Battery Charging With Two Alternators

[B27]

The power that is required to drive a boat through the water is a function of the hull shape and a function of speed through the water.

The power developed by an engine to obtain the speed is mainly a function of the amount of fuel consumed and this can/ would generally be the same if supplied by a single engine or multi engines it the power that is consumer.

There is an efficiency factor which will affect the total fuel consumption.

You have the basic efficiency of the engine / prop / drive chain.

If you use a single engine/prop/drive chain, there could be a difference in efficiency when using twin engines

I think with a sailing boat going slowly, there may be some gain using one engine, because the second engine would be using fuel if it was just idling, driving its water pump etc etc.
But maybe less so if that means there's big fixed prop acting as a brake and trying to may the cat go around in circles.

It might be something to look into if the OP ever gets becalmed on a long trip.

 

[Zing]

…. I want to …. use the existing 60AH engine battery to start both engines, but with the ability to start from the domestic bank if necessary,

Can anyone see any problems? …

Thanks in advance.

Short answer, no.

I do this. Both engine alternators charge the single start battery. It’s worked fine for years, though I do admit it’s a little unconventional. There is a house battery link that works fine too in an emergency, like when the engine start batteries die, as they do every 5 or 6 years ( Caribbean heat).

 

[Neeves]

Running on one engine can save some fuel, but Neeves calculations are off. Running on one engine does not cut the fuel bill in half. If you shut one engine down you either lose speed, or have to increase RPM of the running engine.

Actually, we might have been over thinking this ! You could leave one alternator connected to the "engine" battery and the other connected to the leisure bank and fit a Victron Cyrix 120 between the two banks. The Cyrix is dual sensing, so all batteries will get charged no matter what charging source is active on either bank.

If you still want to use a single battery to start both engines you can, connect the port starter to the engine isolator but disconnect the charging cable from the alternator and connect it to the domestic bank.

Motoring a catamaran on a single engine | Sailing Forums, page 1.

Below, see post 5.

Cruising - 1 engine or 2 - Cruisers & Sailing Forums

Use Google and you find a range of unrelated comments, threads and articles all saying the same thing

A single engine is more fuel efficient that 2 engines for a cruising catamaran. They can, or course, all be wrong.

As said every cat is different - its up to Stemar to run tests and find the most efficient way for him to motor (sails also influence the answer). However there is sufficient background to suggest one engine is more efficient and given the current cost of fuel - he should be intrigued.

Jonathan

 

[Refueler]

Short answer, no.

I do this. Both engine alternators charge the single start battery. It’s worked fine for years, though I do admit it’s a little unconventional. There is a house battery link that works fine too in an emergency, like when the engine start batteries die, as they do every 5 or 6 years ( Caribbean heat).

Interesting .... do you have any intermediary item in the connections ?

 

[PaulRainbow]

Motoring a catamaran on a single engine | Sailing Forums, page 1.

Below, see post 5.

Cruising - 1 engine or 2 - Cruisers & Sailing Forums

Use Google and you find a range of unrelated comments, threads and articles all saying the same thing

A single engine is more fuel efficient that 2 engines for a cruising catamaran. They can, or course, all be wrong.

As said every cat is different - its up to Stemar to run tests and find the most efficient way for him to motor (sails also influence the answer). However there is sufficient background to suggest one engine is more efficient and given the current cost of fuel - he should be intrigued.

Jonathan

Thanks Jonathan, that thread, and in particular post #5 (that you suggested i read) agrees with what i said, running on one engine does not halve the fuel bill, which was your claim.

 

[Zing]

Interesting .... do you have any intermediary item in the connections ?

No, nothing needed as the battery link switch is an emergency only thing, not for continual use.

 

[Stemar]

Motoring a catamaran on a single engine | Sailing Forums, page 1.

Below, see post 5.

Cruising - 1 engine or 2 - Cruisers & Sailing Forums

Use Google and you find a range of unrelated comments, threads and articles all saying the same thing

A single engine is more fuel efficient that 2 engines for a cruising catamaran. They can, or course, all be wrong.

As said every cat is different - its up to Stemar to run tests and find the most efficient way for him to motor (sails also influence the answer). However there is sufficient background to suggest one engine is more efficient and given the current cost of fuel - he should be intrigued.

Jonathan

Thanks for that. Food for thought. As cats go, Jazzcat is pretty heavy, more of a motorsailer, so we do tend to motor a lot. Last weekend, we had the wind on the nose going out then is switched, so it was on the nose coming back, so the sails stayed furled, but the batteries got a good charge, so not all bad!

Short answer, no.

I do this. Both engine alternators charge the single start battery. It’s worked fine for years, though I do admit it’s a little unconventional. There is a house battery link that works fine too in an emergency, like when the engine start batteries die, as they do every 5 or 6 years ( Caribbean heat).

Interesting. How do you charge the domestic batteries?

 

[Neeves]

Thanks Jonathan, that thread, and in particular post #5 (that you suggested i read) agrees with what i said, running on one engine does not halve the fuel bill, which was your claim.

You really never learnt to be gracious

this is a quote:

but at full load the one engine is way more fuel efficient (7.2 vs 2 x 5.7 lph). When motoring we're generally above minimum but a bit less than full load, so somewhere in the middle.

unquote

On one engine he is using 7.2 litres and on 2 engines 11.4 litres, double would be 14.2 litres. Its a forum - the saving is significant. But you don't like the idea and dismissed and ignored it out of hand.

Jonathan

 

[Neeves]

Thanks for that. Food for thought. As cats go, Jazzcat is pretty heavy, more of a motorsailer, so we do tend to motor a lot. Last weekend, we had the wind on the nose going out then is switched, so it was on the nose coming back, so the sails stayed furled, but the batteries got a good charge, so not all bad!

You will find that if you can motor sail you will save more fuel as the sails and engine give you a lift. Not so useful in a strong head wind as then you have seas and possibly want to get to shelter quickly - sails might then be a liability and you might want to use both engines - at revs to be as less uncomfortable as possible - belting into seas and a had wind can generate a lot of water over the cockpit.

The best combination, if you must sail to windward, is leeward engine on at cruising revs, main, maybe reefed and a tight, flat jib and the wind off the bow. If you have to tack to get to your destination, swap engines. This is quicker, more comfortable and cheaper (less fuel) than bashing into seas and chop

In light winds when the motor is increasing the apparent - that's when you will make the savings (just don't flog the sails). You may have to 'tack' to get where you want to go.

We found raising the main 50m^2 a hassle at sea in a breeze and learnt to raise raise in shelter (battens would get stuck under topping lift or lazy jacks), headsail was easy (on a furler). Unless it was 'local' and we were on passage we would keep the sails hoisted - just to use any lift and to minimise aggro from the main.

Jonathan

 

[PaulRainbow]

You really never learnt to be gracious

Not if it means agreeing with incorrect claims.

this is a quote:

but at full load the one engine is way more fuel efficient (7.2 vs 2 x 5.7 lph). When motoring we're generally above minimum but a bit less than full load, so somewhere in the middle.

unquote

On one engine he is using 7.2 litres and on 2 engines 11.4 litres, double would be 14.2 litres. Its a forum - the saving is significant. But you don't like the idea and dismissed and ignored it out of hand.

Jonathan

As is often the case, you selectively quote, to distort figures. You asked me to read post #5, which i did, in its entirety. It says;

"The engine is most efficient (specific consumption) around 2600 rpm. We can motor at ~5 knots on one engine at 2400 rpm, two engines at 1800 rpm. If you look at the fuel usage at light load this is about the same usage (3.8 lph vs 2 x 1.9 lph) but at full load the one engine is way more fuel efficient (7.2 vs 2 x 5.7 lph). When motoring we're generally above minimum but a bit less than full load, so somewhere in the middle.

In the best case two engines uses about the same fuel but also results in twice as many engine hours and in lightly loaded diesels - not their favorite operating point. So, we run one most of the time, and in observation our fuel usage is about 80% of two engines for the same speed."

Note that, in addition to your carefully selected extract, he says "at light load this is about the same usage (3.8 lph vs 2 x 1.9 lph"

He also says "we run one most of the time, and in observation our fuel usage is about 80% of two engines for the same speed"

80% is not half.

FYI, i have never said there is not a saving, i have only said your claim of using half of the fuel is incorrect. I have twin engines and at low speeds i often run on a single engine. I don't have any comprehensive figures, but i do know that to maintain the same speed when i turn one engine off i have to increase the RPM on the running engine by about 30%. So yes, i dare there is a saving, bit it ain't 50% less fuel.

 

[Zing]

Interesting. How do you charge the domestic batteries?

With a generator and solar and a dedicated large engine alternator.

 

[Stemar]

Thanks, so not practical for me.

I think the final outcome is to stick with one alternator for the engine battery and one for the domestics, with an option to add a VSR when the wallet has recovered from buying the batteries.

 

[rogerthebodger]

Thanks, so not practical for me.

I think the final outcome is to stick with one alternator for the engine battery and one for the domestics, with an option to add a VSR when the wallet has recovered from buying the batteries.

Thats the same as my setup except I don't have a VSR as I have a combining switch for emergency starting and/or combine battery charging from my 2 alternators and /or combined solar panels

 

[Refueler]

With a generator and solar and a dedicated large engine alternator.

Confused .....

In post #62 ...

"I do this. Both engine alternators charge the single start battery. It’s worked fine for years, though I do admit it’s a little unconventional. There is a house battery link that works fine too in an emergency, like when the engine start batteries die, as they do every 5 or 6 years ( Caribbean heat)."

You now say that the house battery is by Generator / solar and dedicated large engine alternator ...

So in effect battery banks are charged independently ... its only the start battery that gets both alternators when both engines running ...

 

[Zing]

Confused .....

In post #62 ...

"I do this. Both engine alternators charge the single start battery. It’s worked fine for years, though I do admit it’s a little unconventional. There is a house battery link that works fine too in an emergency, like when the engine start batteries die, as they do every 5 or 6 years ( Caribbean heat)."

You now say that the house battery is by Generator / solar and dedicated large engine alternator ...

So in effect battery banks are charged independently ... its only the start battery that gets both alternators when both engines running ...

Yes.