Mounting an Outboard

Discussion in 'Boat Repair Help' started by Snobal, Mar 1, 2006.

  1. Snobal

    Snobal New Member

    Messages:
    40
    State:
    Canton, OH
    I have 2 questions about mounting my outboard. I have a 1972 Ebbtide tri-hull and just purchased a 1989 Evinrude 28 special.

    1) How far should the "anti-ventilation plate" be below the transom? The owner's manual says no more than 3" and it is sitting about 2 7/8" below right now. The previous motor that was on the boat was a 1970 Wards Sea King 35hp and it had a spacer plate on top of the transom that brought it to about 7/8" below the transom. Is there any type of avg. distance I should be looking for?

    2) The motor did not come with a tach and I would like to add one. Is there a certain type of tach I need or will any one work? Is there a difference between a marine tach and an automotive tach?

    Thanks for any help you can give me!
    Snobal
     
  2. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    You need the cavation plate (the flat plate right above the prop) even with the keel. The bottom of the boat. You can run it about 1 inch above the bottom with out any problems. If the plate is below the bottom (keel) it will create drage which will make your motor work harder.

    As for the tach. look up tiny tach on the net and it will give you a slew of tach that will work and the directions for rigging up.
    I have heard of people that does hook up auto tachs but I have yet figured out how to do.
     

  3. Snobal

    Snobal New Member

    Messages:
    40
    State:
    Canton, OH
    Thanks Bubbakat. The cavitation plate is what the owner's manual is referring to as the anti-ventilation plate. What is meant by the prop may slip or ventilate? I think I had this with the older motor going around high speed turns. The prop would actually leave the water. Would this be prevented by lowering the motor on the transom?
     
  4. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    Yes that is the proper name for the plate. Anti-ventilation.

    If it was doing it in the turns then yes the motor was either to high or trimmed up to high on the notches on the motor. If the plate is even with the keel then it is probably trimmed up to much.

    I have an article some where that explains prop slip and I will try to find it to post it for you.

    The prop ventilates when it is creating more air then water movement there fore you get high speeds from the motor with slow boat movement.
     
  5. Snobal

    Snobal New Member

    Messages:
    40
    State:
    Canton, OH
    The plate should have been @ 7/8" below tyhe keel when the prop was slipping in the turns. It was only sharp turns at high speed though. Slower speeds or gradual turns were fine. I'm not sure where the motor was trimmed at. It did not have power trim and the pin has long since been removed. The crank broke about 2 years ago and it's been off the boat since. Thanks again for the advice and I'm looking forward to that article if you can find it!
     
  6. Bobpaul

    Bobpaul New Member

    Messages:
    3,039
    State:
    Supply NC
    Prop pitch is the theoretical distance a prop will move forward in one revolution. 13" pitch will theoreticly move forward 13". The actual distance it will move forward and difference of the theoretical distance, is how much slip there is.

    Fewer prop blades= less slip and a more efficient prop.

    4 and 5 blade props were designed to get quicker rpm's and get a boat up on plane faster. They should be used in shallow water aplications such as the Fl flats fishing boats.

    Other ways have been used to get quicker rpm's such as drilling the prop hub to allow the water to escape from the hub during excelleration, which eased up on the back pressure and not sacrifice the top end speed by going to a 4 or 5 blade prop, which would create more sli[p and a less efficient prop

    There's alot of things considered in propping a boat correctly. HP, hull design and weight is a big factor.

    If I've left anything out, consider that it's 3:30 in the morning and I've only had 1 cup of coffee.:p