Calculating power requirements for an electric 'shop boat' ...

Discussion in 'Hybrid' started by kengrome, Aug 6, 2008.

  1. kengrome
    Joined: Jul 2006
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    kengrome Senior Member

    I may get a contract to repair boats located in various places around the Philippines (7000+ islands here). In anticipation I am designing a large power boat that has a workshop and living accommodations built into it, complete with everything my employees and I need to do the work where no electric, water, food. materials, equipment or accommodations are available.

    The boat will be a power catamaran that carries everything we need including an electrical generator with power enough for all our equipment. My thought is that since I must buy a generator anyways, why not buy one that's big enough to power two electric motors -- one in each hull -- for the boat's propulsion?

    I'm designing for 32,000 pounds displacement at 20 inches draft. Assuming 500 pounds per HP I will need only 64 HP to push the boat at displacement speed. But these are long and slender hulls with a LWL of 56.5 feet and a BWL of 5.4 feet so I actually have a L/B ratio at the waterline of more than 10:1.

    This makes me wonder:

    - What is the realistic "hull speed" of this boat?
    - Could I reach this speed with less than 64 HP?
    - How fast can I push this boat using twice the HP?
    - What would be a recommended electric propulsion motor?
    - What losses should I figure from generator to propeller?

    If I do not change things I should be able to use an 18 inch diameter propeller -- two of them actually, one beneath each hull -- but this can change if necessary since I'm only just beginning to make decisions about this boat. I welcome your thoughts, suggestions and insights ...


    Attached Files:

  2. Jango
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    Jango Senior Enthusiast

    Speed change with twice the HP (All things being equal - weight etc.) :

    sq. rt. of 2 or 1.4 times mph (knots)
  3. klick
    Joined: Apr 2008
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    klick Junior Member

    There are many motors that would meet you needs. I've been wanting to convert a boat to electric with a high power motor such as this:

    Really any of the large electric motors they have there are powerful enough for what you need. As you can see, the smaller one above is about 28HP continuous and the other is around 35HP continuous, as rated by the manufacturer. However temporarily those motors can be pushed extremly hard, even the manufacturer will admit that 100 to 200HP is easily achievable but it wouldn't be for very long at all, even with water cooling. Crazy EV dragster people have pushed that ADC 9.1" to over 500HP, I think you have to replace the whole brush assembly to push them that far, but i doubt you're looking to do this anyway.

    Regardless, the continuous rating would be what you're looking at, but that would require a very generous generator, and in reality hybrid solutions aren't all that efficient for marine applications, or for any application that requires a constant speed for a long time. You would also need a substantial battery bank to even handle the load that it requires, and that would weigh thousands of pounds, and even at 65HP it would only last 15 minutes, and that battery bank would only last a thousand cycles TOPS, if you buy the most expensive sealed deep cycle lead acid batteries there are.

    I'd still like to do it, despite most of the shortcomings, but for you i'm not sure it's worth the risk. Even if you were really rich and could go lithium ion it really wouldn't buy you much, a million dollars worth of cells still coudln't get you that far, unless you go 5knots.

  4. kengrome
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    kengrome Senior Member

    Hi Ross, thanks for the reply.

    I wouldn't be using any batteries, instead I would simply use a big generator to power the motors directly. There's the AC/DC issue to consider too I guess. Most generators output AC but I presume I would need DC motors if I want forward/reverse propulsion without a transmission -- unless I can get reversible AC motors.

    In this case I might be better off using a small power generator for my shop tools, and a couple of diesel engines for the boat's propulsion ...
  5. SheetWise
    Joined: Jul 2004
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    SheetWise All Beach -- No Water.

    Don't give up so easy. There are solutions to all of these issues -- some of them more elegant than others. Maybe you'll be the one to find the right balance ...
  6. the1much
    Joined: Jul 2007
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    the1much hippie dreams

    Ken,,get ahold of Kaptjer.,, he talked to a company that sells electric set-ups for boats,,,,,make it seem like you wanna buy it from them,,, give them all the #'s ,, and i bet they tell ya what ya need, and that they would be happy to sell you the set-up and batteries for a little under $15,000.00 ,,maybe twice as much if they like ya,,hehe ;)
  7. Guest625101138

    Guest625101138 Previous Member

    You could use a complete AC system but to use industrial components would be very expensive. Probably around three times the cost of separate diesels at the scale you are considering. Any fuel gains would be doubtful as well because even a good electrical system will only get around 90%. If the diesel has a narrow efficiency window then this could be more than made up.

    The EV automative technology does not suit the boating application. There are big gains in the energy recovery with EV systems and they focus on this. So much so that it will be allowed in F1 next year. This will really power the development. Sadly this does not help boats much where regeneration is not required.

    The scale model stuff that is being produced for electric helicopters and airplanes is brilliant but just too small for most boating applications. I figure it will not be long before we see the cheap electronics coming into a wider range of higher power applications.

    These are the sort of motors you should be looking at:

    I have not looked closely at controllers but something like this is required:
    Some prices here to give you an indication:

    If you shopped around you might find something purpose built for traction application but I think you are at the small end of this.

    The gap between the high power hobby stuff and low power industrial stuff is huge at present. You may find something that is bridging the gap if you look hard enough.

    The firm that I found (FEYS) building small boat systems no longer seems to operate. There are some firms offering electric at the top end. If you have solar collection to batteries on a sailing boat then it makes a lot of sense. There are some advantages with electric but I do not think you would appreciate them too much on a work boat. You also have to consider what expertise you have to fix the things when they go wrong. In the worst case I guess you could hard wire the motors to the alternator and effectively have an electric gearbox off the engine.

    Rick W.
  8. Ratch
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    Ratch Junior Member

    I cannot answer your question but I can point out some thing you might want to look into. From what little I understand an electric motor rated hp is not the same as an engine something to do with the electric motor being better because it can maintain equal torque across the rpm range. I too would be interested in learning exactly how they compare.

    For the AC motor look at those used in cooling towers on commercial air conditioning systems. They are designed to work in wet environments and not much more than a standard motor. How this compares to a proper marine motor I have no idea. Teco sell a lot of these where I am so would be a good place to start.

    For the motor speed control you would use a VSD (variable speed drive) this will give you accurate speed control and direction. I know Danfoss make very good ones but there are many manufacturers to chose from.

    Remote control of the VSD is easy they are designed to be run by BMS systems (computer that runs the Air Cond). They will all be able use a 0 to 10 volt input i.e. 1.3v will give you 13% speed. So a 50 cent trim pot and a $2 toggle switch will give you very accurate motor control.

    I am guessing that you would be looking at $5-7k au for each motor including VSD. reconditioned motor would be even cheaper.
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    FAST FRED Senior Member

    ON EV autos the ONLY "gain" is in heavy city traffic , stop & go , one block at a time.

    On the highway the conventional cars do better .

    There is ZERO upside to a direct genset electric motor for a simple boat.

    If you had 15 gensets , a variable hotel load , and variable speed and thruster requirements (AKA a large Cruise Ship , doing without a Berthing tug) it might be worth the engineering and operating costs.

    Pro boat builder went all through the std boat setups , and a cheap shaft , marine gear box and prop will never be beat for a long distance run .

  10. Dieseltwitch
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    Dieseltwitch Junior Member

    Hello, Im new to this forum, I am an avid EV designer. I am also an electrical engineering student here in colorado.

    While going electric may cost you more money it does have some advantages. The amount of tq available is massive! and the speed of the motor can easily out strip any diesel motor setup (10,000 RPMs) AC electric motors are unique in that they can provide a flat tq curve for getting going then transfer that tq to hp to keep the motor running at the most efficient point. what your looking to build with the diesel gen set running the motor is called a Series hybrid. I have one of the planing board right now. I even some day would like to use a turbine gen set as the power source. the site i use for motor and inverters is Metric Mind. hope this gives you a good jumping off point. if you have any question I would love to help you out.

    Also -
    Im not sure as to the power requirements of power to push a boat through the water. in search of this is how i found your post. if any one has any calculation on this mater i would love to get my hand on them.
  11. TollyWally
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    TollyWally Senior Member

    Hey Diesel,
    Interesting link, I bookmarked it. You appear to have a pretty good grasp of an interesting and somewhat misunderstood area of technology. You could be a valuable addition to the forum here. :)

    You need to get your hands on a couple books by a guy named Dave Gerr. One is the Propeller Handbook, the name of the other escapes me at the moment. I can't put my hands on it right now but it will be easy to find at the library or Amazon. Those books will get you all set up in short order.

    I wouldn't put a whole lot of effort into the Grome project. He is long on quick computer renderings and abstract theory, extremely short on follow through.
  12. Luckless
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    Luckless Senior Member

    What about stealing a design idea from farming? Run both the prop and your generator off the same engine? You're not likely to be in high demand of both at the same time after all.

    It would mean a more complex gear system, being able to disengage the generator or the prop at will, but it would likely be more fuel efficient than running an engine to turn a generator to run an electric motor.

    But I'm guessing Power Take Off units aren't all that common on marine engines, I wonder if it would it be faster to add such a thing to a standard marine engine, or easier to take a land based setup and rebuild it for marine use?
  13. Ad Hoc
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    Ad Hoc Naval Architect


    It has been almost a year since you posted your question above.
    Have you made the boat yet, what is she like, are you sailing around the philippines..???
  14. Dieseltwitch
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    Dieseltwitch Junior Member

    Wally - Thanks for the kind words. I would love to answer any questions you might have about electric drive systems. I looked the books and all three by him are on the way! Go Amazon! lol I know that most people on forums do a lot of talking and a lot less actual building, and thats ok! I like going over ideas in my head and some times they are nice to get over with other people.

    Luckless - Im a little lost as to why you would need to worry about disengaging the motor from the prop. If it was designed as a hybrid system the motor/generator could be place any where on the boat. I think I may just be reading your post wrong.

    Ad Hoc - LOL

  15. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect


    Well, reasonable question....did he or didn't he. Makes the debate more realistic then, otherwise everyone is just "philosophying" about, might be not what is or should be by now. Just adds credibility to the posting , that's all :)
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