DIY tunnel drive

Discussion in 'Surface Drives' started by CDK, Nov 29, 2007.

  1. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Baeckmo, double checked and triple checked the rpm gauge.
    2100 rpm is 2100-2150, dependent on the viewing angle, sunglasses and the number of drinks. At sea, both engines have the same rpm at the same throttle setting, in neutral the port engine seems a bit lazy, i.e. needs the throttle a bit more advanced to obtain the same rpm. It also runs more uneven at idle, so I'm gonna take that one out in the autumn. Both reach 4000 rpm without load.
     
  2. baeckmo
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    baeckmo Hydrodynamics

    .....50 rpm variation, seems a rather constant alco concentration!! Fine, then we can proceed without worrying about that. And high idle 4000 seems ok for a 3700 rpm powermax. Btw, what is the inner radius of the existing tunnels?

    Attached please find an example on tunnel arrangement in a crew trsp vessel I designed and built some years ago. That specific tunnel was built to replace a waterjet that didn't meet thrust specifications due to restricted inlet area (classic!!!!). Power is 240 hp VM diesel @ 3600rpm. Max operating speed 31 knots. We had to keep existing 1:1 gear ratio, necessitating a supercavitating propeller within a nozzle to control cavity stability at lower speeds.

    This tunnel is deeper than yours, and rooftop inclination higher due to design restrictions. These drawbacks could be handled because we had exact data on thrust over the operating envelope. A sister vessel had been equipped with a Volvo single prop drive leg for testing purposes. With a precision manometer in the pressure line to the trim cylinder, we recorded thrust at all relevant operating conditions (manometer calibrated against dynamometer at bollard pull). Finally, "real world" data were compared to our calc:s.

    Anyway, you can see the looong soft inlet curvature, the side wings (horizontal in this case), and the radius between wings and tunnel inlet, that I suggest you adopt as well.
     

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  3. CDK
    Joined: Aug 2007
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    CDK retired engineer

    The attached drawing I made for myself last year when I decided to change the tunnel entrance. It is not according to DIN, but it shows the dimensions and the warped ceiling I made.
     

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  4. baeckmo
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    baeckmo Hydrodynamics

    Mmmm, it occurs to me that you, Apex1, Ad Hoc and others, may contitute an acceptance committee, checking out the competence of newcomers! That might make sense to the figures you come up with...... because you are saying that a 14 in dia propeller, rotating 1050 rpm would absorb abt 36 hp! NO WAY!!!! Not even with the stem to the quayside. Possibly if working in quicksilver.....or dragged backwards! If your gearing was something like 1.2 to 1 it might happen, but with the 2.0 to 1 that you said, nope!

    So please, I asked you before: what transmission are you using, can you verify the gearing (note: can be different FW/Reverse)? Is it hydraulic, if so is the servopressure correct, ie no clutch slip? Is the manouvering valve coming all way to full position? Tape magnet (good ones found as closing strips on refrigerator doors; pay a visit to your neighbour)to the shaft. With electric coil as pickup (guess you have better fixes though), compare shaft frequency to net freq. via your oscilloscope if necessary.

    And: can you check exhaust back pressure after the turbine (use transparent plastic tubing to make a waterfilled U-tube manometer. Make connection with more heat resistant material). What does your exhaust system look like (Wet/dry, sound dampers, dimensions etc)?
     
  5. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Gearboxes: Twindisc Technodrive, 2.00:1.00, mechanically operated double cone clutches.

    Exhaust: tangential water injection with 8 mm nozzle in the lower end of the turbo-charger body, 150 mm rubber hose, dia 85 mm internal,to a stainless steel jar, axial input 81 mm at the top, radial output 65 mm dia. at the bottom, approx. 70 cm rubber hose (internal 68 mm) to the 65mm transom feed-through, approx. 120 mm higher than the bottom of the jar. No valves, knees or other obstructions.
     

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  6. baeckmo
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    baeckmo Hydrodynamics

    Check that the wastegates are not stuck in open position; pretty common trouble! Connect manometer (preferrably ~0-2.5 bar) via T-branch to the pressure-control tube and run full load. Any pressure from compressor?

    Is that a watercooled intercooler to be seen on top of inlet manifold (if yes, seawater or engine coolant inside?) or is it a water/water heat exchanger?

    The jar you mention, is there one each engine, or is it one common? Anyway, the 65 mm outlet seems rather small to me, remember there is a considerable volume of steam competing with the exhausts for flow area! The exhaust turbine is extremely sensitive to back pressure; you should use the 85 mm inner dia all the way through!!! Many turboblown marine engines in the 80-100 hp size use 100 mm here. My suggestion here: bypass the jars and go directly to the transom outlet with 85 mm hose for testing; work on one engine only untill we find the snag.

    Max acceptable exhaust back pressure at full load and rpm is ~300 mm H2O, ie 1/3 of what the same engine tolerates when naturally aspirated. With your 2100 rpms you should have just a few cm:s of backpressure.
     
    Last edited: Jul 14, 2009
  7. Ad Hoc
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    Ad Hoc Naval Architect

    baeckmo

    "...Mmmm, it occurs to me that you, Apex1, Ad Hoc and others, may contitute an acceptance committee,.."

    Hmmm, I'm not sure where you get that impression, but nothing could be further from the truth. If anything, I'm always on the receiving end of the "i have a computer program, so you can't tell me anything, even though I've never designed a boat before" and the "I've been on this website longer than you, gotta be part of the club type etc etc"..jesssss, get real. I'm sorry you feel that way from me at least, non intended at all. I think you're making an excellent summary, far better than i could. Especially since it does appear you do this type of investigation far more frequently than I do. I only did this type of investagtion on sea trails, about 2~3 times a year. So your diagnostic skills on this are more current.

    Trouble is, as you can tell from this thread, small amounts of info are given, bit by bit, which makes it very difficult to ascertain the true problem, other than proved "guesstimates", which are all subject to large errors. And as already noted, the problems do not rest in one discipline too.

    The fact you are asking, in some ways, the same questions you did at the beginning, even now, highlights this. Without the answers to the questions, it is difficult to give anywhere near an exact answer.
     
  8. apex1

    apex1 Guest

    May I please just sign that! to make it short...........
    Richard
     
  9. CDK
    Joined: Aug 2007
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    CDK retired engineer

    There is a small plugged tube at the turbo charger's output. When I remove that, there is quite a lot of air escaping, but my manometers are 0-10 bar and higher. I will order a boost pressure gauge 0-1,2 bar. The waste gates are OK, I disassembled and cleaned them at construction time. Since then, the engines made only 20 hours.

    On top of the manifold there are two rubber hoses with an adapter for the difference in diameter. The other stainless steel item is the air intake: just a hollow cylinder with mesh wire at the end to keep the mice out, and a 20mm rubber hose near the end for the crank-case ventilation. There is no intercooler.

    Each engine has the jar mounted under the turbine. Because the water is tangentially injected at the lower part of the turbine, it swirls around the inside of this jar cooling it very effectively. I've used certified exhaust hose with 80 mm outside diameter because that seems to be very common here. With 85 mm inner dia I would need also larger transom exhausts. I'll see what is available.
     
  10. baeckmo
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    baeckmo Hydrodynamics

    Don't worry Ad Hoc, I was joking; I do feel at home here with you guys! Question is if CDK will stand the "Breakdown investigators manifesto" much longer;

    1) Never trust anybody's witness; they all cover their asses and mistakes.
    2) Never trust anybody's calculations; they are all wrong since they are all based on trillion color printouts from buggy computers.
    3) Never believe in anybody's measurements; instruments were always uncalibrated and applied at the wrong place.

    CDK, the Bosch VE rotary injection pumps normally used with the VW engines have a manifold pressure compensation device that adjusts fuel flow according to the prevailing manifold pressure. It is a diaphragm thing on top of the pump housing. Check that the connection to the manifold is intact. To check if it operates, connect to a bicycle pump and pressurize slightly.

    While you are at the pump side, is the surplus fuel return to the tank operating without back pressure (ev. valves open?). The injecton timing is working on pump housing pressure; any back pressure will upset timing.

    These are car engines right? Are they complete with fuel feed pumps, or were they depending on electric "tictac pumps" that were forgotten when you took delivery? The VE pumps must have a feed pump!

    Those turbine housings will radiate an awful lot of heat into your engine compartment. Do you know the breathing air temperature? Are fuel lines at risk, generating vapour locks into injection pumps?
     
    Last edited: Jul 14, 2009
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  11. Ad Hoc
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    Ad Hoc Naval Architect

    baeckmo

    Your 1~3, sounds like the advice given to me by my old chief designer when i was a newbbie. When i presented my "findings", he would always ask have YOU witnessed it or have YOU calculated it, really emphasing the YOU and not ANYONE ESLE!

    I think CDKs patience is grand....but the alternative is to pay a professional to go there and sort it out...so in comparison, I cannot image he can complain too much at the endless questions by "us" for free advice/suggestions.

    Ive no need to chip in since you're making all the right noises :)

    Long road ahead though CDK...hope you're in it for the long haul..
     
  12. daiquiri
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    daiquiri Engineering and Design

    I just love to read this kind of posts. No frills here.
    I feel that I could stand for hours behind your back, watching what you're doing, learning... :)
     
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  13. apex1

    apex1 Guest

    Jepp, it´s a joy. No bantering and no "show me your pictures to proof you´re alive" crap around. And although the topic is too much above my head (was that the right phrase?), to contribute seriously, I enjoy being at least a passive part of it.
     
  14. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    "...I feel that I could stand for hours behind your back, watching what you're doing, learning... :)

    Ditto..
     

  15. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    No there is no diaphragm thing on the pump, nor is there a feed pump.
    On a turbine charged diesel with crude mechanical injection, the intake pressure is known so there is no need to adjust the injection pump for variations. Besides, VW offered their transporter Van with the same engine without turbine and only 10 hp less, so the engine is not pushed to its limit. The turbo version was there for buyers who wanted to pay a bit more. VW used the opportunity to install a very simplistic intake plenum where the air has to conquer 5 square angles instead of the much more streamlined one for the naturally aspiring engine.

    The feed pump is part of the injection pump. In the car they used a small thermostatic valve on top of the filter housing. At low temperatures the return fuel didn't go all the way back to the tank but was allowed in the filter again. I made return connections on the top of the fuel tank, which is very nearby and used a short piece of 4 mm copper tube to create a restriction that prevents the fuel from siphoning back to the tank. VW uses something similar, made from plastic.

    With the water jacketed manifold and the peculiar water injection in the turbine housings they do not radiate much heat, several 100 degrees less than the turbine on my Kia Sorento. I will try to measure it for you.

    Daiquiri: >>I just love to read this kind of posts. No frills here.
    I feel that I could stand for hours behind your back, watching what you're doing, learning... >>

    Thank you for admiring us so openly. Some rep. points perhaps?
     
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