Originally Posted by Surrymark
Of course electric motors powered by generators have long been used on large ships, locomotives, and other transport. It is appealing, the idea of a relatively small motor turning the prop, and a remote
small diesel generator with easy access and power for all electrical
systems. But is anything like that available for yachts? Anybody know?
Diesel electric systems work, but they have limitations... The ideal for a boat would be to have one generator that could do duty for both power generation and house loads... The problem is that the power demands for each system are radically different.
A large complicated boat may use 7kw of power to run all the house loads at a time. Which seems like a pretty substantial generator right?
A typical 40' cruising boat has a ~40hp engine, which works out to a 30kw generator. Add in efficiency loss issues and you are talking about a 35kw generator to drive a propulsion
engine on most boats.
The problem is that a 7kw generator isn't big enough to power an electric motor
the boat would need, and a 35kw generator is too large to power the house loads. There are three ways around this, each with their own problems...
1) a very large battery
bank with the smaller generator which allows the generator to supplement the bank when running the motor
2) a very large battery bank with the large generator. The house pulls down the batteries from an inverter
, and we run the big generator to either power propulsion or to recharge the batteries.
3) a very large battery bank with a mid sized generator. Like 1) but it allows some operational freedom since propulsion can be run direct from the generator at a reduced speed. And while running house loads from the generator a large percentage of the power can be dumped into the batteries
The problem is that no matter what we do you need huge battery banks. Weighing thousands of pounds along with complicated chargers, inverters, auto-start generators, ect... All to run the system.
And while all of this works, it is very expensive, and complicated.
Where this works is where the house loads and the propulsion loads are closer in size, or where there are large differences in needed propulsion power.
So a tug company recently went to diesel-electric. One huge electric motor with three diesel generators. The first generator is sized to the travel load needed by the tug itself, the second and third generators are kicked on based on the size of the tow. This allows them to scale the amount of available power based on demand. But it doesn't work for a sailboat very well, since three 9kw generators would cost a lot more than a 7kw generator and a 40hp motor.