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  1. #1
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    Default Top End Speed Question.. Bigger Prop Needed?

    I have a 19ft Bow Rider, 1985, and the boat weight is 2100 to 2200 dry with a stock 305 / 230hp engine.

    My goal is to have a top-end speed in the 50 to 60 range. I was told that I can get that range with a bigger prop? y/n

    My first thought was to upgrade to a 350 // 260hp motor or bigger hp up to 300hp.

    Just need some clarification here.

    Thanks!

  2. #2
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    Default Re: Top End Speed Question.. Bigger Prop Needed?

    Bigger prop blades on same motor will only give you more weight push or hole shot. Top end speed comes down to rpms, pitch and boat weight/shape. If you go to google and type in "prop slip calculator" you can find a couple of tools that you can use to find your theoretical speed. You can then factor in "average prop slip to get in the ballpark of what you can expect. Increasing power on the same boat would allow you to turn a bigger prop with more pitch would equate to more top speed.

  3. #3
    Captain Philster's Avatar
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    Default Re: Top End Speed Question.. Bigger Prop Needed?

    If you just toss a bigger prop than the engine can turn, the boat goes slower.

    To turn a bigger prop, you need more horsepower.

    You might be able to turn a slightly bigger prop right now if you can exceed the RPM limit for your given engine with you current prop. Since we can't read minds....


    ESSENTIAL DATA REQUIRED
    When posting a question about propellers, you cannot get a reasonable answer unless you provide the following information in your post: Items 1-4 are essential for meaningful response to most questions.
    1) Performance issue you are trying to correct.
    2) Current prop manufacturer, model, aluminum or stainless as a minimum.
    3) Current prop diameter and pitch (required).
    4) Wide open throttle RPM and speed with an average load (very helpful)
    5) Engine/drive make, model, year, and HP
    6) Boat make model, year, length and weight
    Checkmate 2400BRX/Optimax 300xs
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    Alumacraft 17 w/ Merc 40 4S

    Date an I/O, but marry an outboard!


  4. #4
    Moderator Bondo's Avatar
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    Default Re: Top End Speed Question.. Bigger Prop Needed?

    Quote Originally Posted by jmw129 View Post
    I have a 19ft Bow Rider, 1985, and the boat weight is 2100 to 2200 dry with a stock 305 / 230hp engine.

    My goal is to have a top-end speed in the 50 to 60 range. I was told that I can get that range with a bigger prop? y/n

    My first thought was to upgrade to a 350 // 260hp motor or bigger hp up to 300hp.

    Just need some clarification here.

    Thanks!
    Ayuh,.... What's the baseline yer Startin' with,..??
    Any Grease is Better,..... Than No Grease at All.......

  5. #5
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    Default Re: Top End Speed Question.. Bigger Prop Needed?

    Quote Originally Posted by Philster View Post
    If you just toss a bigger prop than the engine can turn, the boat goes slower.

    To turn a bigger prop, you need more horsepower.

    You might be able to turn a slightly bigger prop right now if you can exceed the RPM limit for your given engine with you current prop. Since we can't read minds....


    ESSENTIAL DATA REQUIRED
    When posting a question about propellers, you cannot get a reasonable answer unless you provide the following information in your post: Items 1-4 are essential for meaningful response to most questions.
    1) Performance issue you are trying to correct.
    2) Current prop manufacturer, model, aluminum or stainless as a minimum.
    3) Current prop diameter and pitch (required).
    4) Wide open throttle RPM and speed with an average load (very helpful)
    5) Engine/drive make, model, year, and HP
    6) Boat make model, year, length and weight

    1. Well I have to replace my cracked 305. So I'm trying to decide if I should stick with the 305 and get a bigger prop or get the 350 which is a 30hp difference and pay a little extra. I have been told that the 30hp won't make much of a difference?

    2. Alum, Solas Rubbex 19p

    3. 14.5 x 19 alum Solas Rubbex 19p

    4. WOT is 4200 to 4600

    5. 1985 Merc 305 / 230hp

    6. 1985 Century 19ft BR3000 / 8ft beam, 2100 to 2200lb

    I have attached a pic of what the boat looks like. This isn't my boat but same model & size.

    Attachment 149254

  6. #6
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    Default Re: Top End Speed Question.. Bigger Prop Needed?

    Quote Originally Posted by Bond-o View Post
    Ayuh,.... What's the baseline yer Startin' with,..??
    1. Well I have to replace my cracked 305. So I'm trying to decide if I should stick with the 305 and get a bigger prop or get the 350 which is a 30hp difference and pay a little extra. I have been told that the 30hp won't make much of a difference?

    2. Alum, Solas Rubbex 19p

    3. 14.5 x 19 alum Solas Rubbex 19p

    4. WOT is 4200 to 4600

    5. 1985 Merc 305 / 230hp

    6. 1985 Century 19ft BR3000 / 8ft beam, 2100 to 2200lb

    I have attached a pic of what the boat looks like. This isn't my boat but same model & size.

    Attachment 149254

  7. #7
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    Default Re: Top End Speed Question.. Bigger Prop Needed?

    Based on the numbers you've given, even if your hitting 4600rpm, with a 19p prop your theoretical speed is just over 41mph, assuming your gear ratio is 2:1. Thie gear ratio does play a factor in top speed. Not sure what your gear ratio is but even .01 off could make a difference. What are your rpms running at wide open with the 19p prop you are using right now? If they are near max range right now than you may be able to increase pitch slightly which will give more top speed. But I'm willing to bet it won't be enought to reach atleast 50mph like you want with that current motor.

  8. #8
    Moderator Bondo's Avatar
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    Default Re: Top End Speed Question.. Bigger Prop Needed?

    Quote Originally Posted by jmw129 View Post
    Originally Posted by Bond-o
    Ayuh,.... What's the baseline yer Startin' with,..??
    1. Well I have to replace my cracked 305. So I'm trying to decide if I should stick with the 305 and get a bigger prop or get the 350 which is a 30hp difference and pay a little extra. I have been told that the 30hp won't make much of a difference?

    2. Alum, Solas Rubbex 19p

    3. 14.5 x 19 alum Solas Rubbex 19p

    4. WOT is 4200 to 4600

    5. 1985 Merc 305 / 230hp

    6. 1985 Century 19ft BR3000 / 8ft beam, 2100 to 2200lb

    I have attached a pic of what the boat looks like. This isn't my boat but same model & size.

    Attachment 149254

    Nope,.... That's the Specs, not a baseline...

    How fast does the boat actually Go, at what rpms is a Baseline...

    Btw,... Just puttin' a bigger prop on a boat, Don't make it go Faster,...

    30hp, will make it go faster, but I doubt 60 mph is gonna be seen...
    Any Grease is Better,..... Than No Grease at All.......

  9. #9
    Captain Philster's Avatar
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    Default Re: Top End Speed Question.. Bigger Prop Needed?

    30 HP is worth 3-5 MPH, and that's barely a step up in pitch (probably not a whole step). If you get a bigger engine, try spinning the same pitch prop, but something in stainless steel with more aggressive design (cupping, raking, etc).

    That's it: Same pitch, but stainless and a modern, aggressive design.
    Checkmate 2400BRX/Optimax 300xs
    .
    Alumacraft 17 w/ Merc 40 4S

    Date an I/O, but marry an outboard!


  10. #10
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    Default Re: Top End Speed Question.. Bigger Prop Needed?

    Quote Originally Posted by fishndirk View Post
    Based on the numbers you've given, even if your hitting 4600rpm, with a 19p prop your theoretical speed is just over 41mph, assuming your gear ratio is 2:1. Thie gear ratio does play a factor in top speed. Not sure what your gear ratio is but even .01 off could make a difference. What are your rpms running at wide open with the 19p prop you are using right now? If they are near max range right now than you may be able to increase pitch slightly which will give more top speed. But I'm willing to bet it won't be enought to reach atleast 50mph like you want with that current motor.

    Gear ratio on the out drive is 1.50

  11. #11
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    Default Re: Top End Speed Question.. Bigger Prop Needed?

    Quote Originally Posted by fishndirk View Post
    Bigger prop blades on same motor will only give you more weight push or hole shot. Top end speed comes down to rpms, pitch and boat weight/shape. If you go to google and type in "prop slip calculator" you can find a couple of tools that you can use to find your theoretical speed. You can then factor in "average prop slip to get in the ballpark of what you can expect. Increasing power on the same boat would allow you to turn a bigger prop with more pitch would equate to more top speed.
    I used the link you mentioned with a gear ratio of 1.50, 4600 rpm, 19p, slip of 15% and actual speed of 47

    I assume this is accurate.

  12. #12
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    Default Re: Top End Speed Question.. Bigger Prop Needed?

    I got a theo speed of 55mph, with an actual speed of 47. Did you subtract prop slip from theo speed value. But all this is irrelevent unless we know what your actual wot rpms are with your current setup on the water. After that we can know your prop slip % by comparing theoretical speed to actual speed if you have a way of checking it on gps

  13. #13
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    Default Re: Top End Speed Question.. Bigger Prop Needed?

    Quote Originally Posted by fishndirk View Post
    I got a theo speed of 55mph, with an actual speed of 47. Did you subtract prop slip from theo speed value. But all this is irrelevent unless we know what your actual wot rpms are with your current setup on the water. After that we can know your prop slip % by comparing theoretical speed to actual speed if you have a way of checking it on gps
    I don't own a GPS for boating but that would be cool if the GPS can give the theoretical to actual. I don't know what my slip % so I was guessing with 15%

    What is considered the normal slip % for a good quality prop such as Solas?

  14. #14
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    Default Re: Top End Speed Question.. Bigger Prop Needed?

    Quote Originally Posted by fishndirk View Post
    I got a theo speed of 55mph, with an actual speed of 47. Did you subtract prop slip from theo speed value. But all this is irrelevent unless we know what your actual wot rpms are with your current setup on the water. After that we can know your prop slip % by comparing theoretical speed to actual speed if you have a way of checking it on gps
    WOT range is 4200 to 4600 according to the valve cover sticker.

  15. #15
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    Default Re: Top End Speed Question.. Bigger Prop Needed?

    Still need to know what your ACTUAL rpms are with it on the water, not the range. There are too many other factors that make up prop slip besides your prop. All numbers based off guessing woul be "what ifs". Gotta know your actual running rpms before making any ballpark guesses. If you have a cell phone that is any newer than 4 years old there are plenty of free apps out there that give a gps speed. The one I use on my phone is VERY accurate and matches what all vehicles say and matches the numbers I factor in for my boat to a T.

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