Diesel/Electric Propulsion for Sailboats

If you click 'edit' you'll be given a 'delete' option, I think. WOrked on me just now when my post came through twice.

But:

Why do you want to nuke it?

 

I dont know what "my disadvantage" items you are talking about.

If you are referring to wave_guide's list, I dont consider them a disadvantage. But since you called me on it, I will address them.

This is not a disadvantage. Modern, non-packing gland, shaft seals are pretty reliable. We have had one for over ten years. Not requiring one minute of maintenance.

Electric motors can be reversed almost instantaneously, though likely not in real-time with a marine application, but they can be reversed faster than throttling back shifting a reverse gear. Not a disadvantage.

A Z-drive, while maybe handy and more efficient, does not give a sailboat much advantage over rudder steering, used for centuries. Advantages of Z-drive are easily realized on fast planing power boats however. Not a disadvantage.

I have never heard of a sailboat being struck from behind at the propeller. Most often, it will have to get past the rudder. But, never heard of this actually happening. Not a disadvantage.

No, but you can get very close by separating the thrust from the engine with an Aquadrive, or PythonDrive and going really soft on the engine/generator mounts. Even better in the case of a generator as they are often tuned for optimal output and a constant RPM with minimal noise and vibration. Not a disadvantage.

Murphy's law, yes quite scientific. If you take the law seriously, you should not be stepping out of your house, cause a meteor will strike you.

Not a disadvantage.

 

Another advantage of electric propulsion:

Instant thrust. with the electric motor flat torque curve, you can select a propeller that is efficient over the entire RPM range and would provide more thrust at that low RPM where straight diesel propulsion would otherwise be overloaded.

 

I'm sorry I mixed you two. I don't think there are many real disadvantages in a well done diesel installation compared to any diesel-electric I have seen, atleast on smaller sailboats.

There are some advantages of an electric drive like a wider rpm area a bit more torque at low rpm, faster change of direction etc., but I don't really think these are real problems in sailboats, just minor details.

Most sailboats travel less than 2 kn on idle. Why would it be necessary to go slower without neutral? Slower rpm overall would enable larger and more efficient propellers, but would not make much difference on current size of propellers. A sailor wouldn't like to have bigger propellers causing more drag.

Higher initial torgue and faster change of direction are manouverability issues. I don't see any real problems in manouverability of a well done diesel installation.

If the two above would be real disadvantages, threre are solutions to those without using a diesel-electric system.

The through hull of a shaft or a sail drive is a potential, but extremely unlikely, risk. Even the electric drive needs a shaft or a thick cable through hull and a motor under water is a risk for propulsion.

Since power needed for currently used cruising speeds is beyond the current battery technology the diesel generator and the electric motor would be rather tightly connected. Thus you would need to run the diesel power close to the electric power on anything but short runs. You are not able to use the best efficiency of the diesel much better than in normal diesel installation.

I don't see much problems in using the normal diesel as a generator when thrust is not needed.

In future the batteries needed will probably be cheaper, lighter and less expencive. Then it makes more sence to use diesel-electric on a sailboat.

Joakim

 

On a sailboat, most of the time I prefer to sail. (Yes, I know that you can't always sail. I've done much "sailing" on the chesapeake in July/August.) This makes the propulsion motor an auxiliary device, as it was initially called. This means that most of the time when I run the engine, it's to charge batteries (either under sail or at anchor).

It seems (to me) more efficient to run an engine generator (compared to a propulsion engine with an alternator) to produce electricity. Talk to the cruisers who carry aboard a portable gasoline generator for charging.

If propulsion is another auxiliary load, why not provide it electrically? An added benefit is that, while sailing, the propulsion motor can generate electricity, further reducing diesel use.

Part of the bottom line is range. If I can reduce diesel usage by 25%, I've also increased my range by 25%. I think that's possible by running a diesel generator instead of a propulsion motor with alternator. If I can generate sufficient electricity with the motor under sail to reduce that a further 25%, I have doubled my range.

So, for me, what drives diesel-electric is fuel economy and range.

. The OSSA powerlite system uses no batteries, running the generator all the time the motor is needed (at, I believe 48V but dont' quote me). The E-Motion system relies on a 144V bank. There's also the Duffy electric boat, which has been running on battery alone since 1970 at 48 or 96V. The higher voltage keeps cables to a reasonable size. I agree we need better battery technology, but within certain limitations we can handle the loads with today's batteries. I'm not sure I fully understand what you mean in your last two sentences ("Thus you would need to*…*normal diesel installation"). I assume you mean vary the generator RPM, therefore you are nto able to run the engine at the most efficient RPM at all time. While this is true, there are a few possibilities:

First, if you have a battery bank that will allow you to motor for a short time , you can run the engine intermittently over a longer time, say 15 minutes an hour. Secondly, you can do what we do with direct-drive diesels: for long runs, match generator efficiency to cruising speed (although I haven't seen much on this, and I've a feeling there's something I'm missing here). THird, and most importantly, is that your generator is designed to provide electricity. On a conventional drive, the full power of the engine goes to turn a propeller: electricity is somewhat of an afterthought, with a belt-driven alternator trying to tackle jobs the engine was not designed to do. (If you need more info on this, talk to people who have tried to retrofit a "high output" alternator about design issues, adding pullies to accommodate a second belt, belt slippage, frequency of belt changes. etc. I know a couple who had a professionally installed high-output balmar on their Yanmar. IF the batteries were dead, the belts were sure to go. They ended up with a complicated charging ritual involving a combination of a Honda portable gasoline generator and their Yanmar/Balmar.)

True, for pure motoring, I'm not convinced D-E is any more efficient than a conventional drive. But for a sailboat, I believe there are great efficiencies, especially for long-term sailing. Logically, on a sailboat, it makes sense to me to couple your engine to your house load rather than your propeller.

mickey

 

Just do the math and if the weight and $ costs equal a benefit that is worthwhile to you then do it if not don't... - - no need to get all emotional it suits some and not others...

Just feel rather like being a "grumpy old grampa" for a while

 

My points were

1. If you wan't to be able to run a bit longer at your normal cruising speed (close to "hull speed"), you need a generator, that is about the same power as the diesel you have. It is not feasible to have almost any help from batteries on this. E.g. I use about 3 kW power of my 1GM10 for cruising speed. How much would batteries cost/weigh for say 6 * 3 kWh use. Thus if you want to maintain this posibility, you need a generator quite identical to your current motor and just as inefficient on normal use. A bit bigger boat would like to cruise at 15 kW, which is even much more problematic.

2. If you plan to use sequental charging (generator runs say 50% of time at full power), you need again a huge battery pack in order to take that charging current. Also you loose energy (20%???) each time you charge/discharge a battery.

3. In smaller boats the engine/alternator is seldom the limiting factor. It is the battery pack that is unable to receive the full current at regulated voltage. As well as going to diesel-electric the normal diesel could be designed to be a better generator, when not used for propulsion.

4. Using the propeller as a generator is quite limited. You don't want to do that before you reach your hull speed, since any reasonable power would slow you down dramatically. Even more so you don't want to use it on a beat.

If range is the most important thing, then you should just lower your cruising speed. Dropping 0,5-1 kn could easily double your range.

Joakim

 

Joakim, all good points! Too bad I gotta work now and can't give them the thought which they deserve. Will do so soon

Masali,
I've done lots of math, and I get more confused! If anyone cares to help or peruse, please visit:
http://sites.google.com/a/slowresponseteam.com/sv-bahia/Home/diesel-electric-propulsion
and
http://svbahia.wordpress.com/tag/diesel-electric/

Let me know if I'm missing something

Peace,
oh, and happy thanksgiving to those who celebrate it this past thursday. The rest of you, happy weekend!

Y

 

"Part of the bottom line is range. If I can reduce diesel usage by 25%, I've also increased my range by 25%. I think that's possible by running a diesel generator instead of a propulsion motor with alternator. If I can generate sufficient electricity with the motor under sail to reduce that a further 25%, I have doubled my range."

"So, for me, what drives diesel-electric is fuel economy and range. "

Simply running a SMALLER diesel will give better range at lower cost and with longer engine life.

" you can select a propeller that is efficient over the entire RPM range and would provide more thrust at that low RPM where straight diesel propulsion would otherwise be overloaded."

Propellers are currently made with variable PITCH , unfortuniatly not with variable Diameter.

The prop loading curves show if you can spin the prop at any speed , going slower will not increase torque requirements , no low speed overloading.

If you simply wish to be really efficient a heavily loaded engine will do it with a suitable reduction gear..

The big hassle is most yachty cruising boats for decades have been lowering house loads , so the concept of diesel electric , as used on cruise ships with HUGE house loads doesn't compute.

It is still the least fuel use to have the ability to charge house batts at a HUGE !!! rate (expensive alt , monitoring and gel cells) while the boat is being pushed by the engine.


FF

__________________

 

I would say the least fuel use would be to power through non-fuel sources, like a dragging propeller, wind, and sun
I was under the impression, also, that yachtie house loads were on the increase, what with AC, washer/drier, even trash compactors?!

So far the question has been, "is it worth it to use D-E?" I'll put masalai's question to you, then, as well: Can you show us your math for the cost/benefit analysis? Maybe if we all publish our "math", we can come up with a spreadsheet to see where the charge/motor/regenerate equation calls for direct-drive vs. hybrid.
Y

 

That can be done quite simply and with this formula VxA=W (VoltsxAmps = Watts) calculate the amount of power needed for each item and for each item estimate a fairly generous estimation of each item run time (per day) in hours and a decimal multiply the two items to give watt/hours consumed - - - - Add all these together to give kilowatt/hours and you will have the energy needed to drive your boat....

A "rule of thumb" say 80% time sitting - so house needs only...

So in say 100 days you will have
20 days at house load PLUS propulsion
80 days at house load - some things will only be needed weekly so another category for them (watermaker, watch movies and entertain on Saturday night, process and empty black water, high pressure jet to clean the boats' bottom etc...)

 

80 days at house load would require huge solar area (hard on a rag boat) OR a reduction in std of living , no watermaker , dishwasher , or washing machine.

For an "efficient " noisemaker setup one would have to be able to run whichever units could be run on the gen set AND charge a big house set at the same time.

Sorta gets one into the std marine motorist setup , an hour or two every day AM & PM.Unless the Air Cond is on , then its noise/stench 24/7.

Modest (sailboat style ) refrigeration and house loads can easily (but expen$ivly ) be handled with a std wet cell bat bank.

Depending on the house bat set size (number of days of silence ) you can pay for.

FF

 

Maybe a sobering overview might help.
Watts in Equal Watts out.
No matter how you get it, Watts to the prop are gonna come from some kind of energy.
Keeping it simple, the Diesel and Reduction gear is a lot cheaper than a Diesel, a Generator and an Electric motor.

And one heckuva lot lighter as well as taking less room.

I really dont know why they work out so well in Trains, but the Train's can move real heavy loads for a lot less than a fleet of trucks doing the same job.

 

Very true. No matter how much technology you throw at a problem, the First and Second Laws of Thermodynamics will always win out.

D-E drive in trains is not for efficiency at a steady cruise speed. Indeed, they'd be more efficient- at constant speed- with direct drive from the big diesel to the axles.
The reason D-E works so well in trains is because trains don't operate at constant speed. They need full power and full torque at zero RPM in order to get moving, and they need full power available at any speed with any load. Using a diesel generator and electric wheel motors has the effect of decoupling engine RPM from wheel RPM. If more power is needed, the engine can speed up to whatever RPM it needs to produce that power, while the wheel motors can translate nearly all of that power into torque at any speed.
Transport trucks have enormous gearboxes (12, 18, I've even heard of 27-speed transmissions) to accomplish the same thing. D-E in a train is essentially the limiting case as the number of gears goes to infinity- engine RPM becomes dependent only on power demand, and drive wheel torque can be optimized for the precise conditions of load and RPM. Indeed, even with a 27-speed transmission, a train diesel would probably have a hard time getting more than a few dozen cars rolling; it would just stall, even in low gear. But with D-E, the diesel can rev up to full power before the wheels are even turning, and the electric drives can translate that power into torque at zero RPM.

Now, in a boat, it's a different story altogether. The effect of decoupling engine RPM from prop RPM is still there, and is where the fuel savings (if any) come from- as the boat goes up and down waves, the load on the prop changes, and the engine RPM (and fuel consumption) can change as well even though the prop RPM stays constant. But I'm not convinced that for a typical boat, in typical cruising conditions, the savings (if any) outweigh the added capital cost. Where it does make sense is on vessels such as cruise ships, where the house load is enormous and the propulsion loads highly variable. A typical 75,000 tonne cruise liner might have four main diesel generators and four azipod drives. In port it'll be running one diesel for the house loads only. On a slow passage between adjacent islands it might be running two of the engines, while to cross a big stretch of open water at speed with a full complement of guests, all four might be churning away. And these ships can't draw on shorepower; they need more electricity than most of their ports of call can provide.

I don't think the average cruiser sees this kind of variability. A lot of people, when cruising under engine, seem to pick one speed where the boat is comfortable and reasonably efficient, and always cruise within a couple of knots of that speed. Then, when at anchor, they'll run house loads off the battery that was charged up while the engine was on. A properly sized mechanical driveline with the correct prop strikes me as the more efficient choice here. If you're going to go electric in a cruiser, I think it had better be tied in with enough solar or wind generation capability to make you at least slightly independent of the fuel dock.

 

thudpucker, most of the points are in the post by marshmat, the only one left is cost and for your D/E trains include the cost of the track and track maintenance in the comparison.... Steam also has the capacity to exert maximum torque at zero wheel revs to start and had many drive wheels linked to spread the traction more effectively and there is a benefit in additional weight in the locomotive as steel on steel is not the best at transferring torque into motion....

The cost is huge for recreational boats and I tried to justify it as also for house by using 3.5KW of solar panels so while parked in a beautiful and peaceful bay I would not need the diesel generator - could not justify on that basis either as capital costs were far too great and electric motor controllers and submerged motors or otherwise requiring water cooling??? sorry not a working solution yet....