Hull Speed (Maximum Cruise)

For reference purposes I wanted to publish a table of hull speed versus ship’s water line length. As a rule of thumb, “hull speed” typically establishes the maximum economical cruise speed of a displacement hull as a result of wave making. Hull speed is a speed to length ratio where

Speed in knots / square root of the waterline length in feet = 1.34

I am also including the speed where the ratio equals 2 because this is typically an effective limit on maximum speed.

Like most rules of thumb, there are exceptions, but in most cases, the hull is at least to some degree “planing.”

  • Length     Hull Speed     Speed to Length=2
  • 64                10.72                   16
  • 81                12.06                   18
  • 100              13.4                     20
  • 121              14.74                   22
  • 144              16.08                   24
  • 169              17.42                   26
  • 196              18.76                   28
  • 225              20.1                     30
  • 256              21.44                   32
  • 289              22.78                   34
  • 324              24.12                   36
  • 361              25.46                   38
  • 400              26.8                     40

Not that it matters to the Coast Guard, but when ships get really large, wave making is not much of a problem. For a ship (like an aircraft carrier) that has a waterline length of say 1,000 feet, hull speed is 42.4 knots and they would not reach a Speed to Length ratio of 2 until they are going 63.2 knots.

 

Hybrid Propulsion News Release

We have a news release on the electric portion of the Offshore Patrol Cutter’s hybrid propulsion system. There is not a lot of specifics about the system, but it does provide a rationale for installation of this additional system in a ship that above all else is designed to be “affordable.” The news release is quoted in full below.

ARLINGTON, VA, April 13, 2017 ̶Leonardo DRS, Inc. announced today that it has been awarded a contract by Eastern Shipbuilding to provide hybrid electric drive systems for the U.S. Coast Guard’s new fleet of Offshore Patrol Cutters. The contract, for the first nine systems, is worth $10.7 million. Eastern Shipbuilding is the prime contractor and builder of these nextgeneration Offshore Patrol Cutters.

Under the contract, Leonardo DRS will provide its high-performance, permanent magnet motorbased Auxiliary Propulsion System. This integrated hybrid electric drive system provides capability for the ship to operate much more efficiently at slower speeds, increases mission duration capability, reduces emissions and provides emergency take-home capability in the event of a failure of the main propulsion diesel engines. When coupled to the main propulsion gearbox, the system allows the ship to operate quietly and efficiently during loitering operations while providing superior fuel economy for increased on-station operations and capability.

“DRS is a pioneer in naval hybrid electric drive technologies and we are proud to be able to deliver these advanced systems to the Coast Guard’s newest generation of ships,” said Dianne Howells, Vice President of Leonardo DRS Surface Ships business unit. “Our Auxiliary Propulsion Systems will give the crews of these new ships operational flexibility when they need it, while significantly increasing cost savings in yearly maintenance and fuel.”

The Auxiliary Propulsion System is designed and built by Leonardo DRS, a leader in naval hybrid electric drive propulsion technology. The system includes two of the most power-dense permanent magnet motors on the market today. They have significant advantages in size, weight, efficiency and performance over conventional electric induction motors and produce more torque from the same amount of supplied current. Their smaller footprint allows greater flexibility in engine room design and increased cargo space, and their simpler more rugged construction results in proven reliability and durability.

Using propulsion diesel engines at slow speeds adds significant wear and tear on the engines and increases the potential for coking/wet stacking. By adding this electric Auxiliary Propulsion System, the Coast Guard will have a built-in advantage of reducing not only fuel and maintenance requirements, but total lifecycle costs and increased safety for the fleet.

About Leonardo DRS Leonardo DRS is a prime contractor, leading technology innovator and supplier of integrated products, services and support to military forces, intelligence agencies and prime contractors worldwide. The company specializes in naval and maritime systems, ground combat mission command and network computing, global satellite communications and network infrastructure, avionics systems, and intelligence and security solutions. Additionally, DRS builds power systems and electro-optical/infrared systems for a wide range of commercial customers. Headquartered in Arlington, Virginia, DRS is a wholly owned subsidiary of Leonardo S.p.A., which employs more than 47,000 people worldwide. See the full range of capabilities at http://www.drs.com and on Twitter @drstechnologies. For additional information please contact: Michael Mount Senior Director, Public Affairs 571-447-4624 mmount@drs.com Twitter: @drstechnologies