Nowadays propellers are predominantly used to drive vessels. One might get the impression that this is the most efficient type of drive. This is, however, not the case. Yet more research is required before being able to implement alternatives. For the time being, therefore, we will remain with a propeller drive for the Eco-Trimaran.
    If mounted at the usual position, i.e. on the stern of the floats, the propellers would stick out of the water too often when riding over waves. Since electric motors are to be used anyway, pod drives would appear to be the best choice. Here each of the ship motors are mounted in their own pods which are fastened to
to the keel of each of the floats underneath the vertical pivoting axes, i.e. at the lowest point. At this position the ship drive also does not influence
the passive movements of the float around the vertical axis when breakers hit the side or when making turns. With single-hull vessels, the pod containing the drive is able to pivot around a vertical axis, thus improving navigability in harbour areas. This is not necessary with the Eco-Trimaran; pods may be connected rigidly to the floats. Instead, a bow thruster is planned for the front float. Thismakes it possible to turn in place by positioning the front float at a right angle to the longitudinal axis of the vessel and switching on the main drive of that float.

2. Energy

Electricity is used to drive the vessel. This energy is obtained in part directly from the wind turbine, the solar cells and the wave power plant and in part from the pressure vessels, where it is stored in the form of hydrogen gas. The
pressure of the hydrogen gas is reduced as it is fed to fuel cells, where it is then converted into electricity. We will assume an efficiency rate of 70 % in this case.  The operating scenario assumed here is described in greater detail in the “Energy”  
chapter, while further differentiation in terms of “northern”, “southern”, “worst-case” and “best-case” scenarios may be found in the chapter entitled “Wind”. It is assumed that the vessel will be on voyages a maximum total of 42 days per  year, each time for several consecutive days. We further assume that the motors will run continuously for 6 hours daily during daylight hours, when solar energy is available. During the other 18 hours a day, the ship will lie at anchor on the open sea (possibly using a sea anchor). Short-term storage is replenished during this period. Solar energy is not reckoned with during this phase. A short-term storage facility may actually be physically present in the form of a separate hydrogen pressure vessel or it may be merely virtual, i.e. as part of long-term storage capacity. In any case, the term “short-term storage” is introduced for the sake of simplifying calculations and rendering them more intelligible. In the diagram at the next page, there is a group of three bars for each of the four scenarios:
1. Left bar in each group: energy immediately available to the motors from solar cells, the wind turbine and the wave power plant. 2. Centre bar in each group: potential motor power from (1.) plus energy from short-term storage. 3. Right bar in each group: potential motor power from (1.) and (2.) plus energy from long-term storage. The diagram allows the following conclusions: by directly utilising the energy produced on board in the northern standard scenario, the vessel would be able to travel with an average of 20 kW (27 hp) of drive power. In this case the motors would only be in operation during the day, as long as solar energy is available. The operating range would be unlimited. On-board energy demands are already included in this figure. The corresponding  Motor
                power average for standard southern conditions is 18 kW (24 hp).
    For comparison: The 56 feet solar driven Catamaran “Sun 21”,  with which an Atlantic crossing succeeded in 2007, performs with a drive energy of 16 kW a velocity of  5 – 6 kts.)
    When short-term storage, for which energy is collected mostly at night, is added to this, average drive power of 43 kW (58 hp) results for the north and 30 kW (41 hp) for the south. The motors are in operation in this case for a maximum of 6 hours each day during the day. The operating range of the vessel would also be unlimited in this case.
 For comparison: The 82 – ft catamaran “Solarschiff  Heidelberg”, whose batteries are additionally charged during night by the socket,  performs 2 – 3 kts (average) and 7 kts (max.)
When the energy collected in long-term storage during mooring time is used, an average drive power of 148 kW (194 hp) is obtained for the standard northern scenario and 125 kW (170 hp) for the south. This also assumes a daily motor operating time of 6 hours and a travel period of 42 days.
    If the annual travel period is shortened, more energy becomes available to the motors from long-term storage. This relationship is illustrated in the diagram at right. Here all other assumptions for the standard scenarios remain the same. Only the standard northern scenario is discussed here, since the tendency is the same for the other scenarios. The value originally selected of 42
days of travel per year
travel - mooring (6 weeks) turns out to be a key variable. With shorter travel periods, the amount of power available increases dramatically, while in the case of longer travel periods this figure falls only gradually, asymptotically approaching 43 kW (58 hp), the level already mentioned above, at which no energy can be taken from long-term storage.
  Finally, let’s examine another scenario. The ship lies at anchor near a beach for several days, then travels on to the next beach. Long-term storage is initially empty. The trip lasts only one day, during which the motors are in operation for 6 hours. Sun, wind and wave conditions are the same as those in the standard northern scenario. As may be seen from the diagram at right, at 51 kW (68 hp) the motor power available after mooring for one day is only slightly higher than the value cited above without long-term storage yet with short-term storage. After mooring for one week, 170 kW (228 hp) is available, while this amount is 300 kW (398 hp) after 2 weeks. to fuel

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