Removing impeller Vospower

Discussion in 'Jet Drives' started by RocketDane, Jul 23, 2015.

  1. anthony goodson
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    anthony goodson Senior Member

    The body of your jet is glassfibre composite ,hence the G suffix ,the 90a is aluminium bodied hence the A suffix. Note the internally placed front bearing,the raised handhole,the different transom position and the altered control rods on the G. Some parts are interchangeable, many aren't.
     

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  2. dsckeld
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    dsckeld Junior Member

    VosPower 90G

    Anthony, Baeckmo et al.
    Thank you very much for your help and info.
    After reading this thread, I could also identify the drive as a VosPower 90G drive in the manual we already have.
    We removed the shaft, bearing housing etc. today, and have started refurbishing it.

    There were quite some play in the thrust bearings. We are not sure why, but it looked like the inner races had loosened. We will correct this by moving the seal ring a little aft in order to take up the play. A .8 mm shim should make it all sit tight again. (We have noted the "Glue in the bearing rings" part of the manual/drawing.)

    Looking at the outer race, this also needed another .2 mm shimming in order to be tight. This will remove the play in the axle.
    The bearings at the back has been without oil for a long time it seems, and there are some wear marks on the axle. It has been worn down .25 - .30 mm in places. If we can get the gasket to hold tight again, we will restore it to its former oiled glory. We were considering to turn the axle on the lathe and make new bushings for the bearing. (Noting that it should not have electrical contact between the bushing and the housing.)

    The front edges of the impeller is somewhat marred by the drive eating sand and rocks at some time. We will smoothen the edges again and check the balance.

    Right npw there are 4 shims for a distance of 5 mm in front of the impeller. As we have gotten the nylon hub off now we can easily see how many shims we need to insert. What is the best way to get to tje correct amount of shimming? Remove shims until the impeller gets stuck and then back up one shim, or keep gauging the distance with a set of feeler blades until we get somewhere between .3 and 1 mm (with the impeller touching the other side, for a mean distance of between .15 and .5 mm with the drive completely assembled. Or do you have a better way to do the adjustment?

    Once again, thank you for your very useful answers to our perilous questions.
     
  3. anthony goodson
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    anthony goodson Senior Member

    Hi
    Sounds as though things are progressing well. The rear bearing bushes are not something you can make .They are a DU bush ,made by Glacier Bearings ,and have a teflon lining . Baeckmo made an excellent suggestion here ,convert to a water lubricated bearing WITH A PLASTIC SHELL these are much more tolerant to shaft damage.and since you clearly have machining facilities it should be easy . That shaft was almost certainly metal sprayed and ground in that area and will be hard.




    As regards the impeller shimming ,I posted instructions for this on 13 August 2010 in a thread entitled 15ft Fonda with a jetdrive ,it's not the factory way .just mine,it might be worth looking that up in the archives it was written for a 65 but will work just as well for a 90 .Hopefully we will hear from baeckmo with some hints and tips he has picked up over the years.

    Good luck
     
  4. dsckeld
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    dsckeld Junior Member

    @Anthony:
    I believe you are talking about this post: http://www.boatdesign.net/forums/powerboats/16fonda-jet-drive-34144.html#post388659
    I read the description, and it is a very good match on our drive and (dis)assembly instructions. The instructions on cleaning up the impeller is much what we were planning to do, so good to get a confirmation on that.
    We will see if the bushings in the stator are too much worn (they look like bronze all the way through, so according to your description, they should be replaced. We do have the ability to get a bronze bushing covered with teflon, but it might not be worth it. The shaft should allegedly be chromed, but it doesn't seem to be that hard. We do have a complete machine shop available, so if necessary we could grind the shaft clear. When I removed the stator, I poured some water out of the bushing housing, so the oil is very long gone.
    I will report back with progress (and maybe some pictures) after next sunday's work on the boat.
    The impeller position adjustment sounds very sound, at least as soon as ve have removed the slack in the main bearings, so that we doesn't have to take the main axle moving forward into account upon adjusting.
     
  5. anthony goodson
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    anthony goodson Senior Member

    Be a bit conservative when shimming the front thrust bearings , this shaft ,unlike most propeller shafts ,runs at crankshaft speed. If the bearing preload is too high ,excess heat will be generated ,not a good thing ,especially within a GRP housing.
     
  6. baeckmo
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    baeckmo Hydrodynamics

    Well, not much to add to that instruction...... but since you asked, I use to tape 0.2 mm strips of Alu-foil to the blade tips, then push the impeller with its plastic hub onto the shaft until all blades hit the conical lining. Then I take the distance from shaft end to the hub for final positioning (because measuring the distance from hub to shaft shoulder at front can be a bit tricky; I want an easy-to-reach control dimension).

    Normally I don't bother with adding the stator during clearance checking, because having the specified clearance on all blades will handle eventual eccentricity in stator assembly. Has worked ok so far, so I guess the pudding is proof of the pudding......?

    BTW, where in Denmark are you?
     
  7. RocketDane
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    RocketDane Junior Member

    Some info after we successfully removed and reinstalled the impeller:

    We removed some shims, but the boat is not faster even though the impeller blades are now much closer to the tube walls. Odd. There were some play in the bearing but that is gone now. The boat gets on the plane with two people on board but not three (we are not small guys BTW). With more than two people on board the top speed is 20 knots which is acceptable, I guess, for a 1.700 kg boat with a 190 HP (fuel stop) Yanmar diesel.

    The drive is in better shape after servicing but we did not get the improved power we hoped for.

    Thank you for your help :)

    https://scontent-arn2-1.xx.fbcdn.net/hphotos-xpf1/v/t1.0-9/11855685_10153110743935678_641133703398597324_n.jpg?oh=8d15adab98d72e9e728d3844d063ae7e&oe=5636F556
     
  8. baeckmo
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    baeckmo Hydrodynamics

    Hi again, the performance you get is not ok, you should get something like 27+ with that setup if everything is in shape.

    -Check the boat weight with everything on board; I doubt the 1.7 ton figure mentioned. Also check longitudinal mass center when you lift.
    -Check trim angles at a few speeds in the range 12 knot to max speed.
    -What rpms do you get? Check tacho calibration! (I suppose it is the 4LHA with max power at 3300 rpm!?).
    -Check exhaust temperature (after turbine) and inlet pressure plus temperature after intercooler.
    -Check that there is nothing protruding through the bottom (echo-sounder, pipe fitting et c.) in front of the jet intake. There must be a smooth area ~2,5 x 0,6 m along the bottom.
    -Check that the reverse deflector really is not disturbing the outflowing jet in fwd position.
     
  9. RocketDane
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    RocketDane Junior Member

    Norsafe Mako and Vospower 90G

    Hi again.

    I wouldn't mind 27 knots with 3 people on board. :)

    Yes, it is the 4LHA-DTP engine that tops at 3300 revs and can be run continuously at 3100 revs. For the boat to get on the plane with two people on board it had to be taken to 3400 revs even after the refurbishing of the drive.

    We will try to get an accurate measurement of the mass. The craft seems balanced enough when we lift it in the hook that it uses to be suspended from but it does seem very heavy.

    What do you mean by trim angles?

    What surprises us are the high revs necessary. That must mean the engine is running OK (provided the meter is calibrated correctly as you point out). If it runs OK it must either be underpowered, the boat is too heavy or the drive is not transferring the energy as required.

    Best
    Jeppe

    PS We are close to Silkeborg in Jylland (Jutland).
     
  10. baeckmo
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    baeckmo Hydrodynamics

    Operating the 4LHA at 3400 rpm means that it is running on the fuel downregulating curve and thus not producing full power. This is where the temp and pressure measurements come in; a reduction in fuel flow shows very clearly in low exhaust temp and low charging pressure.

    The reason may be too small impeller pitch or cavitation. I am comparing your figures with my newbuild no B56 a couple of years ago. It is a monohull (loa 7.4 m, bottom rise 18 degrees), equipped with a Vosper 90 jet to a VM 250 hp diesel. With a test weight of 2480 kg its top speed was 32.8 knot. At 3200 rpm the absorbed power was 192 hp and the speed 25,6 knots.

    Now if you check the power curve of the Yanmar, you will see that it is ideal for jet operation, its power curve is absolutely flat from 2800 rpm and upwards, which means that you may select a "coarse" impeller that will give improved cavitation resistance by bringing down the rpms without sacrifying power.

    By trim angle I mean the hull "angle of attack" (or "elevation angle" in artillery terms......), measured as the angle between the keel line and the water surface. This can tell us wether the boat is running in a "normal planing mode" or sitting heavy on its transom.

    And fine, Silkeborg is not that far from my place; might take a trip and get a Gammeldansk and an Aalborg Taffel.....!
     
  11. RocketDane
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    RocketDane Junior Member

    And very welcome you would be :)
     
  12. dsckeld
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    dsckeld Junior Member

    Long overdue report

    Dear Sirs.

    Thank you for your attention to oyr problem. I see my friend, RocketDane have told you some of the stuff we experienced after readjusting and reassembling the drive on our boat.
    I will just add a couple of items to what RocketDane already told you:

    We found that the main thrust bearing inner rings had detached themselves from the axle. This had the effect that the spacer ring (0325 on the drawing) were worn. This were solved by shimming about .8 mm. After reassembly (of course with gluing the bearings in place) the longitudinal play is just shy of .2 mm as per specs.

    Upon assembly I found that with all shims removed the impeller would just scratch the wear ring surface. The impeller were easily rotatable by hand, so I expect the play to be very near optimal, and the scratching being due to the stator bearings being worn (together with the axle).
    I put one shim (around 1.25 mm) in for good measure, and now I had a lot less scratching. But it still leaves us with some .5 mm play most of the way around the impeller.

    This looked to me as if the impeller were too small, although I couldn't determine too much wear of the blade tips. The wear ring on the other hand had lots of wear signs, and the plastic part at the end protrudes into the tunnel so that I can feel a distinct edge here. But not in a range I can measure. What is the original dimension of the impeller? I have seen what missing one inch diameter on an old outboard could do, so this is under suspicion on my part.

    You mentioned something about hull smoothness fore of the intake. We do have an outlet for cooling water protruding a cm or so. But this is original, so I don't expect this to be a problem.

    When we tested the boat, we did try to move people from the front to the back in order to change the trim angle. The results were inconclusive. When we were 5 people on board there were a tendency to run faster when people came aft, as opposed to a small improvement when one of the two people sat at the bow. The latter were with the boat already planing.

    We can try looking into AOA (trim) and weight shortly, but we do have a couple of important projects to take care of first.

    By the way: We had a discussion about how much effect surface smoothness would have. Not "owergrown" roughness, but just "would need a good buffing" gelcoat roughness.

    Best regards,

    Keld
     
  13. dsckeld
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    dsckeld Junior Member

    I would even make sure that the "Gammel Dansk" and "Akvavit" would be on our tab.
     

  14. dsckeld
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    dsckeld Junior Member

    One more thing: The tacho on board reports the max rpm as 3700, with the boat going 21 knots with 2 people on board at 3400 rpm.
    Max rpm gives us just around 25 knots.

    If the max engine RPM is 3300, then this could indicate that the indicator is faulty, or we are running the engine way under load.
     
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