Archive | May 13, 2014

NAIAD Shafts; Common Problems – Unique Solutions

Newly cladded NAIAD Stabilizer shaft

Newly cladded NAIAD Stabilizer shaft

One of the most common problems we see on mega yachts is crevice corrosion and wear on stainless steel shafts, whether those shafts are propeller, rudder or NAIAD stabilizer shafts. Stainless steel shafts can be damaged by excessive wear in contact areas, such as bearings or seals due to long life or misalignment. Shafts can also be damaged from crevice corrosion or stray current corrosion.

The solution to these problems can range from cladding the existing stabilizer shafts or fabricating entirely new units. Cladding or shaft weld-over, is the process of repairing a worn or damaged area on a shaft.  This type of work is done by our on-site machine shop, Straight Line Marine, which is the only shop in Florida that is ABS approved to perform this type of work on propeller shafts.  Replacing a large NAIAD stabilizer shaft would cost thousands of dollars, depending on the size.  However, the cladding repair is less than a thousand dollars.

This was the case on a 145 foot Trinity yacht that we were working on rebuilding its NAIAD Stabilizer System. When pitting or crevice corrosion becomes too extensive, it prevents the lip seal from establishing a tight fit thus allowing sea water in or oil to leak out of the system.

We removed the shafts, cladded them and installed new lip seals in the NAIAD Stabilizer and the owner was good to go.

Upgrading a NAIAD Stabilizer System for better Fit and Performance

NAIAD Stabilizer lower bearing housing

NAIAD Stabilizer lower bearing housing

While performing routine lip seal maintenance on a NAIAD Stabilizer System on a 120 foot Custom Yacht; we found excessive amounts of corrosion on the external side of the housing. The corrosion was so bad we could not remove the plates in order to replace the seals.

An important component to the comfort and safety of any vessel, the principle behind NAIAD fin stabilization is to counteract the tendency of a vessel to roll with an equivalent and opposite righting moment applied in exactly the proper phase and proportion. These righting moment forces are typically generated from a pair of underwater fins, although other control surfaces are also available. Wave forces are thus prevented from aggravating the vessel’s natural tendency to roll.

Specially design hydraulic press

Specially designed hydraulic press

Once the fins were removed from the NAIAD Stabilizer, the corrosion on the shaft and in the bearings was so significant that the shaft could not be removed using the most common methods.  In order to get the shaft out, we built a special hydraulic press to drive the shaft out of the boat. With the shafts out, we then worked with NAIAD to upgrade the system to the next larger size unit.

Upgrading to the next size required a new housing which required fiberglass modifications to resize the hole in the hull.

What started out as a routine job turned into a complete replacement and modification to the yacht in order to guarantee continued high performance and comfort at sea.

Rebuilding BCS Trim Tabs for a Smooth Ride

BCS Trim Tab

BCS Trim Tab

One of the more important features on any Super Yacht is the trim tab system for stability, speed and ride comfort in any sea conditions.

The working principle of a trim tab system is simple. By pushing one of the buttons on the control panel, the electro-hydraulic power unit receives an electric impulse which starts the electric motor. In turn, the latter makes the hydraulic pump start, being connected to it by means of a coupling. As the pump starts to turn, it sends an oil flow towards the side of the hydraulic cylinder corresponding to the desired movement (up or down), making the tab raise or go down. By pushing the button on the other side, the cylinder moves in the opposite direction.

As with any marine systems that come in contact with salt water, the trim tab system is subject to pitting and crevice corrosion.

We recently were hired to rebuild the BCS Trim Tabs on a 100 foot Azmit Yacht. The BCS Trim Tabs on this particularly vessel were not retracted in the up position and were left in the extended position too long contributing to significant crevice corrosion and pitting. When the cylinder is extended the rod is sitting in salt water – when retracted it is safely inside the cylinder in hydraulic fluid.

Crevice corrosion and pitting on BCS trim tab shaft

Crevice corrosion and pitting on BCS trim tab shaft

As part of the job, we fabricated new stainless steel rods in our machine shop, Straight Line Marine and rebuilt the BCS cylinders at High Seas Hydraulics.

After final assembly and installation back in the boat, the yacht is good to go for many more years of smooth sailing.