Jet boat not performing

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

  1. capt littlelegs
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    capt littlelegs New Member

    I wasn't requesting knowledge but you and others sought to hijack the thread with unsubstantiated comments... a reply would have also been helpful to the original author. How am I supposed to listen to reasoned arguments when you don't reply!

    I don't want to open another thread, this is the one you commented on so this is the one you should be replying to but so far all I've heard is drivel! Surely one of you can give a technical reply, it can't be that difficult with all that knowledge and experience! Perhaps this is an area that polite people don't talk about... it's actually a prop in a tube but don't tell anyone!

    Lol, I can just picture you all, don't tell him, we're not going to tell hiim, it's a secret, well we don't know the answer anyway, perhaps he's right, perhaps he'll go away if we keep quiet... Lol the best kept secret in the marine trade, it just doesn't work like that... we don't know why! :D
     
  2. baeckmo
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    baeckmo Hydrodynamics

    Truls, that info is important, thanx. Makes it possible to "backwards" calculation of the jet operating conditions. I'll be back with a few ideas...

    BTW, There is a strong smell of "rule cheater" sticking to this design; do you still have to comply with the length limitation? If so, will anything bolted to the rear be regarded as "hull length", or is it a non-counting appendix? As per today, I guess the outboard parts of the jets don't count?
     
    Last edited: Apr 24, 2010
  3. Wynand N
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    Wynand N Retired Steelboatbuilder

    You obviously are otherwise you will not question experts a but rather contribute in a useful way - sorry for the wake-up call...


    Truls, my view is as stated in post #2.
    I say again I'm a novice with jet drives and I do not believe whatever you do with that jets will help. Unless the hull get sorted out, make peace with the fact that you have a displacement hull with plenty of power but too little to get over the hump to plane. Remember, you are battling with the hull empty to break out, what would you think will happens when she is filled with a good catch:?:
    However, should you by some miracle gets it on the plane, that boat will not be very stable directionally and will slam like no tomorrow - will not want to take her out on the rough though.

    At least in displacement mode you can then run the engines economically by barely ticking over to get to the calculated hull speed of 7.34 knots for 30ft waterline. You mentioned that at WOT you only get to 8 knots - see my argument that with all that horse power available you only gained just over half a knot in speed with all that extra fuel wasted.
     
  4. capt littlelegs
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    capt littlelegs New Member

    You don't know me at all so that's a silly comment. You have a strange logic and a blind faith in someone else you probably don't even know, so, if I'm not an amateur or I don't think they're experts then I'd be right to question and query them, right? I did contribute and was awake long before you...
     
  5. Truls
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    Truls Commercial Fisherman

    Baeckmo -

    Propulsion components and protection for those components as well as "swim steps" are allowed to stick out beyond the transom. By swim steps I refer to hull extensions that extend the bottom. Lots of prop boats in the fishery have these swim steps, but jet boats generally don't.

    I did weld some on that extended about 14 inches across part of the transom, with allowances for reverse controls. However, they gave too much lift in the stern, did not increase speed and made it harder to steer.

    The anchor can stick out over the bow and mentioned appendages at the stern, but otherwise the boats are limited to 32 feet overall. There are lots of boats with cut-off bows. Some builders were pushing the boundaries and then one year the enforcement went around with a tape measurer. Ouch...

    I suspect that my LCG is still a little far forward. There are no scales available where the boat is located, so we pivoted it on blocks to find approximate LCG after all the mods. It may be as far forward as 14 feet, though seemed right in the ball park of Hamilton Jet's calculations.

    There are lots of similar jet boats in the fishery, some as wide as 17 feet, one as heavy as 45,000 pounds. Those with widths in the 14 to 15 foot range tend to perform better. The approach seemed to be early on to throw horsepower on it untill they fly. Then builders realized that keeping weight down was the way to go. The bow on mine does seem more blunt than any others, though I can fish in weather that leaves most other boats at the dock.

    All the Hamilton jet guys use 5 or 6 blade impellers, as opposed to my 4 blade, with a higher KW rating than Hamilton Jet recommend. There is a lot of knowledge aroud based on trial and error, but not very much in-depth knowledge. All the feedback I get is get other impellers or increase horsepower. There is very little weight to loose at this point. I have considered adding a third jet, but there is really no room with the current configuration.

    With three thousand pounds of fish on board all jet boats are 6 to 8 knot boats, but will keep on packing up to 30,000 pounds of salmon. Once we find the fish it is all about keeping the net in the water. The fishery lasts only two months and then most of the boats sit on the beach until next year.

    Recently boats have been moving away from the Hamilton jets toward tractor or other jets. At a cost of $30 k or better each that is not an option for me. Impellers are over $4500 each.

    Props and shafts would probably increase speed, fuel economy, and directional stability, but I need to be able to run over nets and sit on the bottom. I don't think in my case the trade-off is worth it, even at 8 knots. And as far as my experience goes, twin jets are also unparalleled when it comes to maneuverability in tight quarters.

    Though I don't use all my horsepower for speed since there is very little gain past displacement speed, as Wynand pointed out, it comes in handy when pulling on the net. I usually have a towline attached to a stern corner cleat with the net a few feet behind. I will move the net around to intercept fish and sometimes tow the net to get out of a situation with law enforcement or other boats. Prop boats have to pull from the mast and will pull the net out of the water in order to move it around.

    So, even with the obvious design flaws present, I would like to stick with the jets. However, I have to believe there could be improvements to the current situation, and you guys have given me a few things to work with so far. Thanks for spending time on this.

    Regards,

    Truls
     
  6. apex1

    apex1 Guest

    Mr littlelegs,

    your comments and opinions (not only here) have proven your expertise quite well! We do´nt need to know you personally to judge the level of knowledge.

    Actually you are argueing with a professional boatbuilder (in fact two of them), a well proven pro in Hydrodynamics, a boatyardowner, and other quite respected members with hands on experience.

    We know each other for long and have proven each other right or wrong so many times, there is no need to doubt our experience.

    You have shown nothing but opinions and a unpolite behaviour, and YOU are hijacking this thread because you are stubbornly unwilling to accept, that no expert here has the obligation to answer your questions.

    They are handled in various threads over the Forum!

    Just throwing a (wrong) opinion in and assuming one gives you private lessons to correct them is bold.

    There is a Fisherman in trouble earning his living at affordable cost. The ones with the best knowledge about the issue will try to find a solution, be it Baeckmo for the propulsion, Wynand for the "buildability" of hull changes or others amongst us for a redesign of some sections.

    You are not amongst them as far as we found out!

    Feel free to spread your opinions on every thread you like (for that reason we have such platform), but mark them as opinions and tell the auditorium about your level of expertise on that topic, then you act fair and are welcome.

    Senseless attacks of newbies we had quite enough this year..................

    Truls

    be patient! Bodo (Baeckmo) finds some improvement I´m shure, and then there are probably some more options....

    Regards
    Richard
     
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  7. TollyWally
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    TollyWally Senior Member

    I submit this as a question not a solution and don't want to dilute the emphasis on getting a Bay dude's problem solved. Further more I am very much aware of the limitations of a cannery storage yard, the logistical problems of Bristol Bay and the tight time window he is going to be operating with. So now that the disclaimers are in place. :)

    There have been some pretty impressive NC driven quick turnaround sponson and bulbous bow jobs done on steel trawlers and crabbers.

    Could one slice the bottom free at the chines say the last half of the boat, scribe a new line on the transom and weld back together, tying back together with swim steps, net guards, sistered engine rails etc.?

    The idea being perhaps a little rocker aft might pry the bow up enough to get over the hump and up on step.
     
  8. rasorinc
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    rasorinc Senior Member

    I once saw a V shaped ladder frame out of aluminum angle attached to the bow stem running aft about 3/4s of the length of the boat and covered with 1/8' PLATE. I asked whey he had that? The answer was to correct a mistake and be able to get on plane. It provided enough lift so he could. It had a large single prop. Just an observation for you that possibly could lead to a solution. That ladder frame and plate out of aluminum angle should not weigh to much in my opinion and he did not have to tear into the bottom. It was on the Oregon coast. Forgot, he was using counter sunk machine screws thru the ladder frame into the existing plate and frames behind. It was under construction and some 25 years ago. I would think a good knowledgable engineer or NA could put something like that in their computer and see if such might work. Way beyond my pay grade though.
     
  9. baeckmo
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    baeckmo Hydrodynamics

    Truls, you have given a very good picture of the working conditions; I've seen similar limitations elsewhere. Now to your boat, attached you will find the Hamilton thrust/resistance diagram completed (blurred??) with a few extra lines.

    The proportions of your boat is very similar to a standard model that has been thoroughly tank tested, the series 62 model 4665. Plotting the 4665 resistance with 30.000 lbs displacement (red line) together with the Hamilton predictions show that the computer predicition seriously underpredicts the hump resistance.

    This is a known problem with the Savitsky algorithms, even with the Blount/Fox compensation for wetted bow, used by Hamilton. In fact, someone has added a comment regarding this in the margin of your diagram, but someone else chose to neglect it..........!

    If you take a look at the thrust lines for the jet, you will find that for the J1 case, the line is shown dotted from about 22 knots and downwards. This marks that full load operation at 2600 rpm is restricted to 22 kn upwards; below that, performance is reduced due to cavitation! This explains why your engine is making 2800 rpm; the jet is cavitating heavily and the engine is overspeeding with fuel restricted by the centrifugal regulator.

    Knowing the nozzle area, it is possible to calculate the jet flow and inlet conditions. With 400 hp the flow will be ~0.6 m3/s and at the cavitation limit (22 kn.) the required inlet pressure will be about 14 m of water column absolute. From this, the cavitation limit can be calculated for other rpm's and speeds, see the green line. This line in combination with the "undotted" part of the J1 line represents the possible thrust envelope. This is very often neglected by (unknown to??) the jet sales offices.

    The lower crossing between the green and red lines at about 8+ knots represents the working point (where available and required thrust balance) that you get with this setup! You also find the non-cavitating point where the 2000 rpm thrust line is crossing the resistance curve at ~7 knots. Here, each engine is producing ~186 hp. Even if full load cavitation-free operation could be achieved below 22 knots, the thrust margin would not allow stable operation. So, unfortunately there is little chance for radical improvement with this jet!

    On the other hand, with a propeller arrangement it would be easy to make use of full power. A rough first estimate shows that even with non-optimum props a total thrust of ~9000 lbf would be available 16 knots, and a max speed about 23-24 knot. At the 8 knots you have today, the fuel consumption would be reduced by nearly 40 %. Beeing well aware of the operating qualities of jets in your fishing, I am convinced that a well designed propeller/tunnel design would be equally "handy" in addition to its superior efficiency and loading capacity.
     

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  10. Truls
    Joined: Dec 2007
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    Truls Commercial Fisherman

    I figure at this point some more pics are appropriate. Found some that show the bow better, and some of the jets/intake.

    One thing that occurs to me as I look at these pictures is that there is a box built into the transom that houses the outboard part of the jets. This offers some protection from nets and other boats, but may affect performance. I do not recall seeing this box on other boats.

    Thanks for looking.

    Truls
     

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  11. baeckmo
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    baeckmo Hydrodynamics

    ....and yes Tolly, I think you're on track here. It might be possible to reduce the hump peak somewhat by the addition of a "fake nose" up front. But profitwise, a switch to tunnel props is by far the most effective to start with! Even if only to continue operating at 8 knots. As shown above, there is a possible saving, due to the higher efficiency, of about 150 hp at this speed. Say that this boat is working 12 hrs a day during the 2month season. That adds up to 108.000 hphrs. Fuel consumption at low engine load is something like 0.24 l/hph. The saving is thus nearly 26.000 litres over one season. What do you pay for fuel, Truls........?

    I would say there is a very strong case for a set of CP propellers here, any suggestions Richard??

    And for the hull, we need more experienced input on tricks to reduce hump resistance from those of you working with short, fat beauties!! Daniel, what do you say??
     
  12. apex1

    apex1 Guest

    Fully concur Bodo! This seems to be a first grade example for a conversion to cpp setup. And maybe it is worth to think about just a single engine to operate in the upper displ. mode as a maximum. 400 horses should be a sufficient propulsion for that. When the superior maneuverability of a twin setup is NOT on top of the requirements, a single op with cpp would be within a very affordable ballpark.

    look here at this post for a comparison:
    http://www.boatdesign.net/forums/props/controllable-pitch-propeller-summary-30695-12.html#post332962
    this would be already a bit overkill.

    I would like to hear Daniel and / or others comments on that. Mike Johns?? Daiquiri??
    And I´ll mail John on that!
    But I doubt we get her on speed at reasonable effort (and cost).

    Regards
    Richard
     
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  13. Truls
    Joined: Dec 2007
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    Truls Commercial Fisherman

    You guys got a couple of posts by me while I was looking for pictures.

    Baeckmo - I see you have spent some time with this and I really appreciate it. If I still lived in Norway, I would have to send you over some smoked salmon. I am not going to claim to be able to absorb everything you say, but you definitely are convincing in the limitations of my jets.

    Though I spend a total of two months on the boat a year, it actually only fishes about a month, though probably an average of 18 hours a day these last few years. Fuel prices have been between $4-5 per gallon and this last year I burned about 2200 gallons, about twice that of a comparable prop boat as far as I know.

    In the future I plan on spending more time fishing this boat, and fuel economy will become an even bigger issue. At the present time I am involved in other fisheries the rest of the year. It seems more and more that, long term, props is the answer. Would the hull form inhibit performance with props as well, or will the extra efficiency (150 HP) be enough to overcome the blunt bow issue?

    Regards,

    Truls
     
  14. apex1

    apex1 Guest

    Yes, unfortunately your boat does´nt have a "hull form", it is a floating box.

    But if you would be fine with ONE engine and a Controllable Pitch Propeller instead of a gearbox, you might end up with a win win situation.

    We need to discuss this further and deeper though.

    Regards
    Richard
     

  15. Truls
    Joined: Dec 2007
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    Truls Commercial Fisherman

    Thanks for the come back Richard. You know it hurts my feelings a little to hear my boat referred to as a floating box. I love that boat, and it has been good to me over the years. My wife used to refer to it as my mistress.

    However, I have to accept that you are all right. When this boat was originally built the owner was not willing to compromise on anything. He was not a very popular guy with all the people who have been involved in putting this boat together over the years. I believe he was repeatedly told that it was not going to work, probably by Hamilton Jet as well.

    The boat has been an embarrasment for them for a long time around here and I think they were quite relieved when I changed the name and at least made it a successful fishing vessel. That is probably also why they seemingly have not wanted any more money going into this boat. I will give them credit for that.
     
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