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Old 05-08-2015, 06:37   #1
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Why Not Use AC Power for Thrusters and Windlasses?

Fantasizing about having a new boat built, I was thinking about the best way to drive the thruster and windlass.

My boat uses 24v DC power for this, which works fine with a large battery bank and large alternator.

Hydraulic would be better of course, but expensive and awfully messy (!) if you have a leak.

So why not use 230 volt AC power instead? I believe Lofrans can make their windlasses to work with this. Getting a few kilowatts of power to the device at 230v is trivial, compared to doing it at 24v, and I believe that AC motors are more efficient, too. My thruster is 10 horsepower or less than 7kW. At 230v, that's less than 30 amps, compared to hundreds of amps of DC power.

In the past, cruising boats didn't usually have that much AC power on board, but these days we have inverters which could power such devices, with AC generators for backup.

What would be the downside to this? Seems ideal for a boat with robust AC power system, which more and more larger cruising boats now have.
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Old 05-08-2015, 06:47   #2
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Re: Why Not Use AC Power for Thrusters and Windlasses?

Quote:
Originally Posted by Dockhead View Post
Fantasizing about having a new boat built, I was thinking about the best way to drive the thruster and windlass.

My boat uses 24v DC power for this, which works fine with a large battery bank and large alternator.

Hydraulic would be better of course, but expensive and awfully messy (!) if you have a leak.

So why not use 230 volt AC power instead? I believe Lofrans can make their windlasses to work with this. Getting a few kilowatts of power to the device at 230v is trivial, compared to doing it at 24v, and I believe that AC motors are more efficient, too. My thruster is 10 horsepower or less than 7kW. At 230v, that's less than 30 amps, compared to hundreds of amps of DC power.

In the past, cruising boats didn't usually have that much AC power on board, but these days we have inverters which could power such devices, with AC generators for backup.

What would be the downside to this? Seems ideal for a boat with robust AC power system, which more and more larger cruising boats now have.




I guess I'm trying to imagine how much an inverter is going to cost that will carrry 7KW (more at start up). A pure sine wave 1500W inverter cost around 6-700 euros here in DK I shudder to think what a 7Kw will cost
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Old 05-08-2015, 07:04   #3
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Re: Why Not Use AC Power for Thrusters and Windlasses?

I think your dc system is the best. Thrusters need a lot of power for short bursts, well suited to batteries, but if you loaded your full 10 HP of thrust onto a gen set, your gen set would have to work like hell to keep up, then when you released the thrust, the genset would power back down. It could easily overload your Genny resulting in a black out.

Plus you would have a delay in power. Dc-electric motor is virtually instantaneous, but with AC I think you'd have to wait for Genny to spool up every time you want power.

The Genny would bark every time you tapped your thruster, I think it would sound terrible.

I think DC thrusters are the cats meow, I wish I had one.

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Old 05-08-2015, 07:28   #4
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Re: Why Not Use AC Power for Thrusters and Windlasses?

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I guess I'm trying to imagine how much an inverter is going to cost that will carrry 7KW (more at start up). A pure sine wave 1500W inverter cost around 6-700 euros here in DK I shudder to think what a 7Kw will cost
Well, my present boat has a 3kW (nominal) Victron charger/inverter. It practically produces about 2.5kW but is good for 6kW for some seconds at a time.

My new boat (if fantasy ever turns to reality, which is questionable) will be AC-power oriented with electric cooking, and I will use a gang of charger/inverters, like three of ones like the one I have now. That will make 7.5kW continuous and 18kW for startup loads. Should run a 10 horsepower thruster just fine, especially if I go with Lithium batteries. At least I think so. Obviously would need to carefully analyzed.

To your question about cost -- they cost a few thousand each, but they are not just inverters, they are also chargers, and in fact they are the electrical center of the boat -- the bridge between AC and DC power. Worth some investment.

You've spent time on my boat -- you've no doubt noticed the seamlessness between AC and DC power -- there's always AC on tap no matter what, and no one ever worries about power (until the batts start to run down after 24 hours of sailing ). I'd like for the new boat to be like that, too, even more so.
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Old 05-08-2015, 07:33   #5
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Re: Why Not Use AC Power for Thrusters and Windlasses?

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I think your dc system is the best. Thrusters need a lot of power for short bursts, well suited to batteries, but if you loaded your full 10 HP of thrust onto a gen set, your gen set would have to work like hell to keep up, then when you released the thrust, the genset would power back down. It could easily overload your Genny resulting in a black out.

Plus you would have a delay in power. Dc-electric motor is virtually instantaneous, but with AC I think you'd have to wait for Genny to spool up every time you want power.

The Genny would bark every time you tapped your thruster, I think it would sound terrible.

I think DC thrusters are the cats meow, I wish I had one.

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If it would be like what you describe, then obviously no good. It would have to be carefully analyzed.

With the generator running I don't really doubt that it would work fine, since AC power goes through a power boost inverter/charger which limits the current taken from the gennie and adds inverted battery power to make up any deficit.

However, I would not want such a system if it required both generator running and also all the inverters to be functional. I would like for it to be failsafe enough to work with either the generator or inverters, preferably only two out of three inverters in case one fails. Otherwise it would be less inherently reliable than a DC system and so would make no sense. So would need to be analyzed.

Some big yachts use hydraulic power for windlass and thrusters, run off a hydraulic pump mechanically driven off the generator engine. I have spent some time on a boat like that. That scares me a bit, with such a single point of failure able to take down the whole system. An electrical system with diverse power sources seems much better to me.
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Old 05-08-2015, 07:48   #6
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Re: Why Not Use AC Power for Thrusters and Windlasses?

Does anyone currently make 230 VAC bow thrusters, davits, or windlasses? It may be that the cost of doing so is too high relative to hydraulic systems.
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Old 05-08-2015, 07:52   #7
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Re: Why Not Use AC Power for Thrusters and Windlasses?

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If it would be like what you describe, then obviously no good. It would have to be carefully analyzed.

With the generator running I don't really doubt that it would work fine, since AC power goes through a power boost inverter/charger which limits the current taken from the gennie and adds inverted battery power to make up any deficit.

However, I would not want such a system if it required both generator running and also all the inverters to be functional. I would like for it to be failsafe enough to work with either the generator or inverters, preferably only two out of three inverters in case one fails. Otherwise it would be less inherently reliable than a DC system and so would make no sense. So would need to be analyzed.

Some big yachts use hydraulic power for windlass and thrusters, run off a hydraulic pump mechanically driven off the generator engine. I have spent some time on a boat like that. That scares me a bit, with such a single point of failure able to take down the whole system. An electrical system with diverse power sources seems much better to me.
Your system is more advanced than I had imagined, I now think I understand what you are saying. You want an AC thruster to run off your batteries?

I have seen this, I actually managed the project for a while while my friend who designed it took vacation (bum deal- he goes sailing in the Bahamas while I hang around a dry dock in February). It was a really sweet system, and no barking generator. I might be able to find pics on an old blackberry in my home office.

If I remember right, it could maintain 75% of its 40 HP of thrust for 5 minutes. But if I recall- that was it, you couldn't switch over to Genny power, had to wait for the AGMs to recharge.

Unfortunately I wasn't the electrical guy on the team, so I can't give much in the way of details except how cold it is in a Hamilton dry dock in February.

The project also involved cutting the ship in half, just aft of the wheelhouse, and adding 12 feet to the middle and adding sponsors to widen her by 3'. She went from a dining capacity of 50 to 80, big game changer when it came to revenue. It was me who proposed the thruster addition, because I was the skipper and we were using the same dock space. I would only have 6' of clearance on either end (the boat was three decks about 80'). I knew I needed increased maneuverability if I was going to make the lengthening project work, as it turned out, the thruster was a fraction of the cost of bigger engines, so the owner agreed to the thruster.

Traditionally anything commercial, hydraulic is king. You can run your thrusters, your winches your net haulers, your crane- all off your hydraulic system.



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Old 05-08-2015, 07:53   #8
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Re: Why Not Use AC Power for Thrusters and Windlasses?

There are high power AC thrusters on the market-

WESMAR Bow and Stern Thruster for Commercial Vessels

This company also makes proportional control 24 and 48 V thrusters, so that cable size and amperage is reduced. I like the proportional control aspect.

Are your windlass and thruster batteries in the forepeak, which would allow you to limit the amount of high current cable runs from the main bank to the bow ?

Mine aren't but our boat is only 49 feet, and we have two VETUS 12V 50KG thrusters counter-rotating in one tunnel. Each draws 350 amps max, for 700 amps total, 4 minute limit. Maxwell VWC 3500 windlass draws 450 amps at stall.

Electrical thruster HP consumed would be 11.2 hp, but net hp usable as work would be 70% of that I estimate, based on heat and transmission loss. At 4 minutes, that is 46 amp hours equivalent drawn down from the bank.

Family Van is right on the response characteristics of the typical house gen set in providing clean and stable AC.

If you had a 10 kw AC generator (we have an 8kW Onan), you would have a load up time delay problem as the instant on demand of the AC motors would cause some initial oscillations as the gen engine governor settled down at the new load on the gen. A couple of short bursts would be hard for the gen set to regulate, unless you paralleled this demand with a very large (10kW) inverter that could pick up the load instantly and then naturally backed off as the gen came up.
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Old 05-08-2015, 08:21   #9
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Re: Why Not Use AC Power for Thrusters and Windlasses?

A couple things to add about the thruster system I noted above. It was a 48 volt system and had a very large battery bank. It was actually a battery room. I can't recall how many batteries, but maybe if I find my pics.

It was an independent system too. The genset was located in the engine rooms with the engines. The generator ran all AC systems on board, lights, DJ equipment, galley, it also charged the three start batteries (gen and 2 propulsion engines), then it went to another battery charger in the thruster room, where it charged the thruster bank. That is why the thruster had to run off the battery bank, the Genny was only connected to the system via the battery charger. This had the added benefit that you couldn't overload Genny and kill the sound system (and bar sales at the same time).

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Old 05-08-2015, 08:47   #10
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Re: Why Not Use AC Power for Thrusters and Windlasses?

The output from the common gensets installed in smaller yachts is
single phase power.

Conventional electric motors that operate on single phase AC power are generally restricted to 5 HP or less. Anything larger requires a three phase supply, much too involved for a small vessel.

AC motors produce minimal torque when starting, reaching their max. HP rating only at full speed. In an application where maximum torque is needed
at stalling speed (quickly starting a prop rotating in a bow thruster) you cannot beat the cost vs. performance of a DC motor.
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Old 05-08-2015, 08:48   #11
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Re: Why Not Use AC Power for Thrusters and Windlasses?

Am I missing something here but why?

I don't know about anyone else but we have the engine running when we need to use the thrusters or the windlass so have plenty of DC amps on tap. We run on 12v and have 135amp in the house battery (windlass) and around the same for the thrusters (can't check as they are under the bed and junk cupboard).

So I guess my question is why go to all the hassle of having to fire up a generator or plumb in inverters etc to go AC when your alternator will already be spinning and providing plenty of juice to power either or both?

KISS springs instantly to mind

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Old 05-08-2015, 08:49   #12
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Re: Why Not Use AC Power for Thrusters and Windlasses?

Ac is more efficient and you can use much smaller wires etc, larger aircraft all use 115 VAC, three phase 400Hz for power, the weight savings on wiring alone is significant.
But I think your more likely to be electrocuted with high voltage AC than low voltage DC.
Hey, at least your not 12V like most of us
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Old 05-08-2015, 09:06   #13
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Re: Why Not Use AC Power for Thrusters and Windlasses?

Quote:
Originally Posted by Dockhead View Post
Fantasizing about having a new boat built, I was thinking about the best way to drive the thruster and windlass.

My boat uses 24v DC power for this, which works fine with a large battery bank and large alternator.

Hydraulic would be better of course, but expensive and awfully messy (!) if you have a leak.

So why not use 230 volt AC power instead? I believe Lofrans can make their windlasses to work with this. Getting a few kilowatts of power to the device at 230v is trivial, compared to doing it at 24v, and I believe that AC motors are more efficient, too. My thruster is 10 horsepower or less than 7kW. At 230v, that's less than 30 amps, compared to hundreds of amps of DC power.

In the past, cruising boats didn't usually have that much AC power on board, but these days we have inverters which could power such devices, with AC generators for backup.

What would be the downside to this? Seems ideal for a boat with robust AC power system, which more and more larger cruising boats now have.
A problem with motors is they usually have very high start up loads and that peak load determines the system size, so if you have a 7kw power requirement at the bowthruster there will have to be a system peak capability of somewhat more, 10kw to 14kw This pushes up your inverter requirement even further. You can reduce that peak with inverters and / or soft starters at quite a cost for such big motors, but if you go to a DC motor and power it off the battery and if the battery is big enough then the battery takes care of the peak problem cheaply. A useful buffer of power. I'm sure AC would be fine, just I suspect DC is a better choice here. The only downside is the extra DC power cable.

AC shock hazard in a wet area like the anchor locker or ugh, the bow thruster locker fills me with dread.

Hydraulics is fine too. Messy like you say when (not if) it leaks. If you had a thruster powered off the main engine hydraulic pump and fitted a pump to the genset as a back up you could have a redundant system with similar weight and power to the DC system, but with a bit of extra redundancy ie can work with a house battery failure. Still, that said, I don't think redundancy for the bow thruster is too important as you have your redundancy in the form of a dinghy and when you are using the bowthruster you are often in a marina where there are usually marina staff or other boaters to help.
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Old 05-08-2015, 09:10   #14
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Re: Why Not Use AC Power for Thrusters and Windlasses?

The Outback 8048 inverter 8000 watts at 48 Vdc 240 vac 60 hz or 230vac 50 hz. This will do the job. Cost $ 5,000.00
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Old 05-08-2015, 09:37   #15
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Re: Why Not Use AC Power for Thrusters and Windlasses?

I have a 120vac ideal GHWSC windlass (12vdc control) on the bow that is rated for 3000lbs pull, a jk fabrication commercial rb 26 series horizontal drum hydraulic windlass rated for 2750 lbs full drum pull , and a 20 hp 240se 10" bowthruster by sidepower at 24vdc.
A 671 detroit runs the hydraulic windlass, either my 85amp 120v cruising gen off the detroit or my 10kw northern lights runs the bow windlass, and a separate bank of live line batteries with its own 24v charger runs the thruster.
The combination of different power sources and machinery works fine on my 75 ft steel ketch which weighs in at 50 tons.
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