Prop Pitch and Power Management

If you haven’t been following the blog of Kosmos, the Nordhavn 40 that is circumnavigating, this could be a good day to start. On today’s blog (which lags by a few weeks the actual events), they are just starting their Atlantic crossing, traveling alone, with only three people on board.


And on a different topic….


My boat is back at the dock at the Salmon Bay Marine Center, after several weeks of work at the Delta Shipyard. Most of the major work is now complete, although there are a large number of small details that still need completed.




As you can see, it was snowing in Seattle!


While the boat was out of the water, we had the props re-pitched. I mentioned a few weeks ago that the owner of the second N68 did some research which indicated that with some tweaking of the prop pitch we might be able to pick up another 15-20% of fuel efficiency. This sounded overly optimistic, but I decided to have the ship yard take a look at my prop. After a bit of study, they decided my boat was over-propped. This added to my confusion, in that David, the owner of the second N68, heard from the experts that his boat was under-propped. Both of us puzzled over this one. We have roughly identical boats, bought from the same company, and yet the experts are telling both of us that our props need tweaking, and that the fix is the exact opposite for each of us.




We did send my props out for modification, and here is the report I received back:


Kruger & sons report

Report 1 – OSO Class 1 Initial Report Left wheel / Final report after adjustment

Marked Dia 38.5 Marked Dia. 38.5

Measured Dia 38.6 Measured Dia. 38.7

Mk’d Pitch 37 Mk’d Pitch 35

Pitch of wheel 36.45 Pitch of wheel 34.97

Report 1 – OSO Class 1 Initial Report Right wheel / Final report after adjustment

Marked Dia 38.5 Marked Dia. 38.5

Measured Dia 38.5 Measured Dia. 38.7

Mk’d Pitch 37 Mk’d Pitch 35

Pitch of wheel 36.46 Pitch of wheel 35.10


It is too soon to say whether or not the modification to Sans Souci’s props increased fuel efficiency or not, but the first indication is positive. All we have done at this point is a short test run, but here’s what Jeff wrote me afterwards:


“Ken: The sea trial went well. The wind was blowing S/E 25 knots so we have some wind. I went in the wind and with the wind and the engines did not change RPM. I had 1850 on the port and 1879 on the starboard. I have to compare my records but at 1350 I was burning 12 per hr. making 9.5 kts. I think it used to burn 13.5 per hr. at that RPM….”


And lastly…


I’ve now spent four days staring at Sans Souci’s electrical plans. I set what I thought would be some easy goals, but I’m not particularly gifted at studying electrical plans. My goals are: 1) to produce a simplified electrical diagram that I can use to quickly spin others up to speed on the system, and 2) To identify points in the system where I want to add additional current and voltage measurements, so that I can know how much power I have available at a glance. Currently, it is almost impossible to know power consumption around the boat.


This got me thinking about power management….


Power management on a boat is a daily part of life. I don’t want to make it sound tougher than it is, because after you’ve done it for a while, it becomes second nature. Roberta knows that before she starts the dryer she must say: “Is it ok to start the dryer?” There are a number of things around the boat that use a lot of electricity, for instance, the air conditioning, the water makers, the davit, the dive compressor, the electric stove, etc. Before any of these high-current items are started, I need to know about it.


I always have to be aware of where the boat is getting its’ electricity, the state of charge of the batteries, and the approximate electrical load. Mistakes can easily mean blown fuses or dead batteries.


This sounds simpler than it is. Sans Souci actually has four electrical systems; a 12 volt DC system, a 24 volt DC system, a 120 volt AC system, and a 240 volt AC system. Some of these systems have multiple sources of power. For instance, the 240 volt system can be fed five different ways: from two different generators, shore power (directly), our Atlas shore power converter, or our inverters. Each of these different power sources have different limitations, and I constantly need to keep the loads at appropriate levels. Overload means blown fuses, and under-loading a generator wastes fuel and can cause damage.


My electrical panel has digital gauges which help me know what is going on, however the gauges aren’t always easy to interpret, and it isn’t always me doing the interpreting. On our trip to Costa Rica we blew a lot of fuses. This can be fairly annoying when running at night and suddenly all the lights go out.


One of my goals for this winter has been to “idiot proof” the electrical system. I think that if it were just Roberta and I on the boat, this wouldn’t be an issue, but we tend to have a lot of guests and crew on the boat. Guests don’t generally read instruction manuals, and many have no fear of pressing buttons, whether they know what the button is or not. For instance, Sans Souci has electrical heaters built into the air handlers in each room. These electrical heaters use a HUGE amount of electricity, and using them can easily put the boat over the edge to tripping a shore power breaker. Once I tired of constantly resetting the breaker, I discovered that it was possible to lock out the heat strips. This allowed me to become master of the heat strips, and I could adjust the loads as needed to ensure that there wouldn’t be an issue.


I’m approaching this whole issue of idiot-proofing from many different angles. One thing I noticed was that when running our smaller generator (the 16kw), along with the air conditioning, it didn’t take much additional load to blow fuses. We prefer warmer climates, so this effectively locked us out of running the 16kw generator. To ease this problem, I upgraded our generator slightly, to a 20kw. I also swapped our air conditioning chillers to units that would start up with no spike in current demand. Electrical motors tend to use double their operating current, for a brief instant, while starting up. Most power sources are designed to accommodate quick spikes in demand, but I could never get this to work reliably, and I ultimately decided that the best way to deal with spikes is to eliminate the spikes.


Actually, when we built the boat, I did some customizing that was supposed to have made the electrical system simple. Sans Souci has 14kw of inverter capacity, which I believed would be adequate to run the entire boat, exclusive of air conditioning. In fact, my original plan had been that I could even run a limited amount of air conditioning off of the inverters.


However, this was my first experience with a chilled water air conditioning system, and I did not understand the current requirements. The minimum required power for any air conditioning on Sans Souci is around 7kw (Chiller plus Circ Pump plus Raw Water Pump plus Air Handler). I have a large battery bank, but it isn’t large enough to run the air conditioning for long.


With a little bit of math, I can quantify how long I could run the air conditioning off of batteries.


My battery bank of 1,500 amps, at 24 volt. With a bit of math, this equates to 18kw of stored power, implying I could run the air conditioning nearly three hours. Wrong. For maximum battery life, the battery bank should never be discharged more than 50%. In other words, I have only 9kw of stored power. Well.. unfortunately, wrong again. The process of converting power from 24 volt to 240 volt “eats” some of the power. In other words, 9kw in the battery equates to only 8kw by the time it gets converted to 240 volts. If nothing else on the boat were running, I could run the air conditioning for a little over an hour off the batteries.


The bottom line on this discussion: Realistically speaking, air conditioning cannot be run off the batteries. Period. When the air conditioning is running, a generator must be running. I have said a few times over the past few months that Sans Souci is an “Generator Always On” boat, and this is the reason why.


Given this, I asked the technicians to move the air conditioning off the inverter circuit.




When I think about power on Sans Souci, I mentally divide the world into “with air conditioning” and “without air conditioning”. If the air conditioning is going, then I MUST be on a generator or hooked to shore power. If the air conditioning is not running, then power management is completely different. I really want to run the boat off the inverters, with the generator kicking in ONLY when the batteries need charging.


That last half of that last paragraph needs some explaining.


When the air conditioning is not running on Sans Souci, we really don’t use a ton of electricity. Our “normal” load is only around 3kw, and with my switch to LED lighting, I’m expecting that I might shave a full 1kw off of this. There are plenty of spikes along the way, such as when someone fires up the microwave to make popcorn. But, on average, I think we can get the normal burn down to around 2kw per hour. You may recall that back a few paragraphs I mentioned that we have 8kw stored in the batteries. This implies that sitting at anchor, I need to run the generator every four hours, to keep the batteries charged properly. I can lengthen this time by allowing the batteries to drop lower than a 50% charge, but this ultimately shortens the battery life.


Running the generator all the time, even without the air conditioning, is a flawed strategy. We have been doing this on Sans Souci, and it doesn’t really make sense. Let’s look at what happens, and why I say this. Let’s say that I have a load of 2kw, and start a 20kw generator (my smallest). What happens? The answer is that under-loading of a generator by 90% is very hard on the generator. The generator’s life will be shortened. Not good. Historically, we’ve solved this by turning on everything in sight. We turn on the electric heaters, we turn on the air conditioner, even if it isn’t needed, whatever. We need to get the generator load up to at least 50% or more. The downside to this strategy: It narrows the “head room” of available current, increasing the need to do power management, and wastes fuel. My generators burn 1 to 2 gallons of fuel per hour. This translates to money and range. My 20kw generator burns over a gallon per hour of fuel, which isn’t a huge amount, but it does add up. It can easily cost $75 per day, just in fuel, if the generator is run around the clock. This is real money, perhaps not by government standards, but certainly by “Ken” standards.


My latest thinking is as follows (for power management when not running air conditioning):


I have auto-start on my 20kw generator. In other words, it has a mode where it will run only when it perceives a need to be run. I’m thinking I’ll just leave the generator in auto start mode, and forget power management. Without the air conditioning running, I’d have a heck of a time overloading my 14kw of inverter capacity. I’ll just tell the generator to constantly measure battery voltage, and kick in when the voltage seems to indicate that the batteries are in need of charging. My guess is that this means “every three to four hours.”


That said, there are some strange exceptions I’m trying to sort out.


Battery charging on my boat confuses the heck out of me. My house battery bank can be charged many different ways, and there doesn’t seem to be any central brain (except my own) that regulates the charging. The batteries can be charged by: My hydraulic alternators, my main engine alternators, the inverters and a manual electric battery charger. While underway, my main engine alternators put out over 2kw each! This power goes first to the starting batteries, with any extra feeding the house bank. It is possible that my generator never needs to run while I’m underway. I also have the hydraulic alternators which can add another 8kw in battery charging while underway. Before we get underway again, I want to develop a better understanding of the battery charging. With so many devices that are trying to charge the batteries simultaneously, I don’t understand how they don’t get in each other’s way.


Perhaps you can see why I am taking the time to study my electrical plans. This can all be very confusing!


-Ken W


19 Responses

  1. Davo: I did ask how the batteries are strapped in. Jeff said that they are in solidly. There are fittings at the base, plus the cables, and on top will be a shelf which also holds them in place. -Ken W

  2. Tedgo: I have 8kw of hydraulic alternator capacity. I’m fairly certain that exclusive of the air conditioning, I can run the boat off of this. I’m fairly certain that air conditioning will not be required in the Bering Sea and will get my chance to test the theory. – Ken W

  3. Tedgo: I have actually started to type, several times, an email to the mechanics asking them to pull my main engine alternators. I am convinced I could live without them. And, I certainly don’t like the confusion associated with multiple devices all trying to charge my house bank simultaneously. I still may pull them, but really haven’t had the time to focus on it, and know that it would be perceived as controversial. -Ken W

  4. Its difficult to generalise but generator orientated boats seem to have 400 to 1000 amp hr 24 volt batteries with a small 4KW invertors. The emphasis seems to be on keeping the refrigerator and freezers going for 8 hours while the boat is not occupied, that is not wanting to run a generator while no one is onboard, or overnight sleeping in a mild climate with no lights or entertainment running.

    I think your approach to wanting a bit more capacity for the popcorn etc is the right.

    I was surprised to read that your main engine alternators can charge the house batteries, in fact their is a school of thought that the main engine starter batteries should be charged with a normal three stage charger rather than belt driven alternators. The latter can lead to overcharging when the main engines are run for days on end while passage making.

    To my way of thinking if the hydraulic alternators are not capable of supplying ALL of the power while underway then I would not use them, other than once a month to stop them rusting, and isolate them electrically. Ultimately they could be removed simplifying things.

  5. Tedgo: On the European boats of my size, what is typical for a battery bank size (# of amp hours) and what size inverters?

    My boat is set to operate as you suggest … with the generator coming on and off as needed (on its own).

    My battery charging is a mess (or, that’s my preliminary opinion. I’m still studying it).

    There are a lot of different ways to charge the batteries, several of which have intelligent multiphase charging ability, but they don’t know about each other. In other words, there can easily be confusion when multiple devices are enabled at the same time (as is typical). For instance, let’s say that I have the main engines and the inverters going. Turning off the chargers in the inverters is not easy, and yet I have the inverters and the main engines trying to charge the batteries at the same time. Ouch.

    -Ken W

  6. I have always thought that the boat is over propped, the various speed fuel consumption reports in the past have never gone beyond 1650/1700 rpm, whereas the engines are rated at 1800 rpm. Whether this is a conscious decision on your part I don’t know, but if not then it could indicate over propping.

    The prop report.pdf somewhat optimistically indicates that the boat should able to do 12.0+ knots at 1800 rpm, though my own spread sheet calculations don’t agree with that finding. I’ll have to enter the new figures in the next few days and will be interested in the practical results you get in the following months.

    The new props will probably have to turn slightly faster for a given speed, the advantage being that the engines specific fuel consumption is slightly lower. Also the slip should be less and overall efficiency higher. It will be interesting see the results and I suspect that the boat will also move slightly slower at tick over, helping with docking manoeuvres.

    Being a techy sort of person I like forums with useful technical details, its not boring. I have drawn up for myself a simply schematic of your electrical system based on panel photographs and other drawings and discussions. (I have drawn similar schematics for Cape Horn trawlers and other boats).

    The boat is electrically complex, I suppose it comes from Nordhavns heritage of small trawlers, centred around batteries and hydraulics. They have moved into bigger hulls but aren’t thinking bigger electrically.

    With European thinking boats slightly larger than San Souci would be all AC, even including lighting and bilge pumps etc, with two or more generators and shore power. Hydraulics would be confined to steering, stabilisers and bow thrusters.

    AC allows more flexible lighting, overcomes reliability problems with DC motors and generally keeps the whole electrical distribution simple and uniform. Equally European boats don’t have the curious 240/120 Volt arrangement which again simplifies things. DC is kept to a minimum for emergency lighting, navigation lights and keeping the engines running in the event of total AC failure.

    Even in the AC only boat batteries have a part to play in allowing for a quiet period, but as you are finding out its not straight forward. For an 8 to 10 hour quiet period overnight you obviously need 3000 Amp hour batteries, but still without air conditioning. Your battery capacity is still useful, in that you can turn the lights on when you return to the boat and put the kettle on.

    The real problem is that the ideal inverter is not available. The Outback Inverters are relatively crude in that they do not allow the charging current to be dynamically managed, second by second, to keep the generator properly loaded or keep the available shore power within limits. When the air conditioners are on the charge current can be lowered to suit the available supply etc.

    The ideal inverter would be capable of running in parallel with a generator or shore power and could effectively act as a master or slave. The advantage of running in parallel is that there is no need of have a changeover switch on each control panel, so no manual decision/operation is required.

    In master mode the Inverter would supply the load. When the load started to exceed it load rating, or the batteries need charging, it would start a generator. When loads and batteries return to acceptable levels it would turn off the generator.

    In slave mode it would assist the generator in meeting peak loads and at other times would charge the batteries as necessary and help keep the generator properly loaded. This assist mode would allow for a smaller generator to be used. Of coarse their needs to be a second full sized generator which can handle the full load of the boat by itself.

    Victron Energy make better Inverters which go some way to a type of parallel running, but they are still designed for small boats with low KW requirements.

    I think that if I was in a position to buy a boat like Sans Souci I would seriously consider having a Custom Inverter made on the lines I have described. Perhaps its something Nordhavn should consider.

    You raise the query about all the ways you can charge the batteries. The manual changer, the inverter and all the alternators respond to the battery voltage so they don’t interfere with each other, though this is not necessary a good thing for the batteries life.

    The charger and inverter both have three step charging to keep the batteries in good condition, but this assumes that only one is in operation at a time as the charge programs are voltage and time based for optimum results.

    Its not clear to me whether the alternators have a three stage controller between them and the batteries, if they haven’t then that is not good for the batteries life.

    It seems to me that as you have two chargers available the hydraulic alternator are redundant. We have talked before about a 25KW 240 Volt AC hydraulic alternator, no doubt you will have a better understanding of that when you cruise with Seabird.

  7. Ken,
    Did you ever find out if your batterys were clamped down. On the 20/2. blog comments you were going to ask, but as far as I can see, never updated us. Just curious to see what the answer was.

  8. Oh, and on the prop closeup, I think what you’re seeing under the zinc is a cotter pin tail locking the second prop (locking) bolt in place, followed by the prop zinc with the bolt being used as a set screw as Ron suggests. If you look closely at the locking slots on the bolt, you can see the head of the cotter pin just to the left of the bolt on the zinc.

    – John S.

  9. I would think that a difference of .13 on a prop with a diameter of 38.7″ is quite negligible. Perhaps the only time it may be noticeable (and I would even doubt this is the case) would be when a perfectly cleaned hull goes in the water, but once its been there a few weeks and the slime and undergrowth starts to appear, .13 would be the least of ones concerns…… IMHO that is…. 🙂

    – John S.

  10. Yep, a bolt that could be a set screw which remains to be cut to length. The second photo makes it clear that there is nothing bent at all.

    Everybody’s right – even me. I was trying to contrast the final pitch of the two propellers. Perhaps .13 is not statistically significant. I have no clue what the ISO 9100 standard is. I’m inclined to think that with complex propellers such as these, .13 is as good as it gets and has no practical impact.


  11. Unknown: Here’s a better look at the prop zinc: (

    I’m confused looking at the closeup. It looks like there is a bolt, AND it looks like there is something else.

    Keep in mind that you are looking at a “work in process”. My pictures where taken several days before the boat went into the water. I have the utmost confidence in Delta, and assume that we’re not seeing their final work-product.

    -Ken W

  12. John S: You are correct. It is showing the old prop pitch, and the new prop pitch. We went from a 36.5″ pitch down to a 35″ pitch.

    I had actually wrote a bunch for the blog that I decided was boring, and killed. I took the time to compute the “slippage” as the prop moves through the water. Believe it or not, I wrote the prop pitch calculations, decided it was boring, and replaced it all with an infinitely more boring discussion on power management. Somedays the blog just flows, and is good, and other days it doesn’t go so smoothly. Argh.

    -Ken W

  13. Hmmm….I may be wrong, but the way I read the report on the pitch of the props is that the stats are shown as a “Before” and “After” – (Ie – what the pitch was prior to having the work done on them, and then, after the work was done to the props, the resultant pitch is being indicated.)

    Just a thought…..

    – John S.

  14. Ron: I should have asked Jeff about his comment that the two engines were running different rpms, but didn’t. My guess is that he was doing a stress test to see what the maximum rpms were that he could run the engines at, and turned off the auto-sync. In actuality, I would NEVER run the engines at that high of an rpm. The boat achieves hull speed somewhere around 1500 rpm, so any rpm beyond that is just money thrown away. I do run the engines up once in a while, just to give them a workout, but I don’t do it very often, and usually to no more than about 1600 rpm.

    As you indicated, the second N68 does have Detroit Diesels, which have more horsepower. Perhaps that is the difference. The second N68 just had the work done, and isn’t back in the water yet, so we’ll discover simultaneously whether or not either, or both, of us achieved any increase in fuel efficiency.

    -Ken W

  15. Left Wheel:
    Pitch of wheel 36.45 Pitch of wheel 34.97

    Right Wheel:
    Pitch of wheel 36.46 Pitch of wheel 35.10

    Why the difference? Does this account for Jeff’s running the engines at different rpms?

    The second N68 has DD engines. Does she have the same gear ratio as your boat? As I recall, he paid for an engineering study and his props are different from yours.


  16. Ken,
    It was my impression that the biggest risk to the generators was how you broke them in. For example in the first 50 hours they need to be run on heavy and varying loads to break the cylinders in correctly. If you did not break them in correctly with a heavy load then you can potentially cause the life of the generator to be significantly reduced. And even that might not be seen by you but a future owner depending on how many hours you will put in the genset. After that a good break in then the life expectancy will not be greatly reduced. Having said that generators run on the same rpm and like most diesels they like to have a bit of work to do.

  17. “… This can all be very confusing! …” Fried my brain just reading it! smile

    Good Luck Ken! You can it!

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