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Old 03-05-2016, 14:52   #1
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1 diesel 1 electric ?

Hi,

42 foot cat, used daily for 5 hours as a business. I need to change both engines currently have Yanmar 3gm30's and may go with the same. SD will be replaced as well.

Any thoughts on going with one diesel engine 125A alt. and one electric. Diesel would charge the batteries when needed and the electric motor would be used mostly to drive when not under sail.

What size motor and how to connect to the SD 25?

Thanks
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Old 03-05-2016, 19:44   #2
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Re: 1 diesel 1 electric ?

If you want matching power in each hull, then a 18kw electric in one hull and a 36kw diesel in the other driving a 18kw DC generator.
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Old 03-05-2016, 20:14   #3
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Re: 1 diesel 1 electric ?

Why not a large enough diesel to drive the boat, by itself (my leopard 45 cruises easily at 7 knots on one engine at 75% throttle, for example) and then a smallish electric engine for maneuvering around docks, mooring fields and anchorages, and, of course, emergencies. If you put the diesel on the same side that the prop walk wants to turn the boat, the combination of prop walk and the asymetry will drive the boat dead straight, or nearly so. At least it does so on mine.
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Old 05-05-2016, 11:00   #4
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Re: 1 diesel 1 electric ?

This sounds like a great idea. Efficiency and redundancy at the same time. Two birds, one stone.

The only thought against it would be that marine alternators efficiency is actually about max 60%. Some of it can probably be made back through weight savings and the greater efficiency of the bigger diesel engine that replaces the two smaller ones.

Also if there is a large heavy fuel tank in the hull that goes electric that can't be removed. Then some weight savings would be lost.

May I ask what business uses a 42' sailing cat 5 hours a day?


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Old 05-05-2016, 21:35   #5
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Re: 1 diesel 1 electric ?

Maybe a 50hp propulsion motor in one hull (should be able to get a 42' boat up to hull speed in most conditions).

In the other hull say a 10hp generator and a 20hp electric motor with a moderate sized battery bank to keep weight balanced.

Around the dock the 20hp motor should be more than enough for docking maneuvers.
Underway in most conditions, the 50hp motor handles everything except the worst conditions where you can kick in the generator and electric motor. You will have 70hp for a while and then as the batteries wear down, you still have 60hp continuous.
You have a nice big generator and battery pack for house needs.
Electric provides an emergency backup propulsion (short term/instant from battery and long term from generator).

Not sure the current HP or your speed requirements, so the HP needs might change based on your specific boat.
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Old 05-05-2016, 23:22   #6
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Re: 1 diesel 1 electric ?

If you are going down this route you might as well install two electric motors and two generators. A large DC generator for propulsion and a smaller DC generator for house loads and as a backup.
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Old 06-05-2016, 00:45   #7
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Re: 1 diesel 1 electric ?

Do you have a seperate generator to charge the batteries for the motor when the diesel breaks down?
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Old 06-05-2016, 04:58   #8
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Re: 1 diesel 1 electric ?

Quote:
Originally Posted by steve_fpv View Post

Any thoughts on going with one diesel engine 125A alt. and one electric. Diesel would charge the batteries when needed and the electric motor would be used mostly to drive when not under sail.
125A x 12V = 1.5kw
Enough to cause a fire, but not enough to drive your boat.


Just ask yourself: Do you have experts for electric drives around? if not think about who will install it and fix it in case of trouble.
In my part of the world running a boat for business would require all components to be tested / certified /etc. No funny experiments with other peoples lives.


I really like the idea of electric drives but they still have a very marginal market share - so small I consider them experimental. More adventurous natures may disagree but since you are talking about business use I guess your prime concern is reliability and cost.

If you simply want low cost reliable propulsion go for standard diesels.
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Old 06-05-2016, 15:06   #9
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Re: 1 diesel 1 electric ?

Quote:
Originally Posted by earthbm View Post

May I ask what business uses a 42' sailing cat 5 hours a day?


~~~~~~~~~~~~~
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Day sailing and sunset cruises. No overnight charters.

we offer:

10:00-15:00
16:30-18:30

Or...
10:30-18:30
Or...
13:30-18:30

I have decided to go with two diesel engines for safety and reliability.
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Old 06-05-2016, 15:43   #10
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Re: 1 diesel 1 electric ?

Quote:
Originally Posted by earthbm View Post
This sounds like a great idea. Efficiency and redundancy at the same time. Two birds, one stone.
Where's the redundancy? If the diesel stops, the electric won't be far behind....
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