It’s The Generation

It’s the comfort of knowing you are safe. It’s Doppler Radar echoes changing color, showing which targets are a threat to you. It’s instant tracking of up to 100 moving targets. It’s Furuno’s Bird Mode tracking birds to find the best fishing grounds. It’s the “NXT” innovation in Radar, the all-new DRS4D-NXT Solid-State Doppler Radar for NavNet TZtouch & TZtouch2. Target AnalyzerTM function, facilitated by Doppler technology, immediately alerts you to targets (displayed in red) that pose a threat to your vessel. Simultaneously track up to 100 moving targets, displaying their speed & course vectors. RezBoostTM beam sharpening displays higher resolution targets, similar to those from an open array antenna.

that opens the interior to the afterdeck. Walking around the decks offers easy and comfortable transit from aft to forward. The other outdoor area is the flybridge and main helm, which sports a sizeable table with comfortable seating for 12 to 15, a wet bar and grill, plus a king size lounging area.

The Hudson 48PC is everything you’d expect a powercat to be. It will certainly compete aggressively in the market with other powercats based on her fit and finish, including liberal use of marble, walnut, and teak. Powered by twin Yanmar 8LV 370-horsepower engines, Hudson has tested the 48 at 24 knots at WOT and a range of 833 nautical miles at 8 knots.

–Brian Lind

BAVARIA E40

In Europe earlier this year, Bavaria Yachts launched a new single-engine, displacement motoryacht. The E40 has been nominated for the Best of Boats award and the European Powerboat of the Year for 2017. In their marketing of the new boat, Bavaria invokes a number of e-words, calling it ecological, economical, efficient, and emotional. Looking through the layout of this boat, you can see how these five E’s shaped the concept of this new design.

The E40 will make its debut in the U.S. market at the Miami Boat Show this February, in both a sedan and flybridge version. Both will be available in twoor three-cabin layouts. The two-cabin layout offers a large full-beam cabin aft and the three-cabin layout offers two mirrored double-berth staterooms with a shared head.

The engine room is accessed from the after cabin via an easy-access hatch, and appears to have a roomy layout around the powerplant, a single Volvo Penta D4-300. Bavaria has put ecological and efficiency at the front of the design using computer modeling and engine tuning to optimize fuel consumption.

At the midline of the after end of the saloon lies the helm, in a unique arrangement. The helm is raised from the saloon sole in order to give headroom access to the after cabin access staircase. To put it mildly, we will be eager to see how this design functions when the boat arrives for viewing and seatrials this winter. –Brian Lind

Some failures on cruising boats occur gradually, providing warning signs to those who pay attention. Gradually increasing vacuum pressure in the fuel filter warns of a developing fuel quality issue. Climbing engine temperatures point to a brewing cooling system problem. Excessive heat on the alternator warns of an impending failure. Propeller shafts however, fail all at once, with no advance warning. Should this occur, a single-screw vessel will be left dead in the water and calling for a tow. While most twin-screw boats can limp home, both will be facing a expensive repairs, including a haul-out, and a new shaft and prop. Given the risks and costs, it pays to understand why shafts fail and what steps you can take to reduce the likelihood.

Wading into engineering literature about shaft failures on commercial ships and recreational boats leads you into a swamp of metallurgical discussions involving microfractographic evaluation, beach marks, oxides, ions, and much more. Shafts don’t just break—the process turns out to be quite complex. Fortunately, studies point to steps you can take to improve your chances. Let’s begin with a look at why shafts fail in the first place.

At the risk of stating the obvious, shafts fail when the load exceeds the strength of the shaft material. Assuming that the shaft and prop have been properly engineered and safely handled for years of service without breaking, something must have changed: Either the shaft became weaker, the load became greater, or some combination of the two.

Unless you recently made a substantial prop change (increased diameter, for example), load changes are unlikely to be the cause of failure because of the safety factor designed into the shaft specifications. With a weakened shaft, however, loads developed when running in rough seas, or when shifting aggressively while docking might be enough to break the shaft. For these reasons, we want to focus on preserving the strength of the shaft.

METAL FATIGUE

Shafts lose strength due to metal fatigue, stress concentration, or corrosion. Fatigue occurs when we subject the metal to repeated cycles of loading and unloading. If you bend a paper clip enough times, it breaks easily. All propellers apply a rotational twist to the shaft, with the stress concentrated where the shaft enters the prop hub. Steeper shaft angles add another component: bending. The combination of rotational stress and bending leads to fatigue.

What you can do: First, when changing props and increasing diameter, ask the prop shop or your yard to make sure the shaft’s size is adequate. Larger diameter propellers provide more torque and more potential for rotational fatigue. Whether you change props specs or not, the propeller must be properly and precisely installed onto the shaft. Failure to do so can lead to “prop rocking,” a slight cyclical movement of the propeller on the shaft. Assuming movement with each rotation, this might add up to half a million cycles in a day’s run. For an excellent outline of the proper procedure, visit: goo.gl/bj7Gf7. In addition, be certain that the space between the

forward end of the propeller hub and the after end of the closest bearing does not exceed the shaft diameter. As this distance increases, the amount of movement within the shaft becomes greater, leading to metal fatigue. Finally, the clearance between the tips of the prop and the bottom of the boat should equal 15% of the prop’s diameter, at a minimum. Closer tolerances can lead to a variety of loading and vibration issues.

The propeller transfers rotational and bending stresses onto the shaft, and this loading tends to be concentrated where the prop meets the shaft. Ideally those loads spread gradually onto the rest of the shaft. The greater the spread, the lower the likelihood of failure. Anything that concentrates the stress onto a small area must be avoided. Stress concentrations, sometimes called stress risers, put all metals at risk. On an aluminum sailboat mast, for example, all openings cut into the mast must have well rounded corners since leaving them square creates a stress concentration with the increased risk of a crack forming in the corner. In general, sharp edges create stress risers.

As it happens, the area where the load transfers onto the shaft happens to have machining that can lead to stress concentrations. This is called the keyway. The keyway must be carefully machined, with corner radiuses and fillets. The forward end of the keyway should taper so that it ends gradually. “Spooning” the keyway end is the best practice: The keyway transitions into a smooth rounded area giving the appearance of the back of a spoon pressed into the shaft. ABS offers a specification for spooning and most USCG vessels require it. Eliminating these sharp edges and abrupt transitions will help spread the loads and reduce fatigue and micro-fracturing.

What you can do: When replacing a shaft, insist that it be machined to ABYC standards. Check the keyway to make sure all machining utilizes radiuses and fillets with no sharp corners, and that the forward end of the keyway has been tapered to a gradual end.

CORROSION

Stainless shafts suffer from three types of corrosion: pitting, crevice, and galvanic. Most stainless steel does not fare well underwater, and special alloys are required for this application. To protect itself from pests that would eat way at its trunk, a tree forms a skin we call bark. A healthy tree must regenerate any damaged bark or insects will attack the exposed area. A stainless steel shaft survives in much the same way—a thin film of oxygen forms at the surface, protecting the underlying metal from corrosion. Any breaks in this oxide layer will create an imbalance between protected and unprotected metal, starting a chemical reaction that leads to the development of pitting and crevice corrosion and consequently a weakening of the shaft. Not all stainless steel shaft alloys are the same and the differences matter.

Ignoring the various trade names (Aquamet®, Aqualloy®, Nautalloy®), all use the same reference numbers, mostly 17, 19, and 22. The higher the number, the better the resistance to corrosion. For recreational cruisers, the choice comes down to 19 or 22. Grade 17 should not be used. If you are not sure what you have, use a magnet. Only 17 will attract the magnet (some Chinese alloys of 19 or 22 will attract a magnet, and that should also be a red flag). If your shaft attracts a magnet, it would be prudent to pull it before any trip and inspect it for pitting or crevice corrosion.

Numbers 19 and 22 have comparable strength (ductile and tensile), while 22 offers substantially better corrosion resistance. Unlike 19, grade 22 contains molybdenum— a crucial ingredient that fosters regeneration of the oxygen film when it is breached. That comes with a cost, which for a 2-inch diameter by 12-foot shaft, the cost difference amounts to a $700 increase for 22 grade. That’s money well spent, given the risks and the labor dollars spent on the replacement process. For a boat that stays on the move, you could make a case for saving money with grade 19. Boats that sit in the slip for extended periods have a greater risk of corrosion due to the lack of movement that would provide oxygen to starved areas (shaft seals, bearings, stern tubes).

What you can do: Starting with the shaft material itself, it pays to go with the best. Regardless of the alloy, running the