Please help. I think my outboard is too high or too low?

Discussion in 'Boat Design' started by clctrader, Dec 20, 2008.

  1. clctrader
    Joined: May 2006
    Posts: 21
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    Location: Miami FL

    clctrader Junior Member

    Ive got a 85 224 Mako that I have done lots of things to it. I closed the transom of the boat, installed a full platform bracket and added a nice Yamaha 250HP saltwater series outboard to it.

    Well, the bracket is 30" setback and it was not meant to be for this hull. It has 6 degrees and my transom had 13 degrees, so its kind of looking down a little instead of being completely flat. No biggie.

    I put the boat on the water about 2 weeks ago for the first time and it got on plane very fast and was riding nice but once the bow goes down on full plane, it would start kind of hopping a little, and little by little it would bounce even more and more. Kind of like it would catch the reithem and it would get worst and worst. So I trimmed the motor all the way down and that was the only way to make the bow of the boat steady and calm.

    So then I whent and put trim tabs, so I wouldnt have to trim the motor all the way down to act as a stabilizer, and it kind of fixed the problem.

    A mechanic told me the motor was too high, the cavitation plate was 2 3/4" above the bottom of the hull. I brought it down now to 1 1/2 above the bottom and it rides maybe a little bit better.

    Well, I was out yesterday and the sea was as flat as a lake, smooth. The boat was riding ok with the trim tabs, but as soon as I would stop using the trim tabs, it would ride so much faster and better until it starts bouncing little by little until it gets worst and anoying. Keep in mind this bounce thing is very smooth but it starts to bounce higher and higher on the bow until its not cool anymore.

    How can I cure this? by bringing the boat lower ? its already very low in the water. I just dont want to be using the trim tabs all the time like I have to do now.

    Please someone help and sorry for the long post...
  2. Frosty

    Frosty Previous Member

    This is called porpoising --for obvious reasons. There is many reasons for this -one reason may be the extra wieght of the engine, but can be ballanced by placing removable weight on the front to see if this cures it ,--a person for example or a bag of sand.

    ( Some boats that have been stored or trailered incorrectly and have what they call a 'hooked' hull and is concave, this will cause porpoising and is not reparable.)

    Basically you have altered the boat from its original design. You could try another prop, but the wieght distribution will definately affect your problem.

    You could consider moving the fuel tank, battery , skis and other equipment forward for instance.

    As far as lowering the motor I would have thought this would make it worse

    Trimming the boat is necessary and normal. Lots of boats porpoise --yours is quite mild in comparison.

    Can you post a pic of the back end on its trailer so we can see the engine hieght , engine bracket and trim tabs?
  3. clctrader
    Joined: May 2006
    Posts: 21
    Likes: 0, Points: 0, Legacy Rep: 3
    Location: Miami FL

    clctrader Junior Member

    Will post pics tomorow...

    I will post some pics tomorow in the morning.

    I forgot to mention one thing.

    The propeller I have now is way too small for the boat and engine. Its a 17P, and I really have to rev the motor to get it moving, have plenty of RPM and will redline easy.

    I just bought a 19p 4 blade prop that I will try tomorow. Could this help with the propoising problem?


  4. Design_1
    Joined: Nov 2004
    Posts: 71
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    Location: Georgia

    Design_1 Water Logged Hack

    Your prop is not going to effect the porpoising other than slight differences in prop weight. It will effect your hole shot and your top end. Be sure to check your engines max RPM and adjust your props up or down in pitch accordingly to match. This should help you achieve optimal performance for your hull.

    Frostie is right that ballast may be added to the front of your boat to reduce the action. But, a "hooked" hull generally does not produce porpoising. Quite the opposite, it usually creates "bow steering" by forcing the bow down. Propoising is usually created by to much bow rise. This can come from several different issues, center of balance to far aft, a rocker in your hull bottom, a completely flat hull aft section. If your hull has rocker fore to aft on the planning Delta, you can add a slight hook in the last 8" of hull to force the bow down. If done properly, your hull will perform well with minimal loss of speed and eliminate the porpoising. This will also work for a flat section. If your hull already has hook in the aft section, but still porpoises, you will have to move the center of balance forward, as Frostie stated, by adding balast or by shifting equipment forward. You can set a straight edge between strakes running fore to aft to check your hull for rocker or hook. This will let you know what your hulls running surface is doing. Also, take a couple of people to the water with you, or sand bags from Lowe's. Try placing them at the front of the boat as Frostie said. If this reduces the porpoising action, lead ingots glassed into the hull may be your cheapest fix.

    Let us know what you find out.

    Good Luck,
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