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Motor Height Setting

gogittumgogittum Nature CoastPosts: 3,797 Captain
I'm some confused on cavitation plate height.  Posts I've read in various places seem to agree that it should be about an inch above the bottom of the boat, so I put a spacer block on the transom.

A guy who works with outboards a lot looked at it and pronounced it good, but I'm still uneasy.  

I put a straight edge along the bottom and marked it here with the red arrows.  Seems to work fine with the 13 pitch prop, but cavitates on acceleration with the 11 pitch, tho' it's fine when I do get up to speed.

Set up like this, it gives me 25 mph @ 6200 rpm with the 10.5" x 11" prop that came with it and 27 mph @ 5200 with the new 10.5" x 13" pitch Solas prop.  A 1,000 rpm change for 2" of pitch change seems like far too much for that difference.....I would expect around ~300 rpm change per inch of pitch.  I thought/hoped the 13 pitch would bring it down to around 5500 rpm, the recommended max by Johnson.  Surprise surprise.

What do you guys think ??


  • lemaymiamilemaymiami Senior Member Posts: 4,799 Captain
    Here's my simplified starting point for cavitation plates... I set up with it even, or as much as one inch above the bottom of the hull (using a straight edge on the bottom as you have... ).  Then I raise the motor one bolt hole at a time, running it at each setting until it starts to blow out in sharp turns when running on plane... then bring it down one notch and if it doesn't blow out in a turn I'm good to go... I have a bit of an advantage since I'm running a bigger motor equipped with trim & tilt and not a fixed mount the way most small motors are set up... 
    Tight Lines
    Bob LeMay
    (954) 435-5666
  • gogittumgogittum Nature CoastPosts: 3,797 Captain
    Thanks, Bob.  I think I'm about 1/2 way there.  I can't raise it any more and it's rock solid when set right on the transom, so I'll take it out one more time with the 13 pitch prop, which is what I'll prob'ly be using.  If it blows out at all, I'll drop it 1/2" and try that.

    I "may" have gotten a handle on the power, too....I hope.  Today I found where the previous owner had adjusted the throttle butterfly a bit tight under the flywheel on the advance linkage.  I moved it about 1/8" and am now getting about 1/4" more travel on the timing plate at full throttle.  That should translate to a bit more snoose at top end, I think.

    I'll take it for a swim tomorrow or Thurs and see how it goes.
  • CaptjamesCaptjames Pompano BeachPosts: 232 Deckhand
    If you're getting cavitation getting on plane, then drop the motor 1/2".  It's very common for people to have 2 different props, depending on usage for a boat like yours.  The 13" is fine and it's in rpm range which should be 5000-5500 on your motor.  I'd use it if you're fishing solo.  Throw the 11" on there when you bring a buddy and/or have more weight in the boat.  It's also possible for a prop shop to add 1" of pitch to the 11" but might not be worth the cost. 
  • gogittumgogittum Nature CoastPosts: 3,797 Captain
    That sounds right and proper from both.  Thanks.  I'll water test it again to be very sure and will probably lower it back 1/2".  My new timing light will be here today, so that will eliminate one possible problem with power output.

    5,000 - 5,500 rpm is fine, but book says 5,500 is max rpm for this motor and is what I've always gone by in general.  I'd like to see that 55 on this one, too, if possible.  I'd like to see 30 mph flat out, too, but pie in the sky is elusive.  A comfortable cruise of about 22 - 24 mph will be just fine.

    I'd almost forgotten - last time out, I wasn't getting any charging.  I knew I'd put that voltmeter on the panel for a reason and now it's paid off.  I'll jump on that today, before going into town to pick up the meter.  Charging has been fine up till now - I put a new charge module on it when I 1st got it.

    I don't want to spend the money at a prop shop to have pitch checked on both and possibly adjusted until I'm very sure I've got all my ducks in a row

    Thanks again.
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