Hull Deflection Effects on Shaft Alignment
Hull deflection refers to the change in shape of a hull when being transported from its waterborne position to its drydock blocking. It is an often-overlooked root cause for complications experienced during shaft alignment, particularly negative post works outcomes such as vibration. When hulls deflect, it causes shaft line bearing positions to change. This is problematic as these are the reference line that machinery is aligned to. Several factors, including loading conditions, ballasting arrangements, and hull deflection, are essential and must be considered for successful alignment and vessel operation.
What is hull deflection?
Defined as the bending and distortion of a vessel’s hull by induced strains, hull deflection is most affected by loading and off-loading of significant weight (Cargo, fuel, water, equipment, etc.). This phenomenon is more apparent in larger vessels like superyachts with larger equipment, fuel, and water capacities. Similar to the bed of a semi-truck, when “lightship” (low fuel/water), the hull will “hog” in a gentle n-pattern, and when fully loaded, the hull will “sag” in a gentle u-pattern.
Why is it a problem for alignment?
Alignments of a shaft line are most commonly performed while on the hard; however, once back in the water and operating under normal conditions, stresses and strains on the hull could be completely different, causing the hull to change shape. This causes shaft positions and alignment to change. As such, good marine engineering practices dictate the vessel is to have 50% liquid load while in the water, and hull shape under this condition is to be matched when blocked on the hard. 50% is used to minimize the overall deflection of a vessel’s hull in any given condition.
How to rectify the problem?
Traditional methods involve a lengthy procedure using piano wire under tension, individual measurements taken beside the hull, then repeated whilst on the hard. AME uses a Levalign Expert geometric laser system from PRUFTECHNIK to measure deck profiles, as the deck is firmly affixed to the hull and therefore changes with it. A Levalign laser is positioned on the deck, above the forward engine room bulkhead, facing aft. To take measurements, a receiver is placed incrementally along the deck to provide a profile. To measure and compare deck shape while waterborne and drydocked, a reference measurement is collected while in the water and another taken on the hard. If the change in deflection is low, then it is safe to presume the vessel’s hull has retained its waterborne shape. AME has reduced the amount of time required to 2-3 hours of measurements through this method.
Another way is to track measurements live, which is done with the LIVE TREND feature on PRUFTECHNIK’s Rotalign touch. In this method, lasers collect large amounts of data points starting with the vessel waterborne, trending points throughout the lifting process, and finalizing when rested on the blocks.
AME is well versed in performing hull targets. With two locations, fully equipped machine shops, in Fort Lauderdale, FL, one of which is at Lauderdale Marine Center. We’ve got all of your alignment needs covered.