proximity of counterrotating surface drives

Discussion in 'Surface Drives' started by blisspacket, Dec 18, 2013.

  1. blisspacket
    Joined: Jun 2005
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    blisspacket Junior Member

    All right, gurus, I'm piecing together a (hopefully) fast electric and would like to know if any additional thrust derives from twin surface piercing screws being close to each other. I envision P & S chopping down from the outboard side and washing to the center. I can fantasize two pulses of aft-pressured water colliding in the centerline and creating a "squirt" effect.

    That's the hypothesis. Any corroboration or refutes out there, especially with practical experience?? And to continue dreaming, how close might the props be to each other???
     
  2. blisspacket
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    blisspacket Junior Member

    Perhaps "squirt" is inaccurate.
    If one were to change the counterrotations, so that the bottom half of the props, the half immersed in the water, swept from the centerline OUTwards, that would seem to invite cavitation, at least while forward motion was just starting.
    Articulating the drives for steering means you might nott want to have them so close as to create an overlap. On the other hand, volvo and others counterrotate props on the same centerline, to good effect....
     
  3. Baltic Bandit

    Baltic Bandit Previous Member

    Well you are playing with some complex dynamics here. First off, realize that on multi-engine propeller aircraft, the props all turned in the same direction. Given that those designers were doing a lot more calcs than you seem to be, that would be something to think about.

    One of the issues you will have is that if you want a "centerline squirt" those two wave fronts must be synched perfectly other wise you will be getting a "beat frequency" combined with an offset. And I doubt your rotataional speeds are that identical

    secondly, what does this "squirt" give you? once the water has "left" the blade tip, there is no interaction with the boat itself. about the only thing you could hope for is some back pressure from the built up standing wave to increase the immersion of your props - but that sounds dubious
     
  4. blisspacket
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    blisspacket Junior Member

    Thanks for you input, BalticB.
    I do study the threads on prop selection, which confirm my practical experience: seldom is the right screw gained from a set of tables. Matching shaft metrics to hull shapes to performance expectations might involve lines of formulaic equations, and throwing in one misconceived figure renders an unsat prop spec.
    Nope, I'm not in the league of multi-engine aircraft prop spec engineers.
    I can imagine that two props washing towards each other would create the more dense conditions you find in deeper water. Thus the thrust derived in that particular portion of quadrant might be greater. Props spinning at 4000 rpm might cycle or pulse into and out of that hypothetical more dense condition.
    Putting this all into some formula seems madness. On the other hand, executing this in a testtank or on the bay might be something some guru has attempted already....
     
  5. Steve H
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    Steve H Senior Member

    Multi-engine boats and aircraft DO have counter rotating propellers. The Props on a boat can be mounted any distance from one another as long as they don't hit. You will notice on many racing offshore applications that the engines are staggered in the engine room so the drives can be mounted as close as possible. Prop rotation whether turning "in" or “out” will affect different boats in different ways. Most surface drives will turn the props "in" (tops of the propellers moving towards the boat) to counteract the natural tendency of surface drives to lift the stern.
     
  6. blisspacket
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    blisspacket Junior Member

    "Natural tendency of surface drives to lift the stern" is due to shaft angle with the planing surface of the water? And "in" turning props then keep the stern lower...how might that work? Is there data elsewhere, Steve H, available on the web? Thanks.
     
  7. Steve H
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    Steve H Senior Member

    I can only opine from personal experience after spending a boat load of money on SP propellers over the years.

    as I understand it. The vee of the boat leaves a vee shaped wake. Which causes the propeller blades to pull the stern down as it is traveling up hill while submerged on a twin engine boat. In effect providing bow lift. Props with more cup, and or more rake will work even better.

    Now having said that, I can only speak for what works on my boat. I know of a few guys that run turning their props out and they are happy. But they are running low rake props with little or no cup.

    As far as further reading, you might have a look at offshoreonly.

    Steve
     
  8. blisspacket
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    blisspacket Junior Member

    Thank you SteveH. The lead to offshoreonly is great also. I'm a month or more away from water tests. Electric power should give me some measured data that might be worthwhile in all this. On the other hand, I'm playing with 8hp max and transfer to 800 hp rigs is a loooong leap!
     

  9. gonzo
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    gonzo Senior Member

    Surface piercing propellers are ventilated, so cavitation is not really a concern.
     
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