Thread: Prop Theory

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  1. #1
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    Prop Theory

    So... on my Champion 184 with a 2.5 Mercury I've ran a BUNCH of props. I'm drawing a blank on all of them at the moment to give exact numbers. This is more of a theory question than a set-up question. I spent ALL last summer dialing in the set-up. It's as good as it can get before I put the boat... and myself on a diet! LOL Although load doesn't seem to change things other than holeshot. Top end RPM and slip don't change.

    So, out of all of the props I've ran, they were the 13.75" size, regardless of pitch (23, 24, 25, 3 or 4 blade even a 27 Tempest once) I have high slip, into the 25+% range. Example 25 Trophy (4 blade), 1.87 Ratio, 6000rpm at 54mph is 27% Slip.

    I found on a 25M (3 blade) that is the 14.5" size I dropped into the 16% slip range, but also dropped my RPM 500RPM. So I just touch 58mph which is 16% slip.


    My question is this. Obviously the diameter of the prop has an effect.
    I was looking at the Scorpion props. They are bigger yet at 15 1/8"
    By the numbers leaving everything as it is... IN THEORY... this should bring my slip down another 10% which would be my goal. BUT... if that's any indication, I also need to drop a couple pitches because I'll drop another 500rpm or so.
    The reason I built this motor and put it on, I have seen HERE that guys with this boat and motor would hit mid to high 60's. And if I can get my slip down to 10% with a 25p Prop at 6000rpm that is possible.

    The boat is going on a diet this winter, as I'm running heavier than most I've learned.

    What do you guys think?

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    #2
    I get 1 - 2% higher slip when I calculate it, but like you said, either way it is much higher than it should be. More weight = more slip. If I add 2 passengers to my boat, my slip increases from about 9% - 11%. So you are on the right track with weight. More drag = more slip. If the hull bottom is dirty, that would be more slip. If the skeg, lower unit or prop has a lot of nicks in it, that would be more drag. If the engine height is too low that is more drag from the lower unit. Normally a Trophy prop can not be run as high due to the smaller diameter and design. If you raise it too much the slip will increase. I used to have a 24P Scorpion prop and they turned very stiff, the rpm was more like a 26 Tempest. However, I could raise my hydraulic jack plate higher than even my Bravo FS and the drag was still below 10%. If you could turn a 23P Scorpion at 6000 rpm with 10% slip you would be doing 63 mph. You should be able to raise the engine higher than the Trophy to help get the rpm up. Not sure if your combo would need a 22P or 23P Scorpion. How far is the prop shaft below the bottom of the hull with the Trophy prop?

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    #3
    Quote Originally Posted by Pulsare2020 View Post
    I get 1 - 2% higher slip when I calculate it, but like you said, either way it is much higher than it should be. More weight = more slip. If I add 2 passengers to my boat, my slip increases from about 9% - 11%. So you are on the right track with weight. More drag = more slip. If the hull bottom is dirty, that would be more slip. If the skeg, lower unit or prop has a lot of nicks in it, that would be more drag. If the engine height is too low that is more drag from the lower unit. Normally a Trophy prop can not be run as high due to the smaller diameter and design. If you raise it too much the slip will increase. I used to have a 24P Scorpion prop and they turned very stiff, the rpm was more like a 26 Tempest. However, I could raise my hydraulic jack plate higher than even my Bravo FS and the drag was still below 10%. If you could turn a 23P Scorpion at 6000 rpm with 10% slip you would be doing 63 mph. You should be able to raise the engine higher than the Trophy to help get the rpm up. Not sure if your combo would need a 22P or 23P Scorpion. How far is the prop shaft below the bottom of the hull with the Trophy prop?
    I'm 3.5 below pad now with the 25M. That's where it runs best

    The Trophy I ran deep (bottomed out the jack plate).. I ran shallow (lost water pressure at WOT)... it didn't matter.
    Last edited by DieselDood; 11-30-2021 at 02:51 PM.

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    #4
    Make sure the transducer or anything else bolted to the transom is as far from the prop as you can get it. I was having the same high slip issue no matter what I did. After two months of trial I was reading a post about transducer placement. I moved mine and went from 30% slip down to 12% and picked up just over 12mph top end.

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    #5
    I already checked, thought they were ok, but moved them anyway. Now they are WAY outside of the motor.

    It was an easy fix, so I had to try it!

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    #6
    I ran a champion 193 for 14 years. I found the tempest was always the best prop. I ran a 24p tempest with extra cup for best performance. It was a smooth handling prop. Based on your results with 25M I think you have too much prop. I would try a good 23p tempest and start at 3.5” PTP. I also found it was important to keep the front of the champions light. Remove any extra weight you have in the front of the boat and I bet you will see a big difference. The champions need to lift the bow to get best speed. Moving weight toward the back can help, but it will probably hurt the holeshot. It’s best to get it out of the boat if you can.
    Dave
    2021 Triton 18TRX, Mercury 200 ProXS-4s
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    #7
    Quote Originally Posted by got2fish View Post
    I ran a champion 193 for 14 years. I found the tempest was always the best prop. I ran a 24p tempest with extra cup for best performance. It was a smooth handling prop. Based on your results with 25M I think you have too much prop. I would try a good 23p tempest and start at 3.5” PTP. I also found it was important to keep the front of the champions light. Remove any extra weight you have in the front of the boat and I bet you will see a big difference. The champions need to lift the bow to get best speed. Moving weight toward the back can help, but it will probably hurt the holeshot. It’s best to get it out of the boat if you can.
    With any of the small diameter props I spin excessive RPM with high slip. If I drop to a 24 Raker I spin 6500rpm.

    Thats why I named this prop theory though. So far anyone I ask about the diameter, tries to point me back to the smaller diameter props with high slip. No one can explain why the 25M dropped my slip by 10% over a 25 Trophy.

    I spin that 25 Trophy 6000rpm at 54mph.

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    #8
    I’m not a fan of the trophy props on the champions. I tried a few and they always had to be run very deep and always killed the speed for me. I’m thinking you are heavy in the front and that is hurting your speed. Just a guess based on your results.
    Dave
    2021 Triton 18TRX, Mercury 200 ProXS-4s
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    #9
    The link below has some good info for you.

    http://www.bassboatcentral.com/championsetup.htm
    Dave
    2021 Triton 18TRX, Mercury 200 ProXS-4s
    Lowrance Elite FS, w/ Active Target
    Humminbird Helix 9 MEGA SI G4N
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    #10
    Quote Originally Posted by got2fish View Post
    I’m not a fan of the trophy props on the champions. I tried a few and they always had to be run very deep and always killed the speed for me. I’m thinking you are heavy in the front and that is hurting your speed. Just a guess based on your results.

    I've got a LONG thread over on the Champ side about the set-up on this boat.

    My questions are more on the theory of why does the bigger wheel bring the slip down, and at what point will diameter no longer become a factor?

    In theory a 25P prop should be 25 no matter what. At least that's the way guys make it sound.

    Guys running MUCH heavier hulls, lets say a 210 Champion with a 25P Trophy, or Tempest... a 13.75 prop don't have the issues I'm having. Eliminating slip seems to be a black magic that is usually solved with motor height. Motor height in my case has helped VERY little. And the weight I have easy control over makes very little difference as well. (I can drop, Co-angler, no weight in the livewell, and only 10gallons of fuel and only gain 1mph)
    But when I stepped up to that larger wheel, I feel like it's similar to putting a wider tire on the back of a drag car. BUT at some point the width/Diameter won't matter. If I go to that Scorpion prop, which is bigger than the 25M... will I get to the slip level I'm looking for, at 10% or less? Will I drop another 500rpm? If I'm down to 5000rpm, then I should probably drop to at least a 23, but at that point, will I get back into the power band of the motor and be able to pull RPM again?

    When guys say that the Scorpion prop runs "stiff by 1.5-2 pitches" is that actually due to Diameter?

    Hopefully that all makes sense.
    I'm just trying to understand prop theory. Right now it seems like it's just a guessing game for everyone... try this... try that. I'm $3k into props right now and I'm tired of the guess and check. I want to understand.

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    #11
    Usually, it works like you mentioned. A larger diameter prop has more blade area and can run with less slip. Of course if it does not have the right amount of cup and rake it might not be able to lift your bow, and then the drag increases and it does not work as well. Like you said, at one point, if you keep increasing the diameter past what is required for low slip, it just becomes harder to turn on your hull and will slow it down, but might not slow down a heavier boat. Still, like you said, some black magic. There was a post from someone that had a 27P Scorpion modified for more top speed. He said the prop shop measured it as 29.5". They actually cut the diameter down some from 15.125" because his light boat did not require all that diameter and he was able to turn it at higher rpm and gain speed. So, if that is true for most pitches of Scorpion, they appear to have more than the advertised pitch. I am no prop expert, but many props have progressive pitch which changes from the bottom to the top of the blade and the often the advertised pitch is the average pitch, but on a mercury Bravo type of prop, the average pitch is usually 0.75" - 1" less than the advertised pitch. So perhaps Scorpion measures the pitch in a certain place?? They have huge amount of cup and rake, which really helps to lift the bow, but are made out of very soft stainless steel. The Scorpion had low slip on my boat, but so did the Bravo 1, the Bravo FS, and the ProMax. I also used 24P to do the slip calculation on the Scorpion and that of course made it look very good. I did not really know what pitch it actually was, as it was sold as a 24P. If you do the slip calculation using 26P, then the slip is not as good. I have spent too much money on props, just like you did, and certainly would not tell you that the Scorpion will cure your issue, and which pitch you should buy, after what you have been through. All I could say, is if you can find a used Scorpion (new ones cost almost half of many merc props) you might want to roll the dice again??? I ran a 14.5" diameter, 24P, 4 blade, BBlades Blaster and it had 30% slip and only ran 65 mph at 6500 rpm. I recently ran a 14.5" diameter, 24P 4 blade ProMax and it ran 76 mph at 6348 rpm with less than 10% slip. The 24P Scorpion I could only turn to 6000 rpm at 71 mph but it had less than 10% slip if you used 24" for the pitch calculation. So it had more diameter at 15.125" but it did have one less blade. The ProMax probably had similar blade area to the BBlaster but it did not have as much cup or rake. So I have had a bit of a crapshoot with props as well as you. The ProMax was the first prop I have tried under 15.125" that actually worked very well.

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    #12
    Quote Originally Posted by Pulsare2020 View Post
    Usually, it works like you mentioned. A larger diameter prop has more blade area and can run with less slip. Of course if it does not have the right amount of cup and rake it might not be able to lift your bow, and then the drag increases and it does not work as well. Like you said, at one point, if you keep increasing the diameter past what is required for low slip, it just becomes harder to turn on your hull and will slow it down, but might not slow down a heavier boat. Still, like you said, some black magic. There was a post from someone that had a 27P Scorpion modified for more top speed. He said the prop shop measured it as 29.5". They actually cut the diameter down some from 15.125" because his light boat did not require all that diameter and he was able to turn it at higher rpm and gain speed. So, if that is true for most pitches of Scorpion, they appear to have more than the advertised pitch. I am no prop expert, but many props have progressive pitch which changes from the bottom to the top of the blade and the often the advertised pitch is the average pitch, but on a mercury Bravo type of prop, the average pitch is usually 0.75" - 1" less than the advertised pitch. So perhaps Scorpion measures the pitch in a certain place?? They have huge amount of cup and rake, which really helps to lift the bow, but are made out of very soft stainless steel. The Scorpion had low slip on my boat, but so did the Bravo 1, the Bravo FS, and the ProMax. I also used 24P to do the slip calculation on the Scorpion and that of course made it look very good. I did not really know what pitch it actually was, as it was sold as a 24P. If you do the slip calculation using 26P, then the slip is not as good. I have spent too much money on props, just like you did, and certainly would not tell you that the Scorpion will cure your issue, and which pitch you should buy, after what you have been through. All I could say, is if you can find a used Scorpion (new ones cost almost half of many merc props) you might want to roll the dice again??? I ran a 14.5" diameter, 24P, 4 blade, BBlades Blaster and it had 30% slip and only ran 65 mph at 6500 rpm. I recently ran a 14.5" diameter, 24P 4 blade ProMax and it ran 76 mph at 6348 rpm with less than 10% slip. The 24P Scorpion I could only turn to 6000 rpm at 71 mph but it had less than 10% slip if you used 24" for the pitch calculation. So it had more diameter at 15.125" but it did have one less blade. The ProMax probably had similar blade area to the BBlaster but it did not have as much cup or rake. So I have had a bit of a crapshoot with props as well as you. The ProMax was the first prop I have tried under 15.125" that actually worked very well.

    EXACTLY!

    I know fluid dynamics is a difficult concept, but dang! LOL You'd think by 2021 we would have it pretty well figured out!

    I need to try and call Andy at Team Marine in Lacrosse again. He knows Champs and those Scorpions. Maybe he will have the answers for me. I left a message once in the middle of busy season, so I'll have to try again.

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    #13
    Sorry, I noticed a typo in my reply. The mercury ProMax had a lot more cup and rake than the BBlades Blaster did even though both were the same diameter. The BBlades Blaster just did not work on my 24 Checkmate Pulsare / 300R combination with a Torquemaster lower unit. To through another wrench into the equation, there was another Checkmate member that had the same boat I did (just a few years older). However, he had a 400R Mercury with a Sportsmaster and was running the prop near the pad due to the Sportsmaster. He ran 83 mph with a similar BBlades Blaster that slipped so bad on my hull / engine combo. The larger diameter 4 blade Bravo on his 400R / Sportsmaster had similar slip to what that same prop had on my hull??? I know that two identical props can easily turn 100 - 200 rpm different, but the Blaster was totally different on these two applications but the Bravo type was similar. The boats were very similar (his was the Bowrider version, mine the closed deck version but the actual bottom of the hull was supposedly identical).

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    #14
    My triton will run 66-67 at 5500 rpms with a 24p tempest. That’s around 6% slip. I think a lot of the performance depends on the hull design and where the prop is in the water trail behind the boat.
    Dave
    2021 Triton 18TRX, Mercury 200 ProXS-4s
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    #15
    The only prop that you have that is 13 3/4" in diameter would be a trophy. A tempest is 14.5" just like the 25 M and a 24 Raker unless some one has ground them down to fix them. Do you have a jack plate on your boat? You didn't say how high you running prop to pad but I'll bet your too high for that boat, anything over 3" is too high with most running 3.5" under
    Last edited by mdtritn21; 12-06-2021 at 09:45 PM.