Dana 19' mini Jet handeling problem

Discussion in 'Powerboats' started by Brent, Jul 14, 2004.

  1. Brent
    Joined: Jul 2004
    Posts: 3
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: North Fork, Ca.

    Brent New Member

    I have a 1988 19' Dana Mini Jet boat with a 427 big block chev and a berkley JE jet with a jetvrater. The boat was made in the Los Angles area. When I am going above 35 to 40 mph jetvrater up and I let off the power the front of the hull drops and the boat darts to the side in a drastic and un controllable turn, very un safe. I almost throws you out of the boat. In trying to correct the turn, it will dart to the other direction. Can anyone tell me why this happens or give me any suggestions on how to correct this?
    Brent
     
  2. CDBarry
    Joined: Nov 2002
    Posts: 824
    Likes: 57, Points: 28, Legacy Rep: 354
    Location: Maryland

    CDBarry Senior Member

    Sounds like a classic example of planing instability. See Lou Codega's article on the subject in Professional BoatBuilder a few years ago, or the papers by Codega and Blount and by Cohen published by SNAME - its on the SNAME CD or from www.sname.org.
     
  3. Frans X L
    Joined: Jun 2004
    Posts: 18
    Likes: 1, Points: 0, Legacy Rep: 15
    Location: Cape Town, South Africa

    Frans X L Junior Member

    This is probably a broaching tendency or 'bow steering'.
    This is usually caused by the centre of mass being too far forward, ie the boat will float bows down when at rest.
    Another possibility is a bow profile that is too steep and which forms a sort of bow rudder.
    What is probably happening is that when the boat comes off the plane and the bows drop the centre of lateral area is forward of the centre of gravity of the boat . this makes the boat unstable in yaw.
    The quick and nasty fix is to fit a big skeg or fixed rudder at the transom or back. this should move the centre of lateral to the rear and make the boat stable in yaw. This might take some experimentation though.
    If possible move some weight to the rear to let the boat trim stern down at rest.

    A first easy experiment is to place some weight at the back.
    regards
     
  4. Thunderhead19
    Joined: Sep 2003
    Posts: 506
    Likes: 3, Points: 0, Legacy Rep: 21
    Location: British Columbia, Canada

    Thunderhead19 Senior Member

    I worked with a company that was trying to repair an identical problem on a high speed transport-water taxi years ago. In order for the jet drive to suck properly, theres a flat section in the hull. If this flat area has an outline resembling a squat triangle (pointing towards the bow of the boat) What may be happening is that when you let off the throttle, the boat responds to the sudden change in torque and, as you say, the bow drops. This causes the boat to land on one of the edges of the triangular pad, using it as a keel or rudder. Then Zoooom, you go shooting off to one side. There are many solutions to this
     
  5. Brent
    Joined: Jul 2004
    Posts: 3
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: North Fork, Ca.

    Brent New Member

    Thanks I have been told that I need a small rudder on each side of the jet intake,The intake is up in the bottom of boat about 2". I have also been told I need a rudder in the middle of the boat. I have all the weight in the boat back as far as possible. Is their any other solutions ?
     
  6. srs51668
    Joined: Jul 2004
    Posts: 3
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: flint mi.

    srs51668 New Member

    Does the boat turn in the same direction every time you let off? if so you may want to look at the trailer supports on the left and right sides. if the support has broken or changed dramatically on one side that can alter the shape of the bottom of the boat causing one side to try to stop? usually the last 8 feet is most affected. does the boat do the same thing with the jotovator down? if not you may be catching water in the reverse portion of the nozzel.
     
  7. Brent
    Joined: Jul 2004
    Posts: 3
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: North Fork, Ca.

    Brent New Member

    It does it worse with the jetovator down. The last time I was out on the lake it darted to the left, when trying to correct the turn it darted to the right. The tunnel or ridge catches in the front of the boat, and puts it in a turn. From what I have found out is I need to get the bow of the boat up or keep the back of the boat from allowing the front to catch the water. I am not sure how to do this. Maybe I need to cut out the tunnel in front!
    Brent
     
  8. Thunderhead19
    Joined: Sep 2003
    Posts: 506
    Likes: 3, Points: 0, Legacy Rep: 21
    Location: British Columbia, Canada

    Thunderhead19 Senior Member

    Adding appendages would break my heart. If you can't soften the transition between the intake and the v-bottom (lots) then a fin of some kind is probably best, but it may not completely solve the problem. If the sides of the transition were parallel it would completely solve the problem, maybe put a shallow rail of some kind, parallel on either side of the intake. But something as extreme as a rudder is not necessary. A pair of broad, stubby fins
     
  9. SparkyDog
    Joined: Jul 2004
    Posts: 6
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Oregon

    SparkyDog Junior Member

    Sorry to add to your challenge but as for the two fins on the intake of the Berkeley jet, I am about to cut mine off! For my boat (18' Apollo ski boat), during medium to hard turning, the boat slides a bit in the water and the fins then cause the water flow to block and/or go turbulent, which starves the impeller and drops power (thrust) just when you want it most. So adding the fins may just add more problems.

    My 2 cents on the basic problem is yes you have a fundamental problem with boat LCG (Longitudinal Center of Gravity). There is an excellent article about this at:

    http://www.ultradynamics.com/sections/ultrajet/considerations.asp#8

    If it makes sense, great, if not, find an engineer or someone who can explain it to you. But basically, your LCG should be at 40% or less of the length of the boat at the water line. Maybe your boat was not designed properly for the jet or, maybe you have water (or waterlogged wood) below the floor. The fact that the pheom is worse with the jetrvator down shows that the instant you cut power, the nozzle thrust is helping drive the bow down (combined with the suspected incorrect LCG). When you cut power, the boat should continue to plane with the bow slightly up until you are almost at a stop, then the bow should drop nicely into the water. Sounds like this is not the case.

    How to fix the LCG? First, find someone else who has your boat and ask them if the have the same problem. If not, then that's good bad news, because it means there's something weird with just your boat. Then look for what might be doing it - water in the bow, or your stashing 400lbs of gear in the bow or something. If ALL the boats have this phenom, then you will have to add ballast, move engine and/or fuel tanks aft, or something...
     
  10. Thunderhead19
    Joined: Sep 2003
    Posts: 506
    Likes: 3, Points: 0, Legacy Rep: 21
    Location: British Columbia, Canada

    Thunderhead19 Senior Member

    Yes! what is your static trim like???
     

  11. SparkyDog
    Joined: Jul 2004
    Posts: 6
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Oregon

    SparkyDog Junior Member

    I've got the boat completely torn down now, flipped upside down and restoring the bottom - but that is another story.

    Overall length of hull is 18'. Approx length at waterline is 15. In stock config, I estimate the LCG was about 4' from the transom. The fuel tank (35gal) was in the bow and the V8 (Ford 351) was bolted directly to the jet, so CG of the motor was about 2.5'. CG of fiberglass hull was probably right at the middle - 9'. Always felt that CG was just a bit too far aft. When the fuel was almost gone you could really tell the difference doing a hole shot. The thing would seem to stand on the aft and take forever to plane.
    (PS - I too have a jet-o-vator so I can play with trim.)

    Now I am going to replace fuel tank with V bottom and move closer to ideal LCG. Also moving V8 forward about 2'. I want to get the LCG at 5' - 6' with a full fuel tank.

    Hope this answered your question. If not, email me: steve@sparkydog.net.
     
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.