Diesel/electric engine for big cat

[guerreiroazul]

Hi, I am building a 70 feet (22 tones) epoxi sail catamaran for long blue water cruise. I am thinking about auxiliary propulsion, and would like to hear some opinions about a diesel/electric propulsion like Ossa Powerlite.
Thank you,

[CDK]

If the conversion losses from shaft power to DC (20%) and from DC to shaft power again (20%) do not bother you, diesel/electric is a marvelous way to power your boat. No gearboxes, freedom to place the diesel where it is most convenient etc.

With steadily increasing fuel prices I'm not so sure this is the way to go.

[Bglad]

Originally Posted by CDK

If the conversion losses from shaft power to DC (20%) and from DC to shaft power again (20%) do not bother you, diesel/electric is a marvelous way to power your boat. No gearboxes, freedom to place the diesel where it is most convenient etc.

With steadily increasing fuel prices I'm not so sure this is the way to go.

The advantage of diesel electric may lie in fewer diesel mechanical systems since a boat that size would probably need a generator in addition to the propulsion engines. But, there seems to be a pretty big gap between the diesel electric drive systems and other vessel electrical components. Diesel electric systems I have looked at are 48 volts, most vessel electrical systems are 12 or 24 DC and 120/240 AC so the DC propulsion system is another non-redundant system to complicate things especially if you are also planning on storing power in batteries for propulsion too.

It would be nice if the whole boat could be 48 volts or whatever then you could put the generators where they were the most quiet or suited the vessel plan best instead of in the galley or your cabin so you can't talk or sleep when underway or powering large elctric loads. The electric drive systems are very compact and don't smell like diesel or motor oil (well maybe a little like lube oil). Locations for batteries can usually be found without interfering much with accommodations. Work out all the systems to be driven off the same voltage as the propulsion system and it begins to make sense to me for boats in the subject size range. Lots of other considerations for a cruising boat but maybe a good subject to kick off some discussion!

[dskira]

Originally Posted by CDK

If the conversion losses from shaft power to DC (20%) and from DC to shaft power again (20%) do not bother you, diesel/electric is a marvelous way to power your boat. No gearboxes, freedom to place the diesel where it is most convenient etc.

With steadily increasing fuel prices I'm not so sure this is the way to go.

Thank you for the info.
I didn't know the power lost was so great. Now I understand some of the problem of the diesel electric.
I have an other question for you. On a very heavy vessel, what do you think of a generator and three phase AC motor?
Size and weight is not important, the vessel been 150 Tons displ.
Do you see an advantage compere to a classic drive?
I like the electric motor for its ease to change speed and reverse, also not the need to have a shaft brake.
The three phase AC seams to be not as demanding on wires, and safety also.
And I like the diesel running at its most efficient speed all the time.
What is your thought?
Thank you
Daniel

[guerreiroazul]

Thank you! I didn't know about that big loss of 20%... The Ossa manufactory defends the opposite, they say the DE propulsion delivers more efficiency than normal diesel engines.
About the voltage of the system, again the Ossa say there will be no energy through battery bank.
Does anyone has this system in the boat? Is the maintenance hard?

[CDK]

With increasing size, the efficiency can be marginally better, say 15% instead of 20 because the relative air gap in both the motor and generator can be made smaller and the copper windings can be made somewhat more efficient.

If the engine shaft output is 100, the generator produces 85 and the electric motor yields 72 kW or hp without counting cable and controller losses. With these, there will be less than 70 available.

A gearbox wastes approx 10% so brings 90 kW or hp to the prop shaft.

But you could argue that the diesel has only one operating point with maximum efficiency (at highest torque). If that would be say 1800 rpm, it would burn fuel more economically than a mechanically geared one.

That sounds in favor of the electrical drive, but what happens when little or no power is needed and the diesel is kept at 1800 rpm? I think it would consume much more than at idle speed of 650 rpm.

I also think that the cost of the electric drive is a multiple of that of a gearbox. To avoid a shaft brake and gear change, a variable prop seems a more efficient solution to me.

In my opinion electrical drives can be a great solution only if the energy source is producing much cheaper kilowatts than a dedicated diesel can, or if silent running or submerged operation is a prerequisite.

[guerreiroazul]

Thank you. Helped a lot.

[FAST FRED]

A second advantage is , as cruise ships do, run at peak efficiency as many gen sets as the house an propulsion loads require.

Gives lots of redundancy (and maintence) but seems to work as the best solution where loads , power and hotel are very varied.

FF

FF

[WestVanHan]

Back from holidays....

Here goes the D/E debate again-here's what I posted a few months ago.....

"I spent quite a bit of time investigating diesel electric and it is not worth it.
I looked into a Siemens AC setup for a 45' cruiser- $100k plus $20k installation plus.
Then there's the ECUs full of microchips and millions of semiconductors.

I have yet to see a test done,with normal drive-then converted and showing us mileage gains.

So even if it did work...let's say hypothetically a 10% gain which is what was claimed...at $130,000-so lets say $3 a gallon that's ~40,000 gallons you'd need to save to make up that $130k.

So you'd need to burn ~400,000 gallons in order to save that 10% in order to pay for it.

You'd spend the rest of your life continuously circumnavigating just to pay it off.

Spend $10k-get a CPP and get the same benefits."

To reiterate CDK,but being very generous with efficiencies-using high end liquid cooled gen,motors,and controllers (even more stuff to break):

"I have yet to have anyone show me how:
-a generator with perhaps 91% efficiency
-going through a phase inverter/controllers/wires with perhaps 90% efficiency
-then through a motor with maybe 94% efficiency.
-with an overall efficiency of (.91 x .9 x .94)=77%
Is better than a marine transmission's efficiency of 90-95%.

Oh,and if you want a parallel hybrid on a cat that large (with minor range) plan on 30+ tons of batteries.

[gideon]

If using the generator on board we have a total efficiency of better than 77 %, after taking into account the prop losses from a small high rpm prop for a diesel or the slower rotating larger prop for the electric motor the picture changes a lot.
with dual 10 KW motors at 70 % power we run at 7 knots ( 45 ft sailing cat total weight 10 tons metric ) so we use 12 KW or 16 HP and a fuel consumption of 3 liters per hour, the same model boat but equipped with twin diesel saildrives each 30 HP running the same 7 knots we consume 5.5 liters per hour or almost double the consumption.
That is a substantial saving, the real savings come from the possibility to generate while sailing and not use the diesel generator at all for the reasonable short distance that need to be covered ( harbor in or out )
I have tested this setup for the last 4 months and diesel consumption is only needed if no wind is present for a period over 8 hours or if you are in a hurry.

I have attached info on power used versus speed and generating figures while sailing

[WestVanHan]

I had (and know specs of) the most efficient liquid cooled gen set,and is rated @ 14.4 continuous kw 120 VAC.

Seeing as they dont print consumption,I took the kubota engine specs it uses and 16.8 kw continuous and the gen set outputs 14.4 kw @ 120V so the efficiency is about 85%:

5.3 litres an hour to get 14.4 kw= 2.71 kw per litre per hour from the most efficient liquid cooled genset.


"so we use 12 KW or 16 HP and a fuel consumption of 3 liters per hour"
Which is 4 kw hr per litre of diesel (which INCLUDES wiring,battery,controller losses) apparently 50% more like (60%) more efficient than any gen set I know of.

Are you running liquid nitrogen in your generator ?

I assume this 3 litres an hour is starting out with a full battery bank and only recharging the amount you're using.


If you start out with an empty battery bank,I'm sure you won't get very far or go very fast.

Originally Posted by gideon

with dual 10 KW motors at 70 % power we run at 7 knots ( 45 ft sailing cat total weight 10 tons metric ) so we use 12 KW or 16 HP and a fuel consumption of 3 liters per hour, the same model boat but equipped with twin diesel saildrives each 30 HP running the same 7 knots we consume 5.5 liters per hour or almost double the consumption.

Unfortunately that is just another biased attempt and leaves the real world figures out of comparison.

How does a 10kw El motor perform like a 30hp (notice the cheating change from kw to hp!!!) Diesel? Or, as you claim, even better?

The 30 hp Diesel connected to a CPP would outperform your el drive by about 100% better performance or efficiency, that is the fact!!!

[capt vimes]

i must say, i am very intrigued by D/E propulsion systems for sailboats (cruising as a matter of fact)... not if you are looking for performance or performance efficiency but for the simple fact, that on long passages under sail one still has to recharge the batteries every 24 hrs for 1-2 hrs...
and this is absolutely inefficient when done with by the primary diesel!

one has now some alternatives to minimize those useless primary motor runs:
1) alternative energy from solar, wind, draggenerators etc (inefficient and not always available and altogether never enough)
2) drastic reduction in consumption (no radio, fridge, GPS, light and i do not even mention TV, PC and other unnecessary stuff)
3) an additional generator onboard to make up for the electricty one needs on a daily basis for weeks

if one could live aboard (on passages AND at anchor) with the first 2 points - it would be the best for the environment and the purse - if not - the 3rd point becomes pretty viable...
dieselgenerators now have the advantage that they are build for a sole purpose:
run an alternator to produce electricity!
they are way better and way more efficient to do this particular job then the primary propulsion engine which main purpose is to drive the shaft and only as a byproduct generates some electricity...

but if you have a generator onboard, why shouldn't you drop the primary, inefficient diesel altogether?
there are only a few occasions where a real sailor and globetrotter needs shaftpower and while combustion engines do become worse when not started every now and then - el motors could stand for years without deteroriating like diesels do... doesn't it? ;-)

[Alik]

Originally Posted by capt vimes

there are only a few occasions where a real sailor and globetrotter needs shaftpower and while combustion engines do become worse when not started every now and then - el motors could stand for years without deteroriating like diesels do... doesn't it? ;-)

Dreams, dreams...
Strait of Bosphorus and Dardanells - can one pass it with electrical power? How?

[capt vimes]

Originally Posted by Alik

Dreams, dreams...
Strait of Bosphorus and Dardanells - can one pass it with electrical power? How?

depends on the direction...
going from the black sea to the marmaris sea on a sailing yacht... you could make good use of the current there...
if you are heading the opposite way - wait for a southwesterly wind because then the current changes direction and you do not need shaftpower at all...

and btw:
electrical power does not mean from batteries alone... we are still talking diesel/electrical, aren't we?
any modern commercial vessel is running such a propulsion system and i do think that such vessels have passed and are still passing the bosporus by the hundreds on a weekly basis!