Getting rid of alternator?

[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).

[stormalong]

I wouldn't do that. Get rid of the generator. The weight savings would offset any possible fuel savings from getting rid of the alternators.

Ask yourself this: If my heaviest power draw is refrigeration and my battery bank can easily supply that power through the night why do I need to run a generator?

Your alternators only consume horsepower when they are providing power to charge batteries and run your electric loads. If your solar keeps the power up their is only a slight additional load from the alternators -probably not measurable.

On the other hand a generator requires maintenance and fuel, heats up the boat and makes noise when you are trying to relax.

Whatever you do, do not rely on a single power source. Anything can and will fail. Redundancy of two engine driven alternators plus the solar means that any single point of failure is no biggie.

[Dockhead]

Originally Posted by stormalong

I wouldn't do that. Get rid of the generator. The weight savings would offset any possible fuel savings from getting rid of the alternators.

Ask yourself this: If my heaviest power draw is refrigeration and my battery bank can easily supply that power through the night why do I need to run a generator?

Your alternators only consume horsepower when they are providing power to charge batteries and run your electric loads. If your solar keeps the power up their is only a slight additional load from the alternators -probably not measurable.

On the other hand a generator requires maintenance and fuel, heats up the boat and makes noise when you are trying to relax.

Whatever you do, do not rely on a single power source. Anything can and will fail. Redundancy of two engine driven alternators plus the solar means that any single point of failure is no biggie.

This is an excellent answer to your question.


To summarize -- you won't save any fuel, and you will lose key redundancy, by getting rid of alternators.


Modern cruising boats are highly dependent on electrical power and redundancy in the capability of the production of it is very desirable.



I will also add one more reason not to get rid of any alternators: Don't you ever motor? When you're motoring, it can be almost like being on free shore power, if you have reasonable alternator capacity (I have a 2.5kW school bus alternator). "Free", compared to running a generator, because you are using only a very small amount of fuel for the incremental extra power taken from the diesel engine, compared to running a diesel engine solely for the purpose of making power. And "free" because you are not accruing extra engine hours, which means maintenance and depreciation, usually more expensive per hour than fuel.


If you're going to get rid of anything, it would be better to get rid of the generator. Three diesel engines on a weight-sensitive catamaran? If you have enough solar to meet your average daily needs, and you have a heavy duty externally regulated alternator on each main engine, then eliminating the generator might be worth while.

[stormalong]

Dockhead: The OP has a power cat.

[Dockhead]

Originally Posted by mcarthur

. . . . 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?.

Could you provide a link, for where you read this?


An idling alternator should not be consuming that kind of power. Maybe a very large alternator with a double belt system, powered by a very tiny main engine.


The parasitic load of an idling alternator will vary by alternator and belt type, but I would be surprised if any alternator (and belt drive) not producing power would consume more than 100 watts or so (0.013 horsepower) at cruising speed. It's just an unloaded belt drive, cooling fan, and a couple of bearings.



If you have different information, please share a link and we'll look at it.

[Dockhead]

Originally Posted by stormalong

Dockhead: The OP has a power cat.

Yes, then all the more reason to keep the alternators, no?

[Jim Cate]

would consume more than 100 watts or so (0.013 horsepower)

umm DH, I think 100 watts is more like 0.13 hp... still not much, though!

Jim

[mcarthur]

Thank you all, good points!

Essentially the thought process was: we're either motoring or still. In either case, the solar array should (!) take care of all usual charge requirements, with the need for supplementary only in unusual circumstances.

Given that, then any additional system is only needed in unusual circs. So an additional system like a generator only needs to be run in unusual circumstances, which suits it being entirely separate to everything else (ignoring running diesels regularly!). But a system like alternators are there all the time and - so the theory goes - must be taking some power and torque from the engine all the time, even if only a little.

The reference I gave to Tanglewood's data is all I've found that has numbers, with others merely speculating. However, it seems odd that anyone would think that a belt drive to alternator has absolutely zero effect on the output of the motor. Is that what is being suggested - that if the batteries are "fully charged", the alternator has zero effect on the shaft output?

Perhaps what I'm looking for is which would have better impact on power and consumption:

1. removing the weight of the genset (2% of the boat)
2. removing the always-on-at-some-level-of-effect alternators

(BTW, I have no doubt that removing the genset would have better impact on cost of maintenance compared to alternators!)

[mcarthur]

Originally Posted by Dockhead

The parasitic load of an idling alternator will vary by alternator and belt type, but I would be surprised if any alternator (and belt drive) not producing power would consume more than 100 watts or so (0.013 horsepower) at cruising speed. It's just an unloaded belt drive, cooling fan, and a couple of bearings.

Oh thanks Dockhead - that's some thought-numbers I can get my teeth into to compare things.

Do you know anyone who has measured parasitic loads (love the phrase) of alternators?
Would small alternators on large engines be any different to large (e.g. Balmar) on middle sized engines?

[Dockhead]

Originally Posted by Jim Cate

umm DH, I think 100 watts is more like 0.13 hp... still not much, though!

Jim

Of course; typo. Thanks.

[mcarthur]

Originally Posted by Jim Cate

umm DH, I think 100 watts is more like 0.13 hp... still not much, though!

Well, at around 6 knots that's nearly 1% of the power output of our motors. And that's only parasitic, not when generating electricity (which shouldn't be needed with the solar).

[Dockhead]

Originally Posted by mcarthur

Oh thanks Dockhead - that's some thought-numbers I can get my teeth into to compare things.

Do you know anyone who has measured parasitic loads (love the phrase) of alternators?
Would small alternators on large engines be any different to large (e.g. Balmar) on middle sized engines?

I'm sure it's been done -- car makers are chasing extremely small efficiency improvements and are doing huge amounts of R&D. You can probably find some figures if you do some research.



But, as I said, it is only (a) the belt, not transmitting a material amount of power; (b) two bearings in the alternator; and (c) the cooling fan of the alternator. Chump change; absolutely a rounding error and in any case not going to measurably affect fuel economy on a 120hp diesel engine.


Alternators are not "always on"; if the batteries are full and loads are being carried by solar, there is no electrical production (depending on regulation of course, but the whole power system should be design that way).



Relative sizes of alternators vs engines are only relevant in terms of % of power, which was how you expressed it. Bigger alternator has bigger bearings and fan and may have a larger belt or belts (mine has two).



Solar may furnish your power in the day time, but don't you ever move at night? And does solar power PEAK loads, on cloudy days?



I was going to ask why you want to go to hybrid drive -- on a power cat -- but decided not to drift the thread to such an extent.

[Dockhead]

Originally Posted by mcarthur

Well, at around 6 knots that's nearly 1% of the power output of our motors. And that's only parasitic, not when generating electricity (which shouldn't be needed with the solar).

Even if you are able to make 6 knots on 13 horsepower, which I presume would require totally flat seas and no wind or following wind, so not an everyday case, what is the MARGINAL fuel cost of 100 watts more power from the diesel engine? Not the AVERAGE fuel consumption per horsepower multiplied by 0.13, but the DELTA between fuel needed to produce 13 horsepower and fuel needed to produce 13.13. Because that is all you will save. Hint -- it is not indeed even 1%.



Even in this extreme case, I guess this will be unmeasurably little. Or even if you can measure it, it won't even pay for the beer you drink while removing the alternators, not even over 10 years.



Why would anyone chase this kind of meaningless fuel savings and then drag around a 400kg generator representing, what 5% of the entire mass of the boat? That doesn't make any sense at all.

[Jim Cate]

Originally Posted by mcarthur

Well, at around 6 knots that's nearly 1% of the power output of our motors. And that's only parasitic, not when generating electricity (which shouldn't be needed with the solar).

umm again! I think that 0.13 hp is actually around 0.1% of the rated output of your engine(s).

And BTW, on many engines, the same belt that drives the alternator drives the fresh water cooling pump... whatcha gonna do about that if you remove the alternator?

Jim

[mcarthur]

Originally Posted by Dockhead

Even if you are able to make 6 knots on 13 horsepower, which I presume would require totally flat seas and no wind or following wind, so not an everyday case, what is the MARGINAL fuel cost of 100 watts more power from the diesel engine? Not the AVERAGE fuel consumption per horsepower multiplied by 0.13, but the DELTA between fuel needed to produce 13 horsepower and fuel needed to produce 13.13. Because that is all you will save. Hint -- it is not indeed even 1%.

6kn on 26hp (2 motors on the cat, each with alternator so 1% on each 13hp). Flattish seas Little wind

The fuel consumption curve for engines are curves, but the difference in an additional 1% is little so we could assume a linear relationship at that small difference. BUT it's still a difference - why do you assume that it's not 1%?

I suppose I could work out the tangent of the consumption curve at that point to get the actual extra fuel used... hmmm...

Originally Posted by Dockhead

Even in this extreme case, I guess this will be unmeasurably little. Or even if you can measure it, it won't even pay for the beer you drink while removing the alternators, not even over 10 years.

Why would anyone chase this kind of meaningless fuel savings and then drag around a 400kg generator representing, what 5% of the entire mass of the boat? That doesn't make any sense at all.

(2% of entire mass ).

But I agree, if your numbers are right, that the reason for change isn't there (which is why I asked the question).