Motor Position Armstrong Bracket

Discussion in 'Powerboats' started by JOHN P., Jun 21, 2004.

  1. JOHN P.
    Joined: Apr 2004
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    Location: North East

    JOHN P. Junior Member

    O.K. I got the motor to the point where the boat planes out nicely, I have no spray, but I'm getting some ventilation. I posted a previous thread and someone recommended a larger after market cavitation plate to eliminate ventalation. Will this work?? It seems as though I need to drop the motor about 1/2 the distance between the motor mounting holes or about 10mm (.394). Has anyone ever done this and how? It seems like I'm just about there. What should I try next? The advise so far has been very helpful on this boat project.
    Thank you,
     

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  2. nevd
    Joined: Nov 2003
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    Location: Australia

    nevd Junior Member

    Motor height

    Hello John P,

    If you have the time to continue to work on it, you still have many further options.

    Before dropping the motor or installing a large cav plate foil, I would try to run at the current height by dressing the leading edges of the prop by removing any nicks and radiusing any square leading edges so it resembles an aerofoil shape.

    You could also check the remainder of the prop for any damage or manufacturing surface imperfections. A more expensive alternative is to have the prop reworked by a prop "blue printer" who could increase camber or cupping to allow higher running. You could also try a better "quality" prop which is more tolerant to surface piercing. These recommendations assume you operate in an area where you are not likely to damage the prop.

    Your motor bracket will mean that the motor will effectively be deeper in the water if the trim angle increases slightly - is this possible by moving weight without creating other problems? What trim angle is the boat running at the keel and at the chine when you see the ventilation? How far is the cavitation plate out of the water when you see the ventilation and at what speed range do you see the ventilation?

    The transom angle means that the motor will be mounted lower if it is packed back off the transom and with a combination of boat trim angle and transom angle, you may drop the motor 10 by moving motor back as little as 30 or 40 mm.

    If you have not already done it, check and repair any hull damage in front of the prop area which would disturb water flow to the prop.

    Let us know what you find.

    Regards,

    nevd
     
  3. JOHN P.
    Joined: Apr 2004
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    Location: North East

    JOHN P. Junior Member

    Nevd,
    Thanks again for the input. I run a Yamaha prop. I'm trying an aluminum prop until I get into the ballpark. I started with a 17 pitch and went to a 19 pitch. This greatly helped the ventilation problem, but at 5000-5200 rpm in the ocean I will get a bit of vetilation. I can see the tach jump 200 rpm's. Will a stainless prop also reduce ventilation? The prop I'm running is brand new. I dock in the back waters so I'm not sure I would want to put much money into special props or reworking props as It's usually 3 props per summer. What do you mean by moving weight? How or what would you do to space the motor back off the bracket? What material would you recommend to use as a spacer? The boat is a 2004 so there is no real hull damage.
    Thank you,
    John P.
     
  4. woodboat
    Joined: Nov 2003
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    Location: Baltimore MD, USA

    woodboat Senior Member

    The fact that it is Stainless does not mean it will ventilate less but..... Often Stainless props are engineered to be thinner, stronger and less susceptable to ventilation. When I owned a 15FT with a 70 Evinrude, years and years ago, I could run a 19 instead of a 17, much more trim and gained nearly 5 Mph if I remember correctly by simply switching to a high quality Stainless. The tough prop also meant that I never had to buy another prop where I was going through two Aluminums a season. We have a prop shop locally that is VERY good and very reasonable. They surely would have a solution that would work.
     
  5. JOHN P.
    Joined: Apr 2004
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    Location: North East

    JOHN P. Junior Member

    Woodboat,
    Good info. I think I'm looking overall for the best long term solution to my ventilating problem. What's funny is that the boat manufacturer, bracket manufacturer, and motor manufacturer all have different answers. I really don't think they know the answer!!
    John P.
     

  6. nevd
    Joined: Nov 2003
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    Location: Australia

    nevd Junior Member

    Prop selection

    Hello John P,

    I would be surprised if a stainless prop did not solve your ventilation problem, but it may return when the prop is damaged. A 200 RPM increase is not a severe problem particulary if there is no probem in turns.

    Moving weight in the boat is easy if you carry a number of people simply get somebody to move aft and if that decreases ventilation without causing other problems eg porpoising or difficulty getting on the plane, then you need to look for ways to permanently increase weight aft.

    Spacing the motor back could be as simple as installing a ply wood packer against the transom drilled to take the mounting bolts. A more durable material is teflon used in cutting boards. Remember that if your test shows there is no problem with more weight aft, then packing the motor will make a minor move aft for Cof G.

    Good luck.

    nevd
     
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