The hull dimensions and hull form

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The following factors are key in achieving a design which is as efficient as possible, and so wastes a minimum of electricty:
The aspect ratio of the plan view should be a minimum of 1:5 to 1:4 for efficient movement through water:
If the requirement is for two people seated side-by-side, this implies a beam of 4.5 to 5 feet.
This in turn implies a theoretical minimum waterline length (LWL) of 18 feet up to 25 feet.
A vessel for one person (or two in line) might range in beam from 2.5 to 3 feet, giving a minimum theoretical LWL of 16 feet:
Any smaller length and the vessel's hull speed would be so low as to make normal cruising inefficient in terms of battery current consumed.
  Note: other considerations, such as space for batteries, locating the motor & prop. shaft and the fully laden weight, must be included in the design process and will affect the final length and beam.  
  One easy visual indication of likely efficiency is that the bottom of the transom should float about an inch above the water's surface:  
  It should not be immersed, nor should it be much more than 1 inch above the water. This situation needs to hold for the boat's normally laden state.  
Stern 1 Stern 2
  Allied to this is that the vessel's underwater lines leading from mid-ships to the stern should be as smooth and gently curving as possible. Anything that disturbs the smooth, unturbulent flow of water at the stern will contribute to an increase in the stern wash and hence a loss of cruising efficiency.  
  (David estimates that many Broads holiday cruisers, for example, are continually lifting up around two tons of stern water as they move. Clearly, this is an unacceptable waste of energy in an electrically powered vessel)  
  One thing to be avoided at all costs is a design that exhibits any tendency for the stern to be 'sucked down' into the water. This is certain to contribute to significant energy waste.  
  It is important that the boat should float level in the water when it is normally laden and under power.  
  The balance of weight in the vessel wants to be slightly forward of midships:  
  When normally laden and stationary, the boat should not have any tendency to sink towards the stern.  
  This can achieved by carefully arranging the positions of batteries, motor and passengers at the design stage, so that the stern will not be submerged when the boat is normally laden.  
  An interesting comment made by Gavin Atkin regarding efficient hull design is:  
  "...I've thought about hulls for electric power, and my guess is that the key to it is eliminating wake... and one answer to that is to follow the Phil Bolger approach. Bolger's sailing boats are famous for being quick, and leaving no wake, and he has done it in his hard chine boats (he says) by making the curve of the bottom of the boat about the same as the sides. Usually this is a natural curve, (or arc of a circle) in his designs.  
  The reason is that if the curve of each is the same, there will be equal pressure on each side of the chine, and so there will be nothing to cause water to cross the chine and cause loss making eddying..."  
  You can get some idea of the different sizes and general characteristics of electric powered craft suitable for home-build here.  
  Another source of ideas about hull efficiency is available at Moton Ray's site  


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Created: December 2000.