DC vs AC which is better on boats

Discussion in 'Electrical Systems' started by seadreamer6, Nov 6, 2012.

  1. seadreamer6
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    seadreamer6 Junior Member

    I have always wondered why more ac isn't used on boats. My thinking is as follows:
    1. At a given wattage ac wiring is smaller because it doesn't have to carry as much current. For instance 240 watt load at 12 volts would need a wire rated at 20 amps, but at 120 volts ac would only need a wire of 2 amps. If u add up all the wiring on a boat wouldn't the smaller wire save a lot in weight?

    2. I realize that batteries are all dc but the inverter technology has come a long way.

    3. I also read that dc suffers wire loss that ac does not. Is this not a problem on the short runs in a boat?

    4. Marine wire is stranded. Is this because of the vibration loads? It seems that stranded wire has much more surface area that can corrode than solid wire.

    Any thoughts u have on ac vs dc on a boat would be greatly appreciated.
  2. DStaal
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    DStaal Junior Member

    1: That's a voltage issue, not an AC/DC issue - you can run higher DC voltage if it's useful.

    2: But they are still not 100% efficient, and many loads are DC anyway, so you'd have losses at the other end when it converts back.

    3: I haven't heard that directly, but I'm not an electrical engineer either. From what I remember though, that again is more of a voltage issue than anything else.

    I don't know for number 4 - though that seems probable, and there are other advantages to stranded wire. (And corrosion isn't going to be much different, assuming the wire isn't protected.) Most electronics, especially these days, are actually DC. AC's main selling point is that it's easy to change voltages with AC power, which means you can run long wires of really high voltages out to houses, and convert it down to something a bit nicer to work with when you get there. This avoids most of the wire loss, as you have enough voltage to keep it low. There are very few miles-long boats, so that's less of an issue.
  3. masalai
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    masalai masalai

    Neither is "better", determination is (should) be based on need and circumstance - It is easier to transfer to different voltage systems by using "mains" power, (which I have "Floating and NOT earthed as there are issues with electrolysis), and I find that more convenient as cable runs to minimise losses at 12V require very heavy cables (40ft run)...

    I have 12vDC, 24vDC and 2 different inverters on the 24V battery bank which also supplies all the ships-services & navigation systems
    a) 300W (peak 400W) to charge small appliances like cameras, cellphones, the small tablet/laptop, run broadcast radios etc
    b) 2000W (peak 4000W) to deliver power to fridge, freezer and other kitchen appliances, large laptop as well as to charge either of 2 separate engine start 12v batteries etc...

    Then an 800W (peak 1000W) inverter on the 12V systems to supply charge to the 24V battery if needed, or run the fridge & freezer etc., whilst motoring... A simple but slightly "messy" solution that meets my needs and requirements for redundancy and convenience...

    ALL MARINE wiring should be TINNED fine stranded copper and waterproof specifically for marine service... I do not use "pure sine-wave inverters" but the modified electronic conversion is OK even for charging/running both laptops equipped with 24inch separate screens and a shared HP colour-laser printer (HP CP1215)...

    Whilst I am away (not living aboard) from the boat, the engine start batteries are disconnected from the engines but still power the engine bay bilge pumps and are on charge from the 24V powered 2000W inverter system which is in turn charged through the 1500w array of 8x24V solar panels... My AC system is 240V
  4. gonzo
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    gonzo Senior Member

    You can lick a 12V battery and get away with it. 120V will kill you whether it is AC or DC. On wet locations, high voltage is a major danger.
  5. WestVanHan
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    WestVanHan Not a Senior Member

    Also needs to be looked at as a system,power vs sail,how much time is at a dock hooked up to the mains,and what you want to run with it.

    Generating AC and generating DC are two different things,and as the final bit of the charge takes much of the time-your AC genset is winding out with little load while a DC genset's regulator will pull down the rpm.
  6. jonr
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    jonr Senior Member

    IMO, GFCIs make higher voltage about as safe as lower voltage. AC or DC matters less and less as solid state inverters get better.
  7. seadreamer6
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    seadreamer6 Junior Member

    Thanks for your thoughts. Here's what i've learned so far. we'll take the easy one first.

    stranded vs solid wire on boats. Stranded wire is always the standard on boats because of the vibration inherent in a boat. Solid wire does not stand up to vibration loads. The only down side to stranded is that it will be slightly larger in diameter than solid for a given amp rating. This i because the amp rating is based on the total cross section of the conductors. In a stranded wire there are little spaces between the wires so logically the overall diameter of the wire is larger.

    Also, boats have lots of twists and turns where the wiring has to go so the flexibility of stranded is a big plus.

    There is more surface area for corrosion in stranded wire. However, this is not considered an important factor because copper is inherently corrosion resistant, and marine grade wiring standards for insulation and termination take this into account.

    I'm sure this was a nobrainer answer for you experienced folks...but it was an interesting process for me.

    Next post i'll summarize the ac vs dc question.
  8. masalai
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    masalai masalai

    The MOST IMPORTANT is that the wire is TINNED - along its entire length (a requirement for longevity and corrosion resistance)... Solid wire seems to be less able to carry load than fine stranded - but I have no documentation on that just 'other expert' advice and admonishment, which I have accepted from those who should know these things, working in the marine industry...

    An experiment you may like to try - fine stranded copper wire with insulation - induce a little sea water inside and the same for tinned of the same thickness/rating then load it with enough to raise the temperature slightly (so you can detect the warmth by touch), see how long it is before the plain copper strands blacken and arc out to become open circuit...

    PM "Landlubber", or "Pistnbroke", or quite a few others who respond in the electrics/electronics threads - They are better equipped and more knowledgeable...
  9. michael pierzga
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    michael pierzga Senior Member

    DC is perfect for low power applications.

    For high power applications ,the dc motors needed become big, heavy and expensive and the copper conductors that service this dc gear becomes big, heavy and expensive.

    This is one reason you see a mix of ac dc on boats.

    Also think of the impracticality of carrying 1500 amp hrs of batteries to service big dc gear. An AC generator is about the same size and would be more versatile. hot water, cooking , watermaking....
  10. Ike
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    Ike Senior Member

    Lets get one thing straight here. Stranded copper wire is required by law. Tinned wire is recommended but not a requirement. Most marine professionals use tinned wire, but it is not a requirement. If you need the references I can post them.

    Ac is a dangerous commodity on a boat and the AC system MUST be wired correctly in accordance with ABYC or ISO standards or you are floating in something that will easily kill you and maybe people around you as well. Most equipment on boats is DC. However, many appliances that people want to use on their boats, such as air conditioning and microwaves are AC. So AC is a convenience, not a necessity.

    Yes, you could reduce the wire size but you can do the same thing using 48 V DC instead of 12V and not have the danger of 120V AC. Besides on most small boats the wire runs are short so the savings in weight are miniscule. For items than need a long run, like a windlass, you can install a separate battery near the windlass and keep the wires short. Part of the process of wiring a boat is calculating voltage drop and sizing wire appropriately to minimize the drop.

    As Masalai said, neither is better. Each has its appropriate application and should be used as needed. But if AC is not needed (as opposed to wanted) it should be kept off of the boat.

    The above does not apply to large yachts ( anything over 60 feet) These are a whole different game and AC is a requirement for much of the equipment on board. Boats of this size are built to much more stringent standards than small recreational boats.
  11. StianM
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    StianM Senior Member

    AC or DC is both good and choice should depend on the need.

    AC is not more dangarous than DC voltage, but can introduce a disturbance on your hart. Both AC and DC can leave you with burns and damages to nerves.

    A AC line in contact with the hull or the water will not introduce any danger to you or the suroundings, but can introduce reduce the lifespan of your metal hull.
    A DC line in contact with a metal hull will eat up your zinc and hull in no time.

    Inverters produce PWM sine wave and switching of components on and off cost energy while a converter rely on a diode bridge and mostly have loss regarding resistance and not much more.

    If you need AC onboard the best for your fuel economy would be to stick to AC and convert where you need DC. Converting the other way is much less economical.

    Fot the dangers with 120VAC they are mostly ********. I have recived 240VAC lots of times and I'm still not 6feet under. If you get AC voltage trough the body you should get to a doctor to get a EKG or whatever is called. Even you feel fine after a electrical shock your hart could be disturbed and therfore have reduced function and a hart faliure could be the result even 24H after the incident.
  12. michael pierzga
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    michael pierzga Senior Member

    For the original poster if you are building say a 40 footer for cruising , then a good way to accelerate your knowledge about ac, dc, 12v, 24, equipment choices.... is to visit a boat show and examine the electrical solutions taken by first class boat builders on high price tag boats . They always use the best solutions
  13. Frosty

    Frosty Previous Member

    The OP has done a runner.

    On 40 foot you dont have much choice, you wont be fitting 220V bilge pumps and in that size its 12 v or nothing.

    You wont be fitting a 220V radar or VHF.

    A fridge is the only one that has choice to it.

    24V like what? 40 foot wont be using 24 starter. You Dont have much choice at all, and if you were to it would be un- necessarily complicated and parts would be unobtainable.

    Just for some thinner wire?

    Lets KISS. Boats have been a round a long time all this stuff has been worked out for you.
  14. michael pierzga
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    michael pierzga Senior Member

    It is simple. Ac engine room blower or dc engine room blower ?

    Ac refrigeration cooling pump or dc ?

    24 volt anchor winch or 12 volt ?

  15. Frosty

    Frosty Previous Member

    For a 40 foot its all 12 V.

    like I said a fridge is the only choice and you can make this complicated if you want to..

    I don't think you will get a small 24 volt anchor windlass.
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