In Part 1 we learned that powerful alternators can have several negative performance, reliability and safety impacts; however, we assumed basic alternator regulation. But by controlling the alternator output with respect to engine rpm we can mitigate a lot of these problems.
This article covers how to pick our values to input into any regulator with this capability and then how to actually do the programming if we use a unit from Wakespeed, John’s preferred regulator.
Calculate Maximum Power
Let’s assume we have a power-hungry boat and therefore want the most powerful alternator that will be relatively safe for our engine, using the following assumptions:
- 40-foot cruising sailboat
- 50hp (37kW) engine at 3000 max rpm
- Cruise rpm 2000
While this article is very helpful if u intend to use both the Alternator and propulsion power at the same time, I respectfully suggest that that may not in fact be the right answer especially if the Alternator is your BACKUP electrical source. Our solar array is my primary means of keeping my batteries charged. My big Balmar is a BACKUP generator. As such I have a switch on the Alternator regulator ignition power line. This allows me to depower the Alternator regulator when I just want the engine to provide propulsion power. I seldom see the need for both propulsion and power generation, but if that occurs, the techniques described will make that more efficient/ effective. If I need electrical power, run the engine out of gear w the Alternator regulator powered. This gives me complete control over these systems.
Yes I use a DC-DC charger to charge my starter battery from the house. That way my starter is always ready to go.
Hi Edward,
Yes, this article is geared towards people who are trying to maximize charging from alternators and therefore fit large alternators to their engines. There are definitely other use profiles out there and it sounds like yours is one of them. But even those other use profiles can often take advantage of whitespace programming as the reasons for using whitespace all still apply. In your case as it sounds like you have a big alternator, you could do whitespace and then either have a field disconnect or use one of the configurable Wakespeed inputs to shut it down when not desired.
You mention running the engine out of gear with the alternator producing. Unfortunately this is a very inefficient way to charge. It makes me wonder whether you would be able to avoid it sometimes if you were charging when already motoring even if you were not totally sure you need the energy? On the other hand, because of weather variability and load variability, if you don’t need an additional charge source sometimes, you likely have a hugely oversized solar array so you may be just fine (see https://www.morganscloud.com/2023/01/24/balancing-battery-bank-and-solar-array-size/).
I have played around a bit with trying to prioritize solar on our boat over the alternator. For our use case, I found that I should just charge from the alternator whenever the engine is running as if we have a big charge deficit, the solar will not be able to make it up and if it is small, then it is very little extra fuel to do the charging. We do have the bulk and absorption voltages set slightly higher on the solar so the alternator cuts back first but I suspect the total fuel savings from doing that are very minimal for us.
Eric
Hello Eric,
I have a configuration with max. two 4kw alternators on a Perkins 4236. The engine has no less than 230nm torque and all seems to run well. Unfortunately I dont have torque values from the alternator manufacturer.
Up to now, I was more fearing that there might be not enough load for the engine when I am charging at anchor without propeller load. After reading your great explanation here, I am now worrying that I am going to destroy my newly overhauled engine by over torqueing it.
I did all the calculations based on kw in an excel-file, which I would like to show you, but cant upload it here.
I dont feel comfortable any more using both of my alternators now and would like to get a competent opinion. Can you help me with this?
Hi Frank,
Eric explains in this article why there is no way to accurately determine whether or not a given alternator is safe on a given engine other than to stick with the largest alternator the engine manufacturer sanctions and then increase a bit from that using whitespace as described in this article.
Also note that it’s not as simple as matching max torque from the engine to max from the alternator because of side loading and other factors Eric explains.
Full details here: https://www.morganscloud.com/2025/07/06/right-sizing-an-alternator/
If it were me, and given it will be hard to find the max alternator spec for that engine, and particularly given you have just spent money on an overhaul, I would remove one of those alternators and downrate the other one with white space to no more than 3 kW at cruise and 2kW at low rpm.
Hi Frank,
As John says, I think the other article on right sizing an alternator contains the information you are looking for. If it is not clear, leave a comment and I will do my best to chime in.
Unfortunately, propulsion engines make poor generators. You are correct that running a big engine with a light load is not good, both for the engine and for fuel efficiency. Taking huge amounts of power off with a belt is usually a bad idea and unfortunately one where we don’t have a ton of information. That leaves generators and non-belt driven setups on the propulsion engine. Some engines have a PTO which can make it feasible to set something up DIY but most are not well suited and risk decreasing propulsion reliability. All this is assuming that you need to charge off the main engine regularly. For a boat where the vast majority of charging comes from either the alternator while motoring, shore power, or renewables and only very occasional out of gear charging is required, it is not a big deal and installing a lot more solar or a generator doesn’t make sense.
Eric
I’ve been looking forward to this article and I’m not disappointed! Much appreciated.
I had a new Balmar 3kW alternator fitted, together with an ARCO Zeus alternator regulator. Considering how quickly the alternator temperature regulation kicks in, I may be gaining very little over a more moderately-sized alternator, but at least now I have a deeper understanding of the principles behind the white-space programming. Thanks!
Hi Giles,
I am glad to hear it is helpful. It seems that I am hearing a lot of comments about regulators cutting back quite quickly due to alternator temps these days. Have you measured the ambient temperature of your engine room after running for a bit?
Eric
Great article. We are just installing Mastervolt 12kw lithium 12V house bank. We did a parallel Lifeline AGM backup bank after reading your article on redundancy for lithium. It is there for emergency only, and should last a long time with no cycling. The back-up 12v bank, 24V winch bank and 12V starter batteries are charged by DC-DC chargers, or the 6kw Northern Lights driven battery chargers (3). Everything beyond the house lithium bank is always charged. Replacing the engine and 120V fridge compressors and holding plates with a 12V compressor w evaporator plates. Removed the 2nd alternator from the engine, and went with one Arco high output alternator on the standard engine bracket. We will carry a spare. Having just one easily accessible/changeable belt makes life easier. The engine and its crankshaft should be happier without the fridge compressor and 2nd high output alternator hung on the front end. Added 1200 watts of solar to the Bimini, dodger and deck. I have zero interest in davits or davit mounted panels as so many boats do these days. One good Gulf Stream beating would be too much risk. I think we have the right amount of solar, genset, batteries and redundancy with risks minimized. All of your battery, alternator, regulator and solar articles have led to so many questions and leap-frogged solutions. So thank you. My technical network believes the Arco regulators are more reliable than Wakespeed. Time will tell.
Hi Peter,
Sounds like a great and robust system. If memory serves this is for a J/160?
Interesting that your contacts think the Arco is more reliable than the Wakespeed. I have heard the opposite, but, as we both know, that does not make it true, so please advise how you get on with the Arco over time.
One thought, did you consider taking the lithium bank up to 24 volt and running the 12 volt loads of the lead acid kept charged by DC/DC chargers. I’m in two minds about the recent trend to increase the voltage of the main bank and so am interested in your thinking.