Abstract:
A breakaway skeg for the lower portion of a marine propulsion unit is disclosed. The skeg is designed such that the attachment features will fail under certain load or impacts, thereby preventing damage to the lower portion of the marine propulsion unit. The breakaway skeg is attached via shear tabs that include a relief that accounts for at least a part of the volume of the base portion of the shear tab. The shear tabs are designed to fail under certain circumstances before other components of the marine propulsion.

Description:
TECHNICAL FIELD 
     The present disclosure relates to a breakaway skeg for the lower portion of a marine propulsion unit. The skeg is designed such that the attachment features will fail under certain load or impacts, thereby preventing damage to the lower portion of the marine propulsion unit. 
     BACKGROUND 
     Portions of a marine propulsion unit that extend into the water are susceptible to damage by underwater objects. The portions that extend below the hull of a marine vessel are especially susceptible, particularly the leading components such as the nose section and the skeg. A skeg is a fin-shaped feature that extends below the gear case of a marine propulsion unit such as an outboard motor, a stem drive, or a pod unit. The skeg extends below the radius of the propeller (or “prop”) of the marine propulsion unit and serves to protect the propeller from damage from underwater impacts. 
     A breakaway skeg is designed to break away from the lower portion of the marine propulsion unit when the breakaway skeg impacts an underwater object. The breakaway skeg must be designed to account for impacts from the front, rear, side, and bottom. As the breakaway skeg is attached to the lower portion of the gear case, the attachment features of the breakaway skeg must minimize damage to the lower portion of the gear case when it breaks away. Damage to the lower portion of the gear case can make for a very expensive repair. In addition, excessive damage to the lower portion of the gear case can lead to loss of gear case oil and therefore extensive gear train damage. 
     The design of the attachment features of the breakaway skeg may account for impacts from the bottom of the marine propulsion unit. A rigidly and strongly-attached skeg would transmit impact forces upward through the marine propulsion unit and into the vessel&#39;s hull where the forces could potentially cause hull damage or even a hull breach. 
     The attachment features of the breakaway skeg may therefore be designed to provide a strong enough attachment to the lower portion of the gear case to stay intact during navigation but must fracture or breakaway above a predetermined stress in order to prevent damage to the lower portion of the gear case. 
     U.S. Pat. No. 7,435,147 to Eichinger, issued Oct. 14, 2008, entitled “Breakaway skeg for a marine propulsion device,” discloses a breakaway skeg that accounts for an impact force L at locations along the total height of the skeg. The breakaway skeg disclosed by Eichinger, however, does not address impact forces from the bottom that would transmit upward through the marine propulsion unit and into the vessel&#39;s hull. 
     SUMMARY OF THE INVENTION 
     A marine propulsion unit having a breakaway skeg is disclosed. The marine propulsion unit comprises a lower gear case having a lower portion, the lower portion including a nose section, a prop section, and a shaft section having a ventral mating feature. The ventral mating feature includes a mating feature and lateral through-holes configured to accept pins. The breakaway skeg has a shear tab located along the top of said breakaway skeg and configured to mate to said mating feature, wherein said shear tab has a base portion and a tip portion, said base portion including a relief that accounts for at least part of the volume of the base portion. 
     In a second aspect of the current disclosure, a breakaway skeg is disclosed. The breakaway skeg comprises a shear tab located along a top of said breakaway skeg, wherein said shear tab has a base portion and a tip portion, said base portion including a relief that accounts for at least part of the volume of the base portion. 
     In a third aspect of the current disclosure, a marine propulsion unit having a breakaway skeg is disclosed. The marine propulsion unit comprises a lower gear case having a lower portion. The lower portion includes a nose section, a prop section, and a shaft section having a ventral mating feature. The ventral mating feature includes a first and second scallop and lateral through-holes configured to accept pins, and a breakaway skeg. The breakaway skeg comprises a first shear tab located along the top of said breakaway skeg and configured to mate to said first scallop, a second shear tab located along the top of said breakaway skeg and configured to mate to said second scallop, wherein said first and second shear tabs have a base portion and a tip portion, said base portion including a relief that accounts for at least part of the volume of the base portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view of three aspects of a marine propulsion unit installed on a marine vessel according to the current disclosure. 
         FIG. 2  is a cut-away view of a marine propulsion unit according to the current disclosure. 
         FIG. 3  is a view of marine propulsion unit installed on a marine vessel according to the current disclosure. 
         FIG. 4  is a view of the attachment features of a breakaway skeg according to the current disclosure. 
         FIG. 5  is a view of the attachment features of a breakaway skeg according to the current disclosure. 
         FIG. 6  is a view of the attachment features of a breakaway skeg according to the current disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a marine vessel  10  having a marine propulsion unit  20 . The marine propulsion unit  20  includes a lower gear case  30  and a lower portion  60 . The lower portion  60  includes a prop section  90 , a shaft section  70 , and a nose section  100 . The lower portion  60  may be shaped into the form of a torpedo. 
     In one aspect of the current disclosure, the marine propulsion unit  20  is an outboard drive motor comprising an engine contained in an enclosure and a lower gear case  30  extending below the water and having a lower portion  60 . The marine propulsion unit  20  is mounted to pivot on the transom of the marine vessel  10  to provide steering functions. 
     In a second aspect of the current disclosure, the marine propulsion unit  20  is a sterndrive comprising a gear box and a lower gear case  30  extending below the water and having a lower portion  60 . The gearbox is connected to an engine via a driveshaft and transmits engine power at a generally 90-degree downward angle from the engine to the lower gear case  30 . The marine propulsion unit  20  is mounted to pivot on the transom of the marine vessel  10  to provide steering functions. 
     In a third aspect of the current disclosure, the marine propulsion unit  20  is a pod unit, also known as an azimuth thruster. The pod unit comprises an upper pod unit and a lower gear case  30 . The pod upper unit connects to the engine via a driveshaft and contains the transmission  50  and steering functions. The lower gear case  30  extends below the hull and is capable of rotating around  40  degrees to port or starboard in order to provide steering functions. 
       FIG. 2  shows the lower gear case  30  of a marine propulsion unit  20 . The lower gear case  30  includes a strut  40  that extends below the waterline  15 . The lower gear case  30  contains one or more driveshafts  200  for transmitting engine power to the prop(s)  225  via at least one bevel gear set  270 . The lower gear case  30  also includes bearings  260  for supporting the driveshaft  200  and prop shaft  205 . The lower gear case  30  is at least partially filled with oil for lubricating the driveshafts  200 , prop shaft  205 , bevel gear sets  270 , and bearings  260 . The lower gear case  30  may also include an oil pump  250 . 
     The lower portion  60  includes a nose section  100  in the front, a shaft section  70  in the middle, and a prop section  90  at the rear. The lower portion  60  may be torpedo-shaped. The shaft section  70  includes a driveshaft  200  for driving the prop shaft  205 . If the marine propulsion unit  20  includes an rear prop  230  and an front prop  240 , then the shaft section  70  may contain an inner prop shaft  210  and an outer prop shaft  220 . A breakaway skeg  190  is attached to the bottom of the shaft section  70 . The front of the shaft section  70  includes a circular aperture  80 . The inside diameter of the shaft section  70  is formed with threads near the aperture  80  and is configured to threadably accept the intermediate portion  110 . 
     The prop section  90  is located at the rear of the lower portion  60  and includes at least one prop  225  driven by a prop shaft  205 . The prop shaft  205  is driven by a bevel gear set  270  driven by a driveshaft  200 . In one aspect of the current disclosure, the prop section  90  may contain a rear prop  230  driven by an inner prop shaft  210  and a front prop  240  driven by an outer prop shaft  220 . The inner prop shaft  210  and outer prop shaft  220  are driven by a bevel gear set  270  driven by a driveshaft  200 . The use of both a rear prop  230  and front prop  240  increases the thrust provided by the marine propulsion unit  20 . The front prop  240  may have three blades while the rear prop  230  may have four blades. The rear prop  230  may need to be smaller in diameter due to flow velocity at the blade tips. In order for the smaller rear prop  230  to have equal blade area to the front prop  240 , the rear prop  230  may have four blades instead of three blades. In another aspect of the current disclosure, the front prop  240  may have four blades while the rear prop  230  may have five blades. The prop section  90  may also include a propeller shaft anode  50 . The propeller shaft anode may have a parabolic shape that maximizes hydrodynamic efficiency while providing anti-corrosion protection. 
     The nose section  100  is generally bullet-shaped and located at the front of the lower portion  60  and includes an intermediate portion  110  and a nose cone  130 . 
     The breakaway skeg  190  attaches to a ventral mating feature  280  which is located on the underside of the shaft section  70 . The ventral mating feature  280  extends generally from the front to the rear of the shaft section  70 . Through holes  290  are included to accept pins  300 . A mating feature such as a slot  282  or scallop  284  may be included to accept a shear tab  310  and/or an alignment tab  350  that is located on the breakaway skeg  190 . 
     The breakaway skeg  190  is generally fin-shaped and attaches to the ventral mating feature  280  on the bottom of the shaft section  70 . A shear tab  310  is formed onto the breakaway skeg  190  along the top edge. The shear tab  310  includes a tip  320  and a base  330  that attaches the shear tab  310  to the top edge of the breakaway skeg  190 . Multiple shear tabs  310  may be present as is shown in  FIGS. 4 and 5 . In one aspect of the current disclosure, the shear tab  310  may be a single continuous tab as is shown in  FIG. 6 . A through hole  290  in the shear tab  310  is configured to accept a pin  300  that fastens the breakaway skeg  190  to the ventral mating feature  280 . 
     A relief  340  as shown in  FIGS. 4-6  is formed into the base  330  of the shear tab  310 . The relief  340  serves to tailor the stress capability of the shear tab  310  to a predetermined value or range of values. A larger relief  340  can be chosen to decrease the stress capability while a smaller relief  340  can be chosen to increase the stress capability of the shear tab  310 . More than one relief  340  may be included in a single shear tab  310  to decrease the stress capability. The size and/or number of the relief  340  can be chosen to achieve a preselected failure stress for the shear tab  310 . 
     A pin  300  is inserted through a hole  290  in the ventral mating feature  280  that aligns with a hole  290  in the shear tab  310  and fastens the breakaway skeg  190  to the ventral mating feature  280 . The pin  300  is a type that is known in the art, such as a roll pin, shear pin, or dowel pin. A threaded fastener or the like could also be used. 
     INDUSTRIAL APPLICABILITY 
     The breakaway skeg  190  is designed to break away from the lower portion  60  of the lower gear case  30  when it impacts an underwater object of significant mass. When the breakaway skeg  190  breaks away, the lower portion  60  should suffer as little damage as possible. The breakaway skeg  190  can be broken away in the forward direction or when backing down or when impacting an object at some angle to the keel of the marine vessel  10 . The attachment features may be able to withstand normal loads such as steering and navigation while maintaining a factor of safety (FOS) of approximately 1.5-2.0 based on the yield strength of the material. An additional loading condition is a tide-out event on a high deadrise angle hull as shown in  FIG. 3 . This condition can cause the hull to rest on the marine propulsion unit  20 . The breakaway skeg  190  according to the current disclosure is designed to break away but not allow the marine propulsion unit  20  to breach the hull of the marine vessel  10  and cause loss of hull integrity. For the high deadrise angle hull shown in  FIG. 3 , the angle between the upward impact force  360  and the angle of the breakaway skeg  190  may be 19 degrees, or between 15 and 25 degrees. 
     For instance, the breakaway skeg  190  may experience side loads up to 3400 lb during a high-speed steering maneuver of 10 degrees angle at 40 knots. Therefore, the breakaway skeg  190  must be attached to the shaft section  70  such that it can withstand this force, yet break away at a predetermined stress if it impacts and underwater object. A factor of safety (FOS) is useful in defining the stresses that the components of the lower gear case  30  must withstand. The FOS may be defined as a scalar on a continuous range from 1.0 to 5.0. In one example, a FOS scale of 1.0-5.0 may correspond to a von Mises stress of 40,000-0 psi. A FOS of 1.0 corresponds to a stress that is expected be at or beyond the design limits of the components of the lower gear case  30 .  FIGS. 4-6  show relative stresses in the form of shaded areas. Shaded textures correspond to the key on the side of the figures. 
     In one aspect of the current disclosure, an alignment tab  350  is included along the upper edge of the breakaway skeg  190 . The alignment tab  350  does not include a hole  290  for accepting a pin  300 . The alignment tab  350  may be interspersed with one or more shear tabs  310 . There may be an alignment tab  350  in front of and behind each shear tab  310 . In another aspect of the current disclosure, alignment tabs  350  and shear tabs  310  may alternate along the top of the breakaway skeg  190 . Like the shear tab  310 , the alignment tab  350  may be configured to mate to a mating feature such as a slot  282  or a scallop  284  in the ventral mating feature  280 . The alignment tab  350  can be added to the design in order to increase the strength of the breakaway skeg  190  in response to side loads without affecting the strength in response to an upward impact force  360 .