Performance Issues

Discussion in 'Jet Drives' started by Madmax, Dec 7, 2008.

  1. Madmax
    Joined: Dec 2008
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    Location: UK

    Madmax Junior Member

    Hi

    very fast responses today, thank you

    what would be my next step, to be honest, if i could find somebody locally (Weymouth) I would just pay to have it fixed.

    However, nobody around here seems to know much about jetdrives and Vt are taking a long time to answer my emails.

    Just to recap for me, what info do I need to give you in order to solve this problem.

    many, many thanks again for your help, it is much appreciated

    kind regards

    Nick
     
  2. anthony goodson
    Joined: Mar 2007
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    Location: Dorset UK & Murcia Spain

    anthony goodson Senior Member

    These are the performance charts for the 115 if it is a115, Baeckmo will probably be the best source of advice on your hull but as he says he will need more info In my previous post I mentioned impeller tip clearance and air leaks which are both of paramount importance and apart from debris in the jet which you will have seen ,that is about it for the jet Do consider that jets are very weight sensative and all boats grow in weight the longer we own them as there are more and more possesions that we can't do without and we stow them on board, perhaps your 3tons is a little conservative
     

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  3. Madmax
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    Madmax Junior Member

    Hi

    Considering the condition of the impeller, do you think I should just replace it as this to me seems to be important, or is it possible to get it repaired, if so do you know anybody in the Uk?

    thanks once again

    kind regards

    Nick
     
  4. anthony goodson
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    anthony goodson Senior Member

    I always recommend to people that they remain seated whilst enquiring about the price of a new impeller it's safer that way.I can't say that I know of a specialist in impeller repair, but your local prop repairer should be able to do it. Tip clearances are also very important ,and you are very lucky .as these are adjustable on this jet .Make sure your prop repairer understands that the impeller runs in a cone ,and provide him with the dimensions. Dennis at Poole Propellers always did an excellent job ,but sadly they closed down.
     
  5. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Right then, it seems you have a PP 115, which is a bit more promising for you. The impeller (pump rotor, propellor, ....) is looking slightly moth-eaten and is not properly adjusted axially within the conical housing. So it has to come out to see some welding and general refreshing.

    But before that, while the boat is in the wet, it would be of great help if you could perform a bollard-pull test. You need a pulling scale, capacity ~1000 kg (and a reliable bollard.....). Measure thrust from 1200 rpm and upwards with 200 rpm intervals. At a certain rpm there is no further thrust increase; there will be full cavitation. Sounds like a shovel of gravel playing havoc in the jet unit, abort the test at this load. In spite of the impeller being in need of repair, this exercise will give me basic input for a check on the jet/engine matching.

    Then to the hull; unfortunately the picture from up front is not showing the aft, important part of the hull with the jet inlet. Maybe the boatbuilder could be identified and persuaded to supply some transverse stations before you drydock again???

    With a "normal" vee bottom and a weight of 4 tonnes, you should have a speed close to 30 knots with everything ok. My experience with the VP 63 series has shown this engine to produce power around the lower tolerance limit (-5%), rather than the opposite.
     
  6. Madmax
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    Madmax Junior Member

    Thanks anthony and baeckmo, i'll be back
     
  7. Highlanderwj
    Joined: Mar 2009
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    Location: Milford CT

    Highlanderwj New Member

    Jet experience

    Just for information.
    I have been running a twin Hamilton jet boat for 10 years.
    Anything that changes the flow will reduce the thrust to almost zero.
    A few times, a small piece of line wrapped around the shaft infront of the impeller. Many times some seaweed would find its way into the jet.
    Vibration and slow speed are the result. Judging by the photo, you need a good freshening up. Barnicles or a chip or tip clearance too big will cause a problem. You have all three!
    I have my jets serviced every year and love the performance and manuverability.
     
  8. baeckmo
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    baeckmo Hydrodynamics

    Nick, I saw a hull very similar to yours in Cork a week ago. Had a look at it when it sat dry. It confirmed my thoughts that your trouble comes from excessive amounts of aeration. These designs have an inverted "V" where the main hull connects to the sponsons, and here you get a intense mixing of air/water. This foam reduces the speed of pressure waves in the mix, as compared to pure water, which in turn will reduce the slamming pressures on the hull.

    Problem is just to keep the mix away from the jet inlet, as it will spread and follow the bottom. There are two ways; either longitudinal side fences each side of the inlet, or letting the inlet have a "collar", so that it will pick clean water.

    I'm curious regarding your results, so pls "report" ......
     
  9. lyinhell
    Joined: May 2009
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    Location: pender island

    lyinhell New Member

    re;performance

    A prime mover connected to the jet drive can be coupled either directly, or a marine gear can be used to facilitate clutching in/out.
    If you are using a marine gear, check your reduction ratio.
    My 27' aluminum hull is driven by aYanmar 240 hp, twin disc 1.1:1 into a Nomera N800
    Top speed 25knot @3300 rpm nozzle diameter reduced to 7.25" from 8.0"
     
  10. Madmax
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    Madmax Junior Member

    Hi Baeckmo, thanks for not forgetting me.

    I am totally lost at this point, I must admit that I am totally not technically minded. but here is an update:

    I have not been able to locate anyone who can do a bollard pull test, or anyone who could lend me the scales to do it myself.

    I have found a chap who specilises in propellors and impellors and deals with VT, he has looked at the photo's and believes that the impellor is not repaiarable because it is too badly worn, he is prepared to take a look when I get it off, he has stated that he can 'cast' a new one if it's not repairable.

    I also found a chap who can take it off and put it back on again, but I don't exactly trust him because he doesn't come across as all that knowledgable.

    Another chap has said that I should replace the liner aswell?

    VT have moved offices and are taking a long time to get me any diagrams, parts lists etc.

    I plan to take more photo's when I get her out of the water again, but of course I don't want to do this until I know exactly what I am doing. For example can I take off the impellor easily and more importantly will it just go back on again or does it get even more technical? ie the liner and tip clearances etc?

    How do I fit 'longtidunal side fences' or a 'collar'?

    Sorry if I seem a doughnut but I just enjoy fishing.

    Thanks again everyone for your help

    regards

    Nick
     
  11. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Nice to hear you are still with us, Nick! Where in the UK can you be found? Fiddling with the mechanical parts of the PP jets is fairly straightforward, maybe Anthony could give a hint on a decent mechanic? Do you have an owners manual for the jet?

    Impellers may look quite moth-eaten, but are still repairable. It may even be feasible to to have it fixed here; lets come back to that. BTW, did I refer to Mr Brent Voce at VT? He was (is?) involved in the VT jet story, and he is one of those rare characters with excellent knowledge on the product and a nice fellow.

    I'll make a few scetches for you on the fences/collar issue later on.
     
  12. Madmax
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    Madmax Junior Member

    Hi baeckmo

    I am in Weymouth which is on the south coast,

    I have spoken to Brent Voce at VT, but they have recently moved offices and they seem to be in turmoil at the moment, I don't have any manuals but should be able to get one from VT once they get organised. He is a very knowledgable person and very pleasant with it, he is hopeful to get the info from deep within the bowels of VT as soon as he has recovered from the move.

    regards

    Nick
     
  13. anthony goodson
    Joined: Mar 2007
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    anthony goodson Senior Member

    Good afternoon Nick and Baeckmo
    I can't recommend anyone on the South coast I'm afraid, but why not have a go yourself, jets are very simple .it's not like rebuilding a gearbox or a leg.Your impeller should be 317 stainless if it's original ,a good prop repairer should be able to rebuild this even if it is a bit knocked about.When you receive your parts diagrams from VT study them carefully you will see how simple this thing is .As regards the wear ring unless this is very badly scored leave it in. Replacing the wear ring on the aluminium models was commonplace because the aluminium {LM6 ] corroded and "grew" under the ring and pushed it out of round,this won't happen on your jet because it has a grp body.
    I am sure you will forge a good relationship with VT and you may wish to order spares from them but just be aware that although this jet was designed by a brilliant engineer there is a lot of the"cottage industry "about it and many service parts are available from your local bearing supplier.The front bearing is a standard opposed taper roller the rear bearings are DU bushes by glacier and all the seals are standard sizes If you buy the seals locally change the springs for stainless ones Good luck,keep in touch ps I have just remembered some 115's had a cutlass bearing in the stator not a DU bush
     
    Last edited: Oct 30, 2009
  14. anthony goodson
    Joined: Mar 2007
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    anthony goodson Senior Member

    These "fences " are on a cathedral hull similar to yours, you may consider these later when you have rebuilt your jet, Hopefully Baeckmo will be able to give you some dimensions suitable for your hull
     

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  15. gareth w
    Joined: Jan 2014
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    Location: swansea

    gareth w gareth w

    westral24

    hi nick this boat is for sale now in swansea and me and my mate are about to buy this boat but then i seen your posts please can you get in contact with me before i buy my mobile number is 07427560342 thanks gareth
     
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