What is the most environmentally friendly propulsion system?

[dolphin300]

Which is better: a 4-stroke outboard or a diesel inboard?

Is an above the water air-exhaust better than an underwater exhaust, if not for the passengers for the environment? Are converters available to clean up the exhaust?

What about electric? With the double conversion and the extra weight, more overall energy is used plus the dangerous materials used to make the batteries - is the powerplant generating the electricity much cleaner than a small marine engine can be made to be?

[tspeer]

Sail.

[BrettM]

how about an oar?. Biodegradeable of course :)

[Portager]

"Which is better: a 4-stroke outboard or a diesel inboard?"

The diesel inboard because it will burn 1/3 to 1/2 as much fuel to produce the same amount of power.

"Is an above the water air-exhaust better than an underwater exhaust, if not for the passengers for the environment? Are converters available to clean up the exhaust?"

The amount of pollutants produced is independent of the type of exhaust. If properly designed both are equally acceptable for the occupants. The EPA is instituting new environmental requirements for diesels and outboards starting with large engines and working their way down. These regulations have already produced significant improvements. The best you can do for the environment is to get a new electronically controlled engine and maintain it properly.

"What about electric? With the double conversion and the extra weight, more overall energy is used plus the dangerous materials used to make the batteries - is the power plant generating the electricity much cleaner than a small marine engine can be made to be?"

If the electricity is generated by natural gas or nuclear it is better. If it is produces by coal it is worse. The power plant fuel conversion efficiency is typically 40%. The electricity generation and usage efficiencies are about 90% each. Battery charge conversion efficiency is 85% for flooded lead acid batteries, 90% for Gel cells and 95% for absorbent glass mat (AGM) batteries. The net energy conversion efficiency is 28%, 29% and 31% respectively. For diesel engines the fuel conversion efficiency is about 34%.

Cheers;
Mike Schooley

[duluthboats]

What about fuel cell to electric. With a hydrogen fuel cell the waste is heat, water and oxygen. :D
Gary

[Willallison]

Yoy guys are all forgetting Trouty's Neutron Drive!:D :D :D

[Willallison]

Originally posted by Portager
The diesel inboard because it will burn 1/3 to 1/2 as much fuel to produce the same amount of power.


This ignores the difference in weight between the two powerplants. Our 225hp Optimax's can return over 6 nmpg at around 6-7 knots - and that's on a heavy, deep v runabout - hardly the most efficient displacement hull shape. I'm sure that a small diesel could probably better this - but it wouldn't push the boat to 60 knots!
So, I think it really depends on what the application is.

[Portager]

Currently available fuel cells run on pure hydrogen. This is fine is you plan to tow the Hindenburg behind your boat, but I hope you don't intend to go up wind or smoke.;)

To run a fuel cell on higher density fuel you need a reformer. Today, most reformers use natural gas (still not dense enough to fit much fuel in the tank). They are still working on reformers for gas and diesel. In addition to hydrogen gas, reformers produce CO2, CO, NOx and waste heat.:(

Cheers;
Mike Schooley

[lockhughes]

Compromise? Have read some articles about powering an permanent magnet brushless electric motor with a smaller gas or diesel generator. Motor can have maybe one moving part (direct drive to the prop) and can be 80% efficient or more. Genny avoids water cooling, etc and are typically far more efficient (than gas/diesel engines) also...

Lock

[Portager]

What drives the generator? Don't you still need a gas or diesel engine to power the generator? If so then you are starting with the efficiency of the gas or diesel engine and then reducing it by the efficiency of the generator and then reducing that by the efficiency of the motor.

It is like pumping water through a leaky pipe. If you loose a percentage of the water along the way you can't get out more than you put in. In fact you will always get out less than you put in. The way to minimize the leakage is to shorted the pipe. In the case of the drive system the way to minimize the power losses is to eliminate the inefficiencies. This usually means connecting the power source directly to the prop.

The reason there is no enforcement of the laws of physics is because nobody has figured out how to break them yet.:D

Cheers;
Mike Schooley

What drives the generator? Don't you still need a gas or diesel engine to power the generator?
Yup... Dolphin300 didn't specify max.speeds or operating distances, etc. So I was *guessing* he'd want something with more range than just batteries?

If so then you are starting with the efficiency of the gas or diesel engine and then reducing it by the efficiency of the generator and then reducing that by the efficiency of the motor.
You're right. It's just that, as I understand it so far, the newest permanent magnet electric motors are hitting efficiencies something like 80-90% or better. And the gennys are also more efficient than electric or diesel engines - a lot. So combine the two, and you're still well ahead of the straight water-cooled
"traditional" gas/diesel engine?

It is like pumping water through a leaky pipe. If you loose a percentage of the water along the way you can't get out more than you put in. In fact you will always get out less than you put in. The way to minimize the leakage is to shorted the pipe. In the case of the drive system the way to minimize the power losses is to eliminate the inefficiencies. This usually means connecting the power source directly to the prop.
Agree totally Mike. One obvious power loss is vibration and noise - the newer gennys can be whisper-quiet. Electric motors also. So obviously they're more efficient in this regard alone?

Here's an article (sorry, PDF-format) that highlights the inefficiencies of the diesel engine:

http://www.solomontechnologies.com/pdf/Electric%20Wheel.pdf

Turns out they're *so* bad, it's not hard to come up with something better! There are a lot of criteria that might determine what propulsion system is best, but the original post
specified "Environmentally Friendly" as the acid-test. I think Tom had it right with SAIL <grin>... But that doesn't seem to be an option for Dolphin300. (My little boat is sail-only and regularly hits 15-20+kts, but Dolphin may not wish to get so wet?

The reason there is no enforcement of the laws of physics is because nobody has figured out how to break them yet. Cheers;
Mike Schooley

Hehe... true, true. I'm liking the electric approach more and more though, as fuel cell, capacitor and battery technology, and wind and solar generators appear to improve and become more viable over time. I don't see the gas/diesel approach improving much more in the future. The genny suggestion was sort of a short-term solution?

I read your earlier post about the efficiencies of electric power-generating stations, which I'm sure has to be factored into the equation. I'd be *hoping* to recharge on wind and solar as much as possible. Electricity from shoreside fuel cells (electricity from gases) as well, at some point in the future.

Lock

http://groups.yahoo.com/group/QCYCTender/

[Portager]

Lock;

I read "The Electric Wheel" article by David E. Tether with amusement and disbelief. David has a valid point regarding the torque versus speed curve. Some electric motors have a flat torque versus speed curve which makes them very effective at starting up heavy loads. This is why we have diesel-electric locomotives and tug and it explains why his sailboat conversion to electric was successful.

As David pointed out the main advantage of an electric sailboat is the ability to use the prop and electric motor as a generator to recharge the batteries.

The discussion regarding parasitic losses is less technically founded. While it is true that noise is a type of wasted energy, the energy density of noise isn't very high. If it were, I could solve the worlds energy problems by connecting a microphone to my kids. The other parasitic losses that he mentioned, such as oil and coolant pumps, are already taken into account in the engine power rating. By deducting them again he is double booking and inflating them to boot.

On a diesel-electric system the generator and electric motor take the place of the transmission. Boat transmissions typically have a power transmission efficiency of 98%. David claims that the efficiency of their motor is 94%. If the generator is also 94%, then the net transmission efficiency is 0.94 X 0.94 = 88.36%. In my book 98% beats 88.36%.

David's discussion on generators is well I'm trying to be nice, so ... Apparently David is talking about AC generators since he states that they run at constant speed. The problem with AC generators is they must run at constant speed to produce AC power at the right frequency (i.e. 60 Hz or 50 Hz). The thing about engines is they provide their best efficiency when speed is allowed to vary with the load. Running an engine at high speed and low load is very inefficient and hard on the engine. An AC generator is the wrong choice for a diesel-electric boat because you would then need to convert the power from AC to DC which requires an inverter. Inverters are typically about 85% efficient. A much better choice is a DC generator. DC generators are more efficient that AC and they can run the permanent magnet electric motor and recharge the batteries directly. In addition, a DC generator can adjust its speed with the DC load and operate more efficiently and at low loads it is much quieter.

Cheers;
Mike Schooley

[trouty]

:rolleyes: Neutron Drive...sheesh - can't you guys git anything right?:D

I'll say it agin fer ya's so listen up n pay attention coz theres a test on this at the end OK?;)

Now this systems not to be confused with the Fractored Amplitude Reactive Thermistor (FART) drive....OK?

Far from just sounding the part - my system has the guts to leave skid marks in your undies for real.. :cool:

See if you can get this down in one go...

Powered Obtuse Overunity Plasmoidal Induction Scalar Electromagnetic Hydrodynamic Caterpillar Drive. (Poop for short :cool: )

Heres a little schematic and notes to help you all get the hang of this.

Internal wave structure of the scalar potential
http://www.cheniere.org/techpapers/Final%20Secret%209%20Feb%201993/fig1a.jpg

Internal wave structure of the scalar potential (end)
http://www.cheniere.org/techpapers/Final%20Secret%209%20Feb%201993/fig1b.jpg

The overunity principle, the secret of extracting and using free energy!

http://www.cheniere.org/techpapers/Final%20Secret%209%20Feb%201993/fig2.jpg

Too simple really - just hook up the scalar electromagnetic collection overunity device http://www.prahlad.org/pub/bearden/megtitl.jpg to my hydrodynamic, plasmoidal induction drive system, and jacks your aunty, (at least she would be if she had balls) - free energy drive system.

Yup yup yup - this is the most environmentally friendly propulsion system known to man or beast...

Just one problem tho...

The men in black!:rolleyes:

Sneakin it past them takes a whole lotta doing...Let me tell you guys that the Bush oil cabal running this world today aren't too pleased at all about scalar electromagnetics...heck - they arent even one pleased!:p

Luckily - I have Jake and Elwood workin on that as I type!

Lifes pretty straight without a twistee eh?:D

Cheers!

[lockhughes]

Originally posted by Portager
Lock;
I read "The Electric Wheel" article by David E. Tether with amusement and disbelief.
oh boy... ok Mike, burst my bubble <grin>

David has a valid point regarding the torque versus speed curve. Some electric motors have a flat torque versus speed curve which makes them very effective at starting up heavy loads. This is why we have diesel-electric locomotives and tug and it explains why his sailboat conversion to electric was successful.
*some* have flat curves, not all? Don't know what the variations are, but it yeah, I gathered that is a plus for marine propulsion - at least, the kind involving heavy, displacement craft.

As David pointed out the main advantage of an electric sailboat is the ability to use the prop and electric motor as a generator to recharge the batteries.

Yup. got that. I'm interested in a electric-only craft myself. Somehow your post on the EWheel ended up in this thread (most environmentally friendly), and I'm guessing Dolphin isn't looking at the sail option either <sigh>

The discussion regarding parasitic losses is less technically founded. While it is true that noise is a type of wasted energy, the energy density of noise isn't very high. If it were, I could solve the worlds energy problems by connecting a microphone to my kids.
hehe... Makes sense. Though I must say, our old club tender (diesel) shakes and shudders. Noisy, yeah, but the vibrations are *huge*, now that folks like David T. make me think about this stuff. I'm sure the new (diesel) craft our Club is getting will be a whole lot better behaved, but if I look in the engine compartment and see really big motor mounts/vibration dampeners... well, that'll tell me something right there!

The other parasitic losses that he mentioned, such as oil and coolant pumps, are already taken into account in the engine power rating. By deducting them again he is double booking and inflating them to boot.
Darn. Should try and get David T. aboard here - give him a chance to refute (elaborate? obfuscate?<wink>)

On a diesel-electric system the generator and electric motor take the place of the transmission. Boat transmissions typically have a power transmission efficiency of 98%. David claims that the efficiency of their motor is 94%. If the generator is also 94%, then the net transmission efficiency is 0.94 X 0.94 = 88.36%. In my book 98% beats 88.36%.
wait a sec Mike. Shouldn't you be factoring in the efficiencies of the deisel engine into the 98% side of the equation? All that stuff about driving alternators and water pumps, whatever?

David's discussion on generators is well I'm trying to be nice, so ... Apparently David is talking about AC generators since he states that they run at constant speed. The problem with AC
-SNIP-
generator can adjust its speed with the DC load and operate more efficiently and at low loads it is much quieter.

OK, now I'm going to go away and try and read up a bit on generators. Not my primary interest (sail) or relevant to my little project on a club tender (plugged into shorepower/solar/wind),
but might end up being a factor for the tender anyway... thanks
Lock

[Portager]

Originally posted by lockhughes

oh boy... ok Mike, burst my bubble <grin>

Sorry to burst your bubble, but reality can be soooo confining. Where is that portal to the alternate dimension where everything works the way I want it to. O yea, its called dreaming.;)


*some* have flat curves, not all?

Suffice to say that the permanent magnet motors that Solomon Technologies is using have a flat torque speed curve over the range of speeds they are using.


I'm interested in a electric-only craft myself. Somehow your post on the EWheel ended up in this thread (most environmentally friendly), and I'm guessing Dolphin isn't looking at the sail option either <sigh>

The question I was responding to wasn't specific to electric only propulsion. Electric only is OK for short range applications. My earlier post comparing the efficiency of a diesel versus electric using shore power would apply. Due to the inefficiencies involved the diesel is actually slightly higher efficiency that the electric unless you have access to a clean source of power such as hydro-electric, ... The other advantage of the electric option is noise and maintenance (until you have to replace the batteries).


Makes sense. Though I must say, our old club tender (diesel) shakes and shudders. Noisy, yeah, but the vibrations are *huge*, now that folks like David T. make me think about this stuff. I'm sure the new (diesel) craft our Club is getting will be a whole lot better behaved, but if I look in the engine compartment and see really big motor mounts/vibration dampeners... well, that'll tell me something right there!

Modern "light weight" diesels have a problem with vibration. To lighten the engine the manufacturers have lightened the flywheel and greatly reduced the angular moment of inertia. Since 4 cycle engines (most diesels are 4 cycle now days) only fire every other revolution, they have power pulses every 720 degrees/number of cylinders, for example a single cylinder gets a power pulse every other revolution, a 2 cylinder gets a pulse once per revolution, a 4 cylinder gets a pulse every 180 degrees, ... Since each power pulse is essentially a half sine wave, the 4 cylinder will have almost a continuous sine wave power curve going from 0 to max and back to zero every 180 degrees. In a 6 cylinder the power pulses start to overlap and in an 8 cylinder they almost level out. To get a smooth running diesel you want as many cylinders as possible. If that isn't possible, then a high inertia flywheel id the next best thing. So, if your new boat runs too rough get a higher inertia flywheel.


wait a sec Mike. Shouldn't you be factoring in the efficiencies of the diesel engine into the 98% side of the equation? All that stuff about driving alternators and water pumps, whatever?

If we are comparing diesel-electric drive to diesel-mechanical drive then both systems have the same diesel engine driving them and that cancels out. The generator and motor are just an alternative to a transmission.

If you want to compare a diesel-mechanical drive to an battery driven electric drive using shore power then it is a slightly different matter. The shore power generator is slightly higher efficiency (~40%), you have to factor in transmission losses (depends on the distance to the power plant optimistically 0.99 ), battery charger efficiency (multiple by 0.85), battery charge efficiency (multiply by 0.95) and then electric motor efficiency (multiply by 0.94) Net result is 30%. The diesel engine is slightly lower efficiency (~34% which includes the parasitic losses for the coolant and oil pumps) and then you must deduct parasitic losses for the alternator (varies from 0.95 right after start-up to 0.98 once the batteries are charged), transmission losses (multiply by 0.98). Net result is 31.6% to 32.6%.

If I broke your bubble I am sorry, but reality made me do it!

Cheers;
Mike Schooley

[Jeff]

A couple on topic links:

diesel electric propulsion is superior to conventional diesel propulsion for displacement super yachts. <edit: link removed due to site being down>
by Dick Boon and Christian Poorte of Vripack Yachting International Naval Architects B.V. Holland. It will be argued that for various important reasons diesel electric propulsion is to be preferred, by the author, to conventional diesel propulsion. Displacement type super yachts represents at least 95% of the world fleet of super yachts. Concern about complexity, safety, maintenance, efficiency, weight and price have tended to support the "conventional diesel propulsion is best" theory. In this article we will compare diesel electric propulsion and conventional diesel propulsion for displacement type super yachts.

And I'll also insert a link to the thread on Solomon Technologies - "Electric Wheel" electric motor propulsion systems (http://forums.boatdesign.net/showthread.php?s=&threadid=676) since the threads might get separated at a later date.

[Ted1785]

Have you considered pedal propulsion. Good for the environment and good for your exercise.
Ted1785

[Al Aqsa]

Funniest thing I have read in a long time.

Cheers.

:rolleyes: Neutron Drive...sheesh - can't you guys git anything right?:D

I'll say it agin fer ya's so listen up n pay attention coz theres a test on this at the end OK?;)

Now this systems not to be confused with the Fractored Amplitude Reactive Thermistor (FART) drive....OK?

Far from just sounding the part - my system has the guts to leave skid marks in your undies for real.. :cool:

See if you can get this down in one go...

Powered Obtuse Overunity Plasmoidal Induction Scalar Electromagnetic Hydrodynamic Caterpillar Drive. (Poop for short :cool: )

Heres a little schematic and notes to help you all get the hang of this.

Internal wave structure of the scalar potential
http://www.cheniere.org/techpapers/Final%20Secret%209%20Feb%201993/fig1a.jpg

Internal wave structure of the scalar potential (end)
http://www.cheniere.org/techpapers/Final%20Secret%209%20Feb%201993/fig1b.jpg

The overunity principle, the secret of extracting and using free energy!

http://www.cheniere.org/techpapers/Final%20Secret%209%20Feb%201993/fig2.jpg

Too simple really - just hook up the scalar electromagnetic collection overunity device http://www.prahlad.org/pub/bearden/megtitl.jpg to my hydrodynamic, plasmoidal induction drive system, and jacks your aunty, (at least she would be if she had balls) - free energy drive system.

Yup yup yup - this is the most environmentally friendly propulsion system known to man or beast...

Just one problem tho...

The men in black!:rolleyes:

Sneakin it past them takes a whole lotta doing...Let me tell you guys that the Bush oil cabal running this world today aren't too pleased at all about scalar electromagnetics...heck - they arent even one pleased!:p

Luckily - I have Jake and Elwood workin on that as I type!

Lifes pretty straight without a twistee eh?:D

Cheers!

[Wardi]

Assuming you want to carry a certain load at some average speed, (unstated) then a sailing vessel will need a mast, rigging, keel, ballast, appendages and of course sails.
If you add up the extra weight, costs and environmental footprint of that, it may not be any more efficient than a diesel engine. Furthermore, even if the wind is free, the sails wear out and need to be replaced.

A simple calculation for a 6ton diesel powered launch of efficient displacement design comes out at about the same cost as a 6ton sail powered yacht, if the launch runs at an average of 10kts or less.

Even a rowing boat uses energy from food which needs to be grown and transported etc. This is certainly not pollution free either. It begs the question, "What is the most efficient conversion system?"

It would be really interesting to look at the total environmetal footprint and costs for the various methods of propulsion based on moving a given mass at an average speed by water. Anyone looked at this??

[John Perry]

I find that hard to believe but I dont have the figures to disprove it. I rather wonder whether this is a comparison between the fuel cost of the motor launch vs. the replacement of sails on the sailing boat, forgetting that the whole power plant of the motor boat also has a limited life, as do the hulls of both boats. But a reasonably effective sailing boat can if necessary be made almost entirely from renewable resources such as wood, flax etc which is probably not possible with a diesel boat.
John

[Portager]

But a reasonably effective sailing boat can if necessary be made almost entirely from renewable resources such as wood, flax etc which is probably not possible with a diesel boat.
John

An example would be a wooden hull Chinese Junk with stone ballast, unstayed wooden mast, silk “junk” sails and bamboo battens.

Regards;
Mike Schooley

[Wardi]

Hello John,
The main part of the calculation is based on sail costs vs fuel costs. I agree with you that the engine etc are not constructed of renewable resources, however neither are most components of a sailing craft these days ie: fibreglass hull, aluminium mast, synthetic sails etc.

The point I wanted to make is that to make a proper comparison, the total costs need to be taken in to account. Some factor for environmental costs also needs to be addressed. I know this is not an easy thing to do, but I think it would be worthwhile.

I would consider the issue you raised about building from renewable materials as important, but a separate issue, as most people do not do this these days. One reason is cost and performance, but another is serviceability and longivity. This should also be analysed, perhaps we are not really getting value for money?.

Does anywone know of such an analysis being done?

[Willallison]

I recall reading an article not so long ago which broadly covered this very subject. It too suggested that if you take into account all the costs involved in building and running a passagemaker, then power often comes out cheaper. Some of the points made included:
The structure required to support masts, sails, rigging etc are more expensive to build.
Sailboats, still require at least one engine - and this tends to get used quite a bit. As they tend to be smaller, they are run harder and tend to wear out quicker.

Not sure, but the suggestion may have come from something on Dashew's "Unsailboat"...

[joedupont]

Using a battery or a compressed air tank to capture some of the sail power through as propeller is an excellent idea.
Sail boats are beautiful things because of their range.
if you used compressed air. to capture power you would also have a source of "fresh" water.. as in humid conditions air compressors take water out of the air. If done right.. with the right compressor.. that would be very interesting.


Lock;

I read "The Electric Wheel" article by David E. Tether with amusement and disbelief. David has a valid point regarding the torque versus speed curve. Some electric motors have a flat torque versus speed curve which makes them very effective at starting up heavy loads. This is why we have diesel-electric locomotives and tug and it explains why his sailboat conversion to electric was successful.

As David pointed out the main advantage of an electric sailboat is the ability to use the prop and electric motor as a generator to recharge the batteries.


The discussion regarding parasitic losses is less technically founded. While it is true that noise is a type of wasted energy, the energy density of noise isn't very high. If it were, I could solve the worlds energy problems by connecting a microphone to my kids. The other parasitic losses that he mentioned, such as oil and coolant pumps, are already taken into account in the engine power rating. By deducting them again he is double booking and inflating them to boot.

On a diesel-electric system the generator and electric motor take the place of the transmission. Boat transmissions typically have a power transmission efficiency of 98%. David claims that the efficiency of their motor is 94%. If the generator is also 94%, then the net transmission efficiency is 0.94 X 0.94 = 88.36%. In my book 98% beats 88.36%.

David's discussion on generators is well I'm trying to be nice, so ... Apparently David is talking about AC generators since he states that they run at constant speed. The problem with AC generators is they must run at constant speed to produce AC power at the right frequency (i.e. 60 Hz or 50 Hz). The thing about engines is they provide their best efficiency when speed is allowed to vary with the load. Running an engine at high speed and low load is very inefficient and hard on the engine. An AC generator is the wrong choice for a diesel-electric boat because you would then need to convert the power from AC to DC which requires an inverter. Inverters are typically about 85% efficient. A much better choice is a DC generator. DC generators are more efficient that AC and they can run the permanent magnet electric motor and recharge the batteries directly. In addition, a DC generator can adjust its speed with the DC load and operate more efficiently and at low loads it is much quieter.

Cheers;
Mike Schooley

[Portager]

Will;

You’re right, the Dashew’s provided a good comparison of the cost of sailing versus the cost of operating a power boat at http://www.setsail.com/dashew/Operating_Costs.html , however the Dashew’s were into high performance sailing and used high priced sails and rigging. A more cost effective (and lower performance) rig like a Junk sail and an unstayed mast would be much more cost effective and may switch the cost comparison to the sailboats advantage.

One problem with using the prop while underway to recharge the batteries is you can only rechange while underway and the wind is right. I also wonder what the efficiency of the propeller is at producing power. An alternative might be to put a wind mill on the boat. Then you could charge at anchor if there is enough wind.

Joedupont;

I think you would be much better off sticking with batteries instead of compressed air. Batteries have a much higher energy density than compressed air and compressed air tanks can be hazardous if they rupture under pressure.

The condensation water from most air compressors is not palatable. The air/water has trace amounts of petroleum lubricants which should not be ingested. It would be easier to get drinking water from the ait conditioner, but it is hard to keep salt out of that.

Regards;
Mike Schooley

[mattotoole]

As David pointed out the main advantage of an electric sailboat is the ability to use the prop and electric motor as a generator to recharge the batteries.

Well, it's a nice benefit, but not the only one.

It's a very elegant solution for a cruising catamaran, because it eliminates half the mechanical worries -- by replacing two diesels and transmissions with one diesel genset and two electric motors. Plus those motors are likely to be more reliable than any boat transmission.

The greater sail horsepower and speed of a big cat really makes power generation under sail practical. Often the problem with these boats is slowing them down!

[FAST FRED]

A steam engine could be run on chipped yard waste.

Free fuel (for taking it away ) and burning it only oxadizes it faster than rotting on the forrest floor.

Steam has come a long way in the past few hundred years, so you get heat and a gen set power source too.

Steam makes max torque at 0 rpm but would still be efficent at very low rpm , you will either need a big dia prop or a stern wheeler.

FAST FRED