Re: [electricboats] What if...? Hybrid conversion for Trawler???

Agreed - if the diesel engines keep working, there is no reason to replace them.

But what if one or both dies?

The most expensive part of an electric installation is the batteries - especially for big boats, and multiple hours of range.

If I accept running the motors constantly off a generator (up to 3KW each) - What kind of electric motors could drive the large props on this boat (24x26") to at least 1000-1500RPM?

John

On Friday, February 7, 2020, 07:31:28 PM CST, Kev <captainyoung@gmail.com> wrote:

Your 7.6KW generator is equal to 10HP, but there is inefficiencies when converting from AC to DC too. There are a lot of variables, so there is no easy answer to what you would get, but I would guess a speed of 4 knots with that generator. A general rule of thumb is that for each knot increase, doubles the required power. So if you want to go faster, your generator gets big fast.

The only reason I would go with a hybrid system is if you would mostly go on short trips, 2-3 hours, and you had batteries to power that. That battery pack would be big and expensive. Then on occasion you would use the generator to extend your range.

If you are just going to run the generator to power the batteries to move the boat, just cut out the batteries and electric motor, and keep the diesels.

Smaller, lighter boats require much less power, so the batteries can be smaller. As you get bigger, the batteries would have to get very large, and that costs a lot.

On Fri, Feb 7, 2020 at 8:07 PM john via Groups.Io <oak_box=yahoo.com@groups.io> wrote:

The following scenario is a "what ifffff" for a "trawler" class cruising boat.

These boats are generally powered by old diesel engines - typically 120-275HP each (twin engines).

If the diesel engines died for whatever reason, could I replace them with some kind of electric motors, and run the electric motors from the still working generator?   I'd still be burning some kind of fuel, but I'd be rid of two large diesel engines (that I might not be able to get repaired).

Here are the specs for the boat:

Length:  38'

Beam:  13.6'

Draft:  3.6'

Weight:  21,000 pounds

Semi-displacement hull.

Props:  2, bronze, 24"x26"

Gear ratio (if the transmission was kept) - 2.5

Hull speed for the boat should be about 9.5mph

Here are the readings that were taken during a survey of the boat (RPM's for both motors):

700 RPM              3.3MPH

1000 RPM            5.6MPH

1500 RPM            7.5MPH

2000 RPM            8.8MPH

2500 RPM            9.25MPH

The current generator on the boat was a 7.6KW.

If I wanted to be able to attain a max speed of 7.5MPH, at about 1500RPM for each motor, with 24"x26" props, how big would the electric motors need to be?

Can I drive each motor on 3KW or less?

Can this be done practically??

Thanks,

John