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

The big difference in efficiency between the sailboat and the trawler is the turbulence and drag of the transom.  I don't know how much less efficient it is but it's a lot of churning water.  At slow speeds the finer bow will be better but I'm not so sure that will make a lot of difference at such slow speeds.

I haven't left the dock yet having to do some changes to get my charge controller install completed but I have a 38' trimaran that is 22' wide and draft of 3'.  But it only weighs 9k lbs.  So it might take me less time to get to speed and a longer water line may have enough drag to be less efficient than the 40' mono at low speeds. (Not sure about 4kts). But my design is 200ah on a 10kw motor for getting in and out of my slip/neighborhood and then sailing in between ( plus 700w solar panels while I sail ).  I then have two 2kw generators and two 25amp chargers for longer trips I'm hoping will get me 4kts when there's no wind.  But here I have sails for weather and generator for no weather (so to speak). This makes the numbers reasonable for my budget to me.  And I'll soon find out how right or wrong I've been.

From a theoretical perspective I don't think you're in the ballpark in using the generator to push the trawler in any sense that the diesels would.  If the exercise is to think of adding an electric motor as a backup if you lost both diesels on a trip then ok maybe there's something there.  You could mount a belt to on of the shafts of one of the diesels and have the motor as standby.  Then when the diesels are down you could "limp back" at a crawl to harbor for repairs.  If weather and waves are bad you could put out a storm anchor or bottom anchor if it's an option and wait till things calm down.  That would be the only way I see it without having huge diesels to power electric drives which is done all the time on ships of much larger size.  But I don't see the purpose of doing it for a trawler.  May as well use the direct drive.  

All IMHO

I'll be sharing my data when I finally leave the dock lol.

On Feb 7, 2020, at 10:51 PM, Scott Masterson <smasterson2@gmail.com> wrote:



4-5 MPH is more realistic from a 7.6kW generator. Most motor and controller setups will require DC power input. A simple bridge rectifier may make that possible. For that large prop, you will need a large reduction, most likely near 4:1.

Scott Masterson

On Fri, Feb 7, 2020 at 7:07 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