Prop tip velocity

Discussion in 'Props' started by michaeljc, Jun 10, 2013.

  1. gonzo
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    gonzo Senior Member

    They are not capable of adding up the length and height of containers. I think the rest of the math is way over their heads.
     
  2. DCockey
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    DCockey Senior Member

    Containers are 2.44 m wide so two containers next to each other would be 4.88 m wide, and three containers would be 7.32 m wide. Either would go through a 8.6 m wide lock. Given the other constraints up to six 40' containers would fit through the locks and under the bridges.
     
  3. michaeljc
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    michaeljc Senior Member

    Some of us do;)

    The entire time I have been throwing things around here a NA has been doing the calcs for hull resistance on barge unit for the above. I now have the results. I am going to give the same details here as I gave him. If anyone wants to do their own analysis be my guest. The result should equate within (I guesstamate) 15% - which is near enough. If calculation was flawless the results would be exactly the same.

    Sea conditions: Calm
    Wind: minimal
    Hull shape: rectangular, flat bottom
    Bow shape: rounded on vertical axis only
    LWL=14.5m

    B=7.5m

    d=0.9m

    L/B=1.9

    B/d = 8.3

    Output: graphs to show total resistance (in Kn) vs velocity (m/s) up to hull speed - and - Kn vs depth Fhn on ONE unit

    Objective: calculate economic working velocity and required thrust for 3 such units in train formation (one unit is the tug, but should be considered to have the same conditions as above)

    I have also been talking to Tug skippers to get their opinion based on experience.

    Have fun

    M
     
  4. michaeljc
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    michaeljc Senior Member

    Who is this guy? I have had enough of him. How much money does he put into real designs? We have calculated everything except the problem of getting enough thrust into 1 m water depth. I made one mistake, checked it out, found I was wrong and admitted it. Now I know more right? That is how I work.

    This guy could never find the working solution to the constraints of this real world situation. It requires innovation, not just regurgitated theory.

    I would like to put him onto tig in aluminium. I have men who cannot read or write who are masters at this. One such illiterate fool came up with the idea on how to increase the strength of a hollow beam using polystyrene. It is brilliant. On testing we got a 100% increase.

    Thanks to those here with a positive attitude, I appreciate it.

    M
     
  5. michaeljc
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    michaeljc Senior Member

    No one has asked the obvious: What is the hull depth going to be to comply with maritime rules re freeboard under full load? What is the hull clearance when full loaded in 1m of water? How does this effect air draft when carrying one empty container?

    I can play this silly game too!

    And oh: thanks for telling the width of 3 ISO containers. Gee shucks I could never have work that out
     
  6. daiquiri
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    daiquiri Engineering and Design

    Without calculations (you have paid the NA for that) and with no intention to polemicize:
    the economic working velocity will obviously be far below the "hull speed", which is around 9 kts for that hull shape. It will probably be less than half that speed (3.5-4 kts). If your objective is to attain the so-called "hull speed" with a similar vessel and to calculate the prop and engine for that condition (or close to that condition), then you will be working far from optimum at any given speed.
    Cheers
     
  7. DCockey
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    DCockey Senior Member

    Back in post #1 cruise speed was given as 5 knots which sounds reasonable. But as long as the naval architect is doing the calculations it's probably worthwhile to get resistance at higher speeds.
     
  8. daiquiri
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    daiquiri Engineering and Design

    Right, I've missed that one. My fault, I was looking at the last post with design data.
     
  9. michaeljc
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    michaeljc Senior Member

    Yes the graph is showing clearly that optimum speed will be about 3.5 - 4 knots.

    I included the request of up to hull speed only to compare anyone's work with that of the NA. I can make it a bit easier by asking for up to 85% hull speed. This would be OK

    Thanks

    M
     
  10. gonzo
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    gonzo Senior Member

    The maximum beam has to be no more than half the total width clearance on the canals.
     
  11. DCockey
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    DCockey Senior Member

    On all the canals I'm familiar with the maximum beam is limited to what fits through the locks. The rest of the canal system is built so that two vessels of that maximum beam can pass.
     
  12. gonzo
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    gonzo Senior Member

    These are natural canals.
     
  13. DCockey
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    DCockey Senior Member

    Locks on any waterways and canals are built to no more than the maximum beam of boats which can operate on the waterways and canals. Generally that maximum beam is governed by the maximum beam for two boats to safely pass outside of the locks.

    To build locks any wider generally doesn't make sense economically and wastes water. If a higher volume of traffic needs to be handled then two locks can be built side by side, which can handle more traffic then a single lock of twice the size.

    There is probably an exception to this somewhere in the world but above is the situation on the waterways and canals I'm familiar with.
     
  14. michaeljc
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    michaeljc Senior Member

    David is correct in this respect. The canals are natural but I suspect have been widened enough some time in the past such that 2 craft that can fit in the locks can pass.

    These narrow sections where passing will be tight, constitute less than 5 % of total system. This also goes for water depth of 1 m. There are plenty of wide deep sections.

    No Barges are using this system due to water depth and air draft constraints. The locks appear to be at least 30 years old.

    I did an 8 hr inspection cruise of this system a few weeks ago.


    M
     

  15. johneck
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    johneck Senior Member

    Wow, this thread went way off course! Just another thought back to the original point; with shallow draft operations you will need to be careful of ventilation of the propeller from the free surface. Of course there are ways to mitigate or avoid the problem depending on the layout of hull and propeller, but it is something to be aware of. Some river towboats run with tunnels that go above the standing waterline but fill when running ahead. This allows more tip clearance while keeping draft low.
     
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