Patent Publication Number: US-7712424-B2

Title: Multi-hull vessel adapted for ice-breaking

Description:
STATEMENT OF RELATED CASES 
   This case claims priority of U.S. provisional patent application 60/710,111, which was filed on Aug. 22, 2005 and is incorporated by reference herein. 

   FIELD OF THE INVENTION 
   The present invention relates to sea-faring vessels. More particularly, the present invention relates to a vessel having a multiple hulls. 
   BACKGROUND OF THE INVENTION 
   Vessels that are required to navigate through ice-covered waters are typically mono-hull designs. These mono-hull vessels are usually designed to break the ice via a downward force that is applied by a wide spoon-shaped bow. The specially-designed bow presses down on the ice to break it as the vessel moves forward. 
   Very few multi-hull ships, such as catamarans and SWATH craft, are capable of operating in ice. Catamarans, for example, typically have narrow hulls that cannot apply sufficient downward force to break ice. SWATH (Small Waterplane Area Twin Hull) vessels, which usually have two pontoon-like lower hulls that are connected to a catamaran-like upper or center hull via struts, have, with limited success, been adapted for ice-breaking operation. The adaptation is to reinforce the struts, the upper portion of the lower hulls, and the lower portion of the upper hull. 
   The reinforced struts of known ice-breaking-enabled SWATH vessels break ice through a crushing, compressive force. This force must be large, which requires excessive power, since ice presents great resistance to breaking under compressive force (similar to concrete). 
   A need remains, therefore, for a ice-breaking-enabled multi-hull ship that breaks ice in a more power-efficient manner than those of the prior art. 
   SUMMARY OF THE INVENTION 
   The present invention provides a way to adapt multi-hull vessels for ice-breaking without some of the costs and disadvantages of the prior art. 
   In the illustrative embodiment, a SWATH vessel is adapted for upward ice-breaking by modifying its lower hulls and struts to:
         lift the ice along an edge, cutting it from below, thereby enabling it to fracture and break from the force of its own weight in bending; and   to promote separation of the ice from the struts of the SWATH vessel.       

   A modification that is responsible for cutting the ice is the reinforcement of the lower hulls and the addition of a narrow ridge or edge near the bow of each lower hull. As the vessel moves forward, the ice is lifted and force concentrates along the narrow ridge. The ice then breaks along the line of force concentration. 
   A modification that promotes separation of the ice from the struts is to taper the struts such that the widest portion of the strut at the waterline is near the longitudinal mid-point of the lower hull, rather than at its stern, as is typical in multi-hull vessels. The aft-tapering waterline that results from this modification generates a “reamer” effect wherein ice that was in contact with the struts separates from the struts. Generally, the aft-tapering waterline results in a reduction in the frictional resistance of the lower hull/ice interface. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a multi-hull vessel in accordance with the illustrative embodiment of the present invention. 
       FIG. 2  depicts a narrow ridge at the forward portion of each lower hull of the vessel of  FIG. 1 . 
       FIG. 3A  depicts, via a top-view, the waterline of a SWATH craft in the prior art at the struts, wherein the waterline is widest at the stern of the struts. 
       FIG. 3B  depicts, via a top-view, the waterline of a SWATH craft in accordance with the illustrative embodiment of the present invention, wherein the waterline is widest near the longitudinal mid-point of the struts and tapers toward the stern. 
       FIG. 4  depicts a plot comparing the resistance in ice of a multi-hull vessel as a function of trim condition and vessel speed. 
   

   DETAILED DESCRIPTION 
     FIG. 1  depicts multi-hull vessel  100  in accordance with the illustrative embodiment of the present invention. Vessel  100  includes side hulls  102 , struts  104 , sponson  106 , and deck house  108 . The deck house, which is also referred to as the center hull, incorporates a pilot house, and, internally, a (lower) deck for vehicles and an (upper) deck for passengers. During normal operation, side hulls  102  are submerged (SWATH) or partially submerged (catamaran), while center hull typically remains above the waterline. 
   In accordance with the illustrative embodiment, multi-hull vessel  100  has certain modifications, relative to prior-art multi-hull vessels, which make it well suited to ice-breaking. 
   In particular, the upper surface of the bow of side hulls  102  incorporate ridge  220 , as depicted in  FIG. 2 . In some embodiments, ridge  220  is formed as an integral portion of side hulls  102 . In some other embodiments, ridge  220  is manufactured independently of side hulls  102  and then attached thereto. Those skilled in the art, after reading the present disclosure, will be able to design and fabricate ridge  220  for use herein. 
   As side-hulls move forward under ice, the ice is lifted and force concentrates along ridge  220 . The ice breaks along this force concentration. This is a far more efficient way to break ice than reinforcing struts  104  and simply compressing the ice to failure, as performed by multi-hull vessels in the prior art. Upward ice breaking takes advantage of the weight of the ice. That is, in addition to any upward force applied to the ice by virtue of the trim angle and propulsion, the weight of the ice itself is harnessed for the breaking operation. 
   Assuming that vessel  100  is underway with zero trim, as it approaches ice, an initial trim of 2 degrees (bow up) is obtained by moving the center of gravity aft using ballast water. Upon entering ice, the vessel is trimmed forward 1.5 degrees due to the downward force of the ice for a net upward trim angle of 0.5 degrees. The upward trim angle and forward motion of vessel  100  imparts the upward force that enables ice-breaking. 
   A second modification for ice-breaking is related to the shape of strut  104 .  FIG. 3A  depicts a top view of the waterline for strut  304  in the prior-art. As depicted in  FIG. 3A , the width of strut  304  is greatest at its stern  330  when the vessel is not trimmed by ballast water.  FIG. 3B  depicts a top view of the waterline for strut  104  in accordance with the illustrative embodiment of the present invention. As depicted in  FIG. 3B , strut  104  is widest near its mid-point  332 , such that the waterline shows an aft-tapering profile after the vessel is trimmed by the stern. 
   It has been found that the aft-tapering waterline of strut  104  creates a “reamer” effect, wherein ice that is in contact with struts  104  separate from the struts. 
     FIG. 4  depicts a plot showing the resistance, in pounds, of two-feet thick first year ice as a function of the trim and speed of vessel  100 . As depicted in  FIG. 4 , the lowest resistance in ice is in a bow-up trim condition. 
   A problem experienced by prior-art downward ice-breaking vessels is that ice is ingested into the ship&#39;s propulsion systems. But in the case of a multi-hull vessel that is equipped for upward ice-breaking as described herein, the vessel operates at a relatively deep draft with the stern even deeper due to the bow-up trim, reducing the possibility of ingesting ice. 
   It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. For example, in this Specification, numerous specific details are provided in order to provide a thorough description and understanding of the illustrative embodiments of the present invention. Those skilled in the art will recognize, however, that the invention can be practiced without one or more of those details, or with other methods, materials, components, etc. 
   Furthermore, in some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the illustrative embodiments. It is understood that the various embodiments shown in the Figures are illustrative, and are not necessarily drawn to scale. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that a particular feature, structure, material, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the present invention, but not necessarily all embodiments. Consequently, the appearances of the phrase “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout the Specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.