Opinions on the performances of my boat and advices, please

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

Tags:
  1. Jean Marc Delaplace
    Joined: Oct 2017
    Posts: 15
    Likes: 0, Points: 1
    Location: Ermont, France

    Jean Marc Delaplace Junior Member

    Hi
    I have designed a houseboat for inland waterways here in France. It is supported by three cylindrical pontoons an propelled by two, 10 kW, electric pods. Length 15 meters, width 4.6 m, weight 13 metric tons.
    I am disappointed with the top speed which is 8.5 km/h, whereas I expected 12. Any opinion ? What did I wrong? Thanks in advance.
     
  2. rwatson
    Joined: Aug 2007
    Posts: 5,770
    Likes: 259, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    You have designed ... or Built ?

    How do you know that it only has 8.5 km/h ?

    Three cylinders as a hull would be very draggy, especially if the ends are chopped off sharp edged.
     
  3. Jean Marc Delaplace
    Joined: Oct 2017
    Posts: 15
    Likes: 0, Points: 1
    Location: Ermont, France

    Jean Marc Delaplace Junior Member

    I had it built by a shipyard.
    I have measured its speed using the GPS on a canal with no current, but a light wind against me.
    The pontoons have a vertical blade like edge at the front, and an almost flat end at the rear.
    My concern is to know whether a relatively simple modification could improve the speed.
     
  4. TANSL
    Joined: Sep 2011
    Posts: 5,501
    Likes: 152, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

  5. rwatson
    Joined: Aug 2007
    Posts: 5,770
    Likes: 259, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    That's what I thought.
    The first thing to do would be the stern treatment. You could just cut it straight off diagonally as a Prism, but it would be an improvement if you could also make the sharp edges less abrupt.

    In an ideal world, you would have a much more rounded stern, but for a steel pontoon houseboat, one of these two options might not be too difficult.

    Hulls.png

    Of course, there would be buoyancy consequences, so you may need to consider some hull extensions.
     
  6. philSweet
    Joined: May 2008
    Posts: 2,179
    Likes: 128, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    In order for us to tell you what you did wrong, please tell us what it is you did.

    Specifically, we need an accurate description of the pontoon geometry. Their diameter, length of cylindrical portion, length of tapered portion, and depth of immersion. Are all three immersed equally, or is the center pontoon deeper?

    Next, we need to know the drivetrain components from batteries or genset, right on through to the prop. List everything and show us a wiring schematic. What is the location of the pods on the boat?

    We also need an accurate weight and cg location relative to the pontoons. Tell us if the boat sits dead level at rest, or trims a bit. Where did the 13 tons come from, I'm a bit suspicious of that number.

    Most boats of this size are easier to propel as monohulls than as pontoons, unless the pontoon boat is very light for her length (I'd say 13 tons qualifies). I know an old 45' Gibson houseboat with a decent monohull hull used an 80hp outboard to get to about 12 km/hr. I think it weighted about 20 tons.
     
  7. gonzo
    Joined: Aug 2002
    Posts: 13,309
    Likes: 323, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Are the electric motors generating 20kW total? If the propellers have a pitch that is too low or a blade area that is too small, you will not attain your target speed. Is there a wattmeter installed?
     
  8. Jean Marc Delaplace
    Joined: Oct 2017
    Posts: 15
    Likes: 0, Points: 1
    Location: Ermont, France

    Jean Marc Delaplace Junior Member

    The pontoons are made of aluminium. Here is a pic showing the bow:
    Proue.jpg

    view of aft, starboard side (in the shipyard):

    upload_2018-4-8_21-25-40.png

    Detail of one propeller:

    upload_2018-4-8_21-28-19.png

    Both propellers can be oriented independently. There is no rudder. A video showing the operation of the orientable propeller:

    The total length of each hull is 15 m. The length of the first segment (tapered) is 1.25 m. Diameter: 800 mm. The depth of immersion was meant to be 400 mm on the whole length. Eventually, once fully equipped, the depth of immersion is 350 mm at the bow and 500 mm at the aft.

    Using all these values, I calculated the relationship between immersion and displacement, and integrated this function from bow to aft. The total for the three pontoons gave 12,995 kg, with a center of gravity at 8 m from bow, that is, 500 mm behind the middle of the pontoons. This is why the depth is greater at the aft.

    As far as the power, Fischer Panda provided me with a computer that displays in real time the electrical power of each motor. I thus could measure the power at various speed in still water, though with a little bit of wind against me. The curve is the following one:

    upload_2018-4-8_21-42-25.png

    The abscissa is the speed in km/h. The red curve is the average motor speed in rpm (left hand side graduations) and the blue curve is the electrical power in kW (right hand side graduations).

    The propellers have a Kort nozzle, because according to Fischer Panda they provide more thrust than a bare propeller, and also produce no propeller walk when going astern.

    Of course I have no idea of the efficiency of the propellers.
     
  9. gonzo
    Joined: Aug 2002
    Posts: 13,309
    Likes: 323, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Kort nozzles provide more thrust at low speeds. However, they have more drag and less top speed. They are good for towing or dragging.
     
  10. Joao Pedro Silva
    Joined: Mar 2018
    Posts: 3
    Likes: 1, Points: 3
    Location: Rio de Janeiro - Brazil

    Joao Pedro Silva New Member

    The trim angle certainly have contributed for the difference in top speed. The trim angle at rest is around 6º and probably greater when at speed. Maybe the waves created between two pontoons affected the efficiency of the two propellers though. You could try to film the boat at move from another boat to see if the waves generated and the lake's waves are touching the structure above the pontoons too. Just my 2 cents
     
    fallguy likes this.
  11. rwatson
    Joined: Aug 2007
    Posts: 5,770
    Likes: 259, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    While you are at it, film the turbulence at the stern under significant power.

    You might find it easier to create an "add on" stern piece with a decent slow rise profile out of foam and glass, just to test the concept of reducing drag at the stern..
     
  12. rwatson
    Joined: Aug 2007
    Posts: 5,770
    Likes: 259, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Of course, all the high performance pontoon boats have 80 HP plus outboards, up to 300 HP, to get up on high speed plane.

     
  13. Jean Marc Delaplace
    Joined: Oct 2017
    Posts: 15
    Likes: 0, Points: 1
    Location: Ermont, France

    Jean Marc Delaplace Junior Member

    My aim is not speed but fuel economy. So I would like to reduce drag to gain just a few km/h of top speed. The idea of adding appendices at the aft seems intersting, and they would add some buyancy to improve my trim. What could such appendices look like? I would not like to add an extra meter to my total length, could I consider something in the 500 mm range with some efficiency? How could it be made? You mention fiberglass, I could also go back to the yard for an aluminium extension. I do not know which would be cheaper.
     
  14. fallguy
    Joined: Dec 2016
    Posts: 1,697
    Likes: 91, Points: 48, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Prop mod first.
     

  15. fallguy
    Joined: Dec 2016
    Posts: 1,697
    Likes: 91, Points: 48, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.