Piggyback Hatch Cover

What sets a piggyback cover apart from folding and rolling types

Folding hatch covers hinge their panels up and over each other; rolling covers run panels sideways on wheels along the coaming. A piggyback cover instead lifts one panel and stacks it directly on top of the neighbouring panel at one end of the hatch, using hydraulic cylinders rather than a chain or wire haul system. Because the panels simply lift and stack rather than folding through a hinge or travelling on a wheel track, there are fewer wear points exposed to green water and cargo dust, which is why the type is common on bulk carriers carrying abrasive cargoes.

Main components

• Panels - steel pontoon-type sections, one designed to sit beneath and one to stack above at the parked end of the hatch.
• Hydraulic lifting cylinders - raise and lower each panel; power comes from a dedicated hydraulic power pack rather than a shared deck machinery system on most installations.
• Guide rollers and side guides - keep the panel aligned as it lifts clear of the coaming and travels to the stacking position.
• Cleating and compression system - hydraulic or manual cleats that pull the closed panel down onto the coaming compression bar to make the weathertight seal.
• Rubber sealing gasket - continuous gasket around the panel and coaming compression bar that does the actual watertight sealing work.

Selection and sizing

Panel count and stacking arrangement are set by hatch length and the headroom available above the coaming for the stacked panel - a piggyback system needs enough clear height that the stacked panel does not foul deck cranes, hatch coamings, or cargo gear stowed nearby. Hydraulic cylinder capacity is sized to the panel's loaded weight including any accumulated cargo residue, with margin, since cylinders that are only just adequate when new lose margin as seals wear.

Regulations and class

The International Load Line Convention and class rules for hatch cover strength set the required weathertight standard and the load the panel must withstand from green water and stowed cargo above it, where applicable. Class surveys include a hose test or ultrasonic test of the sealing line at each hatch cover and periodic examination of the securing and compression system; IACS unified requirements cover hatch cover design loads for bulk carriers specifically.

Typical faults

• Gasket compression set - the rubber seal loses its spring after years of compression and no longer closes the gap when cleated, letting water track along the coaming.
• Guide roller wear allowing panel misalignment - a worn roller lets the panel drift sideways during lifting, catching the coaming and jamming mid-cycle.
• Hydraulic cylinder seal leakage - slow loss of lift capacity shows up as a panel that hesitates or drifts down under its own weight when parked.
• Corrosion at the coaming compression bar - pitting on the bar the gasket seats against prevents a full seal even with a healthy gasket.

What to look for in a supplier

• Gasket material rated for the trade's temperature range and cargo chemical exposure, not a generic rubber compound.
• Documented cylinder capacity margin above the calculated panel weight.
• Spare parts availability for guide rollers and cleating hardware specifically, the components that wear fastest.
• Reference installations on a comparable hatch size and cargo profile.

A hose test that passes on a piggyback cover does not confirm the cleating force is even across the whole panel - check compression at both ends and the middle, since a gasket can seal at the cleats and still leak at the panel's centre.