Help with calculation for electric catamaran hull: speed, length, with, depth

Discussion in 'Boat Design' started by Jon E, Jun 25, 2016.

  1. sigurd
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    sigurd Pompuous Pangolin

    Thanks Jarmo, for the paper.

    Jim,
    1) I think that because the propeller is so high (as you can see in the video), it ventilates before it has speed to make much waves at all. But yes, I guess that could happen what you said about the frothy convergence - I never had a narrow cat.
    2) Yes, wouldn't be hard to do - what the AWRP paper calls "external asymmetry". Their experiment has "internal asymmetry".
    3) Thanks for clearing it up.
     
  2. Jon E
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    Jon E Junior Member

    Hi Sigurd. I have lowered the engine first 5 cm, then 5 more (10 total). The best speed was original height (just lightly faster), but it ventilated a lot.

    The space between the test foam hulls are only 40 - 50 cm, so i will try one of these days to make it 120 cm (just flip the plywood i sit on 90 degrees). That might give a different result.

    The speed is only 16-17 km/t, so i am disappointed. The engine spray water everywhere, and looks totally ineffective.

    But the hulls has potential, it makes (the hulls!) an extremely small wake behind the boat. Engine (placement, propeller etc. ?) makes all the trouble at the time, it seems.

    Thanks to everybody that still help to keep this thread alive! :)
     

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  3. Ad Hoc
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    Ad Hoc Naval Architect

    Sounds like you need a fence(s) on your strut.
     
  4. Skyak
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    Skyak Senior Member

    FWIW If top speed is produced when the prop is ventilating it indicates to me that the prop pitch is too large for the boat to reach top speed with the motor reaching top power. The ventilating reduces the torque and allows the motor to rev higher to peak power. The electric motor will not have the problematic toque curve.

    The flat drag curves (nearly linear) on all the theoretical calculations indicate to me that the slenderness ratio is so high that waves are not significant and you are down to minimizing wetted surface area. The one caveat is that you are using infinite depth for wavelength -the real depth (shallow?) may change wavelength significantly. If the real use is deep water then check if drag is proportional to wetted area -to speed up the optimization.

    About construction, I favor a simple three panel bottom with large chine radii, and I would recommend you stick with thicker ply that does not need ribs. The flat thick panels are so much easier to keep fair over the life of the boat and a pound or two add up to nothing. All the effort to make an optimal thin hull is wasted if it is not fair and you are pulling a fat boundary layer. I still think the best design would be a trimaran with small floats for stability that don't touch the water when balanced -like your original drawing.
     
  5. Jon E
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    Jon E Junior Member

    How thick ply do you think is appropriate without ribs? Large chine radii? I am not sure i understand. What is a good large chine angle? 10 - 15 percent? Please se the attached image.

    [​IMG]

    Can you please explain. I am an amateur. :)

    I can not take credit for the original drawing. It is Rick W.'s design from an earlier post on this forum - just to illustrate what type of hull i would like to build.
     

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  6. sigurd
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    sigurd Pompuous Pangolin

    I have never experienced anything like that, always it helped to increase the depth if the propeller sucked air. I don't understand what happens in your situation.

    With regards to the AWRP paper.
    I asked my good friend Godzilla what sort of hull he would choose to carry that load at so little length at Fn 0.5.
    He said, 'Toss a pair of two by fours in there'.
    'Wow, I thought you weren't so fond of box sections, and I thought you preferred fine ends', I said.
    'As the length to displacement decreases, all dimensions must be filled out, to allow a narrow hull without excessive wetted surface'.
    I then asked about optimum hull spacing.
    'Best use two monohulls', he replied, and proceeded to hand me this graph.
    He then seemed to drift into dazed pain, and I could no longer make eye contact.
     

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  7. philSweet
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    philSweet Senior Member

    I run the same motor on a small cat dingy and initially had the same spray problems. Before doing anything else, it would be a good idea to get a handle on the spray and ventilation issues. They take a bit of fiddling to solve, but they can be solved. You need to put a thin skeg in front of the motor leg. It doesn't need to be as wide as the leg - 1 1/2 inches should do. It needs to be a very fine wedge tapering at about 4-5 degrees (each side) to a rounded-over nose that's less than 3/8 thick at the front. And it needs to extend down to just below the vent plate on the motor. Don't put any drag on the bottom of the wedge. It needs to be level on the bottom. I ended up bogging a bit of pool noodle onto the back end of the skeg and the motor leg seals against it. You also seem to have the motor kicked under a bit. This make ventilation worse. The motor should be level or a bit out at these speeds. If you get up to 25 knots, the inflow and relative slip is low enough that you can kick the leg further under and get some real advantage, but you have a high slip rate and it won't work for you at these speeds.
     
  8. messabout
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    messabout Senior Member

    Jon E a radius (rounded chine) means that the intersection of the bottom and the sides are rounded in that area. This is often done by using strip construction for the rounded part and ply for the flat parts.

    The reason for rounding the chines is to try to reduce eddy making that a sharp (hard) chine is inclined to produce. Some wizard types, like Hoerner, have investigated this phenomena thoroughly. Any radii is preferable to sharp corners but somewhere in the region of 6 to8 percent of the bottom width is worth considering. There are some favorable structural implications too. You would be diminishing the width of the flat bottom panel and so somewhat mitigating the need for thick flat panels in the bottom.

    Eddy making implies that some input of energy is consumed. Refer to Newton's famously reliable equation, F = ma. In your case you are trying to reduce the total energy consumption that is required to move the boat through the water. Every little bit counts and the sum of a lot of little bits counts largely.

    One of the more than just a "little bits" is the sum of all the wetted surfaces. Your slab sided, boxy sections, are the worst possible summations of wet surface. There are several more, fairly simple, geometries that have significantly less surface and are also fairly easy to build. I do hastily concede that build simplicity too has its favorable points. In this case you are looking for good performance with a very small motive force, so the little bits are worth the sweat.
     
  9. sigurd
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    sigurd Pompuous Pangolin

    philSweet, pics?
     
  10. sigurd
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    sigurd Pompuous Pangolin

    Makes sense to me Skyak. Better a perfect box than a wavy ellipse probably. How about a construction method something like this drawing?
    Carve a rough foam profile, glue the panels to it and put glass over the inside joins, then round and fair the outer corner, then glass the outside. Sounds easy to get right?
     

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  11. Jon E
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    Jon E Junior Member

    I will come with a proposal before the debate on the form becomes too technical and advanced building. Rick (see this link) has for years built fast boats for pedal operation that are clearly effective.

    The model V14 which I have as a starting point - is a very simple construction. It is demonstrably rapid, tried in the real world. I think, why reinvent the wheel...

    I am sure it might be possible to build a slightly faster boat, but is it worth the all the effort? Any comments?

    Rick's V14:
    Design Displacement - 90kg
    Length - 6m
    Beam Overall - 2m
    Beam Main Hull WL - 230mm
    Top Speed - 18.4kph
     

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  12. TANSL
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    TANSL Senior Member

    That's very sensible, in my opinion.
     
  13. sigurd
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    sigurd Pompuous Pangolin

    Seems like a good decision to me - so far the elliptic cat is calculated to about 8% less drag than the box one at design speed 10 knots - you must be the judge. I think the graphs I pulled from Michlet show the difference due to wetted area, but not due to eddy making - and I don't have a feel for that aspect at all.

    To me, the issue with a trimaran (mono with training wheels) as the yellow one, is that it is more difficult to get on board from a jetty, plus it is sensitive to balance - does your dog know how to balance a boat? That is why I favour the single outrigger, where you place some weight on the outrigger.
    But if you want a cat, build a cat. Either way it's going to be a great boat.

    Will you be using structural foam, as Rick did then? I am not sure where to get Klegecell, but I have bought stuff from www.r-g.de in Holland, and they have Airex, which is said to be a tough foam. I like Corecell, but I think the cheapest source for that is in Canada (noahs boatbuilding). If you go that way, maybe we could order some together and save shipping? Otherwise, plywood should still be good with the technique Rick used. Maybe Skyak will recommend a minimum plywood thickness.
     
  14. Jon E
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    Jon E Junior Member

    Video from today. Test.

    First 12 km/t, then full speed (Average 16,5 km/t - 8,9 knots). No waves from the bow. Total width is now 160 cm. No gain in speed. Total weight today is 170 kg. The boat/"test platform" has a price tag at 110 $... + the engine though.

    [​IMG]
     

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  15. Jon E
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    Jon E Junior Member

    I think i will go for plywood with stringers. I will build an aluminium construction to hold the hulls, batteries and engine. Plus myself an the dog. I will guess i could hold the weight of the hulls and the aluminium construction way below 100 kg. probably around 70 kg. + engine, batteries, myself etc. Design weight 250 kg, length 7 meters (3 sheets of plywood). I think it will be great! Thanks, Sigurd - for all your help!
     
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