How much setback is too much?

Discussion in 'Boat Yard Basics' started by WhiteDog70810, Dec 16, 2012.

  1. WhiteDog70810

    WhiteDog70810 Mostly Harmless

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    Here's the plan: Due to the small size of my rear deck, the camber I've added to it and mild to moderate OCD, I don't want a splash well. My plan was to use a Mini Jacker to mount the motor and only drill one 2" hole in the deck for the gas line.

    The Mini Jack gives me 4" of set back. Due to the way I cambered the decks, the highest point is 16.5" above the bottom in the center of the boat. From the pics I've seen of Mini Jackers mounted on regular 15" transoms, the clamp handles just barely clear the back edge of the transom. I am afraid I won't be able to tilt my eventual motor up without a splash well.

    My idea was to epoxy 1-2" thick mounting blocks on to the transom for the Mini Jacker which would correspondingly increase the motor set back, but I don't know if this is advisable. I spoke with Tom C a while back and he suggested 4" of set back for a 9.9 Merc. I assume that 5-6" of setback is probably in the area of diminishing returns, but I also assume that it won't hurt performance, (so long as I redistribute the load to account for the change in CG and my transom can handle the longer lever arm). Is this correct?

    I want to stick with the Mini Jacker or something similar because it is stoopid simple. It will be a pain when I initially tune the trim and height of the motor and pick the right prop for my uses. However, once I figure it out, I plan on never messing with it again.

    Nate
     
  2. Brett

    Brett > PRO STAFF <

    Build a test transom with scrap lumber.
    Remember running angle will be about 5° bow high.
    Set your anti-ventilation plate level with direction of travel
    by adjusting engine trim and then adjust engine mount height
    upwards until the ant-vent plate is level with the rear edge of the transom.
    With a cupped prop, it'll compensate for the top 2 inches of aerated water.
    Then you can tilt the engine up completely to verify the clearances needed.
     

  3. DuckNut

    DuckNut Brandon, FL

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    Nate, I am not an expert but the further back you mount the motor the force exerted on the transom will be exponential. The same goes for distribution of weight. Get the motor too far back and everything else in your boat would have to be up front. It is all a fulcrum (ie: teeter totter)

    With a small motor like you are talking (9.9) I don't think the heigth needed to tilt is substantial. Check it out on the sawhorse. Mount it on the saw horse and tip it up and measure how far it goes below the clamps.
     
  4. lance

    lance I Love microskiff.com!

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    This thread is several months old. . .but what was the out come?  I am having similar thoughts for a skiff I want to build, so I am curious.  I am thinking about a 18 ft. Boggy Creek / Stumpnocker type hull with a 60 HP.  I have a 24 ft T-Craft and a Armstrong motor bracket with swim platform that has a 36 inch set back sporting a 225 OptiMax.  With this set up my cavitation plate is 4 1/2 inches above the bottom of my hull and has allowed me to run much shallower.  I know this might be comparing apples to oranges, but how do you think this type of concept would work applied to shallow water skiffs?
     
  5. Brett

    Brett > PRO STAFF <

    4.5 inches, only with the hull sitting level on the trailer.
    The trick is to remember hull running angle affects lower unit depth.
    Lets make the zero point, the bottom center of the transom.
    and we're going to calculate rise and fall based on that zero point.
    At slow idle, with a properly loaded hull, the keel line should be almost level
    with the zero point of the transom. Increase speed, the bow rises, the angle
    of the hull changes. At planing speeds, optimum hull running angle is about 6°
    from horizontal. That 6° angle provides maximum lift with minimum
    wetted surface/drag. Now remember, our zero point is the bottom center
    of the transom, if the bow goes up then anything behind the zero point
    has to go down. The question is: how much? Well, it's basic trigonometry.
    the tangent of 6° is 0.1051 more or less, multiply times the distance
    from the zero point and you get the difference above or below the zero point.
    So 0.1051 x 12 inches = about 1.25 inches. So at 36 inches behind the bottom
    of the transom, when at planing speeds, the back end of the anti-ventilation plate
    has been tilted down (3 x 1.25 = 3.75) 3.75 inches, in effect you've only gained
    3/4 of an inch lift at 36 inches setback.  :D
     
  6. cwalden

    cwalden I Love my Ghee-not!

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    All of the math makes sense, all of the terminology and measurements dont... to me...

    What is the 0?
    How do you find the fulcrum point?
    How do you measure keel angle? Escpecially when the keel curves along most of it's length.

    I ask all of this because I am thinking of adding a jack plate and trim tabs to me Gheenoe clone. And my friend is thinking of buying a LT25 or a Super 16 and going all out with it.
     
  7. DuckNut

    DuckNut Brandon, FL

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    What is the 0? It was an arbitrary point (center of the transom. Not really arbitrary as you can't teeter totter in the middle of the hull.

    How do you find the fulcrum point? The fulcrum is the point of movement. The transom is the only place the boat can pivot in the water.

    How do you measure keel angle? When up to speed you take a level, making a triangle of a known length and measuring the height of the triangle and then calculate the angle. 5-7 degrees is the unspoken standard running angle.

    Escpecially when the keel curves along most of it's length. The keel should not curve along most of its length. It should only curve in roughly the front 1/3 and then be a straight line.
     
  8. Brett

    Brett > PRO STAFF <

    Quick pic time

    Hull sitting level in water,
    Cav plate level with bottom of hull, normal setup

    [​IMG]

    Hull in planing position, bow rises up due to 6° tilt.

    [​IMG]

    So anything forward of the transom tilts up,
    anything aft of the transom tilts down. Make sense?
    The amount of tilt is in direct proportion to the distance
    fore or aft of the transom.
     
  9. Brett

    Brett > PRO STAFF <

    Same hull with a 36 inch set back, cav plate 4-1/2 inches above bottom of hull
    with hull sitting level it looks like you're really getting up there, don't it?

    [​IMG]


    But get on plane, that bow rise takes effect, and the results aren't so impressive.
    forward of the transom goes up, aft of the transom goes down.

    [​IMG]


    Net result: only a 3/4 inch gain over hanging it on the transom
     
  10. WhiteDog70810

    WhiteDog70810 Mostly Harmless

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    The practical answer to my original question is that the perfect amount of setback is individual to each boat based on its load, trim angle, bottom contour, motor etc.

    What was the outcome? There isn't one yet. I was just freaking out and considering possible "what ifs", i.e. "oh crap, when I tilt my motor up, it will hit the top of the transom". My concern was that the spacers necessary for the motor to clear the transom would also increase the lever arm already created by the a Mini-Jacker sufficiently to damage the transom. I'll let you know it plays out one of these days, but I am now in the "I will worry about that crap once I finally finish this damn thing" stage.

    Hypothetically, you should be able to raise your motor as high as the prop can bite in the water. The distance behind the boat to the sweet spot (the highest point of "bite-able" water in the hump) depends on your speed and bottom contour. In a perfect world, you'd have enough set back to reach the sweet spot and a prop that could bite in beer foam to take advantage of it.

    However, reality loves to stomp the guts out of pretty theories. You must grow, cultivate and harvest that hump of water. For that you need a lot of expensive toys like hydraulic T&T, jackplate and tabs, a cupped prop, probably a compression plate and the horse power to quickly push a war wagon (to survive having a heavy, powerful motor on an extended lever arm attached to its transom) to really see the benefits. It takes a hellacious amount of momentum to get a good bite on aerated water. All those toys add weight and most slow you down, so you need a still bigger motor to feed the rat. You end up herding vectors to allow an outboard that was designed to run in water to instead run in wet air.

    In little hulls like mine, it is a waste of time and money because I won't have enough HP to make the system really sing. A 18' Stump Knocker with a 60 HP might have more luck than my little surfboard. Check out the prop forum. They address this type of question frequently.

    However, it is simpler to just buy a surface drive.

    Good luck,

    Nate
     
  11. WhiteDog70810

    WhiteDog70810 Mostly Harmless

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    Brett,

    BTW, those illustration sum it up well. Also, way to whip out the trig! Sadly, medicine doesn't make me flex those muscles. At best, I can apply the Pythagorean therum on occasion. The rest of the time, I am happy to with using my fingers and the occasional toe.

    Nate
     
  12. jonathanglasser71

    jonathanglasser71 I Love microskiff.com!

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    What an interesting and amusing read !