Getting rid of alternator?

[lordgeoff]

Originally Posted by mcarthur

Question - any reason not to get rid of the propulsion engine alternators, and
Question - would I expect any fuel consumption benefit?

Our position is:

• 50' power cat
• 2 x 135hp NA diesels with original (old) mid-sized alternators
• 1 x 6.5kW diesel genset
• a 12v house bank of SLA batteries
• about 300w of solar
• no aircon, biggest draw is fridge/freezer, radar and autopilot when underway

We're eventually changing to:

• 3+kW of solar
• a 48v LiFEPO4 bank in preparation for hybridization (propulsion and house bank)
• a DC-DC converter 48->12v

Since travel won't be in 100% darkness , the solar will be the primary charging method for everyday living, replacing the alternators (when moving anywhere in daylight) and generator (at anchor).

(to the actual question!) But, since we're taking this in stages I think the first removal could be the two alternators - keeping the genset for now. Are there any reasons to keep the alternators?

As far as I can research using Dr Google, the alternators may be responsible for anything from 2-5+% of horse power from the main engines at cruising speed and some amount of torque. This is even if the batteries are fully charged (can anyone explain?). So by removing the two alternators, apart from not having to worry about two more belts, could I expect 2-5% better fuel consumption?

There have been some interesting posts which are inconclusive, but don't take into account large solar input (like Tanglewood's).

Your cat sounds like a heavy inefficient power cat? Check this link. Maybe a better starting point if you want efficiency.
https://www.sail-world.com/Australia/Foreign-Affair-departs/-12641?source=google

[AndyEss]

Originally Posted by Dockhead

If you didn't notice, his hybrid system will be "good for 6 to 6.5 knots", so he is definitely not replicating the power of the original propulsion system.


I guess you only need 20hp or so to drive an average 50' power cat at 6.5 knots, but you'd better stay away from lee shores in bad weather.

I have side-towed my roughly 30,000 lb, full keel, 38 ft monohull with my 5.5 hp outboard on my dinghy on occasion, similar to many boaters' experiences with uncooperative internal combustion engines.

The dinghy and outboard are necessary in and of themselves to a life on the water.

I don't plan on replacing my 50 hp engine with a 5.5 hp outboard anytime soon.

[Dockhead]

Originally Posted by AndyEss

. . . I don't plan on replacing my 50 hp engine with a 5.5 hp outboard anytime soon.

But just think how much fuel you will save! Motoring at 4 knots in glassy smooth water with that motor, compared to normal conditions with a normal engine . . .

[mcarthur]

Hmm, not heavy and inefficient according to my study of other long range power cats. But if you have some other facts that go further than a bald statement then I'm happy to read them ...

Dockhead, you're entirely right about the speed being lower than we currently get (well, if parallel hybrid then only in electric mode - it would be the same in diesel mode). However, reports from other (hybrid and) electric boats are that the "power" aspect at a particular speed is better, perhaps (!) due to the torque of the not-at-WOT electric engine. ie. the ability to punch through weather increased at the same rpm's.
Of course they are firsthand reports only, not measured values, but - forgive me a little and not aimed at you in particular - no one here has presented the slightest bit of evidence either way about many things being discussed other than "I think", "it must be so", "I'm sure", "my knowledge of X tells me that", "you must be mad", etc. I know I'm not on a science forum but the lack continues to surprise me .

As an example beyond the "I've got years of experience and it must be so" there's papers like this and even this. Of course I always have some doubts about privately published data where you can't see the assumptions and actual numbers, or that haven't been repeated and published by other teams, but at least they are out there with some facts. I wish the most experienced people on these forums would actually writeup their experience in ways that are factual - supported by evidence - because it would be so much easier to discuss: planting one's name against a statement and defending it is part of the concept of science and evidence in general (think cross examination in courts) versus yell and run. Rant mellowing slowly...

[CatNewBee]

Originally Posted by Dockhead

If you didn't notice, his hybrid system will be "good for 6 to 6.5 knots", so he is definitely not replicating the power of the original propulsion system.


I guess you only need 20hp or so to drive an average 50' power cat at 6.5 knots, but you'd better stay away from lee shores in bad weather.

50ft heavy power Cat (fuel, generator, batteries) , 6..6.5kn?

We have 40ft, to get us to 6kn in plain water using the engines we need at least 35hp one engine running on 80%, if thete is swell, wind or current, we need both engines at full speed to sustain 6.5kn, so 80hp.

A 50ft is bigger and has more windage, but a longer hull, that has some advantage regarding swell, I would say you will need 50...100hp to sustain 6...6.5kn, what translates to 38.. 76kW.

Now try to figure out what battery you will need to motor 1h at 6.5kn in dead flat and moderate conditions, it would be 38kWh to 76kWh. A 72V battery would have to have 1000Ah to provide 72kWh, it will be 20 1000Ah LFP cells, total weight 840kg, a generator for 76kW would hardly fit in a compartment of an Onan 8kW.

Fuel efficiency will be worse because of conversions and heat in contrast to using a 76kW diesel engine, or better 2 38kW diesel engines.

I know, you can cruise slower and at 3kn you can extend the range to 9nm under battery, in calm sea even 15nm. Granted. If you live on a small lake, very viable.

[Dockhead]

Originally Posted by mcarthur

Hmm, not heavy and inefficient according to my study of other long range power cats. But if you have some other facts that go further than a bald statement then I'm happy to read them ...

Dockhead, you're entirely right about the speed being lower than we currently get (well, if parallel hybrid then only in electric mode - it would be the same in diesel mode). However, reports from other (hybrid and) electric boats are that the "power" aspect at a particular speed is better, perhaps (!) due to the torque of the not-at-WOT electric engine. ie. the ability to punch through weather increased at the same rpm's.
Of course they are firsthand reports only, not measured values, but - forgive me a little and not aimed at you in particular - no one here has presented the slightest bit of evidence either way about many things being discussed other than "I think", "it must be so", "I'm sure", "my knowledge of X tells me that", "you must be mad", etc. I know I'm not on a science forum but the lack continues to surprise me .

As an example beyond the "I've got years of experience and it must be so" there's papers like this and even this. Of course I always have some doubts about privately published data where you can't see the assumptions and actual numbers, or that haven't been repeated and published by other teams, but at least they are out there with some facts. I wish the most experienced people on these forums would actually writeup their experience in ways that are factual - supported by evidence - because it would be so much easier to discuss: planting one's name against a statement and defending it is part of the concept of science and evidence in general (think cross examination in courts) versus yell and run. Rant mellowing slowly...

You and only you will decide what to believe.


My humble suggestion is merely to focus on the numbers and facts and separate out the marketing stuff -- because there are a lot of whoppers being told to potential buyers of this equipment.



The two papers you link to both look quite sound and objective to me. One talks about the efficiency of different types of alternators (efficiency in producing power, N.B.), and the other about the efficiency of DC generators compared to AC ones.


DC generators are definitely more efficient, and the main reason is not the small difference in efficiency at peak output, but how DC generators handle partial loads. DC generators are not required to run at a constant speed, so the engine speed can be adjusted to suit the load. Running a diesel engine at a constant speed and driving loads which are a fraction of the maximum power available at that speed is very inefficient, and can be harmful. This is the basic drawback of AC generators which you don't have with DC generators.


But this is not an advantage compared to mechanical drive, because with mechanical drive, you are also able to vary the engine speed, and a good propeller, especially a variable pitch one like a Brunton Autoprop, will keep the engine in the best part of the fuel map at a wide range of speeds.



And using a DC generator doesn't change the fact that you have conversion losses in hybrid drive during the two-way conversion from mechanical to electrical and back to mechanical power. If a vendor tells you you can make that up by operating the diesel in a better part of the fuel map, ask him for hard data on that. It's not really possible, and the data is available, so you can see for yourself.


Concerning torque vs. horsepower -- this is much misunderstood. Horsepower is the work done by the engine. You can call it kilowatts if you like -- these are just different units for the same concept. A kilowatt is a kilowatt -- there are not different types of them.


Torque is the force exerted, as opposed to work. Horsepower (or kilowatts) is a direct function of torque x RPM. So if you have two engines with the same horsepower but one has more torque than the other -- these two engines have the same ability to do work -- because that's what horsepower IS. The only difference is one of them does it at a lower RPM.


Whether that is of any value or not in marine propulsion depends. The larger and slower turning a propeller is, the more efficient it is, and the difference in efficiency can be large. For a sailboat, there is rarely any chance to use a much larger propeller. For a power boat, there might be.


But with similar propellers, there is not much advantage to a high torque motor of the same horsepower. At the end of the day, it is horsepower and only horsepower, not torque, which does the work of moving your boat through the water. And a high torque motor with LESS horsepower, can simply do less work, period.





With hybrid propulsion, expect to use more, not less fuel, to motor at the same speed in the same conditions, using power generated with even the best DC generator. If you use a much smaller electric motor and travel at lower speeds, you will save fuel, but you would save even more fuel by using a diesel engine the same amount smaller, with direct drive.


I have a friend with a 60 meter motor yacht with hybrid propulsion. The drive system cost millions of dollars. It uses about 15% more fuel than a comparable direct drive vessel -- operated at the same speeds and the same conditions. I think that will be pretty typical.



Hybrid propulsion can make a lot of sense if you have a source of power which is NOT a diesel engine -- for example, a day tripper boat with a huge lithium battery bank, which can plug in every night. Or a small sailboat that really doesn't use the motor for anything but getting in and out of the dock.



But don't take my word for it -- do the numbers. The numbers don't lie.

[Dockhead]

Originally Posted by CatNewBee

50ft heavy power Cat (fuel, generator, batteries) , 6..6.5kn?

We have 40ft, to get us to 6kn in plain water using the engines we need at least 35hp one engine running on 80%, if thete is swell, wind or current, we need both engines at full speed to sustain 6.5kn, so 80hp.

A 50ft is bigger and has more windage, but a longer hull, that has some advantage regarding swell, I would say you will need 50...100hp to sustain 6...6.5kn, what translates to 38.. 76kW.

Now try to figure out what battery you will need to motor 1h at 6.5kn in dead flat and moderate conditions, it would be 38kWh to 76kWh. A 72V battery would have to have 1000Ah to provide 72kWh, it will be 20 1000Ah LFP cells, total weight 840kg, a generator for 76kW would hardly fit in a compartment of an Onan 8kW.

Fuel efficiency will be worse because of conversions and heat in contrast to using a 76kW diesel engine, or better 2 38kW diesel engines.

I know, you can cruise slower and at 3kn you can extend the range to 9nm under battery, in calm sea even 15nm. Granted. If you live on a small lake, very viable.

You need 35hp for 6 knots? I'm surprised at that. Are you sure you're not looking at the max power curve vs. the propeller power curve? Max power curve is the maximum power the engine can make at the flywheel, at a given RPM; propeller power curve shows the actual power taken from the engine by an ideal theoretical propeller.



In a dead calm, glassy sea, my boat will make 6 knots at 1500 RPM or so. The propeller power curve for my engine (a 100hp Yanmar) only starts at 2000 RPM, and it's 15hp. And I'll bet my boat is heavier than yours.



I have a variable pitch prop, so maybe I'm using more power than the theoretical propeller power curve indicates, but I doubt that even max power at 1500 RPM of my engine is much more than 20hp.

[CatNewBee]

Originally Posted by Dockhead

You need 35hp for 6 knots? I'm surprised at that. Are you sure you're not looking at the max power curve vs. the propeller power curve? Max power curve is the maximum power the engine can make at the flywheel, at a given RPM; propeller power curve shows the actual power taken from the engine by an ideal theoretical propeller.



In a dead calm, glassy sea, my boat will make 6 knots at 1500 RPM or so. The propeller power curve for my engine (a 100hp Yanmar) only starts at 2000 RPM, and it's 15hp. And I'll bet my boat is heavier than yours.



I have a variable pitch prop, so maybe I'm using more power than the theoretical propeller power curve indicates, but I doubt that even max power at 1500 RPM of my engine is much more than 20hp.

I assume the electric motor would use a propeller too to transfer the power into motion through the water with similar characteristics.

My indication indeed are the engine rpm translated to kW output. We have folding props sized to the engines power.

In dead calm sea at 1500rpm we make 4.2kn with one 40hp Yanmar engine (at 3000rpm) . It is a catamaran, 40ft, two hulls more friction with the water, not comparable to a monohull of 40ft.

[Dockhead]

Originally Posted by CatNewBee

. . . In dead calm sea at 1500rpm we make 4.2kn with one 40hp Yanmar engine (at 3000rpm) . It is a catamaran, 40ft, two hulls more friction with the water, not comparable to a monohull of 40ft.

Obviously you will know better, but that sounds weird to me. Maybe prop is not pitched right? Catamaran has some extra skin friction, but it's still related to displacement. Lagoon 40 is like 12 tonnes, light ship. Not going to be more skin friction than my 54' mono at 20 tonnes, light ship. In a dead calm I guess I could side tow my boat at 4.2 knots, with my 3.5hp outboard.

[transmitterdan]

Originally Posted by Dockhead

Obviously you will know better, but that sounds weird to me. Maybe prop is not pitched right? Catamaran has some extra skin friction, but it's still related to displacement. Lagoon 40 is like 12 tonnes, light ship. Not going to be more skin friction than my 54' mono at 20 tonnes, light ship. In a dead calm I guess I could side tow my boat at 4.2 knots, with my 3.5hp outboard.

Dockhead,

It seems impossible to push a 54' 20 tonne boat at 6.5KN with 15HP at the engine. Our 42' 13 tonne boat can barely make 6.5KN with 25HP. Our LWL is much less than yours but displacement also comparably less.

[a64pilot]

I believe it’s the autoprop. Mine anyway and I only have that one to go by seems to load the engine pretty heavy at low RPM, which is good and great for motor sailing.
Loading an engine heavier than what a fixed prop that is chosen to be correct at full throttle does is more along the ideal loading for an engine.
Take a manual transmission car, your cruise gear is quite high and to accelerate you down shift to a much lower ratio.
An autoprop actually operates similarly, during cruise speed through the water drives the prop to quite a high pitch, accelerate the motor and centripetal forces drive the prop to much lower pitch.

In other words I don’t think the thrust output of an Autoprop follows the theoretical curve. It’s very likely using a whole lot more engine power in sustained cruise at lower RPM.
The theoretical curve is a prop that is only correct at full throttle, it’s incorrect at every other throttle setting, the further from full throttle, and the less likely for it to be correct.

The reason why so many people seem to do fine that are way over-propped is that they never go to full throttle, so at their normal cruise speed the engine is fine, it doesn’t get overloaded until the RPM is increased.
An Autoprop from my observation acts at partial throttle once at cruise speed to be over-propped, so at say 1500 RPM a boat will be moving quite a bit faster as the motor is quite a bit more loaded.

[Dockhead]

Originally Posted by transmitterdan

Dockhead,

It seems impossible to push a 54' 20 tonne boat at 6.5KN with 15HP at the engine. Our 42' 13 tonne boat can barely make 6.5KN with 25HP. Our LWL is much less than yours but displacement also comparably less.

Well, there are a couple things at play here --


First of all, depending on your LWL, you might need more power to push 13 tonnes at 6.5 knots than we need to push 20 tonnes. Because at 6.5 knots we are 3 knots below hull speed and so almost no wave-making resistance.


Second, where do you get that you're using 25hp? Does that come from your propeller power curve? Be sure it's not from the max power curve, which has nothing to do with it. Even if that's what you get from your propeller curve, remember, that's a theoretically IDEAL propeller, and yours may be pitched less than that.


I don't really know for sure how much power I'm using because I have a variable pitch prop, so will be likely using MORE power than you get from the propeller power curve. Nor do I know anything about 1500 RPM, because all the curves for my engine stop at 2000RPM. https://www.simplicity-marine.com/De...0)TechData.pdf


But 2000 RPM is about 40hp max at the shaft, and 15hp on the propeller power curve. I would say 15 hp at 1500 RPM is a reasonable, conservative guess.


Remember we're talking about glassy flat water and no wind. The slightest bit of sea state dramatically increases the power you need. And THAT gets us back to the OP -- something he needs to be careful with, if he's planning to radically downsize his propulsion engines.

[Q Xopa]

Originally Posted by mcarthur

Hmm, not heavy and inefficient according to my study of other long range power cats. But if you have some other facts that go further than a bald statement then I'm happy to read them ...

Dockhead, you're entirely right about the speed being lower than we currently get (well, if parallel hybrid then only in electric mode - it would be the same in diesel mode). However, reports from other (hybrid and) electric boats are that the "power" aspect at a particular speed is better, perhaps (!) due to the torque of the not-at-WOT electric engine. ie. the ability to punch through weather increased at the same rpm's.
Of course they are firsthand reports only, not measured values, but - forgive me a little and not aimed at you in particular - no one here has presented the slightest bit of evidence either way about many things being discussed other than "I think", "it must be so", "I'm sure", "my knowledge of X tells me that", "you must be mad", etc. I know I'm not on a science forum but the lack continues to surprise me .

As an example beyond the "I've got years of experience and it must be so" there's papers like this and even this. Of course I always have some doubts about privately published data where you can't see the assumptions and actual numbers, or that haven't been repeated and published by other teams, but at least they are out there with some facts. I wish the most experienced people on these forums would actually writeup their experience in ways that are factual - supported by evidence - because it would be so much easier to discuss: planting one's name against a statement and defending it is part of the concept of science and evidence in general (think cross examination in courts) versus yell and run. Rant mellowing slowly...

Parallel hybrid. Is this what you are looking at doing?
Im interested to hear more please.

[az_r2d1]

Not sure where you travel but the more redundancy on a boat the better. It would be foolish to get rid of the genny or the alternators. You will save none by removing.

[Cheechako]

If your solar is working, then your alternator is not kicking in, so taking very little energy.
Your assumption seems to be the solar is working, so why take the alternator off?