Understanding Hydrostatic Interlock Systems and Lifeboat On-load / Off-load Release Mechanisms

Safe operation of lifeboat release systems is critical to crew survival and vessel compliance with international safety regulations. However, recent onboard drills and training observations have shown that many seafarers—including ratings and even some deck officers—remain unclear about the differences between on-load and off-load release mechanisms on fall-lowered lifeboats, as well as the purpose and function of the hydrostatic interlock system.

This guide is intended to:

• Clarify the core safety principles behind lifeboat release mechanisms.
• Explain the role of the hydrostatic interlock unit in preventing accidental on-load release.
• Highlight key requirements from IMO Circulars MSC.1/Circ.1205 and MSC.1/Circ.1392.
• Provide practical recommendations for onboard training and drill safety.

The aim is to ensure that all lifeboat crew—especially deck officers and ratings—can operate these systems safely, correctly, and confidently, in line with SOLAS and IMO standards.

1. On-load vs Off-load Release: What’s the Difference?

Confusing these two modes of release has led to fatal accidents in the past. Therefore, understanding the release conditions and associated safety interlocks is critical.

2. Hydrostatic Interlock System

Fall-lowered lifeboats are required to be fitted with a hydrostatic interlock unit, typically located at the bottom of the lifeboat. This safety device:

• Prevents on-load release unless bypassed.
• Only permits the release handle to function when the lifeboat is fully afloat (waterborne).
• Often includes a green indicator light inside the lifeboat, showing when off-load release is safely available.

This requirement aligns with IMO MSC.1/Circ.1392, which emphasizes that the interlock must be inspected and tested periodically according to the manufacturer's instructions.

3. On-load Release and the Bypass Mechanism

In rare emergency scenarios—such as when a lifeboat is jammed during lowering or cannot reach the water—the on-load release may be required. In such cases:

• A bypass procedure must be carried out manually and deliberately.
• The bypass mechanism typically requires the crew to:
  • Open or break a protective cover.
  • Manually shift a lever into the bypass or on-load release position.

This extra step is intentionally designed to prevent accidental activation of the on-load release.

Important: IMO MSC.1/Circ.1392 specifically highlights that improper use or misunderstanding of on-load release systems has resulted in serious and fatal accidents.

4. On board Drills and Crew Responsibilities

• On-load release testing is not part of standard abandon ship or man-overboard drills.
• Crew members are not authorized to perform on-load release testing.
• Such testing must only be carried out by qualified personnel and usually in the presence of a certified technician or surveyor, as stipulated in MSC.1/Circ.1392.

However, all lifeboat crew should still be familiar with the location and function of both the release handle and interlock system, including the bypass mechanism (without activating it).

5. Training Recommendations

• Include visual materials from IMO MSC.1/Circ.1205 in onboard training sessions.
• Reinforce awareness of the hydrostatic interlock and its role in preventing dangerous errors.
• Ensure that all officers and ratings understand:
  • When it is safe to release the lifeboat.
  • How the interlock system functions.
  • Why on-load release is restricted and tightly controlled.
• During inspections or drills, review MSC.1/Circ.1392 procedures related to hook system maintenance and test intervals.
• Clearly label bypass covers and controls inside the lifeboat, and brief all relevant crew members on their use only in true emergencies.

BBC – SMR Marine

Training Department

Capt. Dang Quang Dan (+84 0948 067 095)