Kencraft Challenger 215 prop recommendation.

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  • ken2
    replied
    So, if I understand this correctly, the prop diameter controls slip, which in a perfect world means that a 16” diameter prop in a 18 or 21” pitch is going to be 95-97% efficient? If the previous sentence is correct, then this begs the question. Why is it that prop diameter is not included in the various prop slip calculators?

    Prop slip has more to do with the specific prop your comparing.
    I could hand you two different brand/model propellers that are the same diameter and pitch on paper, but over 1000 RPM apart in how they run.
    But if you know what slip ratio to expect from the specific prop your talking about, then you can figure out what pitch you need to get the RPM right if you figure out what speed your expecting.

    The motor is a 2021, 200hp, inline 4.
    While more speed is always nice, I was hoping to find a compromise that would help in the 3500 to 4000 rpm range, without negatively affecting the current top end of 41 mph. As it is now I don’t plane until 38/3900 rpm, and I settle out around 4000 rpm at 24/25 mph. Because the boat is only 21’5” it would help to be able to be on plane at slightly lower speeds, if sea conditions require it. I haven’t used my trim tabs, other than minimally to level the boat from port to starboard, as I didn’t want it to affect the data I was collecting on engine performance. If I can get a decent weather window, I plan on using the tabs to see how well they will help with staying on plane at lower rpms.
    I know the 16x20” prop should help on the top end, but what will it do for me in the 3500 to 4000rpm range?
    Also, for whatever reason, my dealer thinks the 16” prop is going to lug the engine, if that turns out to be true, then would 16”x18.5” prop that Suzuki makes be a good alternative, or would it still have the same lugging issue because of the diameter?


    If you go up in pitch, to the 16 x 20, or the 15 x 21, either will run a little faster at any RPM. Your actual economy I doubt is going to change much, but it will probably get a little better.

    If I’m not mistaken, and I’m not sure how, or if it changes anything, the Suzuki prop in a 21 pitch is a 15” prop versus a 15-1/4” prop. Also, what effect would the change in pitch have on my ability to maintain the boat on plane at lower speeds?

    No, the diameter change is not going to effect things like you asking. There is not an add "X" or subtract "Y" to figure out what is going to happen.
    but if you compare a 21 vs a 19 pitch Suzuki propeller of them same model. (meaning your 15 x 21, vs the same model in 15 1/4 x 19, I'd expect the same slip ratio.

    Leave a comment:


  • Tegin1
    replied
    Sorry for not replying sooner.

    Originally posted by ken2 View Post
    the rules that say "X" change in diameter or pitch will do "Y" are wrong.
    Mainly because its almost impossible to compare diameter, because your comparing two different blade styles.
    (not many props where you could run the same pitch and model prop but have a different diameter to compare)
    and when your running different model/style props, everything goes out the window cause they are different. I could give you two props that are the same diaemter and pitch on paper but they could be 1000 RPM off of eachother in reality.

    as for your RPM, your at 6200, but your stopped by a rev limiter... so what would you be able to turn if it wasn't limiting you.

    I am hitting 6250rpm, but I haven’t tried to hold it long, or trim the motor to see if I can get more out of it, so I’m not sure if I am hitting the rev limiter.

    Basically, I expect about 3-5% slip with the 16 inch diameter Suzuki propeller.
    so expected speed is what really picks the pitch.

    So, if I understand this correctly, the prop diameter controls slip, which in a perfect world means that a 16” diameter prop in a 18 or 21” pitch is going to be 95-97% efficient? If the previous sentence is correct, then this begs the question. Why is it that prop diameter is not included in the various prop slip calculators?


    I am assuming you have the 4 cylinder 200 suzuki, (they did make a 6 cylinder a few years back)
    with a 2.5 gear ratio, turning 6000 RPM, a 20 pitch would be capable of about 43-44 MPH "if you can turn 6000.


    The motor is a 2021, 200hp, inline 4.
    While more speed is always nice, I was hoping to find a compromise that would help in the 3500 to 4000 rpm range, without negatively affecting the current top end of 41 mph. As it is now I don’t plane until 38/3900 rpm, and I settle out around 4000 rpm at 24/25 mph. Because the boat is only 21’5” it would help to be able to be on plane at slightly lower speeds, if sea conditions require it. I haven’t used my trim tabs, other than minimally to level the boat from port to starboard, as I didn’t want it to affect the data I was collecting on engine performance. If I can get a decent weather window, I plan on using the tabs to see how well they will help with staying on plane at lower rpms.
    I know the 16x20” prop should help on the top end, but what will it do for me in the 3500 to 4000rpm range?
    Also, for whatever reason, my dealer thinks the 16” prop is going to lug the engine, if that turns out to be true, then would 16”x18.5” prop that Suzuki makes be a good alternative, or would it still have the same lugging issue because of the diameter?


    If you wanted to stay with the small diameter prop (like you have) go 21. that is the next size up.
    You'll still get your same 8% slip (its the same brand and style prop)
    At 42 MPH, you'd be turning 5750.
    if you have the power to get to 44 MPH, you'd be turning 6000.

    If I’m not mistaken, and I’m not sure how, or if it changes anything, the Suzuki prop in a 21 pitch is a 15” prop versus a 15-1/4” prop. Also, what effect would the change in pitch have on my ability to maintain the boat on plane at lower speeds?

    Either way is fine, your numbers are not bad with your current prop, and if you often load heavy, your really fine with the prop you have.
    But if you want to get a little more speed, you'll have to go up in pitch regardless of which style of prop you choose.
    Thanks for taking the time to help me work through this, I really do appreciate your efforts in helping me understand this better.

    Leave a comment:


  • ken2
    replied
    the rules that say "X" change in diameter or pitch will do "Y" are wrong.
    Mainly because its almost impossible to compare diameter, because your comparing two different blade styles.
    (not many props where you could run the same pitch and model prop but have a different diameter to compare)
    and when your running different model/style props, everything goes out the window cause they are different. I could give you two props that are the same diaemter and pitch on paper but they could be 1000 RPM off of eachother in reality.

    as for your RPM, your at 6200, but your stopped by a rev limiter... so what would you be able to turn if it wasn't limiting you..

    Basically, I expect about 3-5% slip with the 16 inch diameter Suzuki propeller.
    so expected speed is what really picks the pitch.
    I am assuming you have the 4 cylinder 200 suzuki, (they did make a 6 cylinder a few years back)
    with a 2.5 gear ratio, turning 6000 RPM, a 20 pitch would be capable of about 43-44 MPH "if you can turn 6000"

    If you wanted to stay with the small diameter prop (like you have) go 21. that is the next size up.
    You'll still get your same 8% slip (its the same brand and style prop)
    At 42 MPH, you'd be turning 5750.
    if you have the power to get to 44 MPH, you'd be turning 6000.

    Either way is fine, your numbers are not bad with your current prop, and if you often load heavy, your really fine with the prop you have.
    But if you want to get a little more speed, you'll have to go up in pitch regardless of which style of prop you choose.

    Leave a comment:


  • Tegin1
    replied
    Thanks!
    First, I want to apologize for asking these questions, and I really don’t want you to think I’m questioning your expertise, because I know your the go to guy for this stuff. I see so much conflicting information when researching this issue that confuses me and I was hoping you could help me understand.

    When I look at the prop chart from Suzuki, it appears that all the recommended 3 blade props for the 200 hp engine are 16” diameter, which is also your recommendation. Then when I research the effects of prop diameter and pitch I read that a 1” change in diameter is equivalent to 500 rpm and a 1” change in pitch is supposed to be equivalent to 200 rpm. If I go from a 15-1/4x19” prop to a 16x20 which would be a 3/4” change in diameter along with a 1” change in pitch, wouldn’t that drop me ~ 600 rpm, and lower my top end to ~ 5650 rpm?

    Leave a comment:


  • ken2
    replied
    I'd still suggest the 16 x 20 Suzuki prop.
    Your on the limiter with that one. you could use a little more.
    or the 16 x 21 LFS3

    Leave a comment:


  • Tegin1
    replied
    Well, I finally got the motor installed, and the dealer put a 15-1/4 x19, 3 blade prop on, and these are are the results.

    Rpm 650 Speed 1.9 mph Slip 59%
    1000 4 mph 44%
    2000 6.6 mph. 54%
    3000 11 mph 49%
    4000 25 mph 13%
    5000 32.5 mph 10%
    5500 36.3 mph 8%
    6200 41 mph 8%

    These results are with the gas tank full (88 gal), and the baitwell empty (30gal), 1 person on board, and boat 90% loaded with gear. The boat comes on plane at about 3800 rpm, and if the baitwell is full the performance suffers. The max rated RPM for this motor is 6100, and I easily hit 6250rpm as currently propped. The dealer has the same prop in a 20 pitch for me to try, and he is of the opinion that a 16” prop will lug the engine.
    It seems to me that the performance under 3800 rpm is weak and that once I am on plane the results appear to be reasonable. would appreciate any advice you can give me that would help me get better results.

    Thanks,
    Terry

    Leave a comment:


  • Tegin1
    replied
    Thanks for the advice, I need to talk with my dealer on the details and if it doesn’t work out, I will get back with you on the Powertech.

    Leave a comment:


  • ken2
    replied
    If you get a good deal on a Suzuki prop, I'd go with the 16 X 20, and expect about the same speed at around 6000 RPM.
    If you can't get a good deal on a Suzuki prop, you can get a Powertech LFS3 21 pitch from me.
    They are about 4 weeks out on shipping, but they get them out eventually.

    Leave a comment:


  • Tegin1
    started a topic Kencraft Challenger 215 prop recommendation.

    Kencraft Challenger 215 prop recommendation.

    Kencraft Challenger 215, 8’ beam, weight around 3200 loaded, 24 degree deadrise. Previous power was 1993 Johnson 200hp Oceanrunner with 14.5x19 prop, topped out around 5300 rpm at 42-43 mph. Repowering with 200hp Suzuki and was hoping you could give me a recommendation on what prop to go with for new motor.
    Thanks
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