Sea Rayder F16 tries to go left while on plane

Discussion in 'Jet Drives' started by KStro, Jun 4, 2021.

  1. KStro
    Joined: Jun 2021
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    KStro Junior Member

    I saw an old post on this, but with no resolution I could find. I have the same problem with my 97 Sea Rayder F16 120XR. It seems a lot of people do, but no solution I could find. I have to turn 1/4 right to go straight and I can see the jet is shooting right so it is following the steering trying to compensate for the boat trying to go left . So the steering itself seems to be working correctly. Something is pushing/pulling it to the left when on plane. Weight distribution seems to have little effect. I can put so much weight in the left that it pulls hard right on deceleration, but it still pulls left when on plane. Hull looks straight, ride plate looks straight, intake, impeller, jet nozzle look clear. Any help would be greatly appreciated.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Have you tried to move weight fore and aft ?
     
  3. baeckmo
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    baeckmo Hydrodynamics

    That behaviour is the result of an asymmetry in the inflow into the impeller. The inlet shape of the Mercury pump (or most jet units for that matter……) is a compromise between short building dimensions and intake efficiency.

    What happens is that a minor disturbance, fi a step between hull and pump inlet body, will cause a flow detachement from the inlet duct's upper surface. This creates two flow zones with different inlet speeds; a high speed zone at the lower half of the impeller, and an almost stagnant zone at the top half. The high-speed flow passes the impeller “straight through”, very little power added and thus very little rotation into the stator.

    The low-speed flow will “spend a long time” in the impeller channels, and will come out with a high rotational velocity; in fact so high that the stator vanes will stall; ie they can not straighten the flow into an axial direction. It will generate a strong flow vector blowing to port (your pump is rotating clockwise seen from aft), but you won’t see it, because it exits below the straight streamlines from the lower zone. Depending on the degree of the inlet asymmetry, you can get varying effects, even a vertical lifting force. This can happen very quickly and cause a sudden and uncontrolled change of course in jet propelled vessels.

    If you pull off the stator, you will probably find cavitation marks, or even damaged vane leading edges in a sector between 9 and 1 o’clock (seen in the flow direction), while the lower sector is undamaged. Both flow zones are associated with high losses, ie low efficiency.

    If you could provide some pictures from where the jet inlet is meeting the hull bottom, we may find the reason for the anomaly and possible remedies. One issue with this installation is that there is an aeration “channel” around the pump body, and even the slightest height difference or a sharp edge at the wrong place will trig the detachement.

    Btw, I'd like to know the diameter of the nozzle to check the pump capacity.
     
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  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Interesting theory by baeckmo which may be 100% correct, I was oblivious to it being an outboard jet
     
  5. baeckmo
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    baeckmo Hydrodynamics

    It is not an outboard jet, the pump housing is installed in a hull pocket.
     
  6. KStro
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    KStro Junior Member

    Didn't mean to copy my quote above. To answer the first question, moving the weight around any direction (even putting someone big in the bow) seemed to have little effect when the boat was on the plane and this pulling occurs. During deceleration port/starboard weight had a significant effect, but with my limited understanding, I think that would be normal.

    baeckmo: Your post is interesting. I read one from another person with the same boat and a similar problem. He stated that his was OK until he took it in for some work on the jet/pump. When he got it back it had the issue. They took it apart a couple more times, but it remained unresolved when he last posted on the thread. If I am following you correctly, that would seem to fit in with what you are thinking as they likely wouldn't see the issue just looking at it stopped out of the water.

    I attached some pics including one with a tape measure along the nozzle to show the size. One of the bars on the intake grate is missing as you can see. It was damaged by a rope that thankfully didn't go down to the impeller. But the pull problem existed before that incident and seems to remain unchanged after.

    Hopefully these show you what you want to see. Let me know if it would help to see something else.

    It is bad enough that I don't think I could even sell it like it is. It is challenging to use it, I think it is slowing the boat 5 to 10 mph, and I am afraid it may break the steering cable eventually with that pressure on it all the time.

    Thanks to both of you for the interest. I wasn't expecting such thoughtful answers so quickly.
     

    Attached Files:

  7. baeckmo
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    baeckmo Hydrodynamics

    I would like to see the front of the pump housing, where it meets (mates to?) the hull. There is some kind of seal (think it is called a "grommet" in English) between hull and pump inlet. If you could take a pic from the side with a straight ruler along the keel line, crossing the grommet. What I want to see is if there is a smooth transition or some kind of step. The missing bar is of no concern, the remaining ones should be rounded off howerver, but that's the final touch.

    The pictures showing the stator from behind look suspect; the vanes seem to be too short and too straight (and too few) to be of any use. It would help if you could pull off the stator, so we can see its inlet side, I think the vanes are seriously damaged. And yes, the situation is putting high load on the steering components, I think you should not use the boat until this is sorted out.
     
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  8. KStro
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    KStro Junior Member

    OK. I think I am following what you are asking for. I attached some pics. I will be busy next week and I don't think I will be able to disassemble anything before that. But after that I should be good to try to pull apart whatever might help. Thanks again for all the help.
     

    Attached Files:

  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Is that intake grille busted ? Looks like part of it is missing
     
  10. KStro
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    KStro Junior Member

    One of he bars broke when a rope got in there. But this problem existed before that and didn't seem to change after that. I would like to fix it eventually, but get the more major problem fixed first.
     
  11. KStro
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    KStro Junior Member

    It looks like there is a bit of the rubber seal kind of recessed or not completely smooth between the fiberglass hull and the metal (I believe it is called "Ride Plate") as you can see in the pictures above. Is that the seal you were referring to? Would that be enough to cause this issue? It looks pretty tight where the grille meets the metal (ride?) plate. I think most of the rest of it is all out of the water when the boat is planing.
     
  12. KStro
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    KStro Junior Member

    Still looking for ideas if anyone has any. Thanks much.
     
  13. baeckmo
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    baeckmo Hydrodynamics

    Waiting for you to pull the stator, as noted earlier.
     
  14. KStro
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    KStro Junior Member

    Oh, OK. I am not sure I completely understand what I need to remove. I am pretty good with cars, but don't do much boat work. So I don't do the wrong thing, I want to verify. I believe the static area behind the impeller contains the stator veins. So if I understand correctly, I need to remove the nozzle and then remove the center allen screw (shown in picture number 3 and 4 in the first set above) to expose what you need to see. Is that correct?
     

  15. baeckmo
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    baeckmo Hydrodynamics

    Yes, the question is wether the seven vanes seen from the nozzle side in the earlier set of pictures are intact or damaged at the leading edges (facing the impeller outlet). I don't think the center screw should have to be touched; it may hold the impeller to the shaft. Check if it rotates with the shaft. The complete housing aft of the impeller should come out as a unit.
     
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