The persistent slackening of a main engine's holding-down bolts is a serious issue that can compromise the engine's alignment and structural integrity. The primary reasons for this problem are:

• Vibration and Dynamic Forces: The engine's continuous operation creates powerful vibrations and dynamic forces from combustion and reciprocating parts. These forces repeatedly stress the bolts, causing them to gradually lose tension over time.
• Insufficient Tightening: If the bolts were not tightened to the manufacturer's specified torque during installation or maintenance, they lack the necessary pre-tension to resist operational forces, leading to faster slackening.
• Settling of the Chocks: The chocking material—which supports the engine on the foundation—can compress or settle over time. This change in height reduces the clamping force exerted by the bolts, causing them to loosen.
• Poor Surface Contact: Uneven or poorly machined surfaces between the engine's bedplate and the ship's foundation (tank top) can lead to uneven pressure on the bolts. This can cause localized yielding of the material and stress concentration, contributing to slackening.
• Thermal Expansion: The engine and the ship's foundation expand and contract at different rates due to temperature changes during operation. This differential expansion can stress the bolts and cause a loss of tension.
• Material Issues: Manufacturing defects or incorrect material properties in the bolts or nuts themselves can lead to a loss of tensile strength, causing them to slacken prematurely.

Non-metallic chocking materials, such as epoxy resin, offer several benefits for a main engine's foundation compared to traditional steel chocks:

• Superior Contact and Load Distribution: When poured in a liquid state, the epoxy resin conforms perfectly to all surface irregularities between the engine bedplate and the tank top. This creates 100% surface contact, ensuring the engine's weight and dynamic forces are evenly distributed, which reduces stress and minimizes the risk of bolt slackening.
• Vibration Damping: Non-metallic materials have excellent damping properties. They absorb and isolate engine vibrations more effectively than steel, reducing noise and protecting the surrounding hull structure from fatigue.
• Corrosion Resistance: Unlike steel, which is prone to rust, epoxy resin is highly resistant to corrosion from oil, water, and chemicals in the engine room. This prevents degradation of the foundation over time.
• Simplified Installation: The process of pouring and curing non-metallic chocks is much faster and less labor-intensive than the precise machining and fitting required for steel chocks.

To ensure non-metallic chocks are fitted accurately and reliably, the following precautions must be observed:

• Surface Preparation: The steel surfaces of both the engine bedplate and the tank top must be thoroughly cleaned to remove all oil, grease, rust, and moisture. A clean, dry surface is essential for proper adhesion.
• Temperature Control: The ambient temperature and the temperature of the surfaces must be within the manufacturer's specified range for the resin to cure correctly. Avoid drafts that could cause uneven curing.
• Proper Shuttering and Sealing: Temporary barriers (shuttering) must be built and sealed properly around the chocking area to contain the liquid resin and prevent any leaks.
• Accurate Alignment: Before pouring the resin, the engine must be precisely aligned and leveled using temporary steel shims or jacks. This ensures the resin cures in the correct position, maintaining the engine's exact alignment.
• Correct Mixing Ratio: The two components of the epoxy resin (resin and hardener) must be mixed in the exact proportions specified by the manufacturer. Any deviation can compromise the final strength and curing properties of the chock.
• De-aeration: The resin mixture must be de-aerated to remove any trapped air bubbles or voids, which could weaken the final chock.