School me on increasing performance please

Hey Guys, just curious as my Rinker is my first boat that will go fast.  Fast for my standards I should say.  I will use my GPS next time I go out to see how fast she goes.

Right now I am running a 98 Festiva 212,  5.7L,  Bravo1 with a 14.5x19P prop.

Ive been reading around, and I saw where MT stated:  "I used to tweak the drives out just like you did in the video. I'd watch my revs and mph and keep trimming out 'till I hit the max."

What does that mean?  I understand adjusting the trim while watching the speedo for best trim position/getting top speed, but "tweaking the drives" or watching the revs, what is it and how does that and watching your revs help? 

Thanks for the help!


  • Black_DiamondBlack_Diamond Holland, MichiganPosts: 4,489Member ✭✭✭✭✭

    If you watch your rpm/mph as you trim the drive you'll find the sweet spot for best mph...over or under trim and the mph will fall off. 

    2003 342FV "Black Diamond", 350 MAG MPI, 20P 4x4 Props, PC BYC, Holland, MI
  • LaReaLaRea Alexandria VirginiaPosts: 3,287Member ✭✭✭✭✭

    For most boats, you can get close to optimum by doing this:

    1) With the outdrive all the way down, get the boat up on plane.

    2) Put the trim tabs down just a little bit (maybe 10%).

    3) Raise the outdrive until you hear the prop start to ventilate (sudden rise in noise and RPM), or until the boat starts porpoising (bow bouncing up and down).

    4) Lower the outdrive just until it stops ventilating or porpoising.

    After that, start experimenting.  If you raise the trim tabs a little, does it make the GPS speed (or revs) go up, or down?  The sweet spot will vary depending on how the boat is loaded, sea/wind conditions, etc., so optimizing is a matter of practicing and paying attention.

  • 212rowboat212rowboat Posts: 2,205Member ✭✭✭✭✭

    ooooh.. you gotz a bravo... No 300hp ceiling, but a 425hp ceiling instead.... Ooooh... I could have some fun with that... :-)  5.7 says you?..... Oohwee ooohwee ohhh... Rocker rollers pedestal mount, port matched intake-to-head-to-exhaust, smooth out the throttle throats... You'd be adding around 20 to 30 ponies right there, and adding likely 3~4mph... All in an afternoon.. 

    Sad thing, though- if you were to add fifty hp, you'd likely pick up maybe 5~7mph, or 75hp, maybe 7~8mph...  the power added isn't a consistent slope on a graph.. getting the boat on the pad/plate is one thing... Once it's there, adding ponies doesn't equal adding mphs at the same climb.... Drag increases exponentially, not in a steady line. 

    Carb or efi? If efi is it tbi or mpi? ........... 

  • Michael TMichael T Posts: 6,558Member ✭✭✭✭✭
    edited July 2014

    JV, as DA says, that is a nice combination and allows for some up-grades if you wanted to, later. For now to do your first "tweak" of the drive, take your engine up to either the mph or rpm you want. Then take your eyes off the gauges and look ahead. While doing this trim your drives up until the boat starts to porpoise or you feel/hear prop slippage - and either one should happen if you have the correct prop/gear drive ratios. Once it starts to porpoise or cavitate (slip) tap the drive very gently down a couple of times until the porpoising stops. Once you have that "feel" under your belt you can do it in different rpm/mph ranges. Then you can start watching the gauges "with one eye" on the water ahead as you see how the performance adjustments correlate with the readings on the tach and speedo. In pretty short order you won't even have to look at your tach or speedo as you'll feel "in the seat of your pants" that your boat is right near it's best performance. At that time you then can glance down at your gauges to fine tune it. My tachs were always more reliable than my speedo (until I bought a Gaffrig tattletale speedo) so I used my tachs for performance adjustments. BTW depending on your Bravo's gear set and there were, I believe, at least three ratios in that year - not counting high altitude gear sets - you may be under propped at 19" pitch. I often ran 21s and sometimes 23s on that combo. To determine how your prop is doing, take your boat into its WOT (wide open throttle range) - when safe conditions permit!  Where does the rpm fall? If it's right near the top of your engine's rated rpm range then all is good. If it's 200 or more rpm under your top WOT range you can go up to the next pitch of prop - likely a 21, in your case. That would give you the best top end but less hole shot. It would be your preference. I doubt that your boat is over propped with a 19" pitch but - for the sake of information to you - if your rpm fell at the bottom end of your engine's rated rpm range then I'd be worried that my boat was over propped and that I was "lugging" the engine. You should have a ton of fun doing all of this.....and as DA said you have a package that can be - as we used to say - "breathed-on",  meaning - leaning over it with tools and some new gear - a performer  rpm intake and new carb etc. etc. etc. Keep us posted! MT

  • MarkBMarkB OntarioPosts: 3,215Member ✭✭✭✭✭

    jvan, I think you are doing the same thing already. Just different terminology. 

    Boat Name: King Kong

    "Boat + Water = Fun"

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