We received a call in the office from the Sydney jet boat tour operator Harbour Jet the other day, they had been suffering from a plague of jet unit bearing failures occurring every 200 odd hours. We headed down to their operation on the Parramatta river to take a look. Some vibration analysis revealed that the diesel engine was creating excessive torsional vibration at idle and in the upper rev range (2700-2800rpm) and the alignment between the motor and unit was severely out of tolerance with a huge angle present. Whilst cardan shafts can accept large amounts of ‘off-set’, even small amounts of ‘angular’ misalignment results in very poor reliability indeed. Noise, vibration and harshness had also been at very unacceptable levels. We carried out a precision alignment using optical lasers, fitted new engine mounts of correct size whilst assisting in the installation of a new 310kw/415hp Yanmar turbo diesel. Fine tuning on the water using a vibration analyser soon had the engine at optimum idle revs creating a very quiet and vibration free environment for tourists and the jet unit alike. This coupled with correct alignment has resulted in negligible vibration and noise intrusion right the way through the entire rev range, the pleasing turbo whine can now be easily heard above the background noise.

Time for a water test and sea trials to run the new diesel in, a good heat soak at moderate revs and it was time to give the boat a good run. This boat has plenty of power and made short work of the harbour chop, hydraulic steering and reverse bucket also make short work of the large jet unit when going full noise. We soon found ourselves tied up at Watsons Bay Hotel for a well earned drink and the boat is now ready for work. Advanced maintenance tools such as vibration spectrum analysers and laser alignment techniques whilst initially might sound expensive to utilise, they certainly can impact on jet boat reliability and smoothness.

P.S The jet unit bearing has now completed some 800 hours since alignment was corrected!! Anyone with visitors or who are coming to Sydney should consider Harbour Jet for a great trip out on the harbour.

www.harbourjet.com

Matthew Fallow www.asset.au.com