Lighter boat, prop change?

Discussion in 'Props' started by valvebounce, Jun 20, 2018.

  1. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    I have purchased a 1999 Johnson 25hp,2 cylinder 2 stroke outboard,long shaft.
    The boat it was on was an 18ft heavier boat than my boat.The prop is a 10''X 13'' 3 blade.
    My boat is a f/glass 14ft planing hull type,with steering/controls and a dashboard and two seats.
    Apart from the bow section in front of the dashboard,and the splashwell,it is an open boat.
    I estimate the weight to be about 700-800lbs,unladen.
    The usual load will be approx. 2 guys(350lbs)10 gallons of fuel,a 12 volt battery,fishing gear and bait.
    The boat displaces about a foot of water,has a pointed bow,and flattens out towards the stern,with a lateral chine at deck level.
    I am contemplating fitting a 10''X 15'' 4 blade prop,mainly for economy,and to ride at an economical planing speed from A to B.
    The boat will be used for sea fishing(inshore)
    Any opinions on my idea's will be more than welcome.
    "V''
     
  2. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    What you need to do is first get a tachometer. A speedometer is also a good thing but not totally necessary. A tachwill tell you how fast the engine is turning and make sure that you don't over rev it. You want to get the engine to run in the maximum speed range that it is designed for, which is typically in the 5500 to 6000 rpm. The objective is to get the engine into that range so that it isn't overload and bogging down and isn't under propped and overspeeding, which can result in things like burned bearings on the crank. Then you can load the boat lightly and get the engine to run close to redline, and then run it with a higher load and make sure it will get to 5500 rpm. Without knowing what kind of hull each boat is (was) you really can't make a comparison as to what the prop pitch will need to be. If you can find someone with a similar boat and motor you can find out what pitch they are running. Note that different motors often have different gear ratios so you will have to calculate your prop pitch taking the gear ratio into consideration. There are some on line calculators that can be used to calculate what prop pitch you need, but you need to know how fast you expect it to go in the first place. While you can expect to need to increase pitch if you have a lighter, smaller boat, how much is going to be dependent on more than just the weight of the boat, it's dependent on the design of the hull, the weight distribution and other factors, so to do it right you can't really just jump it up 2 inches and say done with it.
     
  3. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    Thanks for your reply YJ,there's some valuable info there.
    I have a remote Tacho to check the WOT.
    With the prementioned loading,the boat lies level when no power is applied.
    At the moment I have 1963 18hp Evinrude fastwin on it,(refurbed and runs sweet)I am getting approx.20-23mph out of it at 5500rpm.(max dia and pitched prop for the 18hp)
    I raised the transom 3'' so the cavitation plate is exactly inline with the bottom of the hull at the stern.
    I bought the 25hp mainly because it's a more modern motor,and the extra power would help to get from A-B.
    The boat cuts thro' the water quite well,and when at anchor is pretty stable.It has a decent freeboard and safety rails.
    Thanks for your advice,it's much appreciated.
    "V"
     
  4. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    G'day V, certainly a tacho is a big help, but how slow the boat is to plane, would be a strong clue, if tardy, you don't want to increase pitch. I'm pretty sure 13" pitch was the standard fitting for that engine, I doubt there is any scope to go up in pitch.
     
  5. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    G'day Mr E,
    You are correct with the standard prop size.I checked out other available prop sizes,they do 10 X 13,10x 14 and 10 by 15.
    The prop shop I use is in London UK,they only stock a 4 blade prop in 10x15.
    My boat was finding it difficult to plane with the 18hp with the 9.25"x11" prop,(which surprised me)I bought a 9x9 and it jumped on the plane instantly.
    I then realized that when I fitted new coils,condensors and points,I had run the new HT leads incorrectly,they were restricting the brass accelerator plate under the flywheel that controls the accelerator/carby.I wasn't reaching anywhere near max revs.
    I changed back to the 9.25x11" prop,ran the HT leads correctly,and the boat got on the plane quite easily.(learning curve,Haha)
    My idea with the 25hp is to cruise on the plane at a lower speed and use less fuel,which will increase the distance I can go.
    I doubt it would get 12000 mls to OZ on two full tanks,although I would love to fish Port Phillip bay in Victoria again.(it's been 54 yrs)
    Have a nice day
    "V"
     
  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    That is a pretty hefty boat, I recall running a 14 ft tin boat with a 13" prop with a 30 or 35 hp Johnrude, and that was the right pitch for the job. You might save a cup-full of fuel per trip, tinkering with props, but another consideration is when you have to back off a bit in choppy water, the lesser pitch is preferred.
     
  7. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    I always ease off the throttle slowly for that reason,don't rocka da boat.Haha.
     
  8. sandhammaren05
    Joined: May 2009
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    sandhammaren05 Senior Member

    Even with the lower geared 1984- OMC 25 you will never need 15" pitch. 13" was too much pitch for the heavier boat
    and it is too much pitch for the load and weight of the smaller boat. The motor needs to turn 5500 RPM to develop it's hp, get an 11" pitch prop. Turning that motor over 6000 RPM is no problem if you use Evinrude or Mercury oil at 25:1. That same powerhead is raced at 7000 RPM and holds up just fine. At 6000 RPM and below 50:1 Evinrude or Mercury oil is fine.
     

  9. valvebounce
    Joined: Dec 2010
    Posts: 447
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    Location: manchester uk

    valvebounce Senior Member

    Thanks for the interest SH,Sounds like a Tacho reading will put me in the picture.
    Just to clarify,the 25hp Johnson is a 1999 model.
    I run the motor at 50 to 1.
     
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