Sea Otter Ballast

[WJM]

If water has no weight when it is in water then how does the open water ballast tank on the hull of a Sea Otter boat work? I understand that the ballast tank is permanently open to the surrounding water - or have I got that wrong?

[Boat&Bikes]

If water has no weight when it is in water then how does the open water ballast tank on the hull of a Sea Otter boat work? I understand that the ballast tank is permanently open to the surrounding water - or have I got that wrong?

 

 

Water weighs as much in water as out surely?

[WJM]

Put a weighing scale on the bottom of the canal, then drop a tonne of water on top of it and...

[BEngo]

If water has no weight when it is in water then how does the open water ballast tank on the hull of a Sea Otter boat work? I understand that the ballast tank is permanently open to the surrounding water - or have I got that wrong?

 

 

Water in Water still has weight but the weight of the (new) water is just balanced by the bouyancy created when that water displaces the water that was there before.

 

The Sea Otter ballast water fills a space that would otherwise be full of air, and so creating bouyancy. The boat then needs greater displacement to provide the neccessary bouyancy and sits deeper in the water.

 

In essence it's controlled part-sinking.

 

When the Sea Otter is dragged up a slip the water runs out, so the towed mass is decreased by the weight of the water.

 

 

N

Edited August 1, 2012 by BEngo

[WJM]

That is not making sense for me - if you put water INSIDE the hull then yes, the density of the boat will increase and the boat will sit lower in the water. But the Sea Otter has a flood-able chamber OUTSIDE the hull. That chamber never provided any buoyancy to begin with.

 

If you hang lead weights off the bottom of the hull that will lower it in the water, but if you hang a 1500kg AIR WEIGHT of water off the bottom of the hull it will have no effect - because 1500kg air weight water weighs nothing when it is in water. It seems to me that the only ballast added is the weight of the chamber structure, because aluminium does weigh something in water - but not a lot!

[Liam]

Think Submarine ballast tanks...

 

http://www.bigsiteofamazingfacts.com/how-does-a-submarine-go-up-and-down

 

The only difference being is that a submarine will use pumps to remove the water from the tanks when trying to surface, where as the boat is pulled up the slipway and out of the water which will run out on its own.

[haggis]

That is not making sense for me - if you put water INSIDE the hull then yes, the density of the boat will increase and the boat will sit lower in the water. But the Sea Otter has a flood-able chamber OUTSIDE the hull. That chamber never provided any buoyancy to begin with.

 

If you hang lead weights off the bottom of the hull that will lower it in the water, but if you hang a 1500kg AIR WEIGHT of water off the bottom of the hull it will have no effect - because 1500kg air weight water weighs nothing when it is in water. It seems to me that the only ballast added is the weight of the chamber structure, because aluminium does weigh something in water - but not a lot!

 

yes, the floodable chamber is outside the hull but attached to it. If you look at a Sea Otter from below, it looks just like the hull of any other slightly V bottomed boat. However, there is a gap about two thirds of the way along the hull which, when you launch the boat, allows water to get into the floodable chamber, which at the start is full of air. The process is speeded up somewhat by opening a valve on the bows which allows the air to escape as water takes its place. When the chamber is full of water, the boat is some 6 inches or so lower in the water and therefore more stable. The wtaer stays in the chamber till you next retrieve the boat. When retrieving , the first haul onto the trailer is just far enough for the exit from the chamber to be out of the water and the ballast then flows back into the canal. This process is speeded up by opening the valve in the bows to allow air to take the place of the water.

Clever little boats!

 

haggis

[WJM]

No - I am not buying the submarine explanation. A submarine's ballast tanks are INSIDE the hull and the only time they are exposed to the outside is when the are flooded or emptied. The Sea Otter ballast tank is permanently exposed to the outside and therefore can never be considered to be inside the hull. If it is not inside the hull it cannot contribute to the boat's density.

 

Because the Sea Otter tank is PERMANENTLY EXTERNAL to the hull it's contents cannot make any contribution to the overall density of the boat. When the boat is in the water the tank is permanently flooded, thus debunking the submarine explanation.

 

 

If a Sea Otter were to heel over at a frightening angle thus lifting the ballast tank clear of the water then for a few seconds before the tank begins to drain the tank will have a ballasting effect - but that is extreme and unlikely!

Edited August 1, 2012 by WJM

[Nickhlx]

The water in the ballast tank, regardless of whether it is open ( by a smallish pipe) will add MASS to the whole, but not weight to the boat if this chamber is all below the water line.

 

If the chamber is below the water line and the pipe(s) that allow it to fill and empty are plugged, so that when the boat is put in the water the chamber stays dry,

the the boat will weigh less than when the tank is filled, by the weight of the water that isn't there, and will sit higher in the water, and have less MASS.

 

Nick

[bizzard]

Old diesel electric submarines also had open bottom fuel tanks, maybe they still do. the fuel being lighter floated on top of the seawater and so didn't interfere with the boats trim no matter how much fuel was in them. Serious sized Fuel-water separaters came into there own though before the fuel arrived at the engines in very rough seas.

[WJM]

"Ballast: Heavy material that is placed in the hold of a ship or the gondola of a balloon to enhance stability."

 

Underwater water is not heavy, it is weightless. And we are not placing anything into the hull.

 

It seems to me that the Sea Otter tank provides inertial dampening, not ballast.

[Chris Pink]

No - I am not buying the submarine explanation. A submarine's ballast tanks are INSIDE the hull and the only time they are exposed to the outside is when the are flooded or emptied. The Sea Otter ballast tank is permanently exposed to the outside and therefore can never be considered to be inside the hull. If it is not inside the hull it cannot contribute to the boat's density.

 

Because the Sea Otter tank is PERMANENTLY EXTERNAL to the hull it's contents cannot make any contribution to the overall density of the boat. When the boat is in the water the tank is permanently flooded, thus debunking the submarine explanation.

 

 

If that was true boats would be incapable of sinking.

[Guest]

My understanding and attempted explanation.

 

It is a dense 'weighty' material contained within a space in the boat - the fact that the material can flow freely in and out of the space to merge with material of the same mass and density does not change that fact that there is defined weight and mass still contained within that given space.....it only wouldn't work if it could flow out but not get back again and was increasingly replaced with air.

 

It would also work in exactly the same way if the tank was 'sealed' except of course when the boat was slipped the water wouldn't escape and you would lose the benefit of the lost weight.

Edited August 1, 2012 by The Dog House

[Pretty Funked Up]

If the water can flow in and out in large quantaties with ease it wouldnt affect it much.

If its restricted (sounds like it is from the fact it holds air ok on the way in) then all the water that cant shoot out the hole would be considered part of the boats mass.

[WJM]

Like on a Sea Otter, I also have 1500 litres of water under my boat. In fact I have a lot more than that, a lot more! My 1500l is not held loosely against the hull with aluminium walls. Instead it can move away from the hull at will. It adds nothing to the weight of my boat. The Sea Otter's 1500l adds nothing to it's weight either. The only difference I can see is that the Sea Otter's water cant be separated from the hull as easily - which has an inertial dampening effect on the boat when it tries to move.

 

So the Sea Otter is not ballasted with water - but it does have its movement dampened by a mass of underslung water.

 

 

A similar effect could probably be achieved by attaching lightweight baffle fences to the hull - though inertia would not be the principal?

[Boat&Bikes]

"Ballast: Heavy material that is placed in the hold of a ship or the gondola of a balloon to enhance stability."

 

Underwater water is not heavy, it is weightless. And we are not placing anything into the hull.

 

It seems to me that the Sea Otter tank provides inertial dampening, not ballast.

 

 

Not wishing to start an arguement but are you sure water underwater is weightless.Doesnt the fact that water pressure increases with depth mean that the more water above you the heavier it becomes?

[WJM]

Not wishing to start an arguement but are you sure water underwater is weightless.Doesnt the fact that water pressure increases with depth mean that the more water above you the heavier it becomes?

 

 

Given the range of depth here (2 feet) I think we can accept that underwater water is weightless for this purpose - just as in-air air is too!

[Pretty Funked Up]

They seem to hold onto their ballast for ages even when totally clear of the water line. I dont understand your confusion I'm afraid.

 

[Guest]

My last go -

 

If my boat sits lower in the water when the water tank is full it demonstrates it is weighing the boat down.

 

If that tank was through out the length of the hull and below the water line not only would it hold more water but it would ballast it. If I filled that tank and then sealed it from the top the boat would sit lower. If I then got in the water and drilled a big hole in that tank so that the contents could mix with the canal the boat wouldn't rise it would sit exactly where it was.

[Wumpty]

Just out of interest, is it possible to get to the inside of the water ballast tank to black it etc? If not, isn't it more suseptable to rust and corrosion?

[Tom and Bex]

I'm with WJM on this and fail to see how it could be described as ballast. Surely if the ballast tank was removed then the waterline would still be at the same place on the hull. I agree the boat may be less stable but that is not the same thing.

 

Tom

[Guest]

Just out of interest, is it possible to get to the inside of the water ballast tank to black it etc? If not, isn't it more suseptable to rust and corrosion?

 

Seat Otters are made of aluminium

[bizzard]

My last go -

 

If my boat sits lower in the water when the water tank is full it demonstrates it is weighing the boat down.

 

If that tank was through out the length of the hull and below the water line not only would it hold more water but it would ballast it. If I filled that tank and then sealed it from the top the boat would sit lower. If I then got in the water and drilled a big hole in that tank so that the contents could mix with the canal the boat wouldn't rise it would sit exactly where it was.

As per my post 10, the open bottom submarine fuel tanks.

[Guest]

As per my post 10, the open bottom submarine fuel tanks.

 

I know - but we are swimming against the tide here bud..

[Chris Pink]

I'm with WJM on this and fail to see how it could be described as ballast. Surely if the ballast tank was removed then the waterline would still be at the same place on the hull. I agree the boat may be less stable but that is not the same thing.

 

Tom

 

 

Here's a simple test;

 

start filling your boat with water.

 

It will get lower in the water, it's called sinking.

 

ballasting with water is exactly the same (except the idea is to stop before sinking)

 

I cannot believe that otherwise intelligent-seeming people can be arguing about this.

 

(that's my Meldrew moment for the day)