Just a rough idea

The diesel generator on deck on a cabin, cooled with radiator, dry stack. Easy to maintain, easy to cool without the ancillaries necessary when the engine is down below. The propulsion is a electric motor mounted classically with straight shaft. No need for any gear, just a shaft brake.
No batteries, the gen set to the electric motor directly.
As for stability, not a great deal, if taking in consideration on the design stage.
Tanks down below.
can be cheap and straight forward, the engine running at almost the same Rpm, the electric motor been also the gear. No batteries to maintain for the storage of the electricity. Just the one for starter.
Lister

 

Why not just skip the gen-set and electric motor? Small diesel to prop.

I'm sure you've got your reasons but it sounds like extra weight, money and hastle to me.

-Tom

 

Unless (even if?) you can duct the heated radiator exhaust above and away from the deck I'd be wary of having an uncomfortably hot boat. Another issue is the considerable amount of noise generated by the radiator fan.
Sorry for being so negative but I'd like to see a succesfull installation before commiting to such a system.

 

If you don't motor all that much per year,and don't expect the same mpg I'd say give it a try and report back.

Try electric fans on the rad,keep an eye on your rad for rot,and you may look into using a liquid cooled electric motor anyways as all the salty cool air being fanned through the motor will rot it out IMO.

I commend you for not trying to make a battery powered boat with 4 tons of batteries to go 30 miles.

 

Lister,

The problem with this set up is that you need a huge generator to drive the electric motor if you want the same performance as you would with a diesel mover.

For example my new boat has a 30kw/ 40hp diesel engine. To switch to an electric motor, I need a 30kw motor... To drive it I need a 35kw generator... The same boat uses a 5.0kw generator to run all house loads, including AC.

So when I am on the hook I have to either have a second generator sized to operate the house loads, or use a massively oversized generator sized to the propulsion needs. A third option would be a 5kw generator sized to the house loads, and a 25kw generator sized for propulsion and run them both while underway....

It can work in some applications, but this type of diesel electric just isn't reasonable for most private vessels, the disparity between generator needs, and propulsion needs are just too high.

 

Lister...if you can get industrial stuff for cheap and don't expect much-go for it.
But you may see my little jab-in requiring liquid cooled motors.
Hint: if you need to water cool the motors,it's not as simple as it may seem.

The diesel electric topic has been beat to death in the hybrid section,and ask catbuilder what he thinks about it..he looked into it very hard.

I'm still waiting to hear the results of the member building a diesel electric boat,with a donated DE drive.

 

I appreciate you in-depth and knowledgeable answer. I learn something.
It was just a rough idea.
Thank you.
Lister

 

It can be done but the problem is it would not be very efficient; Consider that you are doing a conversion first from fuel (chemical) power to mechanical power, than to electrical power, than back from electrical to mechanical power at the prop shaft. Each conversion step is costly in terms of efficiency. Much better just to go from fuel to mechanical power and drive the prop shaft with that.

Even the frictional loss of a transmission is small (less than 10 percent usually) compared to electrical conversion (30 percent or more). Even with very costly rare earth magnet motors and generators, you would be very hard pressed to have losses even approaching the mechanical efficiency.

 

Thank you Petros, I appreciate your explanation.
You all agree on the fact the lost of efficiency is the main problem.
Now I know thanks to you all..
After reading of a prototype of ship with gas turbine generators on deck, and propulsion through electric motor and classic shaft, it gave me the thought why not trying with a classic diesel generator on deck.
But I understand that the turbine need a lot of air to work. So been on deck is logic. And noisy.
Lister