Free charging power for small boats?

Discussion in 'Boat Design' started by Wynand N, Nov 4, 2009.

  1. Wynand N
    Joined: Oct 2004
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    Wynand N Retired Steelboatbuilder

    Something to ponder about....

    Small sail boats - lets say less than 7 meters LOA - are problematic when it comes to charging batteries due to the fact that it is too small a boat to have an inboard diesel fitted. For the moment, lets forget about small portable generators since they cannot supply propulsion and generate noise. A small wind generator or solar panel may help trickle charging, but on a small boat you are limited for space for a decent panel.

    Most small boat have tiny outboard motors and is more effective than similar sized inboard diesels getting a boat to hull speed. And the 4 strokes are very economical as well. Some do have a small charging capacity (5 amps or so) and the majority not. This is quite a shortfall when about 60 - 90 charging amps would be a fair figure for a small vessel.
    And with this said I have an idea how to generate enough power; I believe many had already considered this before, but lets hear from the pro's and engineers on the board if this idea would be feasible and or practical.

    If a small propeller is fitted under the hull with the shaft connected to an alternator via pulley system to get the alternator up to charging speed, what would the following answers be?

    1. Would the drag created by the spinning prop have any noticeable effect on speed when sailing? The surface of the prop would be very small compared to the wetted submerged area of hull.

    2. Again, if boat is under power with the outboard running, would it make a difference to the outboard's performance? Hypothetically, is it then possible to generate a lot of power, lets say 90 amps from the prop spinning in water and the engine pushing the boat with little or no noticeable handicap - IOW, power from nothing when running the OB.

    I know one can't get something from nothing or more out than you put into but many a sailboat have their props free wheeling whilst sailing and that is a lot of energy wasted that could be harvested.
    Or am I missing something:?:
    1 person likes this.
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    90A @ 14.4V = 1296W `1.7HP. That is the drag you will have on the boat. It is about the same power to move a 25 foot sailboat at 2kt with no wind
    Last edited: Nov 4, 2009
  3. wardd
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    wardd Senior Member

    why not an outboard looking rig, instead of an engine driving a prop, the prop drives an altinator

    little or no change to the boat
  4. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    I fully agree with you. Just a "torpedo" (with some air to let it half float) with a cable and electrical cable trailing your boat. An alternator need 1800 revolutions per minute to start supplying some current to your battery. At 3000 - 4000 and your battery is flat, it will supply the nominal current of the alternator. The probability that you have 90 Ampere flowing to your battery is very slim. More likely 10 - 20 Ampere. Thus the status of your battery and the speed of the yacht dictate whether your drag is noticeable. If the "torpedo" is water dynamic made, it is indeed a solution.

  5. apex1

    apex1 Guest

    Old hat mates.

    These towed generators (like a old speed log) are a good and reliable source. And sure, restricted to the size of boat Wynand has in mind.

  6. Guest625101138

    Guest625101138 Previous Member

    Your 90A, or 1.3kW as Gonzo points out, will be achieved with an efficiency around 70% for each the generator and the boat prop - 49% overall. Hence the extra power from the outboard to produce 90A will be 2.6kW. A lot of extra power and half of it wasted in churning water a bit more.

    You can get small outboards with larger alternators but they are nothing like 90A.

    I wonder what are you using all the electrical power for on a 7m boat. Modern LED lights are very energy efficient. Modern electronics do not take much. Maybe microwave and freezer are power intensive. Air conditioners are hungry but not likely on a 7m boat.

    A little water turbine would have similar performance cost to a small wind turbine when under sail but it will not work on a mooring unless you have good tidal flow.

    Rick W
  7. fcfc
    Joined: Feb 2005
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    fcfc Senior Member

    Why nobody spoke of fuel cells for small boats ?

    For small boats, they are virtually unchallengable.

    They can generate electricity at mooring, at night, with no wind and no current, with silence. Solar panel, wind generator, hydro generator (propellers), genset won't do this.

    The setup outboard + fuel cell is about the same price as smaller sail drive diesel inboard. But it will be much much lighter, and much much easier to place in a small sailboat. Noise, vibration and heat when generating electricity only will be incomparable with a diesel running an alternator. The breath and condenstion generated by a fuel cell is about the same of a sleeping man. The servicing / winterization will be much easier.

    Nearly all mini transat 6.50m have this setup. Those who have not this setup use a small 1kw portable gasoline genset for electric generation. But only dream to afford a fuel cell.
  8. apex1

    apex1 Guest

    Cost fcfc,

    they are not cheap, and the fuel is unavailable all world round. And not for free either. At present that is one of the more expensive ways to generate electricity.
  9. tkk
    Joined: Jul 2009
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    tkk Junior Member

    I have planned to make my own genset from an80-90A alternator and a small 4-stroke 2hp engine. The idea is not to have a consumption of 80 amps continuous but to be able to charge the batteries without running the stinking engine for the whole day.

    This exactly is the problem with typical inboard diesels running a small 45 amp alt: you need to idle the 50+hp diesel for hours to have your entertainment batteries fully charged.
  10. fcfc
    Joined: Feb 2005
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    fcfc Senior Member

    Price par Kw generated is around 4€. Clearly, if you need 10Kw.h a day, it is not the way to go.

    But on small sailboats, there is simply no other viable competitors. 1GM10 + SD is over 100kg, dry, without any accessory (soud proofing, exhaust etc ...) . A honda outboard 5hp is 27 kg, Yamaha 8hp 38 kg, the fuel cell is 7 kg.

    Solar panel ? where do you put 3m² of solar panel on a small sailboat. And what is the generated power of a 1/4 m² solar panel on a full day, especially if it is cloudy.
    Wind generator again : where do you put 1.2m rotor ?

    The only real choice is to severly reduce your electric consumption.

    For tkk :

    If you want to charge at 80-90A, you need a battery pack of at least 320-360A, preferably 400-450A otherwise you will destroy your batteries charging them too fast.
    Second, your engine is too small. You need at least 4 hp to run a 100A alternator.

    Propulsion engine + 4hp genset with an alternator + 400A battery pack. This not a small boat.

    For Rick :

    The first significant power hog on a small cruise boat will be a fridge. Not very big power, but alas, it will run 24/24.
  11. Wynand N
    Joined: Oct 2004
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    Location: South Africa

    Wynand N Retired Steelboatbuilder

    I said 60 - 90 amps and I agree it is a bit high for such a size boat but it is nice to have the juice available to charge batteries fast and proper. LED's are very very expensive in my country.

    The towed generators are also high on the feeding menu of some fish and sharks and thus not a good way to go about it Richard.

    I like the idea Wardd suggested to have an outboard rig on the stern to be lowered into the big blue when needed.

    I do not have use of such a charging system and the reason for this post is that many sailors with small boats need more power for charging or to use onboard goodies, and when debating ideas, usually someone will have a brilliant moment out of the ordinary that will benefit many:cool:

    My question remains; if one uses an outboard charging rig like Wardd describes, how would that effect the sailing momentum of a 6 - 7 meter sailing boat when the rig is lowered into the water and charging. Would the effect be noticeable?
    I would not mind loosing a little forward momentum to have all that power at hand if the result would be barely noticeable or not at all.
  12. Michael Tinny
    Joined: Nov 2009
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    Michael Tinny New Member

    Yup, I fully Aggree!
  13. Guest625101138

    Guest625101138 Previous Member

    If your are on a broad reach with plenty of sail pushing the hull beyond its hull speed then the load will be hardly noticed. Sails generating maybe 30kW so robbing a couple does nothing.

    If you are beating in light weather it will knock your VMG considerably. It is more than the wetted surface. Consider it as having a 1.3kW outboard running in reverse.

    Realistically if you are going to use it on a voyage then it does not need to pump out 90A. It could be a fraction of that as long as it is left running.

    If you consider a 3-bladed prop locked under sail it will be robbing maybe 50W or more. With modern electronics you could set up an electric driven prop that would also work as a generator under sail and achieve about 70% efficiency. This would get some useful output instead of just adding drag. It could do a bit better than trickle charge and be as effective as a decent solar panel or air turbine. Of course a sailor would have a folding prop.

    Rick W

  14. Guest625101138

    Guest625101138 Previous Member

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