Rational for a Hybrid?

[billknny]

Is there any rational reason that a diesel/electric hybrid system makes sense for a long-range cruising boat? I see people buying them, but to my engineer self it does not compute.

An automobile has a fundamentally different power use cycle than a boat. They use full engine power only when accelerating or hill climbing. When traveling at highway speeds, the engine is just ticking over. A perfect application for a hybrid because you can install a tiny little engine to keep the car moving at 60MPH, and the electric system handles the short term heavy lifting.

A boat is completely the reverse. The engine is fully loaded only when running at cruising speed--which happens for long periods of time, not brief intervals. Batteries can't keep you going for hours at a time, and you still need to install a diesel engine of the same power output and carry just as much diesel fuel... PLUS the batteries...

What's the appeal--other than being the cool kid on the block with more money than sense?

[jamhass]

One advantage of a hybrid is that you do not have to have the engine close-coupled to the shaft. This can allow you to move the engine to a more favorable location and possibly run the shaft at a more favorable angle, increasing efficiency.

[billknny]

A theoretical advantage, I agree... but given the significant extra cost of a hybrid drive, do you really think that kind of efficiency gain would EVER be paid for? And the extra weight of the batteries might just kill all the gain you ever might imagine, not to mention that conversion of mechanical energy to electric and back again is not terribly efficient.

The kind of arrangement you describe could be done cheaper, easier, and more reliably with the very mature technology of hydraulic drives. The fact that we haven't seen a lot of those, leads me to believe that the "efficiency gains" you propose would result are not real.

There is a special case where it could make sense... if you put one engine in the center of a catamaran, and electric drives in each hull you MIGHT save enough money and weight to make it worth it. Of course most modest sized sailing catamarans don't really have room for an engine on the bridgedeck. My vague recollection is that one manufacturer tried something like this fairly recently... and the technology wasn't really up to snuff for it to work in the real world--yet.

[SV THIRD DAY]

Originally Posted by billknny

Is there any rational reason that a diesel/electric hybrid system makes sense for a long-range cruising boat?

Because it's cool and some people like engineering projects more than they like cruising. Otherwise it's a tough sell.

[Adelie]

Originally Posted by jamhass

One advantage of a hybrid is that you do not have to have the engine close-coupled to the shaft. This can allow you to move the engine to a more favorable location and possibly run the shaft at a more favorable angle, increasing efficiency.

That only works if you go full diesel-electric. But then you pay a weight penalty that probably eats up any gains from optimizing weight distribution plus you pay an efficiency penalty in the conversion to electricity and back to mechanical motion on the order of 15% over the 3% or so of a mechanical transmission.

There are instances where diesel electric makes sense but not for a cruising boat.

[Adelie]

Originally Posted by billknny

Is there any rational reason that a diesel/electric hybrid system makes sense for a long-range cruising boat? I see people buying them, but to my engineer self it does not compute.



An automobile has a fundamentally different power use cycle than a boat. They use full engine power only when accelerating or hill climbing. When traveling at highway speeds, the engine is just ticking over. A perfect application for a hybrid because you can install a tiny little engine to keep the car moving at 60MPH, and the electric system handles the short term heavy lifting.



A boat is completely the reverse. The engine is fully loaded only when running at cruising speed--which happens for long periods of time, not brief intervals. Batteries can't keep you going for hours at a time, and you still need to install a diesel engine of the same power output and carry just as much diesel fuel... PLUS the batteries...



What's the appeal--other than being the cool kid on the block with more money than sense?

I can see it breaking even on cost over the life of the boat and having some functional advantages at the expense of other functionality.

Here's the way I would work it.

I would size the prime mover to get the boat to hull speed in flat water, no wind conditions plus another 10%, say somewhere around 2.5hp/ton. This is significantly less than the typical installation these days, around 4hp/ton. Most engine installations aim to hit hull speed against some wind and waves.

With the smaller engine you can be at peak engine efficiency right near 1.0 SLR which is generally a good cruising speed.

If you need a boost of power the electric motor drawing from batteries can supplement the prime mover but only for some specific period.

So if you want to maintain hull speed against wind and waves you can do it long enough to enter a moderate length canal or lagoon entrance but you can't do it all day.

If the prime mover fails you still have motive power for the life of the batteries which may be indefinitely long if you have significant solar capacity and only need intermittent motive power or only need a very low continuous speed.

The backup motive power comes at the expense of a more complex system.

Some hybrid systems may generate power from vessel motion when under sail.

From what I can figure you need to accept a lower than normal long distance cruising speed. That's fine with me but I don't see most people accepting this.

[44'cruisingcat]

Maybe if you want a big genset anyway.

[CarlF]

The other reason it's inefficient is that energy is lost every time it is converted into another form.

In a conventional system, the energy of the diesel fuel is converted to heat which is converted into rotational energy which is directly coupled to the rotating propeller.

In the hybrid, the diesel fuel rotates a generator instead of the propeller to make electricity which is then converted to the right type (ac or dc, voltage, frequency). More energy is lost in the wire resistance. And then the electricity is converted back to rotational energy to drive the same propeller. If batteries are anywhere in the picture, several more conversions occur costing energy each time.

It's just a guess, but I'd say 30% is lost compared due to these conversions.

[billknny]

Originally Posted by Adelie

I can see it breaking even on cost over the life of the boat and having some functional advantages at the expense of other functionality.

Here's the way I would work it.

I would size the prime mover to get the boat to hull speed in flat water, no wind conditions plus another 10%, say somewhere around 2.5hp/ton. This is significantly less than the typical installation these days, around 4hp/ton. Most engine installations aim to hit hull speed against some wind and waves.
sing speed. That's fine with me but I don't see most people accepting this.

I don't see the logic here for a long range cruising boat. For a daysailer--sure that's a solution that would technically work, although even there the cost advantages seem nebulous at best.

The cost for installation of a hybrid system would be higher than the cost of the larger diesel engine. The amount of time you would save fuel because of the smaller engine are small, and even then difference isn't large anyway. Do you have numbers you are thinking of for fuel savings?

And when you talk about the "life of the boat", what are you assuming for the lifespan and cost of the batteries and how much battery capacity are you installing?

I strongly suspect this is one of the cost calculations done with the heart rather than with realistic data and a calculator.

If wishes were horses...

[Corvidae]

A hybrid diesel only runs at the most efficient speed for the engine, regardless of what speed the throttle is set at. Given a battery bank large enough to move the boat at wake speed for long enough, they can be charged with solar and never run the diesel, or simply shorten motor running time to its exact needed run time.

The battery bank is an integral part of a hybrid system, allowing maximal use of generated energy by stockpiling and offloading as needed. Under long term continuous full load, the only advantage is that there is no gear box to wear down. However if you regularly run your sailboat at full motor speed for long stretches, maybe a sailboat isn't the best boat for you?

[Perna]

Originally Posted by billknny

Is there any rational reason that a diesel/electric hybrid system makes sense for a long-range cruising boat? I see people buying them, but to my engineer self it does not compute.

An automobile has a fundamentally different power use cycle than a boat. They use full engine power only when accelerating or hill climbing. When traveling at highway speeds, the engine is just ticking over. A perfect application for a hybrid because you can install a tiny little engine to keep the car moving at 60MPH, and the electric system handles the short term heavy lifting.

A boat is completely the reverse. The engine is fully loaded only when running at cruising speed--which happens for long periods of time, not brief intervals. Batteries can't keep you going for hours at a time, and you still need to install a diesel engine of the same power output and carry just as much diesel fuel... PLUS the batteries...

What's the appeal--other than being the cool kid on the block with more money than sense?

+1
I agree with you entirely. It makes no sense.

If the engine is not directly coupled to the propeller you can forget any fuel savings. And the thought of only being able to motor against a strong headwind, maybe off a lee shore, for say 5 minutes but not 10 would make me a very nervous sailor indeed.

[Chuck Hawley]

My instincts tell me that the additional complexity could not possibly be worth it, although there are two somewhat related benefits that occur to me:

1. I understand than the torque required by a propeller and the engine's torque curve are different, resulting in an inefficiency unless you're at full power. At least that's what I recall; thus some boats have variable pitch props to eliminate this "torque gap".

2. Related to #1, would this allow you to load up your engine at the optimum fuel setting/RPM/output to squeeze the last bit of energy out of your fuel?

What I don't understand is the thought that you can load up a boat with batteries and get any sort of range/speed out of them, unless its to power silently out of a harbor with a short channel...

Chuck

[Perna]

I think it's not so much the torque requirements but the power requirements which matter.

The classic propeller curve for a displacement hull is very well suited to a naturally aspirated diesel engine which is why you don't need multi-speed gearboxes on boats. A planing hull has the hump at low speed to get the thing out of the water which is better suited to turbocharged engines. But in general a diesel engine has best fuel consumption at say half speed highish load which should match a decent cruising speed and then for more power, hull speed, waves and wind, the fuel consumption is worse, which is also OK or at least to be expected.

[Cadence]

Originally Posted by billknny

Is there any rational reason that a diesel/electric hybrid system makes sense for a long-range cruising boat? I see people buying them, but to my engineer self it does not compute.

An automobile has a fundamentally different power use cycle than a boat. They use full engine power only when accelerating or hill climbing. When traveling at highway speeds, the engine is just ticking over. A perfect application for a hybrid because you can install a tiny little engine to keep the car moving at 60MPH, and the electric system handles the short term heavy lifting.

A boat is completely the reverse. The engine is fully loaded only when running at cruising speed--which happens for long periods of time, not brief intervals. Batteries can't keep you going for hours at a time, and you still need to install a diesel engine of the same power output and carry just as much diesel fuel... PLUS the batteries...

What's the appeal--other than being the cool kid on the block with more money than sense?

I can't see anything but a down side for long range cruising on a small boat.

The advantage of diesel electric is wheel house control. The weight on a boat would not be worth it

[hamburking]

Another advantage is that you don't need to start the engine for those many many times you only need it for a few minutes...like arriving at or leaving a dock or anchorage. For many small boats, that the ONLY time the engine is run, so a hybrid makes a ton of sense.

Also, it separates the engine from the boat. The engine can be placed anywhere convenient. You can still maneuver around a marina with no engine (run on batteries), and you can even remove the engine entirely for service (or replacement) much more easily than if its connected to a shaft. Imagine for instance, a small boat that uses a little portable honda generator. Wow, so easy to maintain (take it home for maintenance) or replace. And so much cheaper too than say, trying to rebuild an atomic 4.

And finally, the hope is that one day we will have another, better power source than batteries. Maybe hydrogen fuel cell, new battery technology, or cold fusion...who knows. By separating the power generation from the propulsion, the boat is ready for whatever may be just around the corner.