Weight or Prop height?

Discussion in 'Prop Shop' started by aharr24, Mar 7, 2011.

  1. aharr24

    aharr24 I Love microskiff.com!

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    All,

    I have just recently finished modifying a 15 ft. aluminum skiff for shallow water.  I have added some weight by installing a front and back deck.  I am seriously considering a hydraulic jack plate by Bob's machine.  Although before I purchase I had a few questions/reservations.

    I am looking at either the Ultra mini jack plate or the Narrow mini jack plate.  (Both have 5 1/2 in. of setback and built in 1 1/2 in. lift)

    Ultra mini: designed for motors up to 40hp, 4 inches of lift, 18lbs

    Narrow mini: designed for motors up to 50 hp, 6 inches of lift, 25 lbs.

    Because of the newly added weight I want to save weight wherever possible.  Will the lighter Ultra mini be sufficient enough with only 4 inches of lift? 

    I currently run Nissan 4-stroke 25 hp. 15 in shaft. I have about 13 in. transom.  The cavitation plate sits about a1/2 inch below the bottom of my hull.  From the Cav plate to the bottom of the skeg is a total of 11 in.  Would the 4 inches be enough to get me shallower or would the 6 inches be a better fit, considering they are about the same price???

    I realize weight distribution along with some simple additions (tiller extender, etc..) can really make a big difference when you are trying to get skinny but am I compromising my ability to run shallower by trying to save weight with the ultra mini with only 4 inches on hyd. lift??

    I guess my question is what is more detrimental to running skinny, weight or prop height?

    Also, would anyone recommend transom reinforcement/backing plates??

    Here is the link, just in case:
    http://www.bobsmachine.com/Products/jackplate_hydraulic_mini.cfm

    Thanks for your help, looking forward to the responses

    ~Loco
     
  2. Brett

    Brett > PRO STAFF <

    Weight is the culprit when it comes to not running skinny.
    Basic rule of thumb, is that your hull draft a rest is equal to the hull draft on plane,
    unless you have enough horsepower to reach speeds where water feels like concrete.

                                                   :eek:

    Doesn't do any good to save 2 inches of draft with a jack plate and cupped prop
    if all your gear, passengers and engine make the hull draft 4 inches more than it was designed for.


    I'd skip the jack plate, get a cupped prop, and see how high I could clamp the engine
    using a handful of assorted shims in varying thicknesses.
    That's how I figured out the best running height for my latest outboard.

    Raise the engine, add a shim, clamp the engine tight, go for a test ride.
    Repeat until the prop blows out, then lose the last shim.
    Just make sure you're clamping to the transom and not a shim.

    [​IMG]
     

  3. aharr24

    aharr24 I Love microskiff.com!

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    Brett,  Thanks for the reply.  (I apologize in advance for my rambling)

    Let me see if I am following you correctly. 

    I have added an extra 50-70 lbs.  the new weight coupled with 2 passangers, gear and engine etc.. will impact my draft however my skiff is rated for a max of 4 people or 600 lbs.  I know the added weight is not ideal but it was really neccesary. 

    I would prefer to have the ability to manuever my prop up or down considering where I am running.  If I am in the shallow mud flats trying to get up (holeshot) will benot as good without a hyrdraulic jack, or at least this is the impression I am under..

    Are you saying rather than a hydraulic jack plate I should jack my motor up or opt for a transom extender jack plate or shims to find the sweet spot??  Will this help get on plane faster and improve my hole shot in the shallows??

    Will a cupped prop do the trick rather than the added weight of a hydr. jack plate.??

    Also, if I go with the hydr. jack plate route what do I gain or lose from having the ultra mini jack or the narrow mini jack.  Will I able to utilize the full ability of my engine in shallow water with only 4 inches of travel  Or would 6 inches have more of an impact.  [FYI: I have a non-tunnell, modified v to flat bottom]

    Granted we are only talking inches here, although it seems every inch counts. 

    Thanks for your hellp, Looking forward to your responses.
     
  4. Brett

    Brett > PRO STAFF <

    I'm assuming you have a flat bottom aluminum hull.
    At speed, the forward motion of the hull creates turbulence
    which aerates the top 1 to 2 inches of water under the hull.
    You can only lift so far before that aerated water affects the prop
    causing it to lose bite. My other worry is that most small aluminum hulls
    really aren't designed for the installation of a jack plate.
    They're set up so that all the load is basically right at the line of the transom.
    The nice thing about the shim trick is that it's cheap
    and you can see just how high you can get before blow-out occurs.
    A jack plate won't really make much difference on a lightweight flat bottom hull.
    I was researching this when I was building my tunnel hull.
    Interesting discussion ensued...

    http://www.microskiff.com/cgi-bin/yabb2/YaBB.pl?num=1235777190/0