the load and ballast on a ship constant without the need for extra tanks. I once saw a barge that broke in two because someone emptied the center tank first and the resulting buoyancy broke her back. It wouldn't have happened if they displaced the oil with water. Filling it back up- you will get oil back into the water and that is a pollution issue. Considering the relative numbers of naval ships using this system, I really won't complain about that aspect.
 
I was never in the Navy, I'm hoping those who actually were in the Navy chime in and teach us something.

As R&W said, it's not really a problem as they've been doing it for years and years. Others have indicated in the thread it reduces trim problems and keeps the load and ballast on a ship constant without the need for extra tanks. I once saw a barge that broke in two because someone emptied the center tank first and the resulting buoyancy broke her back. It wouldn't have happened if they displaced the oil with water. Filling it back up- you will get oil back into the water and that is a pollution issue. Considering the relative numbers of naval ships using this system, I really won't complain about that aspect.

It's not really a problem if the designer and operator accept the additional manpower, maintenance, and equipment requirements. "We've always done it this way" does not necessarily make it the smart way to do it.

I've been a barge engineer responsible for stability of the barge along with all it's systems to keep the drilling rig running. Those systems included fuel & water to keep everything going. My boss was former Navy and we adopted systems that he knew including centrifuges. We used centrifuges because we had fuel quality issues that we couldn't control. Receiving diesel from workboats puts one at a slight disadvantage to say the least.

On another note, consider that ships have historically been allowed to discharge oily water when on the high seas so a little pollution is no big deal for the fuel over water gang.

As for your comment on breaking the back of a barge, all vessels must be loaded correctly. Some vessels are easier to manage than others but weight and balance has to be considered similar to aircraft. I've been to school and performed the calcs for offshore rigs. I'd rather deal with weight distribution problems rather than repairs due to water in the fuel to my diesels...

Now consider that Henning is a license Master with broad high seas experience. Keep his comment in context while respecting his knowledge and perhaps you'll understand that he sees little need for additional manpower and maintenance to deal with a fuel-over-water system. On the other hand, the Navy has historically desired large crews although that is now changing.
 
Ummm, my answer to the OP is for him to float on his back in the pool holding a large, empty washtub on his belly... I will start pouring diesel into the tub...

denny-o
 
It's not really a problem if the designer and operator accept the additional manpower, maintenance, and equipment requirements. "We've always done it this way" does not necessarily make it the smart way to do it.

I've been a barge engineer responsible for stability of the barge along with all it's systems to keep the drilling rig running. Those systems included fuel & water to keep everything going. My boss was former Navy and we adopted systems that he knew including centrifuges. We used centrifuges because we had fuel quality issues that we couldn't control. Receiving diesel from workboats puts one at a slight disadvantage to say the least.

On another note, consider that ships have historically been allowed to discharge oily water when on the high seas so a little pollution is no big deal for the fuel over water gang.

As for your comment on breaking the back of a barge, all vessels must be loaded correctly. Some vessels are easier to manage than others but weight and balance has to be considered similar to aircraft. I've been to school and performed the calcs for offshore rigs. I'd rather deal with weight distribution problems rather than repairs due to water in the fuel to my diesels...

Now consider that Henning is a license Master with broad high seas experience. Keep his comment in context while respecting his knowledge and perhaps you'll understand that he sees little need for additional manpower and maintenance to deal with a fuel-over-water system. On the other hand, the Navy has historically desired large crews although that is now changing.
Again- I'll wait for our naval people to chime in. You make good points but I'm not sure either of us see the whole picture.

As for Henning, the less said, the better. His experience (not his licenses) is dubious to me and should not be used for any reference. I will respect your knowledge and experience, but please don't use Henning as a reference.
 
Are you sure the compensation isn't done using separate ballast tanks rather than putting water in the fuel tank (which has to be the dumbest idea ever)?

Yes I'm sure...I got my Engineering Officer of the Watch on a DDGS. Just the storage tanks are water compensated, not the service tanks. It really isn't a bad setup. The fuel suction is at the top- water comes in from the bottom. As you suck fuel, water fills in pushing the fuel up. Only real problem is when you are topping off at sea and CHENG tries to take on too much- then you start putting fuel over the side.
 
Yes I'm sure...I got my Engineering Officer of the Watch on a DDGS. Just the storage tanks are water compensated, not the service tanks. It really isn't a bad setup. The fuel suction is at the top- water comes in from the bottom. As you suck fuel, water fills in pushing the fuel up. Only real problem is when you are topping off at sea and CHENG tries to take on too much- then you start putting fuel over the side.
JOOC, why do it this way? I can see some reasons for this, Clark1961 mentions why it isn't such a good idea.

Thanks for your service!
 
Why so dumb? Aside from stability and space (no need for a trim tank), there's also little or no air space above the fuel- harder to get a fire started without air. Fuel vapor tends to be explosive. I'm pretty sure these are advantages for a war ship.

I guess with warship maintenance schedules ad budgets you can get away with it, the rest of us use separate ballast and fuel tanks to do the same thing and use narrow longitudinal tanks to limit free surface.
 
The dumb part is fuel in the water going to the boiler or IC engine. Of course the navy probably runs fuel centrifuges and day tanks to help catch any water. These arrangements require extra maintenance and personnel.
FWIW, none of the steam or diesel ships I've been on had water compensated tanks- only the gas turbines. No extra personnel required, just fuel oil purifiers, but you'll find the same equipment on merchant ships.
 
FWIW, none of the steam or diesel ships I've been on had water compensated tanks- only the gas turbines. No extra personnel required, just fuel oil purifiers, but you'll find the same equipment on merchant ships.

Interesting that only the turbine ships were designed with water compensated tanks. Maybe the turbines would tolerate water without damage?

On the other hand, the Navy has watchstanders. So do a lot of merchies. The watchstander is additional personnel when compared to unmanned engine spaces which is where some new designs are going. The Navy doesn't want to go unmanned since they need bodies on board for damage control and historically, labor was cheap. Of course now we see reduced manning on some naval vessels as part of cost control efforts. Is it only a matter of time until the Navy hires Filipinos to man the engine spaces (like just about everyone else on the ocean)?
 
U-Boats used the water displacement method as far back as WWI I believe.
 
JOOC, why do it this way?

Biggest reason is for stability. By keeping the storage tanks pressed up with liquid, you eliminate free surface effect (water sloshing around in a partially filled tank has a negative effect on stability).

The second benefit is that by keeping the tank full you eliminate the problem of fuel vapors collecting in a partially full tank that could explode if the ship took a hit.
 
Don't mind me but why is this still being discussed? Stability, bunkering, limited ship space.... Other than discussing the fine points of systems, what wouldn't pilots already know about this?
 
We're all considering water ballastint our fuel tanks in Cessnas and Pipers. ;)
 
No, it doesn't bother me. But we all should be quite familiar with balance and stability. This thread gives me cause to pause.

Yes, the physics are the same whether it is a ship or a Cessna Piper.
 
U-Boats used the water displacement method as far back as WWI I believe.

Just about the only practical way to store fuel outside the pressure hull is oil-over-water tanks. I think all the diesel boats used external tanks.
 
Biggest reason is for stability. By keeping the storage tanks pressed up with liquid, you eliminate free surface effect (water sloshing around in a partially filled tank has a negative effect on stability).

The second benefit is that by keeping the tank full you eliminate the problem of fuel vapors collecting in a partially full tank that could explode if the ship took a hit.

The DDGs have a long narrow hull so it seems reasonable to be concerned with stability. Although we calculated free surface effect, it was never a big problem on the drilling barges I dealt with. We never ran out of one fuel tank just so we could keep the other one full. Henning might see free surface effect problems on some of the boats he deals with.
 
The DDGs have a long narrow hull so it seems reasonable to be concerned with stability. Although we calculated free surface effect, it was never a big problem on the drilling barges I dealt with. We never ran out of one fuel tank just so we could keep the other one full.

It is true that most ship designs/tank configurations are built in such a way that free surface isn't generally a problem.

When I was Chief Engineer on the LSD (Diesel plant) I didn't have water compensated fuel tanks....but my tanks were fairly small and I had a lot of ballast tanks since the ship had a well deck.

Occasionally, you will find a merchant where either free surface effect or just the loss of weight from an empty tank down low is a problem.

Little piece of maritime trivia - The desingers of Andrea Doria placed a note in the ship's stability book that required ballasting empty fuel tanks to maintain sufficient stability. Engineers generally didn't want to mess with that (the issue with manpower that some have mentioned is the problem of having to clean the purifiers more frequently when running a mix of Heavy Fuel Oil or Bunker C fuel with water through them). And the Captain didn't force them to do it.....that is why the ship capsized after she was t-boned by Stockholm. Had they ballasted the empty tanks, she would never have rolled over.
 
Last edited:
Just about the only practical way to store fuel outside the pressure hull is oil-over-water tanks. I think all the diesel boats used external tanks.


External to the pressure vessel? Yes, but my point was they had the valve technology to do it.
 
Back
Top