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

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

  1. TollyWally
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    TollyWally Senior Member

    Diesel,
    I don't have any specific questions but I am interested in the whole subject.
    I need a little firmer grasp before my questions would do me much good. What I remember from high school physics seems to fly in the face of many people's enthusiasm about electric power. But as always the devil is in the details.

    The torque possibilities of electric motors fascinates me. I look forward to your participation here on various things electric. There are some pretty smart people here who will help you with your various questions with hull dynamics and power once you get enough of the basics down to understand the lingo. Gerr's books will give you a good grounding from which to work.
     
  2. Dieseltwitch
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    Dieseltwitch Junior Member

    Ad hoc -
    I know I was laughing about your passive aggressive way of calling him out. I see a lot of people I wish I could do that to. In school i hear way to much about some ones father or brother that did this or did that or is "gona do". I just think its funny

    Wally -
    Your about where I was at 5 years ago. I hope people don't miss understand my excitement for electric motors as a hate for diesels. Personally I love diesels. Hence the name. All my friends say I have diesel fuel for blood! I went to school for diesel maintenance back in high school and my teacher told me something that got me thinking. he said (yelled), diesels were never made to be put into cars and trucks, they where designed be turned on, run at their most efficient point and then left there for a very very long time. he told me about a car he had designed "on paper" mind you, that would use a small diesel engine to power a large hydropump and then have small motors on each wheel. basically a hydrostatic car. he claimed it would be the "most efficient thing on four wheels!" when I asked him why, he said simply. We produce power in the most efficient way and use it in the most efficient way. gears, shafts and clutches are very in-efficient! he also said that you could increase the overall efficiency by turning off each motor one at a time as you get going. Why drive all four wheels when you really don't need to. later as I learned about AC motors and drive systems I realized that they do exactly what he was thinking about. lets take some work I've done for EV's as an example. I personally drive a VW Jetta TDI for my daily driver. It has a 100HP 4 cylinder Diesel turbo (175 - lbs-ft). When I look at the power curve I see that I can only get max HP around 3500 RPM (only one spot) and there is no way to choose the power of the motor just the speed. When I did the math I found that I only need 14 hp to maintain 70 MPH on flat. but at 70 mph my engine runs at 3500 RPMS, so what happened to the rest of the 86HP? Nothing! its literally blown out the tail pipe and taken away as heat in the cooling system. So this is where electric motors and battery systems come into play. Electric motors have the advantage of being able to have both their speed and power regulated. allowing them to produce the needed power at almost any given RPM. the batteries allows us to store large amounts of energy and use it when needed. this storage ability allows us to use smaller generator to move larger items. think of batteries as a power accumulators much like those found in hydraulics systems. They also act as a buffer allowing us to run our generator at its most efficient point. why make 100HP when we only need 14hp, that wouldn't make much sense. basically the generator will run independently of the the electric motor. When the system comes on line the computer looks at the charge in the batteries if the batteries are below a given set point the computer turns the diesel-genset on and begins to charge the batteries when they are full it shuts off. Now the electric motor can run when ever the drive wants it to, and it will have full power, no waiting for the motor to spin up, having a massive storage tank of power to draw from. the way I am sizing my generator is so that it can handle the power needed to move the car in the worst possible driving situation. allowing me to keep both my generator small and my battery pack small, This can be applied directly to boats, in fact it might even be easer to do. you could have a number of small jet pumps ( cause the least amount of drag when not in use) have relatively small electric motors for each pump. when you are trying to plain out or just get going you could use all of the pumps/motors. once you plane out you could turn off a number of the pumps, allowing the boat to maintain plane and still use very little power. or you can run all the motors and just turn down the power to each one . the net power use would be the same in each case. In the end you use each component todo work at its most efficient point. Doing the most work with the least amount of effort (little waste).

    Note - The diesel engine could even be supplemented with a gas turbine, they are even more efficient then a diesel engine if it can be run at its most efficient point and have that power harvested.

    "Work smarter not harder"
    "Noise is nothing more then wasted power!"

    Gas engines are around 10-20% efficient
    Diesel engines are around 30-40% efficient
    Turbine engiens are around 60% efficient
    Electric motor are around 90-95% efficient (from the power produced at the generator to the end power produced at the motors shaft!)

    Sorry I went on a rant there, hope this gets those gears turning Wally. let me know what you think. Sorry I didn't have any examples of boats, I just got a hair up my *** one day to start looking into making a diesel-electric boat. (still looking for a donor craft)
     
  3. Dieseltwitch
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    Dieseltwitch Junior Member

  4. Luckless
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    Luckless Senior Member

    Well, for a safety stand point I would want a system that completely disengaged and locked down the connection between Engine and Prop. The idea being that you use a fairly standard inboard engine to directly power the prop. When stopped you can switch the engine from powering the prop, to powering the generator.

    Now, I'm not sure about you, but if I'm tied up at some dock, gunning my engine at full speed to power my tools, I really don't want a simple clutch or something being able to slip and engage the props, or someone easily being able to pull a single lever to do it.

    Basically I ask why you would bother to run the props off electric motors, by running a large fuel engine to turn a generator,... Sounds like a fair bit of extra weight to carry on the boat. But I will agree it gives you far more flexibility in placement of gear, just at the cost of fuel efficiency.


    (I once worked on a project to implement secondary backup redundancy for a system. There was a primary with redundancy, a back up with its own redundancy, the secondary back up, and the secondary back up redundancy. After such a project I look at safety systems a little oddly I think.)
     
  5. Dieseltwitch
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    Dieseltwitch Junior Member

    Actually no, you can go with a much smaller motor then you normally would. you use the most power when you accelerate out of the hole, once you get moving the amount of power you need is greatly reduced. this allows you to size a motor to the exact size that you would need to just to keep the boat moving. A small battery pack can provided a buffer to get the boat going and then allow for harvest and storage of the diesels power. Again I'm sorry for the lack of boat examples. but here you go. the hybrid I am designing for my car, will only require a 40kW diesel engine (53 HP), thats half the size of the 100HP engine that I currently have. and half the weight and almost 3 times the efficiency. This is due to the fact that when that motor runs it will ONLY run at its most efficient point! To really understand why electric motor are superior to ICEs in broad range power production you need to look at the power curves. I will try and find some for you. but basically an electric motor can produce 300% its rated tq at 1 RPM and hold that 300% to nearly 60% of its max RPM. while a diesel engine needs time to build up to its max Tq and then once it goes past it, the tq drops off rapidly. thus is it more advantages to harness the diesel power at it most efficient point and then use the electric motor to do the actual work.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Dieseltwitch

    Very interesting. I must point out though, owing to the lack of examples as you state, i'll add one for you. Your suggestion of "..you could have a number of small jet pumps.." is where the "master plan" of falls down. To drive a jet boat efficiently, you need the largest size of jet possible, in the same way as you need the largest diameter prop you can use.

    Ideally the velocity of water coming out of the jet needs to be the same as the velocity of the boat. As the velocity of the water through the pump increases (in selecting a smaller diameter jet), your efficiency decreases. Not to mention problems with increasing cavitation.

    As for being pass-agress....well, where would we be without asking a few questions?? ;)
     
  7. Dieseltwitch
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    Dieseltwitch Junior Member

    True, I was thinking hypothetically. sounds like a jet drive would be best left to high speed craft. What about using a set of Voith Drives? These have intrigued me for some time. Then you could not only reduce the amps at each motor but also steer them independently from a single power source. my only other choice would be some time of pod drive system. Either way you get the idea. I would have to run some simulations to see what drive system would work best for what application using a hybrid system.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    The basic problem with eletric plants for propulsion on boats in a word "weight".
    Great for large cruise liners (as is now the norm). Smaller boats, the weight will kill the design.
     
  9. Dieseltwitch
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    Dieseltwitch Junior Member

    You are right in this! I don't see small run about benefitting from a hybrid system. I would be interested in finding the power need to keep a craft moving at a given speed. I have a method that works to find the power needed to keep a car or truck rolling but have never tested with a boat. if any one is up to the tanks the process is very simple. any takers? all you need is a stop watch and the specs for your boat. ie weight
     
  10. speedboats
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    speedboats Senior Member

    Going back to the original post application, in a multi-hull setup it would be easy then to run 2 drive systems but have a single motor. As was suggested, this gen-set would then power any tools needed for the workshop.

    To isolate the drives from the generator -or power source- would be as simple as an heavy duty battery isolation switch (no energy means no go...)

    As was stated, the best effiency for water propulsion is as big as practical, being large diameter jets or props. As max torque is available with an electric drive from 1 rpm, then electric is awesome for spinning these, whereas say a gasoline or diesel (and we have this problem with larger diameter jets of 12" or more) wants to spin at 3500 - 4500rpm the jet wants to be at 2800 or less.

    The example here was for a displacement vessel, where the power requirement doesn't 'drop off' once the boat is free of the water, although with a prop the amount of energy required to maintain x speed will lower once that speed is achieved, the same isn't always true for the jet. Think of a jet as a water pump (which is all they really are), to acheive 2500 rpm will require the same energy input weather the boat is loaded, unloaded, at speed or stationary. The jet is unaffected by what the vessel is doing.

    I like the idea of electric, which is the reason I've got 'into it' for a potential car project (finances always dictates other things). Not so much the green aspect, but more the beauty of efficiency and raw available power. As has been stated, the need for alot of power in a car is really for short durations, then to maintain 60mph only requires minimum hp to overcome wind drag and rolling resistance. Seeing it in boats (particularly smaller run-abouts) may be a little hopefull. 200lb for the electric motor, plus the gen-set and all the bulk that comes with it would make it prohibitive. In the original example where it would be a vessel with a workshop and a generator would already be onboard to power tools etc, it makes sense to utilise the same generator to run the drive system would save capital expense and weight of more engines for a dedicated drive system

    My 2cents
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    speedboats

    "...Going back to the original post application..."

    That is the whole point. This thread is almost a year old, so, what was selected what was done, since we can talk about X,Y and Z until the cows come home...but what was actually done. There seems to be a deafening silence in this regard!
     
  12. speedboats
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    speedboats Senior Member

    I don't disagree with you, but the topic caught my eye all the same and am interested in comments of others none-the-less. Perhaps, while hypothetical, the concept may have merit and be worth discussion. Peoples ideas (especially about electric) and technology would have and has changed over a year.

    As you say, it is somewhat disappointing not to get a reply about how, if at all, things got on with said project
     
  13. speedboats
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    speedboats Senior Member

    'till the cows come home'. I take it you are not a native of Japan?
     
  14. Guest625101138

    Guest625101138 Previous Member

    The answer is here:
    http://www.bagacayboatworks.com/

    I have not heard from Ken since the end of 2008.

    Rick W
     

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

    speedboats

    well, i'm surrounded by Cows reared for the famous Kobe beef, other than that, no :)

    Rick, thanks...pity, Cebu nice place.
     
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