John,
There are terms and business practices that we have all grown up with and some of us are more technical than others. We must use our experience to evaluate what claims are being made by a vendor. I am not an EE but the "systems" I sell are designed and built to perform within their specifications by engineers.
Vendor
I am now representing Thoosa in a limited geography and support ElectroProp in addition to Electric Yacht, Elco, and Torqeedo systems. I believe that Elco and Torqeedo speak in too glowingly about the capacity of their systems when facing a lee shore. Each named their system to give the market a positive view of the product. That said, when you read their literature they universally point out power input, power output along with maximum constant power output.
You also have to give these vendors a break because they are often selling into a prebuilt boat that may have been overpowered with an ICE that took advantage of propellers to make the geared engine work in an acceptable manner. It is hard to convince a boat owner that his 50hp Perkins can be replaced by a 20kW electric. His buddy at the dock, especially in the US, may be telling him that he needs even more power. Finally, system vendors give you a warranty whereas the DIY will tell you the parts worked on the bench but never tested as a system because it a DIY.
Motor vs System
On this forum we use the word motor to be both a system and a motor. There is a the wide variation of experience in this group. Manufacturers sometimes will use power and efficiency to present their product in the best light. At the end of the project, the system that is installed needs to meet the needs of the boat and the boat owner which may also widely varying as some want power to push to hull speed and others only to get out of a slip.
Efficiency
Efficiency of a motor unit is different than the system efficiency and companies will often use one or the other to make their product look good. My best analysis indicates that most electric motor units are from 90% to 95% efficient. Also that most marine motor systems are between 80% and 85% efficient. My big issue is how much power it take to push a boat at the propeller and at the same time give a reasonable range. As a point of information, the Lynch dc motor is more efficient than the traditional dc motor and when integrated into a system by a marine vendor will likely be about 80% to 85% efficient to the propeller.
Thrust Bearing
All systems need a thrust bearing and they are either built in to the motor as in the Elco induction motor or elsewhere in line with the shaft. There is a weight variation between motors depending on their construction and the application of the amount of thrust required in the application. When we push a 41ft 26000lbs Formosa we do so with a much larger system using common parts in a 48v geared system. When we push a 23ft 4000lbs Pieces we will use common parts but do so with a direct drive system at 24v. The smaller systems form GreenStar are 24v and have a built in thrust bearing. I am sure the heavy built Bell Marine do as well.
Sorry to be rambling but I wanted to try to address your very keen separate observations.
Mike Electric Yachts of Southern California
Posted by: mike@electricyachtssocal.com
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