Jet boat not performing

Discussion in 'Jet Drives' started by Truls, Apr 22, 2010.

  1. anthony goodson
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    anthony goodson Senior Member

    "You would need a transparent inspection plate so that you can see the flow inside the duct. Possibly you will need a strobe light to see the air bubbles."

    The increase in revs ,and the rumbling vibrations under his feet would be a bit of a clue.
     
  2. sottorf
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    sottorf member

    Hi Truls,
    looking at your pictures, I really dont think that the hull is your main problem, The bow wave does not seem that strong that it is absorbing 2x400HP with of power at 8 knots!

    THe AAM boat is proof that you should be able to get quite a bit of higher speed. I think it is definitely work manufacturing a transparant inspection plate for the jet and observing the water flow with a strobe light (you can use an automotive timing light).

    BRgds

    Gunther
     
  3. speedboats
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    speedboats Senior Member

    I see there are alot of straking under the back of the vessel. How far forrard do they go? Are there any which terminate infornt of the intake (even by quite a few feet)?

    The minimal amount of deadrise will tend to trap air, and straking will help to keep more air under the boat. This is quite desirable when trying to reduct friction under the vessel, not so much if any gets into the intake (as has been stated).

    Is there enough straking at the front of the vessel to improve the angle of attack so it can climb out of the hole? Although Nett Profit doesn't seem to be bull-dozing as big a hole as I was imagining...
     
  4. Rampager
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    Rampager Junior Member

    Scanned thru the posts but didnt see any reference to this, if I missed it i apologise.

    Does this model of Hamilton pump have any angle designed into the nozzle? I know on the smaller units you can alter the thrust line by "flipping" the nozzle. Perhaps this may be worth looking at in case it happens to be installed in the "down" orientation??


    Cheers
     
  5. Truls
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    Truls Commercial Fisherman

    I pretty much tore the house apart looking for these old hard copy pictures, and finally found them. Hope the scans show up ok. They show the bow and bottom much better.

    There is not much straking at the bow, as far as I can tell, and I always wondered if what little is there caused problems. Half the time I wanted to cut them off right at the bow and then I thought about extending them further. Never had any in-depth knowledge about it, so just left it alone. The straking at the bottom is actually the keel coolers. These pictures show the intake much better as well.

    To my knowledge there is no adjustments to the nozzle on this model. I believe manufacturing a wedge is the only option there. Please correct me if I am wrong, it has been a few years since I looked into it.

    Regards,

    Truls
     

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  6. sottorf
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    sottorf member

    Speedboats has a good point about the sprayrails: they definitely should not be upstream of the jet intake but from the pictures it looks like they are outboard of the jet and therefor not a problem.

    If air is the source of the problem it is from the low deadrise. I did some tank testing a while back with flat glass plates at planing speeds to observe airflow. it showed that the running trim of the boat had to be more than 2 deg. before entrainment of air under the hull stopped. This is howevr probably not applicable at the low speeds here.
     
  7. anthony goodson
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    anthony goodson Senior Member

    Truls , all these theories and suggestions are very interesting but surely a question you should be asking yourself is ,have you been persistent enough with Hamilton ? I don't think their attitude would be as cavalier in the UK. Perhaps a personal letter to their CEO in NZ might help.You don't a "suit" from the parts dept,office ,or drawing office,you need an experienced jet unit engineer on your boat. He will look listen and feel, and then tell you what is wrong, demand it ,if they were involved in the set up then they owe it to you ,it is their reputation after all.
     
  8. Truls
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    Truls Commercial Fisherman

    Anthony -

    You have a good point about getting further input from Hamilton Jet. It seemed I had exhausted the resources there a long time ago, but it is probably worth a shot. There is a lot of history there from even before I got the boat and the same engineer is still around. He has not been big on ideas, after the fact of a boat that is not performing.

    Regards,

    Truls
     
  9. anthony goodson
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    anthony goodson Senior Member

    Hamilton in the UK have built an excellent reputation over the last ten or fifteen years ,they will not walk away from a problem project if they were involved from the beginning. In fact they are unwilling to supply to anything they are not happy with. I see no reason that Hamilton where you are should behave any differently,if you are not happy with the engineer tell them, they are probably the best resourced jet builder in the world ,they can solve this, but you may have to push them a little .
     
  10. speedboats
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    speedboats Senior Member

    I think he stated that the boat was originally to be fitted with another set-up. Were the pumps bought new from Hamilton Jet or purchased from an entity outside of HJ control?

    I too know that Hamilton won't just sell to anyone without knowledge the project should see success.

    Is the problem the limited amount of thrust from 2 x jet pumps, or the limited amount of available lift from the hull? There does seem to be straking at the aft end of the planning surface, but the strakes further forrard are either of poor design or non-existant. From the pictures supplied the vessel doesn't even look like it wants to lift out of displacement mode. Is perhaps the aft end suppling more lift than the bow. The stated LCG would suggest that this shouldn't be an issue, but we set our pumps back from the transom (in a special box) and these seem to be mounted ahead of the transom pushing LCG forrard. We have built boats down to 8* deadrise at the transom with complete success, using the HJ212. We do however use a warped deadrise which does help reduce air entrapment around the jet intake.
     

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  11. anthony goodson
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    anthony goodson Senior Member

    Hi Speedboats ,yes I take your point about the origin of the jets ,nonwithstanding that I still feel that a good engineer with Hamilton would know instinctively whether it is the jets , the hull or a combination of both that is the problem, but he would need to be there. Given the cost of the other alternatives suggested ,I think I would give it a go first. Beautiful boats by the way, we never see anything like them here. I never did understand why Hamilton did not make a mid-transom variant ,there can sometimes be advantages.
     
  12. baeckmo
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    baeckmo Hydrodynamics

    Sorry guys, but the big issue here is the matching problem causing cavitation at displacement speed. Go back to the diagram attached to #39 note, and Ad Hocs diagrams on hull proportions. I fully agree on the risks with aeration, but we covered that way back.

    Looking at TrulsĀ“later pics, it is amazing how close the wave system corresponds to what you see in the photos from the tank tests of the series 62 model. This increases the strength of Ad Hoc's and my observations. So, Speedy and Anthony, I certainly acklowledge the principal values in these late notes of yours, but let us not lose focus. Any minor changes (like tuning the inlet and impeller for slightly better cavitation performance) to the setup will only have cosmetic value on operating quailities of this vessel, at least with a realistic payload onboard.

    On the next drydocking, remove the grating or every second finger, check impeller leading edges and tip tolerance. And then take a good time to figure out what its earning capacity would be if a speed increase were possible. Then you also have a picture of what investment level that it would carry, resulting in a better "Net Profit" (!).

    BTW, are there any public fundings available for efficiency increase or fuel savings schemes where you operate? If so, this boat is a clear winner of the "possible improvements league".

    PS Just checked the pics showing inlets. Seems that the inlet gratings are attached to a transverse bar up front? This has to be blended into a smoooooooth curvature!!!! The higher the speed, the more important is the inlet roof in this application!! Also, there are eleven fingers, cut away all except no:s 3, 6, 9, counting from the side of the inlet opening. DS
     
    Last edited: May 8, 2010
  13. watchkeeper

    watchkeeper Previous Member

    Hi Truls

    I agree with the comments of baeckmo and Wynard. The hull shapes is just not suitable for pumps, why they were even installed as propulsion system is surprising. I did a lot of work - feasibility and propulsion upgrade on a dual navy project involving Hamiltons, Kamewa and Doens. Jet arre excellent on the right hull and as good as they are they can only be as good as the hull design allows.
    We often install jets on our hulls for clients, 14mt 17ton RHIB 29knot to 47mt 200ton 42knot superyacht and they work brilliantly.
    Rather than go on spending to resolve the existing set up, I suggest it would be better in the long term to convert to shaft or even out boards.
     
  14. hinemoa
    Joined: Jun 2010
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    hinemoa hinemoa

    not a big expert here on boat jets, but did do some time on jetski racing, and after modifying with bits and pieces, the biggest thing that made a significant diference was the intake grill , what we used to do to get more thrust effect for out of the hole thrust was weld intake style scoops into the grill so more water got pushed into the unit thus have more thrust, we did loose a bit of top end but overal speed did rise, i dont know if this was just luck or what but it was a super cheap fix..??
    good luck with it all, im sure once you differ between all the thread and pull out the ones yo can use it will have some effect somehow....
    did you ever play with the idea to change the bow water line a bit to give you the more rise or put some fins on the bottom of the hull like a hydrafoil to help you get the lift at the front??
     

  15. Truls
    Joined: Dec 2007
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    Truls Commercial Fisherman

    Thanks to everybody for continued feedback. I am up here in Alaska working on the boat and getting ready for the upcoming season and to tell you the truth, speed is the last thing on my mind right now. However, come October, November, the need for speed will kick off again.

    Once I get done with all the critical stuff I will probably look at my intake grates and definitely bring them home for the winter. If some modification will give me a little better fuel economy at low speed that will be fine with me. I was all set to cut out some ribs to increase flow through the intake when I heard about a buddy who just brought up a new boat with a traktor jet. He has wider openings in the intake and sucked up a bunch of gravel and rocks while over his first bar. Now has to rebuild his brand new impeller with 30 hours on it. He was in the water less than a day up here.

    There has to be some tuning that can be done to my intakes though. I assume they are designed more for top speed rather than the range I am operating in. And there seems to be a consensus that intake criteria are different at low speed versus top speed.

    I am also thinking, after looking at the boat again, that there could be a benefit in adding a short swim step that is uniform across the stern as opposed to the present configuration (pictures posted previously). It seems a sharp straight edge would help water break away rather than getting sucked up behind the stern.

    It also seems a slight modification to the bow at the waterline would reduce resistance. I am just talking about making the entrace a little sharper without adding overall length to the boat. I have about a foot to work with at the waterline. The lines might not look as good, but it is a workboat after all.

    Thanks again,

    Truls
     
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