Opinions on the performances of my boat and advices, please

Discussion in 'Hydrodynamics and Aerodynamics' started by Jean Marc Delaplace, Apr 8, 2018.

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  1. fallguy
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    fallguy Senior Member

    RW-is that going to get him past the critical Froude number as discussed in this thread attached?

    See post 35. I can’t get him anywhere near to 4.14 where he is drafting at his low speeds.

    Not pushing back; just don’t understand the concept you propose and would like to better. Perhaps you can dumb it down for me!

    btw op .. my friend is going to do the workup for you .. I did not give him your speed .. so you’ll get independent figures

    Please Sanity Check My Catamaran Design https://www.boatdesign.net/threads/please-sanity-check-my-catamaran-design.59969/page-3
     
  2. fallguy
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    fallguy Senior Member

    in the piece I linked, it suggests reducing stern drag by creating a hull shape closer to the WL - is this the goal of your drawing?

    What about the critical Froude value? Does it change or is it still going to be the same, but the design helps with stern drag?

    Hopefully not being a pest..
     
  3. rwatson
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    rwatson Senior Member

    I haven't aimed at a specific Froude number. The OP blew that when he specified using multiple cylindrical pontoons - a bad mistake for low speed cruising. Primitive bow design, parallel hull sides etc - can't be fixed easily.

    I do know that a "chopped off" stern is a huge drag, and it doesn't need any math to understand it. My practical experience was gained around pedal power small boats, sailing dinghys, and the stern design is a huge component of an inefficient hull..

    I think anyone who has played with drag ratios will recognize the validity of the concept.
     
  4. fallguy
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    fallguy Senior Member

    Thanks. Yes. This craft has some issues. The pontoons are not ideal for the weight. I was merely trying to understand if your approach was quantifiable. Experience is nothing to sneeze at!

    There have been many people who thought they could build a houseboat on a pontoon and the common wisdom is to not do it.

    Do you think he could benefit with a trim tab at all?

    If he started over and hooked a different hull under; what would it look like? Your drawing or something different?

    Kind regards.
     
  5. philSweet
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    philSweet Senior Member

    <edit> Deleted power calc. Made a mistake in the model.

    One more question. I haven't looked at the drag implications yet, but is it within the realm of possibility to remove the center pontoon and lower it about 100 mm? I can model that option if it's practical.

    Also, your electric install looked to be first rate from what I can see of it in the picture. Can you post a better view of that for us and describe what you've got going on?
     
    Last edited: Apr 11, 2018
  6. fallguy
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    fallguy Senior Member

    I need the gear ratio at the prop, too. Didn’t see it.
     
  7. fallguy
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    fallguy Senior Member

    How would lowering the center pontoon help?
     
  8. rwatson
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    rwatson Senior Member

    At 12 knots maximum, and that weight, trim tabs would just be drag.

    An ideal hull would be closer to a sailboats, with lots of compound curves, and long sweeping lines. A catamaran without the centre hull, with designed hull shapes for 12 knots would suit his purposes much better.

    Just lowering the centre hull would be a mistake IMHO, both from a performance and a stability point of view.

    Now, if they re-designed the centre hull, and made it more streamlined (like a submarine) , then lowering it could be beneficial for performance, but stability studies would need to be done carefully.
     
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  9. Jean Marc Delaplace
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    Jean Marc Delaplace Junior Member

    The motor can be seen here:
    Home http://www.whisperprop.de/en/easybox/home0/
    it is just a pod with an electric motor inside, no reducer. Max speed 1250 rpm.
    The center pontoon also contains the tanks (fuel, fresh water, gray water and black water) and many tubings are connected to it. So it would be even more complicated than lowering the outer ones.
    As far as speed, I mentioned a top speed of 8.5 km/h, not knots! My top speed is about 5 knots, then. I would be very pleased to have 6.5 knots, actually.
    For those not familiar with inland waterways, the speed limit in canals is 6 km/h (3.3 knots) and in many rivers to 15 km/h (7 knots).
     
    Last edited: Apr 12, 2018
  10. fallguy
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    fallguy Senior Member

    I looked on the F-P site and am a little confused about the motor.

    Some of their motors have torque ratings. Do you know if the easybox has a torque rating?
     
  11. fallguy
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    fallguy Senior Member

    I found it finally. 79Nm
     
  12. Mani Kandasa
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    Mani Kandasa Junior Member

    You could try attaching two small aluminum canoes with covered tops, between your hulls in the aft, to increase aft buoyancy and fix the trim.

    Capture.JPG

    I agree with Watson about the chopped-off stern. A quick fix for the chopped-off stern may be duplicating the bow at the stern too.
     
  13. fallguy
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    fallguy Senior Member

    That is not what he said!
     
  14. Mani Kandasa
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    Mani Kandasa Junior Member

    What is not what who said? ;)
     

  15. fallguy
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    fallguy Senior Member

    I don’t believe rwatson ever suggested mirroring the bow and rereading your post; you did not suggest so (my error), sorry.

    The lost buoyancy doing so would not be good. It is likely the toon would be fully submerged at the stern. Unless I am severely mistaken; you’d be creating a new problem and likely instability in turns. Quite likely poorer performance to most of the toon being deeper yet.

    As to the idea of reserve buoyancy with canoes; putting the motor pods in turbulent water is also not a great idea. How you would do so is not as easy as it might sound. Finding greener water would require dropping down further still and he is already 4” lower than plan.

    I am not a naval architect, but a casual lover of boats. The ideas proposed are all lipstick on a pig and some of mine are downright foolish now that I realize his speeds.

    rw’s idea to mod the stern is probably the best, but the op won’t want to expand the hulls outward for docking.. if I am misreading that, I apologize again, but it looks like the mod would be to add some width to the toon. No dims there; just a wild eyed guess.

    The thing I find fascinating is that FP didn’t offer a workup of new props. The likelihood of them being right is very low.

    The fellow I know is still working on a prop req afaik.

    This is a very interesting thread. On another forum, I have seen the NA decline all requests for houseboat mods of his pontoon designs.

    If the op split his outside toons down the middle and added a flat section say ? 6-12” if he had the space; (3-6” to each half) he might gain some buoyancy and lose his trim issues. Then he could take some of the rw ideas and create a smoother exit.

    All conjecture, but my perspective is that his toons are too small, and his prop is wrong for these. And stern drag is going to be very hard to eliminate due to the trim issue.

    The common monohull houseboats I have seen have sort of wide and flat (ish) hulls. He could probably get closer to that by adding a flat section. Not sure if that can be done practically.

    Interested in comments from wiser men.
     
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