Why do jet drives use impellers instead of reciprocating pumps?

Discussion in 'Jet Drives' started by dapperdolphin, Jan 12, 2022.

  1. dapperdolphin
    Joined: Jan 2022
    Posts: 2
    Likes: 0, Points: 1
    Location: Michigan

    dapperdolphin New Member

    I am an engineering student, and I was wondering why jet drives use impellers instead of piston based reciprocating pumps. Obviously, there are practical concerns, such as salt-water degrading the pistons, but reciprocating pumps widely are used for fluid compression for other purposes, so why aren't they used for propulsion? Reciprocating pumps are more efficient than impellers. Perhaps impellers are capable of higher speeds, but why are there no reciprocating jet drives at all? You'd think they would have some usage, given that they are more efficient at moving water than either propellers or impellers. I know nothing about boats, so I'm hoping someone can help answer this question. Thank you!
     
  2. Blueknarr
    Joined: Aug 2017
    Posts: 1,255
    Likes: 299, Points: 83
    Location: Colorado

    Blueknarr Senior Member

    Volume of flow.
    Continuous flow without pulsation.
     
    Barry likes this.
  3. Barry
    Joined: Mar 2002
    Posts: 1,539
    Likes: 312, Points: 83, Legacy Rep: 158

    Barry Senior Member

    "but reciprocating pumps widely are used for fluid compression for other purposes" As your post is referring to a water pump, it appears by your wording that you are suggesting that a reciprocating pump is compressing water. As an engineering student, you should know that there are two fluids, gas and liquids. For engineering analysis, liquids are almost never considered compressible. So getting by this, you are suggesting why could you not use a recip pump to change the speed of water entering the pump and increasing speed out the back of the boat to obtain thrust.
    Some negatives
    1) with a reciprocating pump a percentage of the energy input into the pump has to be spent in accelerating the pistons back and forth.
    2) small debris may impact the ability of the check valves to properly seal through cycles
    3) The weight of such a pump will impact application. I believe that a Hamilton pump (edit: 212, up to about 400 hp) weighs about 120 pounds.

     
    Last edited: Jan 12, 2022

  4. dapperdolphin
    Joined: Jan 2022
    Posts: 2
    Likes: 0, Points: 1
    Location: Michigan

    dapperdolphin New Member

    You're right, I spoke incorrectly, water is incompressible, which is the very reason that water jet drives provide thrust. Thank you, I did not know that the pumps were so heavy compared to impellers. My thought process was that since most boat rotors are only 70% efficient at transferring force into propulsion there should be significant advantages at low speed to using a direct piston drive, even if there are volume problems.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.