Another Try at an Electric Catamaran

Discussion in 'Boat Design' started by Motivator-1, May 29, 2010.

  1. Motivator-1
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    Motivator-1 Junior Member

    Getting real Close!!

    I have been waiting to come back to everyone with some finalized data, but that is not happening yet. I would truly like to know what Rich has developed with respect to the shape of the 500kg hulls that he used in his calculations. I could possibly move forward from there alone, or at least, understand where I am trying to go. The numbers that he has provided have given me a favorable path to continue to persue in relation to the Energy Requirements.

    Thanks. again, Mental, for the explanations of the power requirements needed to push me along, but as I said, if I cannot get solid effeciency numbers for a Trolling motor, then I'll go with the Outrunner Motors and whatever drives become necessary.(Within Reason for this Boat's application)

    Mental, you felt that fairing the Trolling motors into the hulls might not be a good idea. I felt that the motors could be a part of each of the hulls as in a torpedo, with no additional Appendage as opposed to 2 - 4" diameter Blunt Motors trying to go 12 MPH through the water. Some more thoughts on this subject would be helpful. I did, indeed, plan to have enough voids in the motor embedment to allow for cooling water to be pulled through the surrounding areas around the motor for cooilng purposes.

    Still Waiting for Good News and some Good Propelling Ideas.
    Regards,
    John
     
  2. mental_boy
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    mental_boy Junior Member

    John, looks like Rick went AWOL, so you might be on your own when it comes to the hulls. If you need a 500kg hull you may just have to scale the 400kg hull until the displacement is 500kg at the scaled depth. I'll help you with that if needed when the time comes.

    I don't have any other ideas for mounting the trolling motors. I don't know how much drag the bulb on the trolling motor incurs and I don't know the exact trade-offs. Remember, if you mount the motors in the hull you'll need to use a curved shaft to align the prop with the flow and you'll need to have the prop shaft seal or go above the waterline. Pumping water around the motors eats power too.

    If you use turnigy motors have ~500w per hull. So say your motors are 87% efficient and the prop is 80% efficient, your overall efficiency will be 70% (.87 x .8 = .7). So with 1000w into the turnigy motors you'll have 700w on the hulls. going back to the graph, that's a top speed of 7 kts for the 500kg hulls. You'll also need a 12/1 or greater reduction, which means a really big cog/gear or a double reduction.

    So here's what you need to do:

    1. Make the assumption that you're going to use the 500kg cat hull. If Rick is not available to give you the drawing for the hull we can scale the 400kg hull and assume the power requirements are similar.

    2. Decide how fast you want to go, top speed. Go look at the 500kg power curve and decide how fast you want to go. That's the power on the hull, so your actual power requirements will be 40-60% higher.

    3. Decide what your cruise speed will be. Based on the capacity of your batteries and the data from Rick's graph, pick a cruise speed that gives a range that is acceptable to you. Refer to my earlier post for calcs.

    Once you've done that you can move on to choosing the motors.


     
  3. Jeremy Harris
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    Jeremy Harris Senior Member

    It's a real pity that Rick seems to have left the forum, particularly as he was so helpful to so many.

    I just hope that whatever it is that caused him to unsubscribe isn't too serious.

    One thing to bear in mind with the model aircraft outrunner motors is that they aren't that efficient if run anywhere near their rated spec. They are pretty good if under-run though, often around the 85% mark if the rpm is kep below the onset of serious core loses and the current is kept well below the very, very optimistic Chinese rating!

    A good rule of thumb for these motors is to buy the very biggest ones you can afford. That way you can get pretty good performance out of them for long periods. My modified TowerPro 5330 motor consumes only about 5 watts at full rpm with no load, but that's primarily because I'm driving it with an efficient controller that uses Hall sensors to determine rotor angle (a pretty easy mod). This motor is notionally rated at 2800 watts, but that figure is a real joke. The true maximum rating is probably half that and I'm only running it at a couple of hundred watts maximum. It was less thaan $50 though, so was still a bargain.

    Jeremy
     
  4. mental_boy
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    mental_boy Junior Member

    Ditto, I hope he's doing alright.

    I suspect Rick was just too generous with his time and he decided to focus on his own projects/life etc.

    I wouldn't blame him if that's the case, since he was essentially acting as a naval architecture consultant without pay. He has much to offer the forum, but I don't think he was getting much out of the forum.
     
  5. Motivator-1
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    Motivator-1 Junior Member

    Rick!! We Love You Man!!!

    I'm right with you guys about Rick...I felt from reading all of his posts that I was really getting to know him as a person. I'm very grateful for what he has done for me here, and as you said, Mental, we've had enough help from him to move on from here, and maybe he expects that from us.

    Thanks, Mental for offering, but I can probably draw up the new hull if I need to, but I still am in question as to what determines the best shape for 250kg per hull at 6m long? I have been told by a very reliable source, that Length/Displacement Ratio is the most important parameter. After that, anything else has little additional affect. Considering this, my guess is that by making the ends of the hulls fuller like Rick's hulls appear, that you can achieve a better Length/Beam Ratio, which, other than Draft considerations of which 2 to 1 seems to be best I'm told, that that would be all you can gain. The Double-Ended hulls seem to be the only answer as well. How do you guys feel about this?

    I have looked at the Turnigy Outrunners C80-100-130B or the C80-100-180B. The picture shows one of these motors filling up a persons Handful, and they measure about 3 inches in diameter. They claim that these are the largest. With my 12 mph top speed goals, are these even close to getting me what I am hoping for. I only want top speed for short 5 -10 minute periods. I am aware that the slow speed performance will be less, but probably not by too much. Can I effectively use these motors with reductions along with the Torquedo Props exiting from my hull's Transoms?

    Anyway, I sure appreciate whatever the rest of you can do to help me out. I wish I had something of importance to offer you guys.

    Best Regards,
    John
     
  6. portacruise
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    portacruise Senior Member

    See my post here for info on contacting Rick directly:

    http://www.boatdesign.net/forums/boat-design/ideas-fast-efficient-electric-hull-33336-4.html

    Hope this helps.

    Porta
     
  7. Motivator-1
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    Motivator-1 Junior Member

    Thanks for the link, Porta. I think that if Rick is trying to pull back, then I will respect that and just leave him alone to carry on with what he needs to do for now. Maybe he'll come back to check on our progress in the future?

    Mental,
    Considering your earlier offer about scaling the 400kg hulls to 500kg, I wondered how that works if you are not increasing length at the same time. Maybe I could use your help here?

    Regards,
    John
     
  8. portacruise
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    portacruise Senior Member

    Rick did write me to post his email to this list, but did not indicate anything about returning.

    P
     
  9. daiquiri
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    daiquiri Engineering and Design

    Regarding your hull power requiriments - if you are using Freeship or Delftship, you can evaluate the resistance with Kaper method which is a statistics-based method for slender canoe-like hulls. It should give pretty accurate results for the type of hulls you appear to be considering. It won't take into account the interference between hulls, but for widely spaced it will be small.
     
  10. mental_boy
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    mental_boy Junior Member

    I think the only files that Rick posted of the 400kg hull are in DXF format and Delftship does not appear to have an option for importing DXF files. You could just scale it in a cad program, but checking the displacement might be difficult and you need to get the panels flattened for export in 2d dxf format for cutting.

    So maybe convert to VMRL in another program, import to delfship, scale the hull, check the displacement and export the flattened panels for cnc.

    For scaling the hull I was thinking of simply scaling each dimension by the same amount, so the hull looks identical but has different volume/dimensions. It might not be the lowest drag configuration for 500kg, but I bet you could do a lot worse. I'll also bet that making the ends more full will increase drag.

    Volume goes up with the cube and you need 25% greater volume (500kg vs 400kg). So a good start would be to scale the hull by 7.7% in each dimension since that would give you 25% more volume with a cube. Then check the displacement with the hull sitting 7.7% deeper in the water. This should give you the extra 50kg per hull you need.

    Hull length will go from 18.9 ft to 20.35 ft.
     
  11. Motivator-1
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    Motivator-1 Junior Member

    Good Ideas, Mental. I'll try to use my Cad program to do what you suggest and see what I come up with. Maybe I'll only have to reshape the ends with a Batten Curve to keep the length. My program only saves in DWG, DXF, IGES, STEP, SAT, and 3DM Rhino, other than it's native format. It could be imported to Autocad and then maybe be converted to something else. Does Autocad have a compatible file type? (I don't own Autocad)

    John
     
  12. Motivator-1
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    Motivator-1 Junior Member

    Latest Hull

    Hi Guys,
    I had trouble getting any of my CAD tools to work on the imported Hull file from Rick, so I fudged around to come up with a shape that sort of resembles what Rick drew in a 200kg hull. I attached what I have so far. I would like to have any opinions that anyone has about where I'm going with this. There is one hull shown with more fullness in the ends, and I drew in a Trolling Motor only for reference. Can anyone import this DXF and calculate the parameters?

    I'm still out to lunch on what power source to use, though shallow Draft is important for where I intend to go, and Ilike the twin motor concept. Having Reverse will allow for great manuevering and help to toss weeds.

    I think that I can setle with 9 knots (10+ MPH) Top speed for the most part and with varied speeds hope to average about 40 miles, possibly with the assistance of a 235 Watt Solar Panel. I intend to use 24 VDC if possible to keep things simple. I liked the idea of running a 12 volt Trolling motor on 24 volts. I assume this means 2400 RPM, so I can keep a small diameter Prop and find the appropriate pitch that will keep the amps in order. I previously purchased 2 Motorguide Aluminum Props that I thought could be Re-pitched to some degree to accomplish this. Any Comments?

    Thanks for everything so far.

    Regards,
    John
     

    Attached Files:


  13. portacruise
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    portacruise Senior Member

    On the props, you might buy a handful of dirt cheap RC props to get an idea what the most efficient pitch is by trial and error. That's what McDenny did on this list and also what I have done for my applications. Might give an idea of how much to re-pitch the aluminum props. You would want to get motorguide props that are meant to run at 2400 rpm. Here's some links to troll props, caveats as I haven't tried them:

    http://www.kipawapropellers.com/331655.html

    http://www.youngprops.com/

    Porta
     
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