Jet pump moved forward in hull

Discussion in 'Jet Drives' started by yachtnetwork, Sep 22, 2014.

  1. Barry
    Joined: Mar 2002
    Posts: 1,215
    Likes: 117, Points: 63, Legacy Rep: 158

    Barry Senior Member

    If YN can install an inexpensive shim and raise the bow a bit, he may find that he can turn the boat easier as there is not as much keel in the water to hold the boat going straight.
    I included the reference to the sprint boats to illustrate the result when you have too much keel in the water on a very tight corner and the boat swaps ends, ie lots of keel in the water
    We experimented with a Berkley Jetovator years back and found that the boat responded quicker to turning with the bow raised.
    Similar to trimming out an outboard.
    This could be a simple and cheap way to see if increasing the bow up angle helps steering without grinding the hook or removing lift strikes
    If YN measures the bow up angle with a simple level, we used a staedler with an adjustable angle bubble, 50 bucks , Home Depot, compares it to the table, perhaps a few degrees bow up might cure the problem
     
  2. drmiller100
    Joined: Feb 2009
    Posts: 92
    Likes: 0, Points: 0, Legacy Rep: 2
    Location: Idaho

    drmiller100 Junior Member

    Wow.

    What happens when you chop throttle on a boat on plane? It continues onwards slowing down. Further, shockingly enough, jetboats don't turn without power.

    you should call someone.
     
  3. Yellowjacket
    Joined: May 2009
    Posts: 659
    Likes: 111, Points: 43, Legacy Rep: 447
    Location: Landlocked...

    Yellowjacket Senior Member

    dr, Your lack of knowledge or good sense is showing. Do a search on "dynamic instability" and you will quickly learn that it's no place to be in a boat.

    When you chop the power in a typical watejet, no, it doesn't steer, but it goes straight ahead. That is, the boat is stable and has what is known to those familiar with vehicle dynamics refer to as "stability margin".

    The OP mentioned that he was having problems with stability at higher speeds. While he has plenty of power to push this thing a good bit faster, and would if it was stable, he is encountering stability problems above 35kts or so.

    So if he jacks with the waterjet to lift the nose and that solves his stability problem maybe he can get to 40 or even 50 kts. Sounds great doesn't it? The problem is, if the hull is unstable above 35 knots with no power when the power is cut when you are doing 50 knots the boat will immediately spin no matter what the driver does even if he had steering. It has to get down to below 35 knots to be stable and if it can spin before he gets down to 35 knots it will.

    The inability of waterjets to steer when power isn't present isn't the issue here, it's that the system is dynamically unstable when the power isn't being applied.

    If you tried to sell something that behaved like that you'd get sued in a heartbeat.
     
  4. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,155
    Likes: 394, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    So, to understand this, there is a question mark about stability at speed in this boat, entirely separate from the lack of steering response because of the more forward position of the jet nozzle ?
     
  5. Yellowjacket
    Joined: May 2009
    Posts: 659
    Likes: 111, Points: 43, Legacy Rep: 447
    Location: Landlocked...

    Yellowjacket Senior Member

    In a word... Yes,

    See post # 22, "above 40 mph it starts to get "squirmy""... This is most likely because it is running too flat and most of the hull is in the water. Getting "squirmy" is natures way of politely saying you are running low on stability margin. Go faster and expect it to do nasty things if you don't get it trimmed properly. Using this as a tender also means that people will load it up with a bunch of supplies and could well load the boat even more forward, which won't help the situation.

    As noted earlier, trimming back the strakes from the transom will reduce lift at the back end and that may help the trim angle issue.

    Cut, and test is the only way to find out.
     
  6. drmiller100
    Joined: Feb 2009
    Posts: 92
    Likes: 0, Points: 0, Legacy Rep: 2
    Location: Idaho

    drmiller100 Junior Member

    smiles...... So you are saying change the hull so it goes faster, but don't change the jet angle because if it goes faster due to the hull, that is ok, but if you change the water jet angle when you cut the throttle the water jet angle (with no power) will cause the boat to be unstable. '

    how do you KNOW the boat is unstable when it is not under power? Or are you guessing?

    because the water fairies will know the secret about the shim in the nozzle and push the boat around randomly when it is not under power??????

    I will say IMO none of this matters. If he goes much faster, the boat will lift out of the water, the intake will suck air, and boat will go no faster. He can go faster with existing setup by adding more power, and pushing the nose down harder. This will move the drag point even further forward.

    The boat is probably already unstable at speed because the drag center is too far forward, and the boat is acting like a fishing lure. To cure, move the intake back, get some lift on the nose. Get the center of gravity well in front of the drag center, and it will be "happier" going straight.

    As for sprint boats, they are made to turn. They can be made to go "faster" generally by changing nozzle angle and playing games to generate nose lift.

    High speed drag jet boats (over 100 mph) can have VIOLENT tendencies when power is cut.
    A jet boat under power SUCKS water UP, planting the rear of the boat. The Center of gravity is in front of this drag point, so the boat goes straight.
    When power is cut, the water no longer is sucked UP, and the rear of the boat comes up. At the same time, the nozzle no longer provides bow lift, and the nose of the boat comes down HARD. When the bow comes down, a ROUNDED side of the keel makes contact with the water first.

    The rounded side of the keel creates great drag (bernoulli et al), and the boat's nose gets sucked hard one side or the other. The boat makes a 90 degree turn, and driver is ejected at 100 mph plus.

    Cure? Breakaways. They put a ratchet in the pump so the pump can still spin at high RPM, allowing the pump to "process water", so water still goes UP into the pump, keeping drag at the back of the boat. Optionally, they make a giant "leak" in the intake cleanout pointing up, again so water can process through the pump, and maintain drag at the back of the boat.

    Really fast guys usually run both.

    Hull design can also minimize this tendency to hook sideways - some styles are famous for being evil when the engine blows, some are much more forgiving.

    All of this is 100 mph plus. doubtful this thing is going to reach an honest 50.

    I don't think much at all of the idea of moving the intake forward. For 60 years pump designers have been moving the intake further and further back in the boat. At the transom is now typical.
     
  7. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,155
    Likes: 394, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    The general consenus of the experts here (not me, I know little of the specific idiosyncrasies of jet boats) is that the inlet is not ideally placed at all. Is that a deal breaker that can't really be compensated for ?
     
  8. Yellowjacket
    Joined: May 2009
    Posts: 659
    Likes: 111, Points: 43, Legacy Rep: 447
    Location: Landlocked...

    Yellowjacket Senior Member

    No this is not what I am saying at all and I am not guessing. The hull is right now reaching the limits of stability as speeds go above 40 mph.

    One previous poster suggested that stability at higher speed could be obtained if the waterjet angle were changed to help raise the nose with thrust from the jet.

    I simply said that this is a bad idea because that stability goes away when the power is reduced.

    What I am saying is that enhancing stability by jacking with the waterjet thrust angle may well increase the stability margin to where the boat can indeed go faster, but that when the power is reduced, the stability margin goes away and the boat can become uncontrollable and for that reason changing the thrust angle specifically to enhance stability so that you can go faster is a bad idea.

    Quit being a troll and read what is posted before you go off on a tangent.
     
  9. drmiller100
    Joined: Feb 2009
    Posts: 92
    Likes: 0, Points: 0, Legacy Rep: 2
    Location: Idaho

    drmiller100 Junior Member

    I missed where someone said that. To the OP, did you say it gets unstable when you reduce power?

    Symptoms would be it turns VIOLENTLY sideways with reduction in power, yellow is saying it already does this even at 40 mph.
     
  10. Yellowjacket
    Joined: May 2009
    Posts: 659
    Likes: 111, Points: 43, Legacy Rep: 447
    Location: Landlocked...

    Yellowjacket Senior Member

    GEEZE are you just not reading this thread, or is it a fundamental lack of comprehension? Let me explain it as simply as I can and then I'm done with you.

    The hull is just getting squirrely at 40 mph now. The OP is driving it carefully and has realized that he doesn't have much stability margin. The OP believes that if he goes faster than that the hull would become unstable.

    Barry (Post # 43 at the top of page 4) suggested a fix that most likely will do what he said it would, (raise the bow would improve the stability) but if you do that you stand a good chance of getting into an area where the hull is unstable if you aren't lifting the bow with the jet.

    My response was to that post.

    Strongly suggest that you GO BACK AND READ IT ALL AGAIN!!!
     
  11. drmiller100
    Joined: Feb 2009
    Posts: 92
    Likes: 0, Points: 0, Legacy Rep: 2
    Location: Idaho

    drmiller100 Junior Member

    so reducing power makes it less squirrely.

    so, one idea is to get the nose out of the water by changing the hull. Another idea is to get the nose out of the water by changing nozzle angle.

    You don't like the second solution for personal reasons.

    Got it.
     
  12. yachtnetwork
    Joined: Apr 2012
    Posts: 25
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Florida

    yachtnetwork Junior Member

    Sorry I have been a a way for a while and for some reason my auto notification for post updates has not been sending to my email.

    Anyhow...thanks for all the inputs. We are in the process of making some modifications to the hull at this point (cutting back strakes to right about the ride plate area) and are looking into possibly trying to figure a ship to give a little noise up trim to the vessel.

    Our main concern is not how fast we can get this boat to go. I am sure it can be a 50mph boat however want it to continue to plane quickly and run smooth up to around 35mph. But currently any faster and the boat starts to act differently. (we have all discussed why) so right now we are just wanting to make sure that we have total control from 5-35mph and are hoping that cutting back the strakes will eliminate the straight as an arrow tendency the boat currently has. It can barely be felt at slower speeds but as discussed faster speed equals more feedback in the helm making it harder to break from the straight line. As discussed it could also be the inset for the pump in the hull with the sidewalls of the cut out keeping the boat running straight as well.

    To answer a few questions when you let off the throttle at full speed the boat does not want to spin out of go out of control. It simple stays in an almost straight line and obviously you loose some steering control with minimal propulsion leaving the nozzle. (thats where we had thought about a small fin or rudder system for off power steering help) No tendency to spin out at all. In fact the boat is hard to spin out period. With the nozzle exit moved forward in the hull and from a dead standstill you can punch the throttle while turning and the boat skids out a little bit however it has some different characteristics than other jets we have built and tested. Its kind of fun as it just wants to hook up and go! Also is hard to get it to cavitate unlike some of the williams boats we have run. When you spin them out the pump starves a bit for water, they rev up high and then take a moment to hook up again and go. (short moment) This set up seems to not have that as easily.

    I have another full pump out of a boat including nozzle in one assemble. I will post some pictures soon to discuss the shimming of the nozzle if possible. Only the steering nozzle can be shimmed as the pump nozzle exit itself needs to run in a straight line with the drive shaft. (that is aligned with the engines pto with a special tool and is self destructive if not perfect) If anyone is curious here is a link to the parts for the pump.

    http://partsfinder.onlinemicrofiche...wc&a=20&b=16&c=0-PROPULSION-SYSTEM---EXTERNAL
    Part 670 would be what we would need to ship up. Not much to work with there but perhaps we can grind off the top portion of the connection area and put a washer/bushing or standoff on the bottom portion to get it to tilt up.

    The new pumps offer a VTS (variable trim system) that does exactly that. However they just came out for 2016..add almost $2k to the price of the system and have tons of moving parts we dont want to adapt. Nor do we have room for the actuator that runs it all.

    Thanks again for all the constructive input.
     

  13. drmiller100
    Joined: Feb 2009
    Posts: 92
    Likes: 0, Points: 0, Legacy Rep: 2
    Location: Idaho

    drmiller100 Junior Member

    You can build a tapered shim between part 10 and 550.
    You can build an offset pin to replace parts 680. Usually there are bushings, sometimes plastic, with some give to them which you can screw with to get it to steer for a testing expedition (VERY VERY short term), or build angled bushings for the offset pins.
    You can build a part between 550 and 670, with pivots, which is how a Place Diverter works.
    You can build a complicated gimble system for the nozzle exit, which is how Scott Jets does their pumps, but it is expensive and complicated.
    You can remachine the nozzle to accept an insert, and build an angled insert.


    The first option is by far the easiest. I'd build 3 or 4 of 2 degree shims, and you can stack them to try different angles.

    I guess one question is where is the CG of the boat in relation to the nozzle exit? Normal jets are at teh back of the boat, so you point the nozzle "up" to lift the nose of the boat.
     
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.