Getting rid of alternator?

[guyrj33]

I'd like clarification on some details
in the original post we were told
50' power cat
2 x 135hp NA diesels with original (old) mid-sized alternators

in a follow up we were told it takes 13hp to drive the boat at 6knts.

My questions are
What is the maximum speed of the boat?
Typical cruising speed?
is this power cat way overpowered?
Does the owner typically run the diesels with a 10% load?

[sailingchiro]

IMHO, there is nothing more important than redundancy for airplanes or boats. This is for power (electric) and power (propulsion). On a boat, I would never take the chance of dead batteries nor leave port with out extra fuel filters. Batteries are the life source of a boat. Without them, most systems cease to function. All it takes is for one of my teenagers to drain the batteries with all their electronics before I realize the importance of separate battery sources for the engines.

[transmitterdan]

Originally Posted by steve77

I really like this answer. Try it for yourself and see what the fuel saving actually is. Leave the alternators in place and if you need them for charging batteries you simply put the belts back on and you’re good to go.



Cheers!



Steve

Better be sure the cooling water pump still turns. Just removing belts is bad unless you know exactly how the engine is cooled.

Also, the alternator usually serves as the belt tensioning or “idler” pulley. Without the alternator there may be no way to properly tension the cooling pump belt.

All in all, my opinion, removing alternators is not a good idea for multiple reasons.

[Jetx]

Originally Posted by mcarthur

Question - would I expect any fuel consumption benefit?

Yes, but it'll be too small to notice or measure. You'd be sacrificing a significant amount of resiliency in power generation in exchange for a miniscule benefit.

[kas_1611]

When I first read the Headline I have to admit to going WTF?

I mean why would you remove the alternators from your engines anyway?

However I read on and read the remainder of the thread and the one thing that really leapt out at me was quite simple really. Why are you hauling around a 3rd engine? You've got 2 big generators already so why do you even need a 3rd? Especially as you say you have a huge solar set up. If you ever need a wee boost to the batteries there's 2 huge generators sitting there.

Sell the extra generator and save yourself the weight. Take the money and put it towards custom props that are designed for the kind of cruising or use you want to do. Standard manufacturers props will be a compromise design to suit all needs. Extra speed or extra economy can be gained by properly designed props.

Then make sure your bottom is clean. From my own experience having a clean bum means an extra 1kt at least at cruising revs. As you have a big battery can I suggest Ultrasonic Antifouling in addition to traditional antifouling? Helps keep the beasties off even more for a few amps a day especially if you can leave it running when you are not on the boat.

Just my 2p worth

[Compass790]

Originally Posted by Fuss

if you do this you will burn them out as the field coils will be energized and no cooling will take place with nothing turning

I'm no electrical expert but I doubt that you will burn anything out as an unloaded alternator does not get hot.
Also I have done this as had a 2 alternator setup with one as a spare with no belt & when called into use it worked fine. Can't remember it being even warm when not in use.
If you were worried about it you could pull the field wire off though but good point about waste of energy energising the field.
Got a timeframe for burnout?

[Paul Howard]

When I had the engines on my sailing cat removed and rebuilt, my mechanic bench ran them for me before re-installing.
He insisted on mounting the alternators before starting them so there would be a small load on them when they ran. He said it was not safe to run the engines without the alternators. I do not know if this is correct or not.

[transmitterdan]

Originally Posted by Compass790

I'm no electrical expert but I doubt that you will burn anything out as an unloaded alternator does not get hot.

Also I have done this as had a 2 alternator setup with one as a spare with no belt & when called into use it worked fine. Can't remember it being even warm when not in use.

If you were worried about it you could pull the field wire off though but good point about waste of energy energising the field.

Got a timeframe for burnout?

It depends on the size of the alternator. A typical full field current for a 100A alternator would be about 5A. So that’s about 60W. So yes the alternator would eventually heat up and get pretty hot with no cooling air but I doubt it would burn up.

[CarlF]

You might want to have an easy way to disable the alternators to keep them from charging when the engine in run the morning (before the solar array can do its thing)

This can be accomplished by installing a switch at the helm that disconnects the alternator field wire.

And if the goal is to save fuel, these three things will do much more than removing the alternators:

A very clean bottom and clean props

Run on one engine when possible

Slow down ( on my boat slowing from 8.3 knots to 7.1 knots reduces gph by about 40%.)

[Compass790]

Originally Posted by transmitterdan

It depends on the size of the alternator. A typical full field current for a 100A alternator would be about 5A. So that’s about 60W. So yes the alternator would eventually heat up and get pretty hot with no cooling air but I doubt it would burn up.

Thanks for heads up. Forgot we had a switch on the instrument panel anyway to disconnect field wire of big alternator & ours only pulled about 2.5amps as its a 55amp one.

[Q Xopa]

Originally Posted by Dockhead

Indeed, but note that this is all about saving fuel from the process of the alternator generating power, not from the alternator spinning without a load. Two different things.

Ive spent a fair bit of time, and not inconsequenctial cash, trying to minimise my diesel used per electric usable Watt produced.

Basically trying to maximise my time between fuel dock visits and running costs.

A few ways I have chased it-
Alternator speed. Looking at any Alternator output graph shows the efficiency is better is at lower RPMs. This is counter to what most people do. Sure spinning them quicker gets a higher output, but at a lower efficiency.

So if you can fit/ afford bigger alternators, and spin them slower. Bigger alternators are more efficient, and higher output will reduce charging time (within max C batt charging rate), therefore engine run time. So less engine run is less fuel burned.

If you can also maximise running your motor at its most efficient RPM, ie lowest SPFC, generally the highest TQ RPM. My 4JH4-TE, is 1800-2000. So when the eng is running its is burning less for the power being produced.

So if you can match your engine and Alternator most effic RPMs with the pulley ratio.

To do this with a Generator you need a DC generator. I have nearly finished building my own version of this using a new 2YM15 and a 450A brushless alternator. I had a special adapter plate and drive coupling made. I have a Datakom controller. I am around $8k into it. Its been a fun project, but honestly although Im confident it will work well, now that I've done it, realistically Im thinking its not really worth it.

After all that I will probably just mostly continue to use my Honda 220i. Which essentially work on the same concept and a fraction of the price.

One of the biggest advantages of a DC gen is it will reduce RPM to idle at low loads. This can be significant depending on use profile. Which is what the Invertor generatators like the Honda 220i's, and others do.

Having said all that my preference hierarchy for generating power is-
Solar whenever available, then
Alternators whenever motoring, then
Generator only when others cant keep up. Although I try and minimse using it.

I guess another vote for Alternators over Generators.

[Fuss]

Originally Posted by Compass790

I'm no electrical expert but I doubt that you will burn anything out as an unloaded alternator does not get hot.

Also I have done this as had a 2 alternator setup with one as a spare with no belt & when called into use it worked fine. Can't remember it being even warm when not in use.

If you were worried about it you could pull the field wire off though but good point about waste of energy energising the field.

Got a timeframe for burnout?

true, my comment was a little over the top but it will use battery power the whole time the engine is running ... my alternator needs 80 watts to power the field coils.... the op cares so much about diesel conservation and then wastes energy powering unused alternators.

[Dockhead]

Originally Posted by Q Xopa

. . .

So if you can fit/ afford bigger alternators, and spin them slower. Bigger alternators are more efficient, and higher output will reduce charging time (within max C batt charging rate), therefore engine run time. So less engine run is less fuel burned.

If you can also maximise running your motor at its most efficient RPM, ie lowest SPFC, generally the highest TQ RPM. My 4JH4-TE, is 1800-2000. So when the eng is running its is burning less for the power being produced.

So if you can match your engine and Alternator most effic RPMs with the pulley ratio.. . .

Sure, but in real life both of these would be counterproductive.


If you reduce the alternator output, you will increase the run time for charging. If you are running an engine solely for charging (without using it for propulsion), this will use far more fuel than you save with a small increase in alternator efficiency.


Ditto with the engine RPM. If you use the same power at higher RPM, you fall right down the fuel map into less and less efficient territory. Plus it's really bad for the engine to run a really small load like an alternator, at higher RPM. On the contrary, if you are using a propulsion engine for charging then you want to load that engine as best as you can as a % of maximum power at that RPM. The lower the RPM, the better that will be. So from this point of view also it is good to gear the alternator so that it is doing the most possible work at low engine RPMs, while not exceeding maximum RPM at engine redline.


And if you are using the engine for propulsion, the fuel map also doesn't help you. The savings at higher RPM in fuel burned PER HORSEPOWER PRODUCED is dwarfed by the extra fuel you will consume, by going faster and requiring more absolute horsepower.


So here too -- best fuel consumption will be at the slowest speed you can tolerate, and lowest RPM, even if you are spending more fuel per horsepower produced. Because you need less and less horsepower at lower speeds.

[Q Xopa]

Originally Posted by Dockhead

Sure, but in real life both of these would be counterproductive.


If you reduce the alternator output, you will increase the run time for charging. If you are running an engine solely for charging (without using it for propulsion), this will use far more fuel than you save with a small increase in alternator efficiency.


Ditto with the engine RPM. If you use the same power at higher RPM, you fall right down the fuel map into less and less efficient territory. Plus it's really bad for the engine to run a really small load like an alternator, at higher RPM. On the contrary, if you are using a propulsion engine for charging then you want to load that engine as best as you can as a % of maximum power at that RPM. The lower the RPM, the better that will be. So from this point of view also it is good to gear the alternator so that it is doing the most possible work at low engine RPMs, while not exceeding maximum RPM at engine redline.


And if you are using the engine for propulsion, the fuel map also doesn't help you. The savings at higher RPM in fuel burned PER HORSEPOWER PRODUCED is dwarfed by the extra fuel you will consume, by going faster and requiring more absolute horsepower.


So here too -- best fuel consumption will be at the slowest speed you can tolerate, and lowest RPM, even if you are spending more fuel per horsepower produced. Because you need less and less horsepower at lower speeds.

Yes agreed that the best fuel consumption motoring is going to be at the slowest speed you are patient enough to go at. Although the OP is not asking about motoring fuel efficiency.

Mostly when motoring the Alternator an insignificant proportion of the engine power produced. So my thinking if we are motoring anyway, Alternator power produced is pretty cheap so a bonus.

Yes also agree that slowing Alternator speed will decrease output more than increasing efficiency. Yes this will only practically work if you can get a big enough alternator.

Practically speaking most of are not going to motor at 2 kts or get a big enough alternator to utilise this concept to save diesel.

Im not suggesting this is practical for most of us, more just pointing out concepts that some may want to capitalise on, or atleast be aware of to help make energy management decisions.

[Dockhead]

Originally Posted by Q Xopa

. . . Mostly when motoring the Alternator an insignificant proportion of the engine power produced. So my thinking if we are motoring anyway, Alternator power produced is pretty cheap so a bonus.. . .

I agree.


Remember you don't pay for the AVERAGE fuel per horsepower/hour, you pay for the MARGINAL fuel. Charging while motoring.



And you don't pay for extra engine hours -- maintenance and amortization is usually more expensive per hour than fuel.


So generating power while motoring is for all practical purposes FREE, certainly far cheaper than running a generator.