Inline Outboard Jet, or outboard tunnel?

Discussion in 'Jet Drives' started by Liopleurodon, Mar 23, 2017.

  1. Liopleurodon
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    Liopleurodon Junior Member

    Hello,

    I started a more general thread in the general section, but wonder if focusing a little more and posting here might do a little better.

    I'm basically asking the question of whether it would be reasonable or plausible to combine the inline jet style found on PWCs and inboards with an outboard motor. Picture a PWC jet hooked up in place of a lower unit on an outboard, or picture an engine from a mud motor hooked up to a PWC jet via chain or belt.

    As an alternative intended to make fewer sacrifices at lower speeds, could one fashion a tunnel hull of sorts around an outboard prop, and loosely blend it with the boat hull using flexible flaps, like some do with a jet outboard?
     
  2. Barry
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    Barry Senior Member

    Several years ago, maybe 25, a company did just that. The idea was to improve a bit on the efficiency of the mixed flow outboard pump that just about every outboard manufacturers install and market on their outboard heads.
    They did not make it. Not sure why

    What are YOU trying to gain with this configuration?
     
  3. Liopleurodon
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    Liopleurodon Junior Member

    What I am seeking to gain is better efficiency overall and better functionality at lower speeds. Hence the thought of taking a step further into more of a tunnel prop design integrated into an outboard.
     
  4. jorgepease
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    jorgepease Senior Member

  5. Yellowjacket
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    Yellowjacket Senior Member

    This is being done by the U.S. Navy right now. There are three companies that were awarded contracts to develop a more efficient waterjet for the 55 hp heavy fuel outboard that is used by the special forces. The objective of the system is to be more efficient than conventional outboard jets as well as reduce the acoustic signature of the system compared to propellers or conventional outboard jets. Conventional outboard jets use a radial flow impeller that is mounted on the engine output shaft, then they collect the flow and spit it out the back. This results in a high speed impeller running at 5500 rpm and lot of noise as well as poor efficiency. The design goals for the replacement system are improved efficiency and reduced noise so that the Zodiac boats can be used for mine sweeping or mine laying, where sonar is required. The special forces use this boat and they need a system that has no prop hanging down under the boat. It is expected that the resulting systems would use a more conventional axial flow waterjet operating at lower rotational speeds. The program is an STTR effort that is being done with universities, and the program number is N17A-019. Here is a link to the solicitation. Reduced Cavitation, High Efficiency Outboard Propulsors for Small Planing Craft | SBIR.gov https://www.sbir.gov/sbirsearch/detail/1208623 There were three winners that were announced and work will commence on the program in June, so what you are talking about is most certainly feasible. If you look at the Mercury "Sportjet" drive, this is essentially an outboard motor sitting on a modified lower unit that is coupled to an axial watejet. While it's mounted inside the boat, there is nothing that says this couldn't be done on an outboard. The biggest issues will be ventilation and keeping the unit from sucking up air as you turn or go over wakes at speed.
     
  6. Barry
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    Barry Senior Member

    The most common jet attached to outboards manufactured by Outboard jets do not utilize a radial pump (and I am not sure what radial means as per the previous post, perhaps centrifugal?)
    The impeller in these pumps are an axial flow though to look at the shape of the pump, an axial pump does not jump to mind.
    20 or more odd years ago, there was a company that tried to build a 2 stage axial pump that could be mounted to an outboard but they lasted a couple of years and disappeared.
    BRP, have a current patent on an axial bolt on unit to an outboard but I do not know if its validity has been tested
     
  7. ondarvr
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    ondarvr Senior Member

    The current outboard jet design is from the late 50's and early 60's, with just a few tweaks since then, should be lots of room for improvement. With little too no competition in that market there was no incentive to improve the design.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    I wonder what the real % difference in efficiency between axial and centrifugal pumps is, in this situation. I'd imagine the only reason the military would be interested in that aspect would be for greater range, not saving fuel $.
     
  9. Yellowjacket
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    Yellowjacket Senior Member

    The pumps made by "Outboard Jets" are radial flow (or otherwise know as centrufugal) pumps. If you look at a cross section of the pumping system the water flows in the inlet an then almost radially out of the pump. There are basically three kinds of pumps, axial flow, radial flow and mixed flow pumps. They are categorized that way buy the direction of the flow exiting the pumping element. While they may not be purely radial in their exit flow the pumps made by Water Jets are very nearly radial in their flow at the exit of the impeller. Any momentum in the flow that isn't radial is lost in trying to capture the flow in the scroll, so while it isn't purely radial coming out of the impeller essentially it is so close that it can be called a radial flow pump. The water flows axially into the pump, but the exit is essentially radial. While the impeller inducer section has axial flow, the exit is mostly radial to the shaft axis and therefore the pump is categorized as a radial flow pump.

    The BRP patent, while not past the time limit shows as lapsed in the google database. There are also similar patents that have lapsed due to time.. Even if it was in force the BRP patent would lapse in about 3 years so that really isn't much of an issue.
     
  10. Yellowjacket
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    Yellowjacket Senior Member

    The efficiency issues start with the impeller. The impeller in current pumps is attached directly to the engine shaft and it's turning at almost twice the speed that it should be, which causes high noise as well as increasing losses.The efficiency issues aren't just in the impeller, the radial flow pumps require a scroll, which has high velocity water in it and there are losses associated with the scroll that pull down the efficiency of the whole system. Typical outboard waterjets at 20 knots are between 50% and 60% efficient. An axial jet can get to about 70% if it is well designed and it will get better at higher speed.

    The Navy wants more speed as well as more range from the efficiency. They have speed goals and if your efficiency is good you should be able to achieve them. Those goals are based on a heavily laden zodiac, like you would see with a boat full of burly guys carrying battle gear...
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Maybe a reduction gear at the powerhead could help, already adopted in some prop outboards. ResizedImage600611-DF200A-Geat-Ratio.png
     
  12. ondarvr
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    ondarvr Senior Member

    Suzuki's make a good motor for a pump because they use the reduction with the offset shaft.
     
  13. Yellowjacket
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    Yellowjacket Senior Member

    I wasn't aware of the offset geared outboards, that looks like a way to balance a heavy 4 cycle powerhead with all that weight in the cylinder head (cams and valves and intake and exhaust) on a tower and not have too much weight hanging off the back end of the tower.... Weight and cost tends to compound itself.. There's a reason 4 cycles motors weren't used for eons as outboards, only the desire to be green and ignore weight would let you come up with a solution like that.

    For the Navy program the requirement is to use the Evinrude 55 hp heavy fuel engine. There are lots of gear ratios available for outboards in that power class, all the way from 1.64:1 up to 2.84:1 So you should be able to get into a reasonable impeller speed range with gear sets that are available even without the additional reduction. If you are going to a larger size class than 55 hp you may want to slow it own more and scale up the pump accordingly, but that's a different animal.
     
  14. ondarvr
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    ondarvr Senior Member

    When mounting a pump to an outboard the original gear ratio of the motor is irrelevant, you're removing all of those components and connecting directly to the crankshaft.

    (unless you go with a Suzuki)
     

  15. Yellowjacket
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    Yellowjacket Senior Member

    I don't think you quite understand the concept. The idea isn't to connect an impeller to the output shaft of the engine, it's to create a replacement for the lower unit that uses an axial watejet.

    Centrifugal jets make high pressure and need to turn faster to make that higher pressure. But thrust is mass flow x the change in velocity (change in momentum) so with a centrif pump you have higher pressure, and higher exit velocity but you have lower flow at the same power. But propulsive efficiency is based on the flow velocity. Higher velocity difference between the jet exit and the boat gives you low efficiency.

    For higher propulsive efficiency at lower boat speeds you want more flow at a lower pressure, which is what an axial jet gives you. An axial jet to work right needs to be mounted with the drive shaft in the horizontal position and it needs to turn at a lower speed...

    Here is a patent drawing from the BRP patent. The idea is to use a gear set to turn the drive horizontal and then put the impeller on the shaft similar to the layout of a conventional outboard gear set. It would look something more like this...

    And since you now need a gear set you can use that gear set to get the speed you want for low acoustic signature.. Hope that makes more sense now..


    [​IMG]
     
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