High power alternators vs Genny on a Cat

[boom23]

Originally Posted by Cheechako

You have weather helm on a cat? My Lagoon 42 TPI seemed to have no helm feel at all, it was a bit distracting for an old mono sailor.

I did on my ex L450 with square top mainsail.

[Thalas]

So is the consensus that HP alternators don’t put enough load on a main engine at full 100amp+ output for longevity? I thought these alternators would vary load based on needs (e.g. pull more generation when charging is needed, but would lighted up greatly when prop shaft power was needed).

[rom]

Originally Posted by tuskie

We have installed a small diesel genset to allow us to sail.

Sounds stupid, but I'll explain. We have a good solar/lithium set up that usually adequately supplies our house load at anchor. We have lots of fridge/freezer space of the inefficient kind, as well as entertainment systems, lights, computers. We're "power pigs". It was cheaper to increase the power supply side of the equation than to replace the inefficient refridgeration. At anchor, were fine and self sufficient. Day sailing is also fine, even if the sails shading in panels means that we run the bank into further deficit.

The real issue comes when we do overnighters or longer passages. We'll start out with a bank that's a bit depleted from running the house load overnight, and then sail all day perhaps further depleting the bank if the panels are shaded. At night the power loads increase with additional draw from lights, electric winches and increased use of radar. Sooner or later, the battery bank will need a recharge.

Do we start an engine and get a small charge from the incorrectly labeled 80 A alternator. Or do we pull out a portable genset in the dark, sit it on deck in salt spray and plug it into the mains socket? Or do we push the start button of a properly installed diesel generator and engage the real 80 A lithium battery charger for a couple of hours and keep sailing? We chose the latter option.

From the example you describe it seems to me you fit pretty well in the category where "high power alternators" is a good option, which is the question of this thread.

My humble answer to the OP is I would do the math, how much power do I need, when, how often. Do that for a 365 days period, as your needs may vary upon seasons. Personally, I would then do my best to avoid having a third diesel onboard.

[Tellie]

Originally Posted by rom

From the example you describe it seems to me you fit pretty well in the category where "high power alternators" is a good option, which is the question of this thread.

My humble answer to the OP is I would do the math, how much power do I need, when, how often. Do that for a 365 days period, as your needs may vary upon seasons. Personally, I would then do my best to avoid having a third diesel onboard.

Indeed, first things first. I'm always amazed at how many people know all about alternators, solar, gensets etc., and still have never done the math. This math will quickly separate you from the "You don't need" and the "You gotta have" and get you faster to the "That fits MY needs and the boats capabilities"

[Dockhead]

Originally Posted by Bean Counter

. . . An alternative is to run a main propulsion engine, which when at anchor is an inefficient way to charge batteries even with high amp alternators. The main engines are not designed to run at 1500 rpm for hours in neutral - it wastes diesel and will probably prematurely wear out the engine, detracting from its prime purpose. The hi amp alternator comes in to its own when under way using motor(s) which in a cruising scenario is quite a low time percentage to warrant the upgrade cost.

First of all, the high amp alternator is always a great thing -- you get a proper charge whenever you motor, whatever type of batteries you have, but especially if they are not lead and have a high acceptance rate. A car-type alternator is not made to produce bulk power.

But concerning charging with high amp alternators at anchor -- I don't think you can make a blanket statement like that. It depends on the size of the alternator, the pulley ratios, the output curve, vs. the RPM you are operating it at and how that loads this or that particular engine.

Properly configured I think it could be fine. My alternator produces about 2.5kW of power and needs about 5 horsepower from the main engine. That's a reasonable load for a 2000cc engine at 1200 RPM or so. A bigger EchoTec alternator on a smaller engine would be even better.

I do also have a separate generator -- a heavy duty 6.5kW Kohler. But if I had a catamaran with two engines, I would be loathe to add a third diesel engine. I would try to properly engineer something with very large alternators.

LiFePo batteries also help a lot, because their huge acceptance rate means you don't have to run the engine for long, if you have a big enough alternator. If you have those, then using large alternators instead of a separate generator makes even more sense.

With a LiFePo bank which could accept 10kW of charging (say), I might even be inclined to add a very large alternator, 10kW or more, driven by a jackshaft from one of the mains, and not to be used when a propeller is engaged. In addition to normal school bus alternators on both mains which could be used with propeller engaged. That would make one of the mains kind of dual use -- main and generator.

I really don't think you need THREE diesel engines on a boat less than 100 feet long.

Originally Posted by Thalas

So is the consensus that HP alternators don’t put enough load on a main engine at full 100amp+ output for longevity? I thought these alternators would vary load based on needs (e.g. pull more generation when charging is needed, but would lighted up greatly when prop shaft power was needed).

High output alternators will NOT reduce power just because the engine's mechanical power is needed for the prop. So you have to be careful not to overload the engine -- the engine manual will tell you how big an alternator you can use while the prop is engaged.

But as to the first question -- see above. It depends.

[CatNewBee]

it is not an adequate replacement at all. a generator provides no just 12v charging, but also plenty of 220v/110v power. An alternator only delivers 200A DC.

[john61ct]

Originally Posted by Thalas

So is the consensus that HP alternators don’t put enough load on a main engine at full 100amp+ output for longevity? I thought these alternators would vary load based on needs (e.g. pull more generation when charging is needed, but would lighted up greatly when prop shaft power was needed).

My understanding is the "low load - cylinder glazing" issue can be handled by ensuring higher temps are reached for an hour or two every ?10-20? hours

The engine load depends on amps actually generated, what's demanded by the bank.

As I said 400+A @28V actual output (12 kW) will load 25-30HP.

You need to turn down (or off) the alt current in order to have those HP available for propulsion. A field current switch at the helm is an easy solution.

[john61ct]

Originally Posted by CatNewBee

it is not an adequate replacement at all. a generator provides no just 12v charging, but also plenty of 220v/110v power. An alternator only delivers 200A DC.

Some alts deliver a lot more.

Inverters can handle most appliances.

Some owners have little use for AC.

[CatNewBee]

some do not need electricity at all...

[CatNewBee]

the question is, can a high amp alternator replace a generator on a cat. most cats having a gen installed, have it on purpose (e.g. for the AC units, diving compressor etc.) a DC alternator cannot deliver enough juice for those. To replace a 9kW Onan you will need an alternator with 12v / 800A.

[john61ct]

Originally Posted by CatNewBee

the question is, can a high amp alternator replace a generator on a cat. most cats having a gen installed, have it on purpose (e.g. for the AC units, diving compressor etc.) a DC alternator cannot deliver enough juice for those. To replace a 9kW Onan you will need an alternator with 12v / 800A.

It's not that straightforward, can't just use a single conversion formula, all depends on the owners' needs / vs desires, how big the LFP bank is, how long the big loads run, etc.

Many can get by with just solar too.

If you can get by for an average 24 hours just using batteries, likely you can get by with just a good alt setup.

Also if you can motor while the heavier loads are running.

Aircon for 18 hours a day, the engine/alt setup would need to be designed specifically from the beginning to support that, not practical for most, but could be done.

[CatNewBee]

One of the reasons buying a sail boat is to not have to use / run the engines. Running the engine on anchor just to charge the battery is the second best solution. A generator is optimized for this job and the better choice. Solar is of course better than anything else, it is quiet.

[malbert73]

Lots of new boats seem to rely on AC and genset rather than well designed ventilation. We’ve found this on charter
That said, we have a genset (came with boat) and in summer on Chesapeake the ability to run AC on a hot humid windless day has saved our sanity

[catsketcher]

For all the lurkers who are thinking that they need huge alternators to generate massive amps or they won't enjoy cruising - don't worry. For hundreds of years sailors have had this "problem". When it is hot, sail somewhere cooler, go swimming, snorkelling, or throw your bean bag in the water and paddle around. It is a highly sociable thing to do. We also like to run fast and dive off the bow and then try to hang onto the anchor chain. It is called playing in the sea - it is one big reason we go cruising.

To get air through the boat, open the hatches and put the sun cover up. Save money by anchoring out, don't go into marinas unless you absolutely have to, they are airless places and you get nice cooling breezes anchored just outside.

As 44C says, AC is not a must have. In Australia very few boats have it, except for those who live aboard in a marina. If I wanted to be shut up with the doors closed and have the AC on I would stay at work or commute in the car. There, just saved you about 20 grand on your next boat.

I realise I am like a mono guy in a multi thread but I shake my head at the endless upping of comsumerism on cats. I go sailing to get away from incessant consumption and to realise how little we need to be happy.

[Barra]

^^^ couldn't agree more! Those sitting in their air conditioned boxes really have missed the point of going cruising!

Just our humble opinions of course eh