Jack plates, yes or no?

Discussion in 'Boat Yard Basics' started by Brett, Feb 27, 2009.

  1. Brett

    Brett > PRO STAFF <

    Good question. I don't know enough about the use of jackplates to
    comment, yet. Do they let you run shallower? Do they make you run
    faster? Do they improve handling, holeshot? Does the extra load and
    weight being pushed back behind the transom affect draft?
    How should a motor be set up on one? Procedure for adjusting?
    Hydraulic, electric or manual? I'm going to be digging all over the
    web pulling up websites and reading all I can find. Those of you that
    run them on your boat...your comments would be appreciated.
    What I find I'll post.

    One use that requires a jack plate is the installation of a jet outboard.
    It's the only way to fine tune the height of the leading edge of the
    intake to where it won't ventilate. You have to be able to adjust to the
    last 1/8th of an inch to get the pump to run it's best, without throwing
    spray back over the top of the transom and filling your hull.
    For a small jet outboard a minimal setback manual jack plate is all
    that's needed. Once set, no further adjustment is necessary.


    I have a problem with that statement. After all the research I've done
    on pocket tunnels, the amount of rise of the water off the bottom of
    the transom at planing speeds, is not that much. I call BS on that quote.

    No BS here, just basic set up info:










    Because a standard template is used to drill the bolt hole pattern
    through the transom, a jack plate would allow correcting the height
    of the engine installation to the true shaft lengths.

    The basics of jack plates as read are these:

    Used to adjust the height of the motor to it's highest running position.
    The smaller the amount of your lower unit in the water, the less friction/drag is produced.
    Less drag means better speed and fuel economy.
    Moving the prop further back from the transom allows time for air bubbles to rise up.
    That means cleaner water for the prop to bite into.
    At idle speeds the prop and skeg are higher up than on a standard transom mount, run shallower at idle speeds.
    Does add weight to the transom, which means slightly more draft.
    Increases static rotational torque to the transom, if you don't have a strong hull, don't use a jack plate.
    The maximum height of lift is limited by the location of the water intake and the type of propeller.
    Conclusion: If you're looking for the absolute best performance from your outboard, get a jackplate.
    If you just want to go fishing, they're not necessary.

    Trim tabs: Any hull large enough to run open water safely,
    will benefit from the use of remotely adjustable trim tabs.
    Trim tabs allow you to adjust the way the hull runs, fore to aft,
    port to starboard, to fit sea conditions and vessel load, while under way.

    A very nice setup:

  2. beavis

    beavis Well-Known Member

    yes, they work awesome

    but the most incorrect statement that I see a lot of people make is:

    "For every 6 inches of setback, it makes your engine load act like it weighs 50% more."

    so wrong

  3. Brett

    Brett > PRO STAFF <

    Agreed, load is load, what should have been said was:
    The greater the setback, the greater the static rotational torque.
    The basic concept of leverage: Force multiplied by Distance.
    Transom needs to be strong enough to resist the increased torque.
    The shift in the center of gravity of the hull would not be much.
    Incorrect statement removed. Thanks for the heads up Beavis.
  4. beavis

    beavis Well-Known Member

    Brett, you didn't need to remove it. I thought those were basically all the notes you were writing about them. Trying to get feedback on everything you were finding. I have just seen that statement too many times when people are arguing against jackplates.
  5. Brett

    Brett > PRO STAFF <

    Yeah, but wrong is wrong. The idea is to have the correct info shown.
    So anyone deciding whether or not to install a jackplate,
    makes the decision based on the best info available.
    I was finding stuff till late last night, and reading that one this morning,
    after you pointed it out, was definately mathmatically incorrect.
    The sum of the down forces is the hull load.
    Rotational forces at the transom only apply to the transom.

    Bad info removed.  ;)
  6. Brett

    Brett > PRO STAFF <

    Verifying what I read through scale drawings,
    came up with these images which explain the advantages of a jack plate...

    Note: The water level off the bottom of the transom
      is extended past the propeller to allow labeling.
    Not to indicate that it remains at that level!!!!


    Normal installation of outboard,
    cavitation plate 1/4 inch below bottom of hull
    when hull setting level on trailer


    Normal installation with hull on plane,
    cavitation plate is actually further below the surface of the water
    because of the hull running angle.


    With a jack plate, the motor is installed so that the top of the water intake
    is just below the bottom of the hull with the hull sitting level on the trailer.


    But with the hull at running angle there is room for more adjustment upwards,
    caused by the difference in trimming the motor out to compensate for the angle change.


    At best adjustment the tips of the prop and the top of the intake
    are just below the surface of the water as it comes off the transom.


    At idle speeds, with a hydraulic or electric jack plate, the engine can be elevated to a point
    where the intake is just below the natural water level allowing the engine to be run in shallow water.

  7. marshman

    marshman just try to follow me....i dare ya...

    so...??  you are saying that the water doesnt "rise" coming out from "under" the boat??
    have you ever looked over the transom in a moving boat??  

    bare with me, im not a physicist(see, i cant even spell it, maybe i did, i dont know, lol), but from someone who owned and operated many different types of boats(i grew up in boats...), experimented with various jacks and tunnels and the like...

    man, a boat  "displaces" water as it moves forward....correct nautical terms would be "displacement hull" and "planing hull"....i understand we are dealing with planing rather than displacement, but....even though you are on "plane"  you are still displacing water...no boat rides completely up on the top of a water surface...the water parts in the front, goes down the sides, under the bottom, and out the back...

    move your hand or a toyboat real  fast in a bathtub or whatever, you will see the "hump"....so...no, the water doesnt actually rise higher than surface of the lake, it just rises to go back to "level", immediately behind the boat....but there is a "hump"....(a deeper running, heavier craft, this hump will be farther behind the boat, hte water has farther to travel to get back to "level"...a smaller, lighter boat, the water pops up right behind)

    now, tunnels....different topic...

    jacks, i think can help most any craft...required??, nah...personal preference, yeah...a  runabout on a lake, i wouldnt bother...a boat built for any kind of specific purpose that the readers of this would be using, probably can benefit from a jack...again, required?? nah...

    now, im going to the bar...im thirsty... ;D ;D ;D ;D ;D
  8. HighSide25

    HighSide25 Well-Known Member

    dont forget, if you add trim tabs, you can tilt your motor out, jack it all the way up, and trim the tabs all the way down. the tabs will push water to the prop and intake while also keeping the bow down allowing you to get on a plane. this is how you can get in some real skinny stuff, but your speed will deminish by at least 30%
    No Bait / Lures Only likes this.
  9. Brett

    Brett > PRO STAFF <

    As stated at the beginning of this thread, I don't know enough
    about jack plates to be considered an expert. The images were
    posted to illustrate the advantages of a jack plate.
    Water displaced by a hull, returns to its natural level
    after the hull moves on. I don't know the speed at which
    water returns back to it's natural level. But looking at
    the bottom of the transom, while a hull accelerates forward
    shows that it does take a certain amount of time for the
    water level to "bounce back". The faster the hull is moving
    the further back the return point is. At speeds over 20 mph,
    in a small boat, that distance is about the length of the hull
    or more. For the purpose of showing the relation of the water
    inlet to the level of the water coming off the bottom of the hull,
    I extended the lines out past the motor for labeling purposes.
    Not to indicate that the water remains below the natural level.
    It obviously does return to the natural level.

    That make sense? :)

    Note added: The water level off the bottom of the transom
    is extended past the propeller to allow labeling.
    Not to indicate that it remains at that level!!!!


    As I don't use a jack plate, any pics out there that show your setup,
    a side view please, at it's best height for running. Show the whole
    motor, transom and jack plate. I'd like to see the "sweet spot" as it
    relates to a hull. I'm learning as I go.

    Install a nosecone with the water pickup on the underside of the
    leading edge, and a surface piercing prop, and you can jack the
    motor up to where the water level coming off the underside of the
    transom intersects the lower unit at the midline of the bullet.
    Intake is still under water, prop blades rotate half the time above water level:


    But even with this setup, you'd still have to contend with hull draft on plane
    and the draft of the lower unit from the midline of the bullet to the bottom of the skeg.
    Still need about a foot of water to run.
  10. topnative2

    topnative2 Well-Known Member

    One has to remember that you get blow out on turns when using a jackplate to its full potential-----especially manual jacks :eek:
  11. Tom_C

    Tom_C Well-Known Member


    Brett, draw this set up:

    8" setback
    Cav Plate 5.5" above hull
    2 degrees from horizontal

    With a 3 blade Aluminum prop on my classic with this setup I can run strait or very wide turns. To run at max speed I need to drop the motor down so the Cav Plate is 3.5" above hull and 4 degrees from horizontal.
  12. islander1225

    islander1225 Well-Known Member

    i have a bob's cavitation plate and a tsg electric jp and can run the motor all the way up with no blow out, and also i can take full turns with it all the way up with no blow out.



  13. Brett

    Brett > PRO STAFF <

    Alex, is that the motor running position or do you trim the motor
    out to level the cavitation plate with the direction of travel?
  14. Tom_C

    Tom_C Well-Known Member

    Bertt, Thanks for the drawings.

    I need to say I am not a Hydro Dynamic Engineer. This my opinion of why/how jack plates work.

    I think Brett has a good point about the hull running angle. The hump theory is an over simplified explanation of JackPlates. Reality is the water will not raise fast enough when you are going faster than 10 or 15 mph. When you add the hull running angle plus water rise into the equation it become clearer how you can raise your motor, decreasing drag, and increasing your speed.
    I agreed with most hull having a running angle of 4 to 6 degrees from horizontal my hull running angle is less, closer to 2 degrees because of weight distribution. The drawing below show 4 to 6 degrees running angle, but are close enough to show the relationship of hull running angle plus water rise.

    I have a Gheenoe Classic 15”6” it has the following setup:

    1. 3 blade aluminum prop. When using a jackplate a SS prop with cupping is recommended.  Cupping a prop helps to hold water to the prop decreasing prop slippage.

    2. JackPlate with Trim, This unit has 8 inches setback and raises my motor 2 inches at minimum height. If my running angle was 4 to 6 degrees you could add a inch to my numbers.

    3. Extented travel TrimTabs, most TrimTabs have 2 inches of travel. My TrimTabs have 4 inches of travel. When my Trim Tabs are up they are out of the water when the boat is on plane.

    4. Thrust Director, Cavitation Plate attachment. A Cavitation Plate attachment help to prevent the prop from ventilating, (drawing air down through the water causing the prop to lose grip).

    Without a Cavitation Plate attachment I can raise my motor so the cavitation plate is 2.5 inches above the hull. You can see below if the water did not rise, my prop would be out of the water. In reality my prop is about 2 inches below the surface of the water.  If I try to raise the motor any higher the prop will ventilate,


    By adding a Cavitation Plate attachment I can raise the motor higher. This is possible because the Cavitation Plate attachment helps prevent the prop from ventilating. You can do the same with a SS prop with cupping.


    By lowering the Trim Tabs I can force the water higher at the prop . Not only is the water higher at the prop, my stern is higher decreeing the trim of my boat. With this setup I can raise the cavitation plate 5.5 inches above the hull. My motor RPM increases by 400 RPM but I lose 20% of top speed do to the prop slipping and I can forget about tight turns.

  15. Flyline

    Flyline Won &quot;Do More With Less&quot; Award!


    Now if I want a trim tabs with a riserplate (no setback) on my gheenoe LT15 with 25hp 2-stroke and my cavitation plate is 1.5 inches above the bottom of the hull.

    If I'm going to add a trim tabs and add a 4 blade heavy cupping S.S prop. then raise my motor 6'' above the bottom of the hull without need a jackplate and more weight in the back.

    I'm trying to save weight in the back and run skinny as possible wihtout need a jackplate. I'm just going to see if this works without a jackplate but if not then a electric jackplate on order to run very skinny! :)
  16. Tom_C

    Tom_C Well-Known Member

    WARING Do not attempt raising a water cool motor without a water pressure gage!!!
  17. Brett

    Brett > PRO STAFF <

    You explained it well Tom, thanks.
    I see your point about needing a water pressure gauge,
    running the jack plate at the level needed for speed and control,
    or higher, places the water inlet at the level where just a touch
    too high, would allow air to be pulled in. Run at that level,
    where air instead of water is being drawn in, you'd overheat the outboard.
    With the gauge, as soon as you saw the pressure drop,
    you'd know to drop the jack plate down a bump to regain pressure.
  18. Tom_C

    Tom_C Well-Known Member

    Brett, That’s correct. With a cup prop and cavitation plate attachment you may be able to still run with little or no water cooling. Looking back to see if your motor is still peeing is not good enough. When running a 5.5 I lose 25% of water pressure. My intake is still under water, but by using the Trim Tabs to push water to the prop I am creating turbulence in the water flow to the motor water intake. Without the setback of a jackplate at lower speed your motor may have proper water flow to the intake, but as speed increases it is very possible that the water will not have time to rise, before the water intake and it will not take long for your impeller to be destroyed at high speed.
  19. marshman

    marshman just try to follow me....i dare ya...

    my motor runs "quite" high...i dont have a water pressure gauge....i guess i been "putting my motor in harms way" over these last 4 years by using the "pee gauge"....hmm

    my 25--

    oh, and this one (not micro)has a water pressure gauge, and maintains 20PSI running that high...the top intake holes are plugged, to prevent sucking air...


    ***not arguing...just bragging, maybe...
    No Bait / Lures Only likes this.
  20. Tom_C

    Tom_C Well-Known Member



    It looks like your cavitation plate is an inch or two above your tunnel. This is the same as having a boat with the cavitation plate an inch or two above a non tunnel hull.

    My comments were to directed towards those that think just because you mount a jack plate you can raise the motor 4 inches plus and all is good.

    So to the newbie wanting to run a JackPlate to the Extremes the two things I recommend are Water Pressure Gage and a Tack so you can see your RPM.