Floating your boat

This topic is empty.

Viewing 20 posts - 1 through 20 (of 20 total)

Author

Posts

18 November 2012 at 08:53 #37644

Bob Abell 2

Participant

@bobabell2

I've been thinking about the theory of what makes a steel boat float?……..As I wait for the glue to dry!

Suppose a naval architect designs a ship weighing 20,000 tons…..How does he know it will float before it is launched?……..Very important!

Looking on the net, it says……"A 20,000 ton boat floats because it displaces 20,000 tons of water "

Assume the ship is a box shape for easy calculations and is the size of The Great Eastern……..ie……..800 feet x 60 feet and according to the records, it displaced 22000 tons of water……..Which must be the ships weight?………The net also says, GE weighed 11,000 tons…………Which clouds the issue somewhat!

This is the way I see it………It may not be correct, but it is an interesting challenge!

We need to know what volume of air represents ….20,000 tons of water?

Will post this, while I go away and work this out

Bob

Edited By Bob Abell on 18/11/2012 09:05:35

Advert

18 November 2012 at 08:53 #3923

Bob Abell 2

Participant

@bobabell2

18 November 2012 at 09:07 #37645

Colin Bishop

Moderator

@colinbishop34627

Bob,

If it helps, I seem to remember from primary school that a pint of water weighs a pound and a quarter….

While you are at it, you could work out why airships float in the sky…..

Colin

18 November 2012 at 09:23 #37647

Bob Abell 2

Participant

@bobabell2

Thank you for the info, Colin, but I've cracked it!

22,000 tons x 2240 = 49,280,000 lb

A cubic foot of water weighs…..64 lb

Volume of water = 49,280,000 lb divided by 64 = 770,000 cu ft

Area of GE = 800 x 60 ft = 48,000 sq ft

Waterline depth = 770,000 divided by 48,000 = 16 feet!………..QED!

I find that very satisfying!……..I have always wondered about this

Bob………Naval Arch. ONC

18 November 2012 at 09:37 #37648

Paul T

Participant

@pault84577

Bob

A few things for you to consider:

Fully laden the GE topped 32,000 tons

Salt water and fresh water have different densities so the displacement of the ship will change between river and sea.

The density of both salt and fresh water is subject to changes in ambient temperature and air pressure.

Paul

18 November 2012 at 09:47 #37650

Bob Abell 2

Participant

@bobabell2

Thank you, Paul

With a mass of 32000 tons, the waterline increases to 21 ft………Just about floating

They actually had different sizes of paddlewheels in extreme cases, which must have been a time consuming nightmare to change! How they did it, is still a mystery

My calcs were for fresh water, so sea water ain`t a problem?

Bob

18 November 2012 at 12:15 #37654

Paul T

Participant

@pault84577

Freshwater was a huge problem for the GE as they could only take on coal at a deep water harbour and the changing salinity of the water on incoming and outgoing tides affected the buoyancy of the ship with the obvious risk of grounding.

18 November 2012 at 12:32 #37655

Bob Abell 2

Participant

@bobabell2

Hi Paul

Is this difference in water densities shown on the Plysol line?

Probably is?

Bob

18 November 2012 at 13:23 #37656

Dave_P

Participant

@dave_p

Yes it is Bob.

Also difference in winter

Edited By Dave_P on 18/11/2012 13:26:09

Edited By Dave_P on 18/11/2012 13:32:37

18 November 2012 at 13:55 #37657

Bob Abell 2

Participant

@bobabell2

Thank you, Dave

Now it all makes sense!

When did this system come into force?

bob

18 November 2012 at 14:56 #37658

Dave_P

Participant

@dave_p

Samuel Plimsoll finally managed to get his bill passed in parliament in 1876 but made many enemies of shipowners who were sending ships well overloaded to sea at great cost to seamans lives. The act allowed shipowners to paint the lines where they thought best resulting in some painting them on the funnel! In 1890 they were forced to paint them as intended.

This Plimsoll line is for the starboard side of a vessel; on the port side,the markings are reversed. The centre of the disk is placed at the middle of the loadline. The lines are one inch thick.

The letters indicate cargo, season and location:

LTF Lumber, Tropical, Fresh
LF Lumber, Fresh
LT Lumber, Tropical
LS Lumber, Summer
LW Lumber, Winter
LWNA Lumber, Winter,
North Atlantic

LR Lloyds Register of Shipping

TF Tropical Fresh Water Mark
F Fresh Water Mark
T Tropical Load Line
S Summer Load Line
W Winter Load Line
WNA Winter Load Line, North Atlantic

Hope this helps further Bob.

Cheers

Dave

Edited By Dave_P on 18/11/2012 14:57:23

18 November 2012 at 15:01 #37659

Dave_P

Participant

@dave_p

Just realised, this was after Great Eastern was launched.

18 November 2012 at 18:16 #37662

Bob Abell 2

Participant

@bobabell2

Let`s see how a submarine works out

About 6.5" square section x 84" long and weighs 60lb

For it to float it displaces 60lb of water………….6 galls

60lb of water is about 1 cu ft……..1728 cu ins

Area of sub = 6.5 x 84 = 546 sq ins

Depth of waterline =1728 / 546 = 3,2"…………Which is exactly on the hull centreline

So in order to submerge, it has to take in 60 lb of ballast?

Is this correct, Paul?………..Doesn`t seem right somehow

Bob

18 November 2012 at 18:42 #37663

Gareth Jones

Participant

@garethjones79649

You are correct Bob, the submarine would need 60 lb of ballast to submerge, which is about 6 gallons, or around half the total volume of the vessel, assuming you flooded it with water.

Gareth

Edited By Gareth Jones on 18/11/2012 18:42:51

18 November 2012 at 18:54 #37665

Bob Abell 2

Participant

@bobabell2

Thank you, Gareth………That`s nice to know

Taking on 60 lb of water will take some doing and expelling it too

The dimensions are roughly based on Paul`s M1 venture, just out of interest

Bob

18 November 2012 at 19:19 #37666

Colin Bishop

Moderator

@colinbishop34627

You can buy submersible bilge pumps intended for yachts which can shift quite a lot of water quickly.

Colin

18 November 2012 at 19:37 #37667

Bob Abell 2

Participant

@bobabell2

Thank you, Colin

I found the calculations very interesting and the Plymsol line was a vital breakthrough.

When GE was loaded to 32000 tons…..They wouldn`t have known it was safe or not safe to sail!

Now then, about that airship……..zzzzz

Bob

18 November 2012 at 19:39 #37668

Paul T

Participant

@pault84577

Now that we have discussed buoyancy, displacement and getting it submerged how are we going to stop it from going all the way to the bottom without bobbing back up again?

Paul

Edited By Paul T on 18/11/2012 19:40:14

18 November 2012 at 19:48 #37669

Bob Abell 2

Participant

@bobabell2

Go forward

Or pump some water out?

How do the calcs match up yo yours?

Bob

18 November 2012 at 19:55 #37670

Paul T