GL: Tankless Electric Water Heater

[Ken Bloomfield]

 Rich,
Your desire for the endless nice warm shower water is admirable and
understandable, but I think you really are underestimating the kind of
wattage that would be required by electrical heat to achieve any kind of
water flow that would be useable.  There is a friend at our marina that
uses the tankless "heat on the fly" and he uses a Rinnai heater (despite
them saying that they are not for use on boats and RV's) and it works
very well.  It is mounted on an outside wall of his houseboat, and he
powers it with a three tank propane supply and electrical from his
inverter.  I believe the electrical is just for the controls.

The specification for 5 gallons per minute of water is 150,000 BTU/hour
and this is relatively easy to achieve with the propane source of heat. 
The modern day low-flow shower heads run at 2.5 gallons per minute, so
the BTU demand should go down to around the 75,000 BTU/hour level during
a shower with no other hot water faucets running.  Again, this is easy
with the propane.  However, look at the electrical equivalent:

  1. 1 kilowatt of electricity transfers 3348 Btu/hr to the water, so to
    achieve the 75,000 BTU/hr you would need to supply 75,000/3348 =
    approx. 22 kilowatts of electrical power.

  2. You are not going to do this off any battery/inverter combination,
    and you will have to have a mondo generator to supply this.  Doable,
    but very large and expensive.  I think prohibitively so.

Here are the specifications for the Bosch AE115 (albeit slightly less
than above due to the flow-rate/temp rise differences, but close enough 
for jazz and atom bombs):

Energy Factor = 0.9
Efficiency = 0.9
Weight = 20 lbs.
Dimensions = H: 15.25" W: 15.5" D: 4.5"
Water Connections = 3/4" Male NPT
Volts = 240/208V
Kilowatts = 17.25/13 kW [note: heat delivered to water, NOT power
consumption which is higher - KB]
AMPs = 80 (2 x 40 AMPs) [note: the power consumed is 80 amps @240 vac =
19,200 watts or 19.2 kw - KB]
Wire Size = 8 gauge (4 conductors & ground)
Phase = Single
Flow Rate at 450F Rise = 2.6 GPM
Flow Rate at 500F Rise = 2.3 GPM
Flow Rate at 600F Rise = 2.0 GPM
Flow Rate at 700F Rise = 1.7 GPM
Flow Rate at 800F Rise = 1.5 GPM
Recommended Service to Home = 150 Amps

Much as I hate to say it, I just don't believe that the physics support
the idea of an electrically heated tankless heater on a boat.
All the best,
Ken Bloomfield
m/v Tellico Lady, Marine Trader 50'
Maryville, TN

  My objectives are to replace my "standard heater" with a waterless electric
  system and plum my outside water (salt or fresh) through a water purifier into the
  waterless system and on into the boat's needs, thus providing endless hot water and
  fresh water on the boat. That will eliminate all those "watch the hot water" times
  since the water itself is also endless and gray water goes over board.
  
  Rich
  Day Dreams - 43' Carver