Jet boat not performing

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

  1. Truls
    Joined: Dec 2007
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    Location: Seattle

    Truls Commercial Fisherman

    Now, say money was no object. Would a couple of TJ 431HH Traktor Jets work in place of the Hamilton's? They would have a bollard pull of almost 5000 pounds each. From what I could grasp out of Baeckmo's chart, I am trying to overcome about 7,000 pounds. Off course, it might not work like that. The link is posted below.

    http://www.marinejet.com/view.php/id/26

    What is the difference with these jets other than larger diameter? They do not show any internal components in their online drawings.

    Even if it did work, I would probably be in it about a $100k, which is not an option.

    Truls
     
  2. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Truls

    You've gotten some first class advice from baeckmo/Apex et al.

    Before you do go any deeper, perhaps a review of what you have is in order, to ascertain a way forward.

    With the figures you posted, assuming the HP/Ton indicates a best speed of around 22knots and worst around 18knots. That is of course assuming everything is fine (which it isn't:( )

    Your L/B ratio is roughly 2
    Your Length Displacement (L/D) ratio is roughly 3.8

    These two figures are the main killers.

    There is an excellent paper by J.Robinson on power prediction of chine/round bilge hulls. He has taken real data from real sea trails of many vessels. For the hard chine some 103 no less, to arrive at a "constant" for predictions.

    As you can see, in their regression analysis your L/B ratio and L/D ratio fall way outside the "norms". (your boat is the red box)

    Clearly this just reinforces that which has already been mentioned, sadly you have a floating box!:(

    So, what can you do, whilst staying inside those nasty rules?

    Firstly, the pictures of the inlet grill...wooahhh. Way too much grill. If you can, remove the grill and note the difference. On large jet boats there are no grills. I realise this is a much smaller jet, but, worth noting the effects. Looks like too muchy blockage of the inlet by the grills.

    Try and improve the planning surface. If you can run spray rails, from bow to stern, port and stbd, you may (or even then again may not), be able to encourage an improvement on your L/B ratio.

    Other than lengthing the boat (which you can't really do - to stay legal) or reducing the lightship weight significantly (ie around 25%), there is not much that can be done hydrodynamically. Hence baeakmo's suggetsion has serious merit.

    Not much real help...but perhaps a few more ideas/thoughts....
     

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  3. TollyWally
    Joined: Mar 2005
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    TollyWally Senior Member

    Now I'm not even at a kindergarten level of understanding jet pumps but those traktor units have always fascinated me. I remember the first time I saw them at the expo in Seattle in the late 80's. Their first units were quite crude. I don't mean that in a bad way but they looked home made with hardware store parts. You could really see how things were put together. It left you with the feeling that I could make one of those myself.

    Now I'm not suggesting you fab up a unit or anything else. I've just been fascinated with those for 20 some years. The big difference as I recall is the traktor pump centers around the idea of large volume and lower pressure and more conventional pumps are lower volume higher pressure.

    The original traktors I remember were designed for easy service. I think the individual vanes on the impeller were simple and cheap to replace. These are foggy recollections, I'm certain someone can speak more accurately than I on the topic.

    I still regret losing the original literature. It too was crude and homemade looking but very revealing, very much more how to. In subsequent expos all of their stuff, units literature, guys at the booth, was much more slick. Much more corporate and much less revealing.

    I understand 100 grand not being an option. Nothings cheap, nothings easy. But nobody probably understands that as well as you.
     
  4. dskira

    dskira Previous Member

    Short Fat Beauties is my real love (in boat)
    Unfortunately I have no trick. I just can tell you what I designed and built for the fisheries.
    Bluff bow, absolutely but I find that it was necessary to have a deep forefoot, and the rake of the keel was close to horizontal. I designed with a displacement/length very high, in the 600 to 700 fully loaded, since we where more interested in motion and ability to carry cargo at a reasonable hull speed.
    Pushing the water in front of us like a snow plow, never alarmed me as a defect.
    We try to have some with cutaway fore foot, like in the picture, but I finded the vessel difficult to handle, and the difference in motion between loaded and empty was to great.
    I have to tell you that since built of wood, the lines were very fair due to the material. Bending a 2" plank in a bluff bow was quite an organization, but no possibility to be really subtle in the lines. The twisting was aalready almost ninety degree on the garboard plank. It is more difficult to built with a deep forefoot, since you have some time put a stealer.
    The aft end under the trasom, buildown, same story.
    I took much more care to the aft end. I always find that the exit of the water along the hull was more important.
    Interestingly for a yacht, Van De Stadt was the precursor of the contrary. The entrance was the important.
    My problem was to give the prop a good amount of clear water, and I find it far more efficient to put two relatively large rudder in the side, the prop dead center. Alway one engine one prop, and one or two genset.
    I did that for the high rolling period with huge waves and fully loaded. Can be to 50 degree. The helm was somewhere more easy to handle. But I question this decision now. I will not do it, due to the difficulty to link and keep linked at the right angle.
    I tryed as far as possible to balance the lines, so in great roll the boat stay on course without killing the helmsman.
    What I try to say with this is: I don't have a specific trick. It is all in the unity and interactions of all the components.
    I did some god, some bad, and some really ugly, but each one was a learning lesson.
     
  5. capt littlelegs
    Joined: Apr 2010
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    capt littlelegs New Member

    It doesn't work like that... sorry.

    This is what I said:

    So is this a wrong opinion then? I can find other examples... which is much the same I said!

    Or is this a wrong opinion, from the same person responding to my comment?

    ?? Seems to have changed his opinion! There was no comment either way about either post from anyone else or your mate, glad to see you mind him so well, you sound like a salesman or a spin doctor! You jet people obviously feel threatened in some way when challenged, well it wasn't even that, I just wanted to know what didn't work like what, a simple reply might have cleared up any misunderstanding but I found out you don't do that, you're too far up yourselves!

    <removed>
     
  6. anthony goodson
    Joined: Mar 2007
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    anthony goodson Senior Member

    capt littlelegs
    I assume that I am now refered to as "his mate".I apologise for my absence,I have been offline en route to my home in Murcia where I am now ,meanwhile you seem to have been busy attempting to start WW3.My original comment was not intended as arrogant, it is common practice in these forums to suggest searching the archives to avoid duplication and to stay on topic It was fairly clear that your original posting described an impeller as behaving in the same way as a big propeller in a tube ,indeed that is how you later described it. It doesn't and I was grateful to speedboats for also pointing this out. The intended purpose of the jet will govern the percentage of maximum thrust available at zero speed ,some large diameter jets designed for bollard pull are nominally 100 per cent ,faster jets are designed to run with a small amount of inlet feed. The percentage here is very small ,and in no way compares with the behaviour of a propeller ,in a tube or otherwise and is certainly of no help in this scenario .Perhaps if you calmed down a little and looked with awe, as I do, at some of the knowledge that has surfaced on this topic you would get a bit more pleasure from your visits.
     
    Last edited: Apr 25, 2010
  7. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Truls, thanks for the salmon offer; I would like to share it with the others here over a cold beer...really look forward to meeting some of them irl.!

    Generally speaking on jet propulsion, the losses in the inlet is what limit their use at speeds <25 knots. The problem is A: a lousy efficiency due to the inlet losses, and B: full power cannot be used due to cavitation, again a result of energy lost in inlet wake and turbulence.

    "Tractor Jet" is using increased diameter (and twin impellers) to increase flow, which is positive for low speed thrust. Since impeller tip speed has to be limited in order to avoid cavitation, they always operate with lower rpm's and need a reduction gear. You can see that TJ use a max tip speed of ~20 to 25 m/s, while Hamiltons work with something like 42 m/s in the non-cavitating full load range, and about 30 m/s at bollard pull. As with all the other alternatives, TJ thrust is falling with increased speed. From the advertized bollard pull it follows a negative incline, the steepness of which we don't know, other than that it is steeper than the Hamilton line. The most important design difference though, is that the TJ inlet is designed for low speeds, while the Hamilton and most others inlets are designed for planing speeds!

    As an example, your jets will ingest water from behind and from the sides when trying to climb the hump. Looking at your picture it is obvious that this inlet is in dire need of modification to work in the low speed range, as noted by others here.

    So what difference would another jet make? A tractor jet would accept full power all the way, but its efficiency is still lower than for a propeller. In your case, it would greatly increase weight, probably to the extent of nullifying the thrust gain. The same goes for the bigger Hamilton; you will enter a negative thrust/resistance spiral! This is the reason why I turned to propeller alternatives to find solutions, instead of digging into detail changes of the existing equipment, that are not giving more than cosmetic improvements. The real culprit is still the peak resistance at hump speed; either shave it off by some means, or use a propulsion method that generates the necessary thrust to get over it!

    Moneywise, to make your fat mistress less heavy on your purse (!) a tunnelprop arrangement will be good value for the investment. I agree with Apex that a single arr. might be a good alternative, but since draft is a main concern here, this is easier to control with two smaller props; you get the better redundancy for free.
     
    Last edited: Apr 25, 2010
  8. Truls
    Joined: Dec 2007
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    Location: Seattle

    Truls Commercial Fisherman

    Thanks again, Baeckmo. You have given me a lot of information and a much better understanding of the limitation of my jets. I think it is finally starting to make a little sense. The comments about the inlet injesting water from behind and the sides is a bit of an eye opener, though it seems so logical in retrospect.

    I might make some efforts to improve the inlet grate. A stainless grate with a larger effective inlet would not be that big of a deal to fab up. I will also research entry at the bow and how the water exits at the stern, based on Dskira's comments. Any recommendations for a qualified professional in the Seattle area? I guess it might not have to be limited to Seattle. Other than that I will probably be satisfied with the status quo. I can see twin screws in tunnels at some time in the future, but I would definitely have to bring the boat down south for that.

    One last thing that has been in the back of my mind, now that it is established that I have a slow boat. What would happen if I removed the nozzle all together, and replaced the impeller with a Kaplan style propeller? Less restrictions than the current configuration - higher efficiency at low speeds?

    Regards,

    Truls
     
  9. apex1

    apex1 Guest

    Truls start with just removing the inlet gratings, we´ll see further then.
     
  10. capt littlelegs
    Joined: Apr 2010
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    capt littlelegs New Member

    No I didn't refer to you as his mate. It is arrogant and patronising to automatically assume from just one post that I need educating instead of a simple clarifying reply.

    My original post was a simple analogy, nothing more, at no point did I describe an impeller as behaving in the same way as a big prop in a tube, you should all read more carefully!

    I later said a jet, described by Speedboats as a big hydraulic pump could also be (and sometimes is crudely) described as a (big) propeller in a tube as it's not even technically an impeller anymore, it's a screw or turbine and call it a pump if you like but so is a prop but that doesn't mean that they work in exactly the same way as you all assumed I'd said!

    Unfortunately I won't get more pleasure from any visits, from what I've seen already in only a week, it feels very negative and hostile with too many idiots, inflated egos, point scorers, conclusion jumpers, boat and judgement critics and offensive control freaks, not to mention the daft reputation score! I'd look in awe if more respect were shown.
     
  11. CDK
    Joined: Aug 2007
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    CDK retired engineer

    The picture showing the grate is no prize-winner, but to me it looks as if the grate body covers 40% of the intake hole and is made from rectangular steel pipe. It is impossible to estimate the size: if the intake opening has the same size as a cross section of the duct, it is a bottleneck where water velocity must almost double to cope with the jet pump's demand. That, together with the enormous surface area of the grate itself, disrupts the flow and presents the impeller with turbulence and starvation.

    I also do not see guides on the sides of the intake, so that part of the hull sucks in more than one way. Removing the grate will be a very rewarding experiment and may solve the propulsion problem. It may not be a permanent solution though, the environment seems unfit for an unprotected intake.
     
  12. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Nope, it does'n work that way. As was said early on, the limitation for increased flow, and thereby thrust, lies in the inlet. The jet "metering" nozzle is used to tune the flow, so that the static pressure in the impeller inlet is kept above the cavitation threshold. Increase nozzle dia, and the flow will try to increase, inlet losses increase, static pressure is reduced and cavitation is a fact; the impeller is working with vapour instead of water and performance is ruined.

    The engine power is translated to hydraulic power in the jet pump. Hydraulic power is FLOW x PRESSURE. In your jet, the metering nozzle will give a flow of about 0.6 m3/sec. With a pump efficiency of ~0.83, the impeller gives a pressure increase of roughly 407 kPa, or 4.2 bar, resulting in a velocity through the nozzle of about 28.5 m/s (inlet conditions neglected for the sake of clarity).

    As a comparison, a reasonable size propeller for this application, say 25", would pump over 3 m3/s at 16 knots; its pressure increase a modest 80 kPa.

    So, again focus on the inlet; follow the advice and get rid of that grid. If you want to have some protection use only 3 fingers, equally spaced. The edges of the inlet in your pic seem a bit oddly twisted; is this really the original Hamilton weld-in insert? But until there is a solution to the hull's hump resistance, changes to the inlet will give marginal speed improvements. The upside though, is that cavitation damage (including bearing overload due to vibrations) will be reduced.
     
  13. Truls
    Joined: Dec 2007
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    Truls Commercial Fisherman

    As far as I know, hamilton inserts were used. They are cast aluminum and definitely not home made looking. I believe the grid is aluminum as well. I have seen a bunch of boats with these older hamilton jets and have not noticed anything different in the intake. The HJ322 may have a different look.

    There is rumored to have been a pair of Kamewas installed before the Hamiltons. Some bigger engines too. There is evidence of a different, longer foot print though all smooth. Supposedly the guy never paid for the stuff and they were cut out for later to be replaced with hamiltons.

    Thanks for all the input here. I will check some stuff out this summer and give an update in the fall.

    Regards,

    Truls
     
  14. sottorf
    Joined: Sep 2007
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    sottorf member

    Hi Truls, having read the thread here, I would propose you approach the problem a bit differently. Even if your hull is not perfect, you cant really change so you have to find a way around the problem. Changing the propulsion system is not first prize either...

    If you could post a pciture of the boat running at it s maximum speed here it would help to try and figure out the problem.

    For a 33' boat your boat is quite heavy but not that heavy that you should be stuck at 8-10 knots. You should be able to get up to 20 knots or so IF it can plane. I suspect your problem might lie elsewhere.

    As your hull has a flat deadrise and you say there is a large bow wave that has a tendency to entrain air in the flow under the hull. Even if you have 10% air being sucked into the jet there will be 50% thrust loss as waterjets are very sensitive to this. However confirming if air is your problem is hard to do. 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. If this is indeed the case you may be able to eliminate it with some small changes to the hull.Hamilton also have good experience using jets with air entrained and they might be able to give you an alternative impeller that can handle air a bit better.

    Changing the grid as Ad Hoc proposes will be a good start. I would propose a few trials without the grid to start with.
    Regarding fuel economy, waterjets are not the right choice ( as quite few people have pointed out) as they are not fuel efficient at low speeds as their efficiency is very low. Attached is a typical figure of waterjet overal efficiency compared to other propulsion systems. You can clearly see they only get efficient at high speed. Waterjets are only useful for low speeds if you need high thrust, good maneouvering and shallow draft - but not for fuel consumption.

    My suggestion is if you cannot get the waterjets working, you modify it with a pair of simple, inclined shaft, fixed pitch propellers in half tunnels. This will be a cheap and effective solution to get the best speed and fuel economy albeit with some added draft and a problem with those tides you have there. Controllable pitch propellers are probably to expensive for this application.
     

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  15. Truls
    Joined: Dec 2007
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    Location: Seattle

    Truls Commercial Fisherman

    Thanks for your reply, Sottorf.

    Here are a couple of pictures that I believe is with a lot of horses applied. I had one of the guys take these a few years back to get a better idea of what happens. Nothing jumped out at me at the time, but then again I don't really know what to look for. I think this was 7 to 8 knots, so full speed for me.

    On another note, I talked to a couple of guys from All American Marine today, and they are working on a very similar looking boat as mine for the same fishery. 32 feet by 15 feet and three inches wide. About 25000 pounds and with a single 525 HP Scania and a Traktor Jet.

    They are hoping for 27 knots light and 10 to 12 knots with 20,000 pounds of fish.

    Here is the link
    http://www.allamericanmarine.com/monohull/PXXX_TopHouseBay.html

    Thanks,

    Truls
     

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