is my engine is mounted too high?

Discussion in 'Powerboats' started by the brain, Oct 6, 2016.

  1. the brain
    Joined: Sep 2016
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    the brain Senior Member

    I've completed the I/O to OB conversion.

    ?1.is my engine is mounted to high?

    ?2.what prop is recommended.
    I currently have a 14 1/2" X 24 SS prop. at WOT she won't go past 4500RPM at 45 MPH.

    looking for a mixture of performance and fuel economy I expect to go 55MPH.

    engine 2004 carbed 2 stroke hull weight 1700LB engine weight is 383LB.

    https://www.youtube.com/results?search_query=run2wtrim
     

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  2. gonzo
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    gonzo Senior Member

    Yes it looks too high. When it is trimmed all the way down, the bottom of the anti-ventilation plate should be about 1" above the keel per feet distance to the transom. Use a straightedge on the bottom for alignment. That is the first rough approximation. You can then tinker with it a bit up or down.
     
  3. ondarvr
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    ondarvr Senior Member

    It didn't look like your AV plate was out of the water, which would typically mean no, it's not too high. Was it ventilating when you trimmed it up, if it wasn't then you're fine, but it will eventually ventilate if trimmed up high enough. It's trial and error on motor height, you need to move it up and down to see where it works the best for how you want to use it.

    If the motor is running correctly you're about 1000 RPMs low, a lower pitch prop would help.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Over-pitch prop for sure, I'd say. 19" pitch tops. You can expect 55mph, but it won't happen.
     
  5. PAR
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    PAR Yacht Designer/Builder

    Agreed, a 175 on that size boat and a 55 MPH expectation is pretty unrealistic. What's the weight of the boat? What's the distance from the ventilation plate to the bottom of the boat at the transom? It does appear to be mounted high, but there's a far bit of latitude in this area.

    Assuming a 3,500 pound boat, with a 175 HP two stroke and a little slip, you'll be lucky to hit 40 MPH, likely the high 30's, with any chop, wind, currents, etc.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    40 mph does seem realistic, as a top speed.
     
  7. PAR
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    PAR Yacht Designer/Builder

    When I do a quick work up in my head, I get about 37 - 38 MPH in a best case situation, at 3,500 pounds.
     
  8. mydauphin
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    mydauphin Senior Member

    Yep, this hull superstructure is not very aerodynamic, not a big deal at 30 knots, but a big deal at 50. It will double your hp required to get there.
     
  9. the brain
    Joined: Sep 2016
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    the brain Senior Member

    I have 18" of setback so I was thinking it was a tad high. I attach image of engine at it's highest point it has been lowered 1 1/2" since this image. I can lower another 1 1/2" that it.
    Thanks STB

    please excuse my ignorance I'm not sure what ventilating is I assume it's the water pumps impeller peaing? if yes I will aim the camera farrer back next shakedown to observe the pea.

    during my first shakedown a month ago there was a boat mechanic (he owns a repair shop down the road) he was tuning a boat on the opposite side of the dock I asked him how doe's the engine sound (I was concerned my engine mite not be hitting on all 6 cylinders because the engine had been sitting for over a year) he said it sounds fine.

    I'm already going 45MPH on GPS w/ the correct diameter and pitch w/ additionale 1K RPM (I want to be at 5500RPM at WOT.
    how much additional speed will I gain?

    it has been suggested a 14 1/2" by 21" aluimin prop.


    it has been suggested a a 15" X 17" or 15"X 18"

    I want decent performance and decent fuel mileage and don't want to be that Guy that's constantly swapping props.

    I'm getting closer to choosing the prop but still need advice the Mercury prop selecter was way off for me they where suggesting pitchs w/ the engine would be at 4700RPM I did the selecter three times.
    Thanks STB

    well she's moving at 45MPH right now w/ the wronge pitched prop. are you saying w/ the correct pitched prop. she will be slower?

    PAR the boats hull weight is 1700LB engine is 383LBs why are you thinking the weights is 3500LB?

    The boat actually gets on plane quickly as is
    couple more ?s
    1.will the boat achieve plane quicker w/ stansionary trim plates?
    2.will the boat achieve plane quicker w/ hydrofoil on the engines cavitationplate?
    ?3. will the boat handle any better w/ trim plates/hydrofoil .

    remember main purpose of boat is cruising trolling having good top speed is a small %.

    Thanks STB
     

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  10. gonzo
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    gonzo Senior Member

    Ventilation is when the propeller is sucking air. The plate over it prevents it. However, the plate needs to be touching the water.
     
  11. the brain
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    the brain Senior Member

    this ventilation doe's it feel like the prop is slipping and not pushing the boat as it should?



    ?1.w/ one engine must I use a right rotating prop.?or will a counter rotation left turning prop work?

    ?2.I think I’ll try a 14 3/4” X 21 pitch what do you Guy’s think?

    still waiting for response to ?s 1.2.3 in previous post

    thanks STB
     
  12. ondarvr
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    ondarvr Senior Member

    1. Yes, it will perform better with trim tabs

    2. You don't have a cavitation plate, it an anti ventilation plate, but the wrong term is used so often people think it's correct. A foil is a tool that may help in some situations, it allows you to raise the motor higher and not ventilate when accelerating. People think it's designed to drag in the water and hold the bow down, it may sort of do this, but if it drags in the water at speed it may slow you down and may cause handling issues.

    3. Trim tabs generally help handling when adjusted correctly and have few negative affects. Foils may, but can also create problems if set up wrong, sometimes scary handling.

    The AV plate does not need to be in the water when running at high speed, it should be above the surface....but....every boat and motor combination is different. You mounted it on a bracket, this lets you mount the motor higher, but it also changes some other aspects of the setup, some of the questions can only be answered with trial and error on your part.

    Dropping 1 inch of pitch will get you around a 200 RM increase, and not all props are created equal, so different brands will perform differently, not all 20" props will give the same RPMs. Many shops will let you try a few of props until you get it dialed in.

    Your speed may go up a little, but maybe not, that boat is not designed for performance, so don't get set on seeing 55MPH.

    The pics are at the wrong angle for us to see the relationship of the AV plate to hull, they should be directly from the side or from behind, right at hull level.
     
  13. gonzo
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    gonzo Senior Member

    It will make the engine over-rev and the boat will slow down, because the blades are turning on air instead of water.
     
  14. PAR
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    PAR Yacht Designer/Builder

    With the weight and better images of the boat, my quick mental math says 42 MPH is about right. The hull is a warped bottom with fairly wide chine flats. This is why you're getting up on plane quickly, but is also what will restrict much better top speeds. In the mid 40's you run smack into a drag wall, because of hull shape and you just need a whole bunch more power to push through it.

    A ventilation plate foil will help you pop up on plane faster and the prop will stay engaged longer in turns, but it will also shave top speed a touch. Trim tabs will help in some speed ranges, but unless adjustable from the helm, will hurt in other speed ranges. As a rule, warped bottom hulls don't get all that much from trim tabs, unless top speed is your only goal. Prop selection is a hit or miss sort of thing, once you venture very far from the stock configuration. You might manage a few more MPH, but pushing a warped bottom into the high 40's, no matter how you do it, will completely defeat any hope of efficiency and good fuel usage. There's also a risk of longitudinal instability issues once you near 50 MPH on this type of hull.
     

  15. ondarvr
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    ondarvr Senior Member

    The AV plate doesn't need too be touching the water at speed, the tips of the prop can even be out of the water, and different props can handle this better than others. At low speeds the AV, and a foil, which is just a larger AV plate, stop the prop from pulling air from the surface. Once on plane the AV plate and foil should be out of the water for best performance. Again, every combo is different, so its just general rules that apply here.
     
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