Tunnel drive performance issue

Discussion in 'Boat Design' started by CDK, Aug 7, 2008.

  1. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    there must be something to focus on at about 6-8 ft ..cabin ..deck rail bottle of wine .........etc
     
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    I've asked a holiday guest to make pictures from the wake, but he came up with just this....
     

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  3. Guest625101138

    Guest625101138 Previous Member

    CDK
    You have a quandary. You have built something in between a surface drive and a tunnel drive.

    At low speed it operates in tunnel mode. The water is able to negoriate the severe transition from the bottom into the tunnel and stay attached. The prop is working as a conventional submerged prop.

    I gather it now does 10kts at 2800rpm on the engines. It has 2:1 reduction so the prop is doing 1400rpm. The prop is 14 X 17. Each prop will absorb 30kW in this condition. Required torque is 204Nm at the prop or 102Nm at the engine. My bet is your little VW struggles to get 102Nm at 2800rpm. Hence it cannot get into its power band. You are stuck.

    If you somehow managed to find a very steep wave that you could coast down to get on the plane the water will not remain attached to the tunnel and the prop will go into surface piecing mode. Once it gets air it will momentarily lose thrust until it picks up more rpm. If you are lucky it will be able to generate enough thrust to keep on the plane.

    I think you need to decide if you want a boat with tunnels or a boat with surface props. My personal view is that tunnels are better for your relatively slow speed application.

    If you have the performance data from the original merc - prop size, gear ratio and known speed at known rpm it can be used to confirm power predictions. The weight of the boat would also be handy just to see that it all stacks up.

    Right now if you assume the boat will do 20kts with each motor pumping out 60kW then the pitch needs to be taken down to 15".

    Reducing the pitch might just enable you to get the boat on the plane.

    You can work out how to achieve attached flow through the tunnel but right now I think its entrance has serious problems. It needs some means of funneling more water into it. Once it gets air you are back to surface prop mode. Your little motors do not have the low down grunt to break surface props free when they are fully submerged.

    Rick W
     
  4. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    Thank you Rick. I believe you hit the nail on the head.

    The follow-up on this story is in:
    http://www.boatdesign.net/forums/surface-drives/diy-tunnel-drive-20433-new-post.html

    Baeckmo is presently teaching me diesel basics. It is not exactly what I had in mind because it is a field I followed colleges in for several years and even graduated, but I let him go on without complaining. Eventually we will arrive at the hydro-dynamics, where my knowledge is insufficient.
     
  5. Guest625101138

    Guest625101138 Previous Member

    I saw that thread and the power calculation is low if the prop is 14X17 doing 10kts at 2800rpm on the engine. Means you are barking up the wrong tree chasing a problem with the motors.

    What is the rated engine torque at 2800rpm. My bet is around 100Nm.

    I thought about how you might determine the acceptability of the angle of the backplate in the tunnel. I have found that water will not stay attached through a sharp 45 degree curve at the speeds you are considering. It will stay attached at 30 degrees.

    If you get a high pressure hose and aim it along the hull toward the tunnel so it hits the bottom ahead of the tunnel it should curve up into the tunnel if it is going to stay attached. You can do a simple test with the back side of a spoon in a tap stream to understand what I mean by the flow attaching.


    The velocity ratio will be 1.35 on the 14" prop if you can get the boat to 20kts with 60kW on each motor. This means the inlet area that water can enter the tunnel needs to be at least 35% larger than the area of the prop. Otherwise the prop is being choked and the tunnel will go negative pressure. Water will come in from the sides as well if attached flow is possible between bottom and sides. THese need nice curves as well.

    Rick
     
  6. Guest625101138

    Guest625101138 Previous Member

    CDK
    I hunted up some data on the Golf 1.9l TD and the reference gave 184ft-lbf (249Nm) at 1900rpm. If this is the same as your engine then it should not be bogged down with 100Nm at 2800rpm. It should have huge torque reserve.

    Something does not add up.

    Rick W
     
  7. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    By now, we have established beyond reasonable doubt that the rpm indicators lied. There is 2100 rpm with the throttles a little over half position, resulting in 10 knots. Pushing them all the way makes no difference.

    VW used this engine block with different heads and fuel systems, so there is all kind of mis-information on the internet. My engine type is ABL, used in the VW transporter and Vanagon between 1996 and 1999. The performance is given as 55 kW at 3700 rpm, torque 127 Nm between 1700-2500 rpm.
    Labeled AAZ, with very minor modifications like a higher boost pressure and an aneroid on the injection pump, there should be 60 kW and 135 Nm.

    At the same time VW installed a new head and direct injection. The info you found belongs to the 1.9TDI, but that is (unfortunately) not what I have.

    The high pressure hose experiment is a brilliant idea. When I take the boat out this winter, I will certainly try that.
     
  8. Guest625101138

    Guest625101138 Previous Member

    CDK
    I have done two independent sets of calculations.

    The curve is based on the drag of the hull.

    The table is based on the power absorbed by a 14X17 prop running at 1050rpm.

    You can determine that the power required to drive the hull at 10kts with 1050rpm on the prop is under 10kW for each engine in both cases.

    It seems your motors are indeed underperforming by a long margin.

    Rick W
     

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  9. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    "...The curve is based on the drag of the hull..."

    Which hull?
     
  10. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    Please explain how you obtained the hull drag data.
    In your graph, drag and speed are in an almost linear relationship. That would mean that with unlimited engine power, any speed between 10 and 22 knots would be possible. I would have expected a sharp dip shortly after 10-12 knots.
    But with the original motorization, at a fixed throttle setting the boat kept moving under an awkward angle at around 10 knots, or slowly climbed over the hump and doubles its speed. Semi-planing at speeds between 10 and 20 knots was only possible with one hand on the throttles to continuously adjust them.
    I normally pushed the throttles all the way forward, waited until the boat returned to the planing angle and pulled back to a position that gave 3500 rpm and approx. 22 knots. Trimming the stern drives out increased the speed a little bit, but made climbing out much more difficult.

    With the tunnels and transom extension, the waterline increased to 26 ft.
     
  11. Guest625101138

    Guest625101138 Previous Member

    The drag curve provided assumes displacement of 2.5t, level trim and clean hull. If you have some idea of the trim at 10kts I can give you an adjusted value at that speed. I have no accurate way to correct for fouled hull.

    The point is both the drag data and prop data are not very different. The motors are only producing around 10kW at 2100rpm.

    If the motors were labouring you should be seeing black smoke from poor combustion. If this is not happening then there is likely to be a problem in fuel supply or fuel control.

    Another way to confirm the power from the motors is to look at fuel consumption. You should be able to find data for the diesel you have that will be accurate. If you cannot then a typical figure is 180g/kWh. You can back calculate the power produced based on fuel consumption at a certain speed.

    Rick W
     
  12. apex1

    apex1 Guest

    Rick is´nt it possible to keep your mouth shut when you have no clue what we´re talking about?

    >>>>>The drag curve provided assumes displacement of 2.5t, level trim and clean hull<<<<<<

    which hull Rick??? Any? `cos that fits your playstation?

    The power is well above 20kW @2100rpm and the fuel consumption at half load is never close to 180g/kWh, more likely 230g and above.

    Go reduce my points again as you do everytime one contradicts you here! What a sad individual........................................
     
  13. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Rick

    True to form you don't answer my Q's, so why not answer CDK's?
    "...Please explain how you obtained the hull drag data..."

    We would all like to know where you plucked these values from?
     
  14. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    ".....Go reduce my points again as you do everytime one contradicts you here! What a sad individual..."

    Oh, you too??....yes very sad. Like a child!
     

  15. apex1

    apex1 Guest

    No Sir!
    Children act polite and even wise sometimes, the hunger for revenge is not a childs emotion, thats a smart a.s´s one!
     
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