Diesel/Electric Propulsion for Sailboats

[masalai]

From my very inexpert understanding, the electrical advantage is basically that max torque is available right from zero revs whereas a diesel needs to rev up to about 2000 revs... - The electric motors I like are maxed at around 1000rpm (African Cats) and will regen when immersed else lift up out of the water - therefore you can use efficient propellors and save heaps by not needing feathering etc for sailing efficiency....

[mp459]

Originally Posted by masalai

From my very inexpert understanding, the electrical advantage is basically that max torque is available right from zero revs whereas a diesel needs to rev up to about 2000 revs... -

Right. To hear E-Motion tell it, you can drive a bigger prop with a lower-HP electric motor, because a diesel would stall at low RPM. But this doesn't explain why you would want to compare an DE with an underpowered conventional drive to get "apple to apples" comparison, as is suggested in the proboat articles. Either:
a) You can drive a boat as well with a lower-hp electric motor as you can with a higher-hp traditional-drive diesel, which is what the DE companies are saying
or b) you can't: you lose a lot of driving power in the lower-hp DE motor (which seems to be what the proboat articles suggest), so you should compare it to a de-powered direct (ok, with a trans/reduction) diesel drive,
or c) you can drive teh boat just fine with a reduced-hp electric motor, or a de-powered direct diesel, or the standard diesel (which would mean that we've all been overpowering our boats for the past several decades).

Now, Mr. Calder has forgotten more than I'll ever know about this stuff, not to mention I have a _lot_ of respect for him. So my question is: What gives? Which is it?

In other words, if a lower-hp DE setup drives the boat as well as a higher-hp traditional configuration, how is this not apples-to-apples ?

I'm guessing I'm missing a glaring piece of the puzzle somewhere.

[masalai]

You gotta "read between the lines"- - You Yanks developed Hype, learn to live with it.... Them marketing people have a lot to answer for in the corruption of many things (selling, politicians, commodities, fools and so on he he he or )

[Guillermo]

Originally Posted by mp459

...I have heard nobody advocate for reduced power in a conventional engine as a means of gaining "efficiency".

Well, in fact most conventional propulsion systems mounted aboard are overpowered from the propulsion point of view. As diesel has been cheap for many years, recreational use-time of boats through the year is usually very short, price of diesel engines has been reducing over time in relative terms, more and more home appliances are mounted aboard, as well as an speedy feeling has grown up among most boatowners, installed power has increased unnecessarily most of the times.

To efficiently run a boat for many miles on end to an S/L ratio of 1.1, let's say, only a relative few HP are needed, usually much less than the nowadays installed power. This forces diesel engines to be run for many hours sometimes even well under 50% of their nominal HP, which is no good for them.

Many years ago auxiliary cruising sailboats used to have the power/displacement ratio under 1.5, but nowadays that figure is commonly located in the 2.5-3.5 range or more, sometimes rocketing up to 7-8, like it happens with some examples of modern light and fast motorsailers.

In my opinion to mount a conventional system with power being over a ratio of 2.0-2.2 is a waste of money. This kind of power (for most cruising sailboats) is more than enough to beat dead to winward into a force 10 in sheltered waters.

Cheers.

[mp459]

Guillermo,

No arguments from me there. I remember when I was getting certified for NITROX diving, the first thing the instructor said was something along the lines of, "We're used to limits with diving, and the only thing that NITROX does is introduce a couple more limits." I feel the same way about increasing HP on a displacement hull: It increases the limit of what you can power through by a small percentage. If a low-HP motor has to work at capacity a small percentage of the time to power you through difficult situations, the more you increase the horsepower, the more you decrease hte percentage of the time when you need that extra power. And the bottom line is you still have that limit, excpet now it increased by a knot or half a knot of current, say, or a few extra knots of headwind.
But my point is not adding more power to the engine. My point is that the claims from DE manufacturers is that a lower-hp motor will handle as well as a conventional system, whereas the claims from other sources is that you are significantly de-powering the boat.

[Wave_Guide]

The conventional design, essentially unchanged since the invention of the steam boat, has several inherent disadvantages:

1) It requires a through-hull connection of the propeller to the engine. That connection is usually below the water line, making the connection vulnerable to leakage and a high maintenance headache.
2) The propeller rotation cannot be reversed without some form of transmission (most internal combustion engines can't run in reverse rotation).
3) The propeller thrust cannot be yawed (vectored), requiring the boat to maintain enough velocity through the water to achieve rudder steerage.
4) The propeller shaft (going through the hull) is vulnerable to being struck from behind -- turning it into a harpoon aimed at the engine.
5) Coupling of engine vibration to the hull cannot be completely eliminated because of the through-hull connection. (Also, anything that snags or suddenly stops the propeller will damage the engine or transmission because it is directly coupled).
6) You have to run the engine to get propulsion. In an emergency, Murphy's Law dictates that that is the most likely moment when the engine will fail to start. It's also difficult to silently sneak into a slip at 4 AM without waking your neighbors (or parents... I remember the night long ago when I decided to shoot into a slip under sail alone, just a little too fast, to avoid alerting my parents and... you can guess the results...).
7) Internal combustion engines with reasonably sized flywheels have a minimum rotational speed (lowest possible RPM). Just try running a diesel engine at 10 RPM. Electric motors on the other hand can turn at dead slow speeds without complex transmissions.

Some of those above disadvantages are the reason we have diesel-electric train engines and you rarely see anything else pulling trains. Can you imagine the size of the transmission required to reverse the rotation on one of those monsters? Instead, they simply reverse the polarity to the electric motor. All that's required then is an electric polarity-reversal switch.

The production of high torque at low-rotational speeds is also an advantage with diesel-electric trains since they must get enormous masses moving from a dead stop -- but less of a concern when moving a fluid medium with a propeller. The authors here that got hung up on the power issues miss the crux of the advantages of diesel-electric designs: replacement of rigid drive shafts with flexible copper wire (and now the boat doesn't need a hole in its transom through which must pass a rotating drive shaft), easy reversibility of rotation, steerable propulsion, de-coupling of the engine from the load (propeller), and ease of maintenance where only the propeller-motor assembly is exposed to the elements (...ever try to replace a propeller shaft while the boat was still in the water?).

Now imagine this design: A DC motor attached directly to the rudder and rudder linkage as a single unit. When the rudder is turned, the motor-propeller assembly turns with it, thus providing yaw authority well below rudder steerage speed. Power is supplied to the motor via just two wires that feed from a through-hull well above the water line, completely eliminating the need for leaking hull penetrations. And if you provided batteries for the motor as a power source, in addition to the diesel engine (that would turn an AC generator with current rectified to DC), you could sneak around like a submarine for short runs without even starting the engine. And the engine could now do just one task: charging batteries -- instead of being in its usual dual role of also turning a propeller. Now you can mount the engine on really nice vibration dampeners and not have to worry about any drive shaft alignments to a propeller. Heck, you could even put the engine someplace other than in the stern!

Do you know how we start turbine engines on jet airplanes? We spin them up with a DC motor. And do you know what we do with that DC motor once the fires are lit in the engines? We reverse their polarity and use them as generators to recharge the batteries and run all the other electrical stuff in the airplane. DC motors are just as happy turning mechanical energy into electrical energy as they are turning electrical energy into mechanical energy. (We don't use AC motors because it's difficult to control their speeds, you can't use series-wound motors for enormous amounts of torque, and it's inefficient to power AC motors from DC batteries. Please don't ask me why we don't use "AC batteries" because I will refer you to your 7th grade science class teacher for remedial training.)

So here's one other little added advantage: If you unfeather that propeller while under sail, and if you don't mind a little drag (if you're already moving along at hull speed, what's wrong with putting out a little extra sail?), you can let that propeller turn the motor in order to charge the batteries. Instant sail-derived electrical power! No more noisy stinking engines charging batteries while you're under way.

But then, who am I to buck tradition and try to redesign the steam boat?

[Wave_Guide]

I'm obviously new to this blog software...

I need to nuke post number 156, and I'm baffled on how to do that. Any ideas?

[Joakim]

Originally Posted by Wave_Guide

1) It requires a through-hull connection of the propeller to the engine. That connection is usually below the water line, making the connection vulnerable to leakage and a high maintenance headache.
2) The propeller rotation cannot be reversed without some form of transmission (most internal combustion engines can't run in reverse rotation).
3) The propeller thrust cannot be yawed (vectored), requiring the boat to maintain enough velocity through the water to achieve rudder steerage.
4) The propeller shaft (going through the hull) is vulnerable to being struck from behind -- turning it into a harpoon aimed at the engine.
5) Coupling of engine vibration to the hull cannot be completely eliminated because of the through-hull connection. (Also, anything that snags or suddenly stops the propeller will damage the engine or transmission because it is directly coupled).
6) You have to run the engine to get propulsion. In an emergency, Murphy's Law dictates that that is the most likely moment when the engine will fail to start. It's also difficult to silently sneak into a slip at 4 AM without waking your neighbors (or parents... I remember the night long ago when I decided to shoot into a slip under sail alone, just a little too fast, to avoid alerting my parents and... you can guess the results...).

Very few of these advantages are related to electric drive itself, which most often also has a traditional shaft. Most of them could be equally solved with a sail drive, which could rather easily be made turnable, if that would be necessary (like Volvo IPS).

A well isolated diesel is quite quiet. I think that the worst noise from my Yanmar comes from the beeper check before starting it.

Joakim

[FAST FRED]

For max cruising efficiency all one needs is a set of batterys that can accept a huge charge rate , and an alternator capable of charging at about 80% of the battery BANK capacity.

Either a bus alt, 12v 250 to 300A /or 24v same amps , with a set of AGM batts would fill the requirement.

The cost of std Wet cell batts and a std but oversized alt would have a longer life as a house set ,and lower overall costs per year , but the BIG DC setup could provide shorter charge times.

What are you willing to pay for added silence? Rational electric propulsion is still waiting to get to the drawing board.

FF

[westsail42]

Originally Posted by Joakim

Very few of these advantages are related to electric drive itself, which most often also has a traditional shaft. Most of them could be equally solved with a sail drive, which could rather easily be made turnable, if that would be necessary (like Volvo IPS).

The downside of saildrives is that you cant install them on all hull forms. Example, full keel forms may not be able to take a sail drive. At least not without considerable modification.

Originally Posted by Joakim

A well isolated diesel is quite quiet. I think that the worst noise from my Yanmar comes from the beeper check before starting it.

Joakim

Now that is saying quite a lot! Some of those (now older) Yanmars sounded like a chattery box of rocks!

[westsail42]

Originally Posted by FAST FRED

What are you willing to pay for added silence? Rational electric propulsion is still waiting to get to the drawing board.

FF

This is one of the most common misunderstandings about choosing D/E. No one should consider D/E for the "added silence" alone. If you are talking retrofitting a straight diesel setup, the costs and risks from adapting the necessary systems is too much.

But, considering D/E in the context of, and integrated with, the entire electrical power requirements of the entire boat, and all the geeky electrical doo-dads we put on our boats, I believe it makes sense. More efficient? not sure, but I think it has a shot.

I also think that an integrated D/E system has a chance of even simplifying boat systems overall, depending on how the design is approached.


As a concept, I think D/E is sound, but for me, the jury is still out on reliability of the current small boat D/E offerings.

[Joakim]

Originally Posted by westsail42

The downside of saildrives is that you cant install them on all hull forms. Example, full keel forms may not be able to take a sail drive.

Now that is saying quite a lot! Some of those (now older) Yanmars sounded like a chattery box of rocks!

What kind of electric drive can you install on hulls that can't have a sail drive?

I have a Yanmar 1GM10C. I don't think it is much different from 1GM from early 80's. It is noisy unless you have a good installation, which is easy to do with a sail drive. And the beeper is loud.

Joakim

[westsail42]

Originally Posted by Joakim

What kind of electric drive can you install on hulls that can't have a sail drive?

Any straight shaft installation could be retrofitted with diesel electric. There are a few makers out there (search for "ossa powerlite" or "emotion hybrids").

Originally Posted by Joakim

I have a Yanmar 1GM10C. I don't think it is much different from 1GM from early 80's. It is noisy unless you have a good installation, which is easy to do with a sail drive. And the beeper is loud.

Yeah, we have a mid-90's era 3GM straight shaft installation. It has been noisy since we installed it. And I installed sound insulation! In our case the engine mounts needed to be pretty stiff so I think much sound/vibration gets telegraphed to the hull.

Yes, Yanmar beepers are LOUD.

[Joakim]

Originally Posted by westsail42

Any straight shaft installation could be retrofitted with diesel electric.

Certainly, but then you still have many of your disadvantage items.

Joakim

[mp459]

With a saildrive, you still have the through-hull. There's an interest thread on the Volvo saildrive on a cruisers forum, bot I forget which. I like the idea, in principle, of an electric motor attached to the rudder, but I see other issues, including sizing a rudder to take the weight of the motor, sizing rudder fittings for the extra weight, and not least configuring a rudder with a bulge big enough at the end, plus a weird propeller protrusion, would be aesthetically weird. Most sailboat hulls taper off at the end, one way or the other. Also traditional hull forms encapsulate the propeller in an appeture to protect it.
I guess it's feasible to encapsulate a motor, up to say 6 inch diameter,in the rudder. Have the motor "face forward" (so the shaft comes out forward of the rudder). Then you could still put the propeller in the traditional aperture. Neat idea.

Another issue, of course, is that you now have a submerged motor. There's several ways of getting away with this, but they all add complexity.

However a lot of the drawbacks listed are still eliminated, even with simply replacing the engine with an electric motor:

2) we can reverse the motor without transmissions
4) "propeller shaft vulnerable to being stuck from behind": now sure how anything can eliminate this…except that with an electric motor, wherever it is placed, striking the shaft harms only the motor, not the engine (generator), batteries, etc.
5) we can better insulate the engine
6) we can propel with battery only
7) we can turn the prop "real slow".

and additionally,
- we can generate electricity using propeller drag
- we can "simplify" the boat systems, by looking at the propulsion as "just another load" on the electric system
- we can save some gas in the process
- we can move the engine anywhere
etc.

Y