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Old 03-08-2013, 11:41 AM   #1
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Generator Sizing and Boat E Loads?

I do not yet own a boat but am looking at 50 foot Trawlers for my wife and I to perhaps live aboard and move to WA state and cruise the Orcas and Inland Passage.

I am very, very well versed in electricity, Amps, Watts, RF, current etc.

Do these boats really have that large of an electric load requiring a 20 KW generator?

My entire house without the spa or AC draws less than 3KW as measured by me. I use an Amp probe, Kill-0-Watt meter and my new PGE smart meter displays current load.

I have four freezers, an 1800 Watt microwave, four ceiling fans, a large entertainment center, washer, dryer, dishwasher, 28 outdoor 75 Watt motion lights and more. If I were to include the AC and spa with pumps and E heat on add 10KW for a total house draw of roughly say 13KW.
But this never happens as we do not use the spa when running the AC.

I notice the boats I am looking at include a 3-4 KW inverter that sounds about right for a boat at anchor unless it has electric cooking then I understand the need for some more juice but not 20 KW.

So tell me please what systems do you require for a day at anchor from say 3 PM to 8 AM.

The bridge stuff looks to be all DC, so there is efficientcy saved there.
Most have converted to LED lights.
Some stuff going on in the engine room, OK, what is it and what does it draw?
Air handler/fan?

So, a refrigerator, freezer, a radar, VHF radio, safety and mood lighting, entertainment center OK, but still no where near 20 KW. (My 3 ton AC draws 60 AC Amps or 7200 Watts.

I assume charging all batteries takes place while underway from an Alternator or dockside power. If and when I purchase a boat I will consider a small wind generator.

So why a 20KW generator?

No flaming please, serious questions.

Thanks,
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Old 03-08-2013, 11:59 AM   #2
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Greetings,
Welcome aboard. Good question. I've wondered as well about just the same thing.
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Old 03-08-2013, 12:00 PM   #3
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Bragging rights?

We have 6500 watts and we never work it hard. No AC but an all electric galley. For the size of boat you are considering I think 6 to 8 kw would be ideal. And some limited solar capacity to float the batteries at anchor.
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Old 03-08-2013, 12:33 PM   #4
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We will easily draw 10 kw out of our 12.5 unit when (all electric) cooking, washing, drying, AC or heat pump heat cycling, water heater and charging batteries. When cruising in the PNW managing the power so the above loads can be safely achieved is pretty easy.

It is not uncommon to find a MV with two gensets, one big and one small for fluctuating load needs. It can be frustrating, knowing too much about volts and amps and trying to make sense out of boat loads.
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Old 03-08-2013, 12:45 PM   #5
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The big generators are all about AC (air conditioning, that is, not alternating current). The AC load is about the same as everything else put together. This is why in some bigger boats people have 2 gen sets, one big and one small.

Oh, and heat too if you use reverse cycle AC and/or electric heat.
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Old 03-08-2013, 12:47 PM   #6
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It all depends on how the boat is equipped and what all you are using simultaneously. I have a 20kw genset, while we have never pushed it to the limit, we have loaded it up pretty good. On the other hand, we have tripped the breaker when running on a single 240 volt 50 amp shore line (12kw theoretical)the boat has two, but some marinas can't accommodate them or we didn't feel we needed to hook up tow for whatever reason. Here's most of our stuff:

Full size electric stove, electric range, microwave, toaster, 6 AC/heat units of varying sizes. 30 gallon water heater, washer and separate dryer. A full size Sub Zero side by side reefer, plus a wine cooler and a separate ice maker. Full sized Dishwasher. A shallow well 230 volt fresh water pump that augments the DC pump. A few TVs, stereo system, some computers and some AC lighting. So there is a cadre of "household" stuff, with the biggest variable being how hot or cold it is outside, how hot or cold the sea water is, and how desirous of a temperate interior you and yours are.

Up in the PNW, you might be able to avoid using the AC/Heat system altogether by getting diesel heat and tolerate the occasional heat wave by relying on ventilation and wind. 20KW would be unneccessary in that case on our boat. The next biggest being if you are cooking, drying clothes and running dishes at the same time. But wait, there's more...

Various battery chargers and charger/inverters, which are something you don't have at your house. After a long period of not running the engines or the genset, overnight in most cases, these can have big power demands. The better ones in fact will scale their power to the boat's other demands, taking more time to charge at lower amps so as not to trip the breakers or cause overloads.

And you must consider start up surges for things like pumps and compressors

So, to run the whole boat without worrying about overloading, when it is really hot out (or really cold), 20KW is about right for us, certainly not too much. I have often considered getting second, much quieter generator of about 10kw, which my boat is pre-wired and switched to accept, for running on hot nights and when the loads do not justify the genset. We lived full time on moorings for months at a time, and anchor out a lot. And look forward to living on moorings again in the future.

Obviously there are many alternatives to having a very AC dependent boat, such as the diesel heater or use of propane or diesel-fired appliances, DC refrigeration (though that will increase battery charging requirements), all DC lighting (POs of my boat substituted way too much AC for OEM DC which some day I may get around to rectifying, it's livable now, especially since I put in a big inverter).
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Old 03-08-2013, 01:00 PM   #7
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We been a live a board for 17 on our 58 ft boat in the Puget Sound and we can barely get by on 50 amps which the usually dock power. We have to gen sets, one is 10 KW and the other is 5 KW. However we replaced the electric stove range with propane, and installed a Webasto diesel heating system to reduce the amps required to under 50 amps. 50 amps on a 50+ ft boat is not much.

However the gen set might size might not be based on KW but on HP required for other systems like hydraulics. If our 10KW needs replacing I will size by the HP rather than KW as gen set power the hydraulic for the bow thruster and get home. So I will replace with the biggest engine hp gen set that can fit in the space.

Have to look at the total requirement of the boat
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Old 03-08-2013, 01:12 PM   #8
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Stock gensets size the HP to the KW demand so they will be directly proportional to each other.
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Old 03-08-2013, 01:23 PM   #9
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Many, maybe most boats oversize their generator.

Here is a good analogy. Your house that never draws more than 10 KW at any one time, probably has a 200 amp panel. That is 48 KW. Do you need a 48 KW genset for emergency power?

Many, maybe most boats size their generator the same way. Boats of the size you are considering like caltexflanc said above have two 50 amp 240 V shore power cords, so that means that they need a 24 KW genset to supply equivalent power. Most could get by fine with a 8 KW genset by simply managing their loads intelligently, like not running the washer/dryer when you are using the stove to fix dinner.

On the other end of the scale from yours, my Mainship Pilot 34 has a 3.5 KW NextGen genset which will supply a bit less than 30 amps fully loaded. The 12,000 BTU A/C draws 12 amps, microwave 10 amps, the water heater 10 amps, the stove 8 amps per burner and the coffee pot 8 amps.

The Freedom 20 inverter/charger can draw 15 amps or greater, but I set its maximum shorepower (or genset) draw setting to 30 amps and it will automatically cut back to stay within that parameter.

So I can only run one additional appliance while running the A/C. This is actually good because it keeps the geneet loaded. I first let the inverter/charger use as much available power as it needs until the batteries start getting full and the current draw drops off. Then I turn on the water heater.

If I need one of the other appliances I turn off the water heater for a few minutes and turn it back on when I am finished.

This takes a bit of work and understanding of the load requirements, but it works. There are automatic load shedding devices that can be installed on a bigger system that do the same thing so you don't have to watch current draw and flip breakers yourself.

A 20 KW genset will rarely operate at 10 KW and often at much less. If you use the A/C at night it will be almost zero much of the time. Read the thread on underloading to see the consequences of severely underloading a diesel.

MasterVolt published a nice analysis of generator efficiency lvs loading. Fuel consumption per KW drops dramatically at low loads because it takes significant power just to keep the machinery turning whether it is producing power or not.

David
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Old 03-08-2013, 01:56 PM   #10
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Quote:
Originally Posted by caltexflanc View Post
Stock gensets size the HP to the KW demand so they will be directly proportional to each other.


If the gen set is sized by the HP then it probable will have a large KW. Actually, I am looking at a bigger hp engine and coupling it with the existing generator. To me there are two separate parts to a gen set 1) engine and 2 generator. Our smaller 5 KW generator is a cruise genorator powered by the main 671 engine. When out and about we use the cruise gen most of the time. We presently could make it by with just the 5 kw genorator, but the engine should to be about 75 hp for the get home application.
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Old 03-08-2013, 02:00 PM   #11
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Thanks all

I wrote this post because I was confused reading about the vast difference in the two power sources listed on many of the boats I am looking at. A 20KW genset and a 4KW inverter. Big differnce.

I like quiet and am able to manage my power useage as suggested by a previous post.

I like the idea of using Diesel to cook and heat with much like the Earthroamer motorhomes.
EarthRoamer Ľ EarthRoamer Xpedition Vehicle Models

That in itself could eliminate at least 5KW I am guessing.

Anyway cool forum, thanks for the polite responses. That in itself is something I am not used to.

My son is on a ship that has un limited everything except food.
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Old 03-08-2013, 02:48 PM   #12
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Just to piggy back on David's excellent post, we used to charter a very nice GB 49 in the PNW, which had as I recall, a 20kw OEM genset. He had added a 4kw second set. Boat also had diesel heat and propane cooking. We had it once in a week of 80 degree weather and never ran the big genset, just the little one now and again.

I think boat makers over size the genset in many cases because owners tend to add additional loads over the lifetime of the boat, rather than subtract them. And, it is convenient to many (including my wife) not to have to fuss with load management any more than they had to do on land.
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Old 03-08-2013, 03:12 PM   #13
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Keep in mind that there may be times when you are running every or nearly every electrical appliance at one time. Also keep in mind that the startup current of many appliances is less than the running current.

If you have a livaboard boat, you might have an electrical clothes dryer, electrric oven and range, electric water heater, airconditioning or electric heat, you might be making coffee in an electric coffee maker while ironing clothes with an electric iron while your wife is using an electric hair dryer and your daughter is vacuuming the carpit with an electric vacuum cleaner.

You might be willing to see to it that all these appliances aren't running at once, but some boat owneres might not think about it or understand the problem.

The bottom line is, you are the buyer, don't buy what you don't think you need.
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Old 03-08-2013, 03:33 PM   #14
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Originally Posted by caltexflanc View Post
Up in the PNW, you might be able to avoid using the AC/Heat system altogether by getting diesel heat and tolerate the occasional heat wave by relying on ventilation and wind.
And you must consider start up surges for things like pumps and compressors
When we are running the genset or at the dock we enjoy reverse cycle AC much more than diesel heat. Dry heat on the water helps to dry out things after a wet week or two. Two or three heating systems in the PNW is nice to have.
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Old 03-08-2013, 04:01 PM   #15
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As liveaboards, we do the "panel dance" of electricity occasionally . We have 2 30A panels;

panel 1 powers all outlets, battery charger, aft cabin heater, water heater, and the fridge.
panel 2 powers the washer/dryer, mid and forward cabin heaters and has plenty of room for additional circuits.

Since I swapped out the electric stove for propane, the only time things become dicey is when the lady powers on her hair dryer right after a morning shower, when the water heater and aft cabin heater are on. Otherwise, we do fine. Things will get better when I install a diesel furnace.

We have a 12kw genset and a 15w inverter for the TV/DVD/game console.
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Old 03-08-2013, 04:01 PM   #16
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We have an all electric boat including clothes washer, no AC and a 9kw generator.

We never have to shuffle loads, and have never overloaded the generator.

If we had AC we might have to move to a 12 or a 15 kw generator but that would be about it.
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Old 03-08-2013, 04:15 PM   #17
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Thanks everyone. I wrote a response earlier but received a message about it needing approval??
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Old 03-08-2013, 04:20 PM   #18
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Thanks everyone. I wrote a response earlier but received a message about it needing approval??
Hmmm...see that thing just under your username?

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Old 03-08-2013, 04:42 PM   #19
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?

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You tell me. What does that mean?

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Hmmm...see that thing just under your username?

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Old 03-08-2013, 05:04 PM   #20
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Thanks everyone. I wrote a response earlier but received a message about it needing approval??
I fixed it for you. New members who post hyperlinks need moderator approval. Unfortunately we have a lot of spam and this is one way we control it. In a few more posts that will not happen to you again.

Sorry for the inconvenience and welcome to the forum.
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