Has anyone tried a variable grid plate

Discussion in 'Jet Drives' started by jorgepease, Feb 12, 2012.

  1. jorgepease
    Joined: Feb 2012
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    jorgepease Senior Member

    I am about to start building the XF 20 Flats Boat. I wanted to modify the boat a bit and install a the propulsion system from a PWC for two reasons, I want the motor out of the way and I want to protect the prop.

    However in the keys we have lots of sea grass, I am not worried about the stuff floating around, I can usually avoid that. What would happen if you modified the intake to either be much larger or pull from a different angle while at idle speeds, is that possible? In other words instead of pulling from the bottom of the boat have it pull from the sides horizontally?

    Has anyone played with this? Also what happens if you just create more than one intake, wouldn't that spread out the suction force so its not concentrated enough to yank grass off the sea bed.

    Thanks for any ideas. The boat I want to build is designed with a tunnel for a jacked up outboard which gets shallow enough but I really would like to have the motor out of the way.
     
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    My first experience with a Berkeley jet was when everything was installed, sealed and connected.
    I started the engine for the first time and within seconds the jet had pulled in my mooring line, wound it around the shaft until there was a solid lump of rope and the engine stalled.

    The line wasn't under the boat but it was pulled there because without hull velocity, water is drawn from all directions.
    I cleaned out the mess with a knife, removing the lump in small fragments for several hours.
    Subsequently I installed intake grates, thin vertical metal strips covering the intake from beginning to end, with 3/8" gaps between them. That was somewhat detrimental to the flow of water, but proved effective against large objects entering the intake.

    My guess is that sea grass in small quantities doesn't harm the jet. Large parts with roots will not get past the grate.
     
  3. speedboats
    Joined: Jun 2006
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    speedboats Senior Member

    Weed grates normally have the bars spaced well apart and have a gap at the end. This allows the weeds to pass through the pump sort of like a big waste disposer in the kitchen sink.

    Rock grates normally have the bars closer together and span the full length of the intake. While these help to keep larger FOD out weed can build up on the outside and effectively block the intake.

    Stomp grills help by having every second bar in the rock grate lever down when 'stomped' on so pushes the weeds away from the intake and also clears anything jambed between the bars.
     
  4. jorgepease
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    jorgepease Senior Member

    I think what I have to do is go out to the keys and rent a PWC and idle around in the shallows to see the effects on the sea grass in my area. Here is an image of the type of grass I need to NOT SUCK off the bottom

    [​IMG]
     
  5. Milehog
    Joined: Aug 2006
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    Milehog Clever Quip

    Horsepower isn't the issue here, what is the duty cycle of a PWC drivetrain and is up to the task of propelling a lightweight 20' boat?
    The only commercially made fishing boat with a PWC jet I've seen, which BTW was heavier than the XF 20, didn't stay on the market long. The boat's demise may or may not have been due to the PWC propulsion system, I don't know what happened.
     
  6. jorgepease
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    jorgepease Senior Member

    Yeah I have heard that too, though I would be using the larger 4 stroke yamaha skis which are heavier than the boat and have lots of reliability testimonials .

    However I have been on lots of PWC forums and some answers are starting to come in and they all concur in that if you run shallow in this grass and sand you are probably going to plug the intake and suck up sand and debris.

    There are some benefits to the jet running wide open in that I will never harm the prop but a jacked up motor on a tunnel is almost completely protected and I can still idle around without worrying about sucking up protected sea grass. so maybe no jet is better
     
  7. villacose
    Joined: Dec 2011
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    villacose Junior Member

    when i used to run small creeks in my Waverunner, I installed stainless steel screen on the intake grate, about 1/16 mesh, and this eliminated any problems with rocks getting sucked up. it significantly reduced power and speed, but going up small creeks that twist and turn, you don't need a lot of speed. this would work on seaweed as well, you would just have to kill the engine and wait a little for the weed to float off or dive under and pull it away.

    outboard jets can handle rocks and other debris pretty well, but PWC jet units are fragile. they have to be protected. not to mention the fact that if you got a bunch of crap sucked into them they are a hell of a lot harder to clear.

    the bad thing about most outboard jets is that they are mega loud. a 50 hp 2 stroke OB jet is almost unbearable, you need ear plugs.

    BTW, glad to see another Yamaha enthusiast, they are the best IMO. I have heard some horror stories about PWC's and seaweed, you would be well advised to be careful and have a backup plan when testing anything.
     
  8. jorgepease
    Joined: Feb 2012
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    jorgepease Senior Member

    Yes, I scrapped the idea of a PWC propulsion system. I may one day go for a real jet but for this project I found my motor

    .. the new four stroke Yamaha 70 is amazingly light and has had good reviews. I think it's going to push my light xf20 as fast and as shallow as I would want to go ... though one day I may try a jet because I really like the idea of having a wide open platform, no motor in sight.
     
  9. villacose
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    villacose Junior Member


  10. jorgepease
    Joined: Feb 2012
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    jorgepease Senior Member

    Fun! I'd like to try it someday when I have some extra money!
     
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