Ships draught

[Sailfree]

I can remember reading somewhere that a ships draught alters slightly between static and underway.

Dr Google has no info that I could find .

Can anyone offer any clarity please?

 

[dom]

Static draught does indeed vary from dynamic draught and there are many factors:

• Shallow water/Squat effect (becomes material around 2.5x ship's draught)
• Hogging and sagging (bending of ship in middle depending on load distribution, etc.)
• Water temp
• Roll/Pitch

Kukri will no doubt have the dets, but the Shipping Co.s all specify a worked-up dynamic draught + a margin which is sometimes left to the discretion of the pilot.

 

[Sailfree]

Thanks.

Yes it was the squat effect I vaguely remembered from fluid dynamics lectures 50yrs ago! Could not remember the term to look up though!

 

[newtothis]

I remember seeing some footage of a cruiseship trying to get under a bridge with limited air draft somewhere in Scandiwegia. The trick to ensure clearance was to floor it to gain some advantage from the squat effect.

 

[Sailfree]

I remember seeing some footage of a cruiseship trying to get under a bridge with limited air draft somewhere in Scandiwegia. The trick to ensure clearance was to floor it to gain some advantage from the squat effect.

Was that the captain of the Costa Concordia!!

 

[RJJ]

Static draught does indeed vary from dynamic draught and there are many factors:

• Shallow water/Squat effect (becomes material around 2.5x ship's draught)
• Hogging and sagging (bending of ship in middle depending on load distribution, etc.)
• Water temp
• Roll/Pitch

Kukri will no doubt have the dets, but the Shipping Co.s all specify a worked-up dynamic draught + a margin which is sometimes left to the discretion of the pilot.

Squat effect - Wikipedia

 

[RJJ]

I remember seeing some footage of a cruiseship trying to get under a bridge with limited air draft somewhere in Scandiwegia. The trick to ensure clearance was to floor it to gain some advantage from the squat effect.

Mentioned in the above link.

 

[Stemar]

I remember seeing some footage of a cruiseship trying to get under a bridge with limited air draft somewhere in Scandiwegia. The trick to ensure clearance was to floor it to gain some advantage from the squat effect.

I happened to read about that a couple of days ago. Here's a news article

 

[DJE]

I seem to remember the QE2 hitting a rock somewhere off New England and some of the blame going to squat effect increasing the draft.

 

[sarabande]

Plus salinity, which varies summer to winter, and around the seven seas. And especially when entering a river, from the high seas.

Hence the multiple marks on the Plimsoll line.

 

[Bilgediver]

I remember seeing some footage of a cruiseship trying to get under a bridge with limited air draft somewhere in Scandiwegia. The trick to ensure clearance was to floor it to gain some advantage from the squat effect.

Brave people........Can do a lot of damage to the bridge when you get it wrong.

 

[MADRIGAL]

Plus salinity, which varies summer to winter, and around the seven seas. And especially when entering a river, from the high seas.

Hence the multiple marks on the Plimsoll line.

And going the other way. A bulk carrier fully loaded with grain at Thunder Bay, Ontario (fresh water) would not be fully loaded (i. e. would draw less water) when she gets to the port of Quebec (salt water), 2 000 nm further downstream. Even in fresh water there are seasonal differences in draft depending on water temperature.

 

[Bilgediver]

And going the other way. A bulk carrier fully loaded with grain at Thunder Bay, Ontario (fresh water) would not be fully loaded (i. e. would draw less water) when she gets to the port of Quebec (salt water), 2 000 nm further downstream. Even in fresh water there are seasonal differences in draft depending on water temperature.

Wonder if they still have beautifully polished brass Hydrometers on board. A necessary tool when loading grain for Europe in Fremantle on the mouth of The Swan River. More tons more dollars

 

[Frank Holden]

And going the other way. A bulk carrier fully loaded with grain at Thunder Bay, Ontario (fresh water) would not be fully loaded (i. e. would draw less water) when she gets to the port of Quebec (salt water), 2 000 nm further downstream. Even in fresh water there are seasonal differences in draft depending on water temperature.

Loading in in a fresh water river where the waters outside were a in summer zone you would load to your F ( fresh water ) marks - seen on the left ... and by the time you reach salt water the ship will have - as if by magic - risen out of the water and will be floating on her summer marks - S - seen on the right.

In Thunder Bay you would be doing the same sort of thing but working on the basis that you would be on your W ( winter) or WNA ( winter north atlantic) marks upon reaching Quebec. This example conveniently ignores bunkers and fresh water consumed while covering that 2000 miles.


u

 

[capnsensible]

Draft Mark's are taken hourly on berthed submarines for safety. They can be ignored once at sea though.

 

[Bilgediver]

I remember seeing some footage of a cruiseship trying to get under a bridge with limited air draft somewhere in Scandiwegia. The trick to ensure clearance was to floor it to gain some advantage from the squat effect.

The canal Haugesund to Stavanger. ?? He does it twice or more a day . and you feel as if you can reach out and touch the banks as well as the steel bridge at one point.

Squatting in canals is not good as the ship can develop a mind of its own as the depth under the keel reduces as some colleagues of mine found out in the Suez canal back in the sixties when they emulated the recent debacle of the Ever G. They were more lucky and delays to others minimal. Their first indication of a problem was the helmsman reporting no steerage. I think they did a left.... right....then left and nose into the west bank all at about 8 knots. No damage and carried on to Europe to discharge best Australian Wheat.

 

[scottie]

I recall watching the effect of the Belfast Friday evening ferry from Glasgow as it passed Bowling harbour especially at low tide

the water disappeared slightly before the ferry arrived dumping some boats on the harbour bottom and Rushed back as it passed definitely a Moses and the Red Sea effect