slow boat.. my fault?

Discussion in 'Boat Design' started by ub3r l33t dud3, Sep 13, 2012.

  1. ub3r l33t dud3
    Joined: Sep 2012
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    Location: capac michigan

    ub3r l33t dud3 Junior Member

    hi! new to this site.

    my question is about my boat that i just purchased. it is a 10' g.w. invader that was converted to a jet drive from a o/b by someone, first off im pretty sure the whole conversion was based off an old funjet, it has a 60 hp Suzuki engine and it runs great, the be fiberglass work isn't to bad and whoever did the conversion did a good job reinforcing the area. when i first got the boat the carbs were plugged by mud-dabbers and after i got them cleaned i took it out and found that it was porpoising uncontrollably g(probably from the added weight). we tried adding weight to the front and that helped but to solve the problem we added a foil under the jet drive ( just temporary). i took a GPS to see how fast the boat was going and it only got to 25 mph.

    im wondering if the foil we made is pushing to much water away from the nozzle and the exiting water has nothing to push against. also the intake for the pump could be not placed right since it was a homemade conversion. il try and add a picture of the foil as well as the underside of the boat. thanks in advance for any replies :) and pardon my spelling
     

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  2. ub3r l33t dud3
    Joined: Sep 2012
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    ub3r l33t dud3 Junior Member

    here is a picture of the foil. its adjustable by means of a threaded plug that pushes on the jet and holds the foil at that angle. the foil isn't set at the angle in the picture that was just after we made it, it sits at much less of an angle.
     

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  3. Commuter Boats
    Joined: Oct 2006
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    Commuter Boats Commuter Boats

    "im wondering if the foil we made is pushing to much water away from the nozzle and the exiting water has nothing to push against."
    The problem is not the foil, the nozzle wants to be clear of the water in order to gain any efficiency, can you post a picture of the bottom to include the intake?
     
  4. ub3r l33t dud3
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    ub3r l33t dud3 Junior Member

    yes i can il go and take a few pictures right now.
     
  5. ub3r l33t dud3
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    ub3r l33t dud3 Junior Member

    couple of pictures here, sorry the boat is on the trailer and it was getting dark

    the bottom of the boat is pretty flat it only has a slight v. the intake is maybe an inch or so higher than the rest of the bottom of the hull.

    [​IMG]

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  6. Commuter Boats
    Joined: Oct 2006
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    Commuter Boats Commuter Boats

    The water flow into the jet intake should be very smooth, it would seem where the metal surrounding the intake fairs into the hull is rough enough to cause water separation which would add to pour jet performance. The same is true immediately around the intake.
    If you could fair the transition from the halls to the intake in such a way that the water stays attached I believe you would improve performance.
    Gerald
     
  7. ub3r l33t dud3
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    ub3r l33t dud3 Junior Member

    ok thank you very much for the reply. i study aerodynamics so i can see where you are coming from.
    im going to take the boat out in about an hour to look for where im taking in water. water is shooting straight up the side of the boat on plane and i think it is coming in the rub strip when it dose that.

    the entire hull seems to need modification to gain efficiency, then again i don't have much experience with small speedboats, only larger boats that i grew up with so it could be acting normal
     
  8. ub3r l33t dud3
    Joined: Sep 2012
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    ub3r l33t dud3 Junior Member

    ok, so after replacing a wheel bearing on the side of the road after the wheel came off :rolleyes: i finally got it in the water and agree with what you say about the un-even hull, when i go around a corner it gets water to the intake and i can hear the motor slow down under load.

    my next question is should the intake be so far up compared to the hull of the boat ? it is nearly 2 inches above the bottom of the hull, and if it is too high how could i solve this problem?
     
  9. Commuter Boats
    Joined: Oct 2006
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    Commuter Boats Commuter Boats

    I think you're going to have to live with the elevation of the intake and it's probably what is causeing your purposeing. You seem to be able to control the porpoiseing with the trim tab you installed, oversimplified, you've lost lift in the area of the intake and then provided lift further aft to compensate for it.
    Jet boats, by their nature are burdened to carry the weight of the water that they use for thrust, your boat is further burdened by a reduction in lift in the area of your intake.
    If it could be done without adding much weight you may see benefit in fairing the entire intake area with an eye towards regaining lift yet maintain a smooth flow of water to the pump.
    Gerald
     

  10. ub3r l33t dud3
    Joined: Sep 2012
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    ub3r l33t dud3 Junior Member

    thank you , i never even thought about the weight the water in the jet. also have a "water in the boat" issue that is probably not helping.
    I just solved some electrical issues this morning and repacked the other bearing on the trailer. In the proses of making two trim tabs that will be more permanent.
    Smoothing out the hull will have to wait till just before winter as there are many things that need fiberglass work and i would like to get the fiberglass on and then have the winter to sand and prep for paint.

    Perhaps il start a build thread for the poor boat. thank you for the advice! :)

    [​IMG]
     
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