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Old 28-02-2014, 11:27   #31
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Re: Boston Boat Owners!!!

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Originally Posted by sailorchic34 View Post
Oh dear, I must not know anything at all.

Honey The difference between shore power and generator sizing is inrush current. Any EE will tell you you don't size the generator for the connected load, You size it for the starting inrush of the connect load.

Motors when started need 150% or more of rated amps to start the motor, depending on motor windings, etc.

On land the power plants deliver that inrush, no problem. It only happens for a second or so, and all breakers will handle the momentary surge. For emergency power and shipboard power, you must factor in the starting inrush when sizing the generator.

That is why I said it sounded about right. 12kw with inrush is 18kw or perhaps a bit more. To reduce the starting inrush, one can use a soft start VFD that limits inrush current on inductive loads. EE's love VFD's on large electrical loadsand even require them in many cases where standby generators are installed.

Don't factor inrush into the generator sizing and you will burn out the generator somewhat quickly.
Heathing is not the issue, its the air conditioning compressors and other electric motors that are the problem.

.
Your inrushing statements are interesting, but it only applies as you stated if someone is dumb enough to have all their electric appliances turned on when they switch power sources and I don't know anyone (actually did know just one but I think/hope he learned from his mistake) that stupid. If you routinely download before switching power sources, then turn on one item at a time with the items that have high surge loads such as electric motors and air conditioning compressors being turned on first and things like electric lights and battery chargers last, then you don't need anything close to a generator that could handle the total surge load that would occur if you left everything, including all the air conditioners turned on and just selected your genset as the new power source. You do need a small margin beyond the maximum amps you plan to draw because once it's all loaded up things like air conditioners will cycle off and on periodically, but if they're on separate thermostats, that won't happen simultaneously so you only need enough margin to account for the surge of one or maybe two air conditioning units, not the whole load.
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Old 28-02-2014, 11:42   #32
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Re: Boston Boat Owners!

I don't think her comments were insulting at all. They were very well presented.

Your response makes one very large assumption: that the changeover would be manual.

You simply don't know that.

It could well be that her (GG's) installation will include an automatic transfer switch to avoid that very manual changeover. It's a large enough boat and installation to consider incorporating that feature.

In that case, sailorchic is correct.
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Old 28-02-2014, 12:27   #33
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Re: Boston Boat Owners!

Proper design would allow for human error and simultaneous operation. Really with air conditioning a 60+ foot boat, your looking at three/ four heat pumps or a larger chiller/ pump combination with multiple fan coils. In any case it is quite possible to have multiple compressors kick on at the same time.

Many 40+ foot powerboats are all electric. Most I've been on can't run everything at the dock at one time. Most have larger generators then the shore power rating. Just how it is. You must factor in inrush ratings of the probable number of motors running and starting automatically at one time.

Many a time, I've had to give a list of motors with voltage/max start amps/ HP to the EE's and determine what was the real worst case load. How many 1000 HP chillers would run, etc. On the big loads I've done, everything had VFD's/ soft start which helps reduce starting loads lots. No not everything running, but it could be alot of things running. Resistive loads don't have an inrush, But motors do have inrush and power correction factors to boot.

On 30 to 40 foot boats, power requirement is not that high But on GG's big boat with 7 people aboard, its quite possible to have an electric range/oven on while washing /drying clothes, 2-3 A/C units cycling, air handler fans running, battery charging, water heater, water pumps, etc. Too many automatic loads for manual switching.

You must assume with automatic cycling loads that once in a while they will all cycle on at once. Above a certain size one can take a diversity reduction on the total load. But GG's boat size is not there yet.
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Old 28-02-2014, 13:16   #34
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Re: Boston Boat Owners!

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Originally Posted by Stu Jackson View Post
I don't think her comments were insulting at all. They were very well presented.

Your response makes one very large assumption: that the changeover would be manual.

You simply don't know that.

It could well be that her (GG's) installation will include an automatic transfer switch to avoid that very manual changeover. It's a large enough boat and installation to consider incorporating that feature.

In that case, sailorchic is correct.
I didn't think they were insulting either, that's why I said interesting! Time to dig out the reading glasses?

Yes, an auto transfer to the genset wouldn't give you the chance to download, and that's one reason why it would be very bad practice to install one in that direction. Another reason is that most people prefer to check that the volts and cycles are correct before subjecting all their sensitive and expensive electric and electronic items to an unchecked power source. It seems like severe overkill to size a genset based on the assumption that you will have everything turned on and surging at one time when that extreme case is so easily avoided by taking just one very common sense precaution. When you transfer power sources, always download first!
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Old 28-02-2014, 13:29   #35
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Re: Boston Boat Owners!

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Originally Posted by sailorchic34 View Post
Proper design would allow for human error and simultaneous operation. Really with air conditioning a 60+ foot boat, your looking at three/ four heat pumps or a larger chiller/ pump combination with multiple fan coils. In any case it is quite possible to have multiple compressors kick on at the same time.

Many 40+ foot powerboats are all electric. Most I've been on can't run everything at the dock at one time. Most have larger generators then the shore power rating. Just how it is. You must factor in inrush ratings of the probable number of motors running and starting automatically at one time.

Many a time, I've had to give a list of motors with voltage/max start amps/ HP to the EE's and determine what was the real worst case load. How many 1000 HP chillers would run, etc. On the big loads I've done, everything had VFD's/ soft start which helps reduce starting loads lots. No not everything running, but it could be alot of things running. Resistive loads don't have an inrush, But motors do have inrush and power correction factors to boot.

On 30 to 40 foot boats, power requirement is not that high But on GG's big boat with 7 people aboard, its quite possible to have an electric range/oven on while washing /drying clothes, 2-3 A/C units cycling, air handler fans running, battery charging, water heater, water pumps, etc. Too many automatic loads for manual switching.

You must assume with automatic cycling loads that once in a while they will all cycle on at once. Above a certain size one can take a diversity reduction on the total load. But GG's boat size is not there yet.
I strongly disagree that with many things operating at once, they will all cycle on at once...ever! Since the time involved in an air conditioner compressor surge load is only about 5 seconds, seriously, what are the odds of more than 1 or possibly 2 of her 4 or 5 air conditioners cycling on during the very same 5 second length of time, AND during a time when ALL of her other electrically operated toys are also operating at full capacity? I think the answer is just about never. Also, you don't need to download the smaller loads like battery chargers to avoid exceeding your maximum during a surge of one or two air conditioners.

I didn't just dream this up, when you change power sources, it's common practice to download, always! Not optional.
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Old 28-02-2014, 14:03   #36
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Re: Boston Boat Owners!

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I didn't think they were insulting either, that's why I said interesting! Time to dig out the reading glasses?
Glasses now on. Sorry, you're right, I was wrong.
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Old 28-02-2014, 14:40   #37
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Re: Boston Boat Owners!

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I strongly disagree that with many things operating at once, they will all cycle on at once...ever! Since the time involved in an air conditioner compressor surge load is only about 5 seconds, seriously, what are the odds of more than 1 or possibly 2 of her 4 or 5 air conditioners cycling on during the very same 5 second length of time, AND during a time when ALL of her other electrically operated toys are also operating at full capacity? I think the answer is just about never. Also, you don't need to download the smaller loads like battery chargers to avoid exceeding your maximum during a surge of one or two air conditioners.

I didn't just dream this up, when you change power sources, it's common practice to download, always! Not optional.
I'm not talking switching power sources, but total load on the system running on the generator after its been switched over and running. Transformer and motor induction spikes can be 30 times operational amps, for 3-5 Cycles. Incandescent lamps and switching power supply also have inrush spikes.

All Motor's have power correction factors where a 1 HP motor may need 1000 watts to make 1 hp. Depends on the motor winding efficiency , but most boat electric motors will not be premium efficient type.

Now start the motor and for the initial 3-5 cycles you've got a 30 times spike, after the rotor starts to move the inrush starts to drop over a second or two from 300% to 100%, till the motor is up to speed. Start a few loads like that with the generator 60% loaded and your going to start stressing the generator windings. Do that enough the the windings will burn up.

My quick calc, would indicate a peak load of roughly 14 -16kw on a 60' boat. Ok its a guess, but an educated guess. Generators generally list their peak rating. Continuous ratings will always be lower then peak ratings. So you never put a 12KW generator to handle a 12KW load, You have to upsize it so that the continuous rating is at or below the peak electrical load.

This is why GG's boat has a 20kw genset. The load is not 20KW its somewhat less then that. Quite a bit less normally. But the peaks will fry the windings over a short time if you go too small a genset.

Here's a nice little writeup about generator ratings.

http://pdf.cat.com/cda/files/4252018...EXE0549-00.pdf
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Old 28-02-2014, 14:52   #38
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Re: Boston Boat Owners!

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Originally Posted by Stu Jackson View Post
Glasses now on. Sorry, you're right, I was wrong.
No problem at all, I just didn't want you to think I was getting insulted by someone expressing an honest opinion, though it's one I disagree with.

It may be appropriate to size a power source to match the maximum surge of everything on the circuit surging at one time in large industrial applications or when price/weight/size is no object, but I have LOTS of experience with auxiliary generators on boats and airliners and I've never heard of any knowledgeable person not downloading before switching to an aux power source such as a genset. An auto transfer installed on a boat this size has SO much potential for causing mischief that I can't imagine why anyone would do it. Besides potentially putting out bad electrons to your electronics ( I realize that some check the output automatically before allowing a transfer), and causing everything capable of surging, to surge at once, you also will be asking your genset to go from no load to nearly max output instantly which is very hard on any diesel engine no matter how big it is. Much better to do it manually, check the power, then add the high load surging items one at a time, and then gradually the steady, smaller loads like water heaters and battery chargers, etc. while watching that the load stays below your maximum by a reasonable margin, one that will accommodate the surge load of one or two of your air conditioners. On a boat where size/weight/fuel consumption/cost are all a factor, there's just no need to size a genset to be capable of 150% of its maximum expected load.
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Old 28-02-2014, 15:14   #39
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Re: Boston Boat Owners!

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Originally Posted by sailorchic34 View Post
I'm not talking switching power sources, but total load on the system running on the generator after its been switched over and running. Transformer and motor induction spikes can be 30 times operational amps, for 3-5 Cycles. Incandescent lamps and switching power supply also have inrush spikes.

All Motor's have power correction factors where a 1 HP motor may need 1000 watts to make 1 hp. Depends on the motor winding efficiency , but most boat electric motors will not be premium efficient type.

Now start the motor and for the initial 3-5 cycles you've got a 30 times spike, after the rotor starts to move the inrush starts to drop over a second or two from 300% to 100%, till the motor is up to speed. Start a few loads like that with the generator 60% loaded and your going to start stressing the generator windings. Do that enough the the windings will burn up.

My quick calc, would indicate a peak load of roughly 14 -16kw on a 60' boat. Ok its a guess, but an educated guess. Generators generally list their peak rating. Continuous ratings will always be lower then peak ratings. So you never put a 12KW generator to handle a 12KW load, You have to upsize it so that the continuous rating is at or below the peak electrical load.

This is why GG's boat has a 20kw genset. The load is not 20KW its somewhat less then that. Quite a bit less normally. But the peaks will fry the windings over a short time if you go too small a genset.

Here's a nice little writeup about generator ratings.

http://pdf.cat.com/cda/files/4252018...EXE0549-00.pdf
First, I'm not advocating that she buy a 12kw genset to handle a 12kw load. I also think your estimate of 14-16kw is pretty generous for a 60' boat. Of course an electric oven and clothes washing machine and more air conditioners would add a considerable load to the things I'm used to powering. My 47' boat has a 5.5kw genset that powers 2 air conditioners and the water heater and battery charger all at once. If the batteries are real low, I shut off one of the air conditioners until the load subsides a little and then turn it back on. Coincidentally, I had the same model Northern Lights 5.5kw genset on my last boat, an '89 model, and when I sold it in 2007 I had done nothing to it but change the oil regularly and replace a cracked exhaust elbow (while standing on my head!). I know the guy I sold it to and the generator is still going strong at 24 years old. This generator has about 7500 hours on it and is almost always loaded to within 20% of its maximum output. The windings seem to be doing just fine so far.
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Old 28-02-2014, 15:44   #40
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Re: Boston Boat Owners!

No question sailboats need smaller gensets. But I'm assuming that the 60' motor boat will be all electric, which is quite common, as the 20kw Genset would sort of indicate. So figure 6-8 tons cooling, electric range/ oven, microwave, water heater, battery charger. A big fridge freezer with a second small one on the fly bridge, Ice maker, washer /dryer. It adds up pretty quick.

Think you could get to 14 -16 kw pretty quick. Add surge requirements and 20kw is looking about right.

I'm always amazed how much electrical even a 40' powerboat has, and how much space. Not quite the same as a nice Tayana sailboat. BTW in the 60-65' range 20kw generators look to be pretty standard. (I looked at yachtworld). Lots O toys to power on the big powerboat boats.

20-30 percent off nameplate rating is about where continuous ratings lay.
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Old 03-03-2014, 10:32   #41
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Re: Boston Boat Owners!

First, the gen set needs to be able to deal with a reasonable demand, not the worst case scenario. Living on a boat requires that people manage their resources somewhat carefully. There is no logical reason why every appliance on the vessel needs to running at the same time at anchor. When berthed alongside, there is more flexibility provided you have 2x50 amp inlets available (not usually the case at most marinas). The problem with over sizing the gen set is that it will run cold and die young. We were told to always load up the genny, even when all we needed was to charge batteries. So there is a trade off between too big and too small - what is the Goldilocks size??
Second, regarding MA sales tax, we kept our trawler in RI for 2 years before moving to MA. No sales tax and no "use" tax. I believe that 6 months is adequate, though there should have been "no intent" to simply circumvent the MA sales tax. We had intended to go long distance cruising and legally moved from FL to RI without any tax due. When we moved to MA we received confirmation from the MA DoR that no tax was owed.
Lastly, I think GG would be best served by getting work done in Portsmouth, RI. NE Boatworks is equipped to do pretty much anything she needs and RI has all of the boat trades, equipment suppliers and services.
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Old 03-03-2014, 13:00   #42
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Re: Boston Boat Owners!

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First, the gen set needs to be able to deal with a reasonable demand, not the worst case scenario. Living on a boat requires that people manage their resources somewhat carefully. There is no logical reason why every appliance on the vessel needs to running at the same time at anchor.
People that buy 60 foot and large powerboats generally aren't that concerned with conserving resources. They want their creature comforts.

I'm not talking about everything running BTW, just the typical high end power sources. When sizing gensets, one MUST factor in starting inrush under the probable worst case. On a 60' boat that would be in my eyes, (I'm only a ME not an EE) two AC compressors and a fridge compressor vplus base load. They start and stop automatically. There will normally also be a second mini fridge on the flybridge or back deck along with an Ice machine.

If you check yachtworld, you'll see that a 60'+ motorboat many times comes with a 20kw genset. You don't think the manufacturer installed a 20kw genset just for kicks and giggles. Its there to handle the load of an all electric boat at anchor.
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Old 03-03-2014, 13:08   #43
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Re: Boston Boat Owners!

We have a 50ft boat with a 12.5KW genny that is oversized for our needs!!
It can run the hotwater heater, frig. 2 freezers, icemaker, 2 reverse AC units, charge batteries, etc. I have not run the watermaker and the washing machine at the same time, but might also be able to do that!!

My point is that firstly it is not good to oversize because the generator should be run near capacity not at 10% load. Second, 2 generators might be better suited than one large one for the same reason. And lastly, unless tied up in Newport, the opportunity to tie into 2x 50amp dock outlets is few and far between in popular locations.
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Old 03-03-2014, 13:17   #44
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Re: Boston Boat Owners!

Breathe deep. Please. What you are discussing is a basic design criteria.

Design criteria 1: I don't want to have to think about anything when I'm running my appliances. I may be in hot sweltering weather, want to AC to run full blast while I'm doing my laundry and drying clothes. This is true for ATS or manual changeovers.

Design criteria 2: I can think on my feet and recognize that I may have to turn some loads off before I engage the generator.

Each will provide a slightly different result.
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Old 03-03-2014, 15:43   #45
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Re: Boston Boat Owners!

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We have a 50ft boat with a 12.5KW genny that is oversized for our needs!!
It can run the hotwater heater, frig. 2 freezers, icemaker, 2 reverse AC units, charge batteries, etc. I have not run the watermaker and the washing machine at the same time, but might also be able to do that!!

My point is that firstly it is not good to oversize because the generator should be run near capacity not at 10% load. Second, 2 generators might be better suited than one large one for the same reason. And lastly, unless tied up in Newport, the opportunity to tie into 2x 50amp dock outlets is few and far between in popular locations.
So is your boat a sail or power boat. I assume you have a gas range/ oven. Just curious.

Stu is of course 100 correct. Either choice works.

Generator sizing, depends on many variations. Most generators should not be operated continuously above 70 percent of rated kw, unless they are rated for continuous operations at 90 or 100% load. Most oddly are not. Just as a 30 amp shore cable is not rated for 30 amps continuously.

Really operating a generator from 30 to 70 percent does not adversely effect the generator. On the other hand running a 12kw generator above 70 percent may exceed its design capacity and cause the windings to heat excessively. This as most generators are not rated for continuous 80 to 100 percent load. Read the link that I posted earlier. This per my discussions with cat and various EE's, by the way. So its not me saying it, it's engineers that design generators saying it.
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