Diesel Electric Propulsion

Sounds like you'd like to do slow cruising on the electric motors and only run the generator when the batteries need it so the generator would run at an efficient level with the motor load and charging the batteries. I guess you'll add solar panels and maybe a wind powered generator while you are at it?

It's only a small step to add a sail and .....

Marmot, all your arguments are 100 % correct. And for a 30 ft full displacement hull, I would agree with you, it is not worse going for a DE setup.

But his trick is downsizing without any reserve power.

On a 30 ft displacement hull with a typical hull speed of 7 Kts, he would need about 9 KW of mechanical diesel power in order to reach hull speed (no wind and current). With an high efficient (85 to 90 %) electric drive he would need only 40 to 50 % of electrical engine power because of the different type of torque curve of an electric motor. If he wants to go at 6 Kts, this is 85 % of hull speed and amount of power needed. If he calculates the size of his variable speed DC-Diesel-Genset to this 85 % plus the losses, he would need only a very small genset. The higher consumption during acceleration could be buffered via the batteries (called hybrid propulsion). The same battery bank serving as the house bank and this little gen working also for the house load, is a very simple setup. But 15 NM per Gal is still very ambitious and this setup ist most likely more expensive and maybe heavier than a pure mechanical setup.

But he is talking about a multi hull design with lower wetted surfaces and much lower hull resitance and therefore even lower power requirements. This might change the calculation towards an economical DE setup. The examples of small DE boats, I have seen in the Netherlands and here in the Hamburg harbour (tenderboats and smaller sailing catamarans) proved his ideas. The cost may still be prohibitive.

On bigger applications things are turning bad for DE. Unless dictated by other reasons like construction, naval architecture, enviromental concerns, noise or vibration reasons, DE is the second best solution in the range between 35 to 40 ft and 300 ft. Only with a Sub, there is no other choice .

This is the setup, he is talking about (and this can be further downsized to one Genset only). Whether he uses inboard or outboard motors does not make much of a difference.

I think there is a lot of efficiency in diesel/electric for just cruising. While I was still working I conducted a fuel burn test on the dynamic positioned drill rig I was serving on. Displacement was 32,000 tons, propulsion (6) 5000 hp electric thrusters, main engine gen sets were (7) 4.6 MW. On a day with almost slack currents and almost no wind, our power needs were 3 MW, hotel and drilling loads were 1 MW of the that load. Typically with those light loads we would be running (2) gen sets. I asked the engineer to go to just one after I informed the drill floor not to bring on any heavy loads, and give me a fuel burn number. Then for this same small load I had the engineer bring on all 7 gensets and calculate fuel burn. Huge difference, but that was to be expected as it takes a lot of energy to spin the rotating mass of these engines and to pump coolant that is capable of removing the heat.

When cruising in a small boat in steep seas with electric drive, voltage determines the speed and the controller will ramp up amperage for the work done. As you climb a wave, voltage is constant, amperage is high, then when you surf down the backside of the wave voltage still is constant and amperage is very low to zero. This is how you make back up lost efficiency in the conversion process. If you average out all the peaks and valleys of electrical energy draw in this scenario and conclude the average to be 6 KW for a constant cruise speed of 6 kt, that is where diesel/electric really shines. The electric motor has all the torque at 1 rpm as it does at 2000 rpm (unlike an ICE), so as it ramps up power demand climbing the wave the battery bank supplies this and as it surfs down the face of the wave, speed is held in check and some regen occurs. All this time a small diesel (around 12 hp) is running at a constant load of 80% of its rated output.

I've done sail, used to have a Cal 40. On a boat as small as what I'm considering (28'~30' cat, < 9000 lbs) not as much room for solar as I would like, but I think I can fit 2000 watts of panels. This will more than handle hotel loads on the hook with enough left over to slowly top off the batteries after a passage. The last time I was cruising I found that I only moved about 10% of the time, the rest was swinging on the hook. With 36.4 KWhr of LiFePO4 cells and only needing at slow cruise 400 w-hr per nm, I figure after enjoying some anchorage for a week of sunshine, I could motor 80 nm on just the batteries. Unlike just 1 year ago, 36400 w-hr of LiFePO4 cells is now only $9000 through my supplier. It was this huge drop in cell cost that got me thinking in this direction. The high powered electric outboards that are currently on the market are charging $30,000 for 23 KWhr of lithium batteries.

OK, get in really huge following seas and you might be able to refill the bunkers.

What is it about sailors and electricity that keeps these visions of near perpetual motion alive?

Consuming a gallon of diesel for 15 nm doesn't sound like perpetual motion to me.

I used to consume 120 tons of fuel per day but someone else was paying the bill.

If sized for a speed range of 4 to 10 kt, and not sizing it for the 40 kt that market segment seems to want, I don't see why a < 9000 lb powercat can't achieve 15 nm per gallon at 5 to 6 kt. 6 kt average for a 24 hour run is ballpark for a lot of sailboats.

I don't agree with that at all, never said that. For the application the OP proposed, it is a lose lose in all aspects. It is a boating fantasy.

No, but your "surfing regeneration" sure does.

Several of us here have sailed on ships that burned more than your little drilling rig and we did it in the engine room where we had every kW and HP monitored and documented for every kind of propulsion from reciprocating steam to diesel-electric so there is a small chance we know what we are talking about.

I for one, started out crossing the Atlantic on a diesel electric hybrid vessel and know very well how much energy it took to turn the screws or charge the batteries, or both.

If you are convinced you have a better mousetrap ... build it and prove us wrong.

It probably is a boating fantasy. But what's wrong with that? I have no doubt that it would be more cost effective to build the same boat using four stroke outboards with high output alternators. Maybe 15-25hp? But that's easy. More fun to try the impossible. Of course using gas engines you will never achieve the holy grail of an unlimited range powerboat that you might have with solar / electric.

Don't stop believing!

Never have, never will ... I will be a tinkerer and experimenter until I die but neither I nor anyone else will ever get more out of a system than we put into it.

There are energy losses at each and every conversion, it requires X amount of power to move Y mass in Z amount of time. In a marine application, the source of that power is (in this world at this time) a tank full of some kind of fuel. Each time the energy in that tank is converted to some other form; heat, electricity, chemical form, state of matter, or mechanical motion, some portion of it escapes the system and is no longer available to perform work.

Anyone who believes that multiple energy conversions will in some magical way lead to increased system efficiency is likely benefiting from the latest changes in the laws regarding hallucinogenic herbs. To the best of my knowledge the laws of physics have not changed to meet current marketing campaigns or user expectations.

I accept that your an engineer and I'm just a deckie, but will you at least give me this....

1) Efficiency isn't the same lightly loading a large engine for just 1% of its rated output vs loading a much smaller engine at 80% for the same output.

2) A large diameter prop with greater pitch turning slow vs a smaller diameter prop with less pitch turning fast for the same speed.

3) The torque required to start to turn that large diameter, large pitch prop is better handled with an electric motor's low rpm torque characteristics, i.e. rated torque available at 1 rpm.

I know my last ship just had those (7) small by your standards 4.6 mw gen sets, but I have served on SeaLand container ships where the main engine was producing 60,000+ hp at 118 rpm direct drive and since the hp was obtained at that rpm then torque would be around 2,700,000 lb/ft, but even so, it still had a torque curve. Scale down to an engine the size I'm considering and that torque curve at idle would stall the engine if direct drive to a large, high pitched, more efficient prop.

I'm not completely green at this, even though a deckie when at CMA, I crammed in as many engineering classes as I could and still managed to graduate 3rd in my class. Didn't cost anymore other than one B grade that lowered my GPA, and gave me some insight as to what is happening below decks.

OK, got a few minutes to play so ...

Surely you don't think that the microscopic amount of fuel burn represented by a very small increase in efficiency at the higher loading means anything in the real world? So what if the engine burns 1 percent less fuel per HP at 80% load than it does at some smaller load ... the total amount burned at the lower power is so much less that the tiny efficiency gain is lost in the noise.

You must have left something out ... all I read is the equivalent of "A sequoia vs a cabbage on a hot day." What are your trying to say?

So? Doesn't that get back to what I said about the benefit of using electric propulsion in boats that maneuver nearly constantly vs the type of operation you claim to pursue? Do you want a cruising boat or a ship assist tug?

Why do you say that? Very low horsepower direct drive, direct reversing engines used to be the norm in small boat propulsion. They started and ran without benefit of clutches or gearboxes and performed quite well.

Is that large high pitched prop really the most efficient for the size of boat and speed you desire or may be obtained with the power you propose to use?

Kneeling before the idol of "efficiency" and calling that God the only God is a foolish endeavor.

On props all I said is a large diameter slower turning prop is more efficient than a smaller faster turning prop for the same speed.

But on to the meat of this discussion. From your post you are telling me there isn't much difference in a large engine loaded at 1% vs a small engine loaded at 80% for the same work done?

Just the energy it takes to spin the rotating assembly combined with coolant pumps sized to remove the heat when at 100% means there should be a sizable difference in fuel burn rates. A 300 hp engine vs a 10 hp engine for producing 8 hp of work, my money is on the smaller engine.

I don't have that much time ...

That's OK, wouldn't want you to waste your time. I know a few engineers that prefer to keep the old man out of the loop. Most though were willing to share their knowledge.

As per HTM09's suggestion I contacted Fischer Panda and Barry Fower advised me the complete system of (2) 10 kw motors, controllers and their 6 kw DC gen set would run £33,000.00, a bit pricey but not out of the question as a pair of 250 hp outboards would run $40K USD. To prove proof of concept (and since the boat is set up for outboards and I'm not a fan of more thru hulls) I'll just start with lithium batteries from my source and inverter for AC loads and go with the pair of Torqeedo 4.0s. If my concept doesn't fly, then all that I am out is the loss on resale of the Torqeedos, as I would set up for hotel loads this way no matter what the propulsion will be. I currently use LiFePO4 cells and will never go back to lead.

I finally got through this entire thread and want to thank all the posters for their sharing of information and links on this subject. It was a good read.

28 - 30 foot cat

Have you thought about a pair of oars on the mega yacht as big what you are proposing.

Sorry deckofficer but why bring 300 hp diesels or 250 hp outboards into the equation.

Have you thought of the weight you are going to put into that boat, stick a couple of throw away outboards on the back and invest your money.

Seriously look at the specific fuel consumption of different engines, the large slow speed diesels you referred to are up to 50% efficient these days whereas when I started 30 % made ship owners very happy.

Anyway do what ever you feel is right for you, I tried putting a four speed transmission into a boat to see what would happen many years ago and it nearly killed me, then the transmission people introduced two speed units and I missed out on another fortune.

Good luck.

PS. I was the chief engineer on the worlds largest diesel for 6 months, 12 cylinders & 88,000 hp and once we set up the main engine fuel burn we spent as much time ensuring the crankcase & cylinder lubrication consumption was what the company wanted. Mind you that was when we had to use hand drawn indicator cards and tweaking the flow through a site glass to the cylinder quills. Yes long before common rails & computers. Lol

I'm not bringing it into the equation Yachtjocky. 175 hp to 250 hp twin outboards is what the boat is spec'd for, I'm wanting to try electric outboards and a DC gen set (small 6 kw).

The Laws of Thermodynamics say:

Zeroth Law: "You must play the game."
First Law: "You can't win."
Second Law: "You can't break even."
Third Law: "You can't quit the game."

Do you wanna Play Thermodynamics ???????? I forgot "you must"

"Keeping the old man out of the loop" is hardly my reason for not wishing to start from scratch and teach someone the fundamentals of thermodynamics and marine propulsion in an online forum. There are many texts that will do a far better job than I can or have the patience for.

I am quite happy to discuss the pros and cons of reasonable ideas but when I read a foundation statement such as "I think there is a lot of efficiency in diesel/electric for just cruising" then it tells me that the person making that statement has the vocabulary but not sufficient comprehension of the issue or the context to continue a discussion. I read that as you have already decided (incorrectly) that your idea has some merit.

I don't see much hope or even a reason to put much effort in changing your mind. It might provide a few moments of entertainment for other readers but I don't think that is why you came here, is it?

I told you that the suppliers are delighted to sell you anything you like. They make good money feeding the fantasies of imaginative boaters. Their products work well in several specific and limited applications but, by your description, yours is not among them.

Let us know how that works out.