New diesel-electric hybrid installation - how to size propellers?

Discussion in 'Hybrid' started by RayThackeray, Nov 10, 2011.

  1. RayThackeray
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    RayThackeray Senior Member

    Gonzo, I don't have "green customers" - we are an organization that desires to operate in such a manner as best we can. Though we could operate exclusively by sail if we like, for this boat we have a system we are going to install and make good use of. Your sniffy attitude is completely unhelpful, so you may as well butt out, like I have asked Ad Hoc to do.
     
  2. Willallison
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    Willallison Senior Member

    Ray,
    You - and your 'organisation' may well have the very best of green intentions, but as you said yourself you "really can't figure out where to start in this application". This is naval architecture 101... a propeller cares not one ounce where the power is coming from... be it internal combustion engine, diesel electric or nuclear. The simple fact is that - as others have tried to point out to you - 50kW is simply insufficient to drive a 50 ton boat.
    A number of years back I did a preliminary design for a 50ft sailboat that had a similar set up. http://imaginocean.net/contents/en-us/d13.html It weighed 17 tons and I considered the 2 x 30kW Fisher-Panda gensets marginal.
    You may not like hearing what Gonzo, Ad Hoc, etc are telling you but rest assured it is the truth.....

    In so far as your question is concerned, the simple answer is that the largest diameter prop with the least number of blades is the most efficient. It is never as simple as that, or course. A two bladed prop can, for instance, cause vibration as the blades make their sweep past the hull bottom.

    It's just one a of a myriad of decisions that must be considered in producing an outcome that meets your objectives. Just the sort of thing that those who's replies you don't like make every day...;)
     
  3. RayThackeray
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    RayThackeray Senior Member

    I'll repeat - so how is it, motorsailing (most of the time) over 21,000 miles, I've been able to happily motor an average of over 100 miles per day in a pretty heavy displacement 51 foot boat, running my knackered old diesel at just about 1100 rpm, barely above idle speed?

    But don't worry for me, I'm satisfied that this will work in my application and when I report the sea trials information in February you can gloat all you like if I'm wrong. But I don't think so.
     
  4. Willallison
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    Willallison Senior Member

    Nobody is suggesting (well I'm not, and I doubt the others are) that you won't be able to potter along at 4 knots with the sails up and the engines acting as an assist. But a prudent designer would always point out to his / her client any shortcomings of a decision taken that is likely to compromise the vessel. For most, the inability to be able to move off a lee shore (using just one example already noted) as a result of having insufficient power to do so, would be considered a safety risk.
    If you were going to jump in a plane and fly across the atlantic would you be happy to do so in the knowledge that the plane could do 50 knots, but that you might very well encounter a headwind of 60?
    Nobody is here to gloat.... we are all boating enthusiasts: both proffessional and amateur. It's a public forum, so you will read some stuff here that is perported to be fact that borders on the insane. But consider that you have had a number of knowledgeable folk (again, both proffessional and amateur)suggest to you that you might be making a mistake and none that have backed it as a sensible powering option.
    At the end of the day, it's your boat... if you are happy with the significant compromises that specifying an undersized powerplant involves then that's your call.
     
  5. RayThackeray
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    RayThackeray Senior Member

    All I ask is that people PLEASE read the VERY FIRST POST in this thread:

    "Please let's not discuss whether a total of 50kW is enough for such a medium displacement trawler, I'm well aware that I'm not going to be able to power this vessel at hull speed. This organization has made a very strong "green" commitment, and this actually fits the application of a long-range cruiser very well. 5 knots is likely the maximum speed when not motor-sailing. I'm very happy with over 100 miles a day...."

    I've already stated interminable times that I'm happy with the compromises. All I'm trying to do is size the propellers to best match this configuration. Hearing the same tired old lecture is a waste of my time and yours.
     
  6. Willallison
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    Willallison Senior Member

    Matt has already shown you the way. There are more complex means of arriving at a solution, but this will be a very good start.
    But the reason why nobody has directly answered your question is that there is no one answer... at least not one that can be given based on the info you've provided.... it is something that must be established in conjunction with a whole host of other paramaters.
    I understand your frustration at not being able to get a straight answer, but as I said, it's not a simple yes / no question. But also consider this from our point of view. Imagine if instead of being a designer, I was a surgeon. You come to me and ask if an 8 inch long knife is sufficient to sever the head off your wife. Do I say..."well..yes...". Or do ask whether chopping off her head is really the best solution....
     
    Last edited: Nov 23, 2011
  7. UK AMAT
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    UK AMAT New Member

    Ray,

    Call FRANK & JIMMIES in Fort Lauderdale. I personally have had them size/pitch/cup the props on over a dozen boats with fantastic results.
     
  8. UK AMAT
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    UK AMAT New Member

    F & J FLL 954.467.7723. Make sure you have height measrement shaft (at prop) to hull. Approx distance of props to COG. Distance of shafts from centrline. Deadrise angle. also, do vdc motors run direct from genset at output amperage or do vdc motors run from battery bank which is charged by genset? (big difference in the short run calcs). about time you stop taking beating and get accurate info.
     
  9. DCockey
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    DCockey Senior Member

    Not quite as simple as that IF the shaft speed is constrained. In that case maximum efficiency occurs at a finite diameter which is dependent on blade area, section profile, etc, and larger diameter propellers will have lower efficiency.
     
  10. gonzo
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    gonzo Senior Member

    A "green commitment" does not change the laws of physics.
     
  11. Willallison
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    Willallison Senior Member

    True, but doesn't alter my statement
     
  12. FAST FRED
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    FAST FRED Senior Member

    "and larger diameter propellers will have lower efficiency."

    A larger diameter will probably be needed if the prop is a 2 blade so the boat could be sailed with out towing a huge speed brake.
     
  13. gonzo
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    gonzo Senior Member

    Larger than what? Unless you offer specific dimensions, the statement makes no sense.
     
  14. Joakim
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    Joakim Senior Member

    What would be the power requirement from the safety point of view? In what kind of weather would this ship lose control? What would be the first problem? Not to have enough power to move at all headwind or not to have enough speed for steering in side wind.

    I think this ship may reach 8-8.5 knots at full power in calm and still be able to make 6 knots to rather heavy (~15 m/s) head wind at 2*25 kW power.

    Many of the old Swans seem to originally have 2-3 hp/tonne. Is that enough for safety? http://www.classicswan.org/swan_by_ss.php

    The current models seem to have 5-6 hp/tonne. Is this for better safety or just for faster cruising speed requirements? http://www.nautorswan.com/

    My sailboats have had 3 hp/tonne and that has always been enough for me, but I haven't tried to motor against more than 10-12 m/s and even that only for a short time. Cruising speed has been about 1.5 hp/tonne and about 1.15 speed length ratio.
     

  15. gonzo
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    gonzo Senior Member

    He is talking about .6 hp/tonne. That is 1/5 of the power your boats had; proportionally.
     
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