Need prop recommendation for 20 CC Seacraft

Discussion in 'Props' started by chipresistor122, Dec 2, 2020.

  1. chipresistor122
    Joined: Dec 2020
    Posts: 6
    Likes: 0, Points: 1
    Location: Stamford

    chipresistor122 Junior Member

    I have a 20ft seacraft - 1986.

    I restored the entire boat top to bottom . Currently I have a 175 hp Suzuki.

    Boat runs around 44 knots wide open . I am looking to increase top end speed.

    Right now I have a 17 pitch 3 blade, stainless .. any recommendations for replacement?

    Thanks so much
  2. bajansailor
    Joined: Oct 2007
    Posts: 3,099
    Likes: 1,207, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    Are you able to achieve the maximum operating revs that the engine was designed for?
    If you are exceeding the maximum, then that is not good, and you need a propeller with more pitch.
    If you are not able to reach the maximum speed, then you need to come down in pitch a bit.

    I have to ask though - if you can do 44 knots 'as is', surely this is plenty fast enough for most purposes?
    missinginaction likes this.
  3. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,036, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    I would seriously doubt you are getting 44 knots, or near it, with your present set-up, but regardless of that, what bajansailor said is apposite, it gets down to what your engine will rev out to at maximum throttle, and it needs to be well into the range specified by the manufacturer. This appears to be a moderate vee boat that would need a flat calm to be going that fast, without "killing" those on board. If as I see specified, the gear ratio is 2.5:1, then with a 17 inch prop, to reach 44 knots you would be way over red-line, and I surmise the engine has a rev limiter.
  4. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,036, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    As a rule of thumb, your outboard will drop 200-300 rpm in full throttle revs, with each increasing pitch size. If you are getting to mid-range in the specified range for that engine, going higher in pitch is not indicated, if your engine is going higher than the top figure, then a step up in pitch is advisable, as the engine is running in too low a "gear" for best results.
    bajansailor likes this.

  5. missinginaction
    Joined: Aug 2007
    Posts: 1,097
    Likes: 248, Points: 63, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    The first question you need to answer is this: What is the maximum wide open throttle RPM for my engine? You'll need to look to your owners manual or the people at Suzuki for that answer. Once you have the maximum RPM spec for your engine you can move to the next question. What RPM is my engine attaining at wide open throttle out on the water? Look at bajansailors post #2 above as to what to do depending on the answers to the two questions.

    I'm not familiar with your boat but would say that if you're getting a true 44 knots (50/51 MPH) on a 20 footer with a 175 hp outboard you're doing pretty good. Short of an engine swap I'm not sure that a prop change is going to help you much. So, in that respect I agree with bajansailor.
    bajansailor likes this.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.