# Damage height versus waterline when ships collide

Science Advisor
Gold Member
I am curious. There have been two collisions recently between a military ship and a large commercial ship. Both times, the military ship was damaged at the waterline and the other ship was damaged far above the waterline. How does that happen?

sophiecentaur

## Answers and Replies

anorlunda
Staff Emeritus
Wow, that's difficult to answer. The tanker's hull may have been stronger at the waterline thus undamaged even if struck.

The destroyer may have rolled over during impact.

Lots of speculations are possible.

FactChecker
Vanadium 50
Staff Emeritus
Science Advisor
Education Advisor
Collisions at sea are not 2-D.

russ_watters and FactChecker
A.T.
Science Advisor
I am curious. There have been two collisions recently between a military ship and a large commercial ship. Both times, the military ship was damaged at the waterline and the other ship was damaged far above the waterline. How does that happen?
Multiple contact areas? Smaller ship lifted up on impact?

FactChecker
Science Advisor
Gold Member
On second thought, I think all these responses are right. I saw a photo that seemed to show that the tanker had a large protrusion from the nose at the waterline -- almost like it is supposed to protect it. And the military ship looks like it was damaged in between some much stronger horizontal lines (floors?).

This thread can be closed.

jim mcnamara
Mentor
Unlocked. Please, no speculation.

It is common for tankers and similar large cargo ships to use what is called "bulbous bow," which is a large, rounded extension of the bow below the waterline. This apparently improves the hydrodynamic performance of the hull, although I do not fully understand why. This means that the absolute front of the tanker is quite aways in front of the visible bow. Warships do not use this bulbous bow. When we look at the photos of the hole in the side of the McCain, it looks almost exactly like a female mold of a bulbous bow.

This would explain why the damage to McCain is at and below the waterline while the damage to the tanker is all much higher.

russ_watters, FactChecker and berkeman
Science Advisor
Gold Member
It is common for tankers and similar large cargo ships to use what is called "bulbous bow," which is a large, rounded extension of the bow below the waterline. This apparently improves the hydrodynamic performance of the hull, although I do not fully understand why. This means that the absolute front of the tanker is quite aways in front of the visible bow. Warships do not use this bulbous bow. When we look at the photos of the hole in the side of the McCain, it looks almost exactly like a female mold of a bulbous bow.

This would explain why the damage to McCain is at and below the waterline while the damage to the tanker is all much higher.
In this most recent accident I have seen simulated reenactments where the tanker hits the side of the McCain with the bulbous bow.

anorlunda
Staff Emeritus
Unlocked. Please, no speculation.

Um, but before the final accident report, anything that anyone says is speculation by definition. What room is there to say anything that is not speculation?

davenn
Science Advisor
Gold Member
It is common for tankers and similar large cargo ships to use what is called "bulbous bow," which is a large, rounded extension of the bow below the waterline. This apparently improves the hydrodynamic performance of the hull, although I do not fully understand why. This means that the absolute front of the tanker is quite aways in front of the visible bow.

some quick research and reading gives all the answers

Warships do not use this bulbous bow.

this is simply not true -- particularly for the larger ships including aircraft carriers

from wiki ...
A bulbous bow is a protruding bulb at the bow (or front) of a ship just below the waterline. The bulb modifies the way the water flows around the hull, reducing drag and thus increasing speed, range, fuel efficiency, and stability. Large ships with bulbous bows generally have twelve to fifteen percent better fuel efficiency than similar vessels without them.[1] A bulbous bow also increases the buoyancy of the forward part and hence reduces the pitching of the ship to a small degree.

Bulbous bows have been found to be most effective when used on vessels that meet the following conditions:

• the waterline length is longer than about 15 metres (49.2 ft)
• the vessel will operate most of the time at or near its maximum speed [2]
Thus, large vessels that cross large bodies of water near their best speed will benefit from a bulbous bow. This would include naval vessels, cargo ships, passenger ships, tankers and supertankers. All of these ships tend to be large and usually operate within a small range of speeds close to their top speed.[3] Bulbous bows are less beneficial in smaller craft and may actually be detrimental to their performance and economy. Thus, they are rarely used on tug boats or recreational craft like powerboats, sailing vessels, and yachts.

and it isn't a new thing ... warships as far back as 1925 started using a bulging bow

berkeman and fresh_42
jbriggs444
Science Advisor
Homework Helper
and it isn't a new thing ... warships as far back as 1925 started using a bulging bow
In fact, the Arleigh Burke (DDG-51) class of which the John S McCain is a member has an upgrade package which includes a bulbous bow. The article referenced here is a bit of a sales pitch, but seems reliable:

http://www.globalsecurity.org/military/systems/ship/ddg-51-upgrade.htm

DDG-51 - Other Upgrades
The US Navy is a recognized leader in the development of hydrodynamic technologies for improved ship power and fuel savings. Stern flap and bulbous bow are two technologies that have demonstrated cost and fuel savings. The application of the stern flap to naval destroyers is a recent innovation. The stern flap originated from stern, or transom, wedge research conducted in the 1980s. Stern wedges or flaps have been installed on naval destroyers to create a vertical lift at the transom and to modify the distribution of pressure on the after portion of the hull. The Navy reports better fuel efficiency, higher top speed, and reduced emissions. The cost of implementation, $170,000, can be recouped within approximately one to two years. The Navy also found that refitting a bulbous bow on a DDG-51 Class Destroyer results in tremendous fuel savings from reduced ship resistance. Although the original funding for this project was$3.4 million, savings for 50 ships in the DDG-51 Class are estimated at \$200 million. The bulbous bow concept has been well received, and as a result of the great potential for cost savings, bow designs for future ships are being reexamined. The success of the bulbous bow retrofit has resulted in the Navy aggressively pursuing spin-off technologies with the potential for similar fuel savings.

davenn