3 and 4 blade help; Tohatsu 30 on LT25

Discussion in 'Prop Shop' started by Stonehenge, Mar 20, 2017.

  1. Stonehenge

    Stonehenge I Love microskiff.com!

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    I have a 30hp 2003 Tohatsu short shaft on a standard layout gheenoe LT25. Tohatsu states 5250 - 6250 rpms on the specification sticker.

    Currently I am running a 12 pitch Solas aluminum prop at 5700 rpm. I am considering trying a new prop.

    What pitch would be good in four blade stainless? Currently there is a power tech 4 blade stainless 11pitch asking price of $200 I can pick up used and am wondering if it would be a good choice.

    Also, if I went stainless three blade what recommendations would you all have?

    Thanks in advance!
     
  2. devrep

    devrep Well-Known Member

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    too many unknowns.
    is your solas 3 or 4 blade?
    what model PT are you looking at? Some are double and triple cupped, others have little cup at all.
     

  3. Stonehenge

    Stonehenge I Love microskiff.com!

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    Currently running Solas aluminum 3 blade.

    I was considering the 11 pitch 4 blade power tech SRA4P11PTN30.

    I also just found a used three blade stainless powertech 12 pitch sra3r12ptn30 at a better price.

    Looking to reduce my trial and error costs as much as possible so any input is greatly appreciated.
     
    Last edited: Mar 22, 2017
  4. Stonehenge

    Stonehenge I Love microskiff.com!

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    Ok, so I got a powertech SRA 12 pitch 3 blade stainless. Running 5,500 rpm. I really like the performance over the aluminum prop but want to get to 6,000 rpm. Should I look for a 10pitch?

    If I do drop pitch it will increase rpms. Considering that, would I expect to see increase,decrease, or the same speed if I go with same prop lower pitch at a higher rpm?


    There is an 11pitch 4blade SRA but 4 blade right now available used. Waste of money trying it out??
     
    Last edited: Apr 10, 2017
  5. Shane Sloane

    Shane Sloane Active Member

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    If you drop the pitch to get higher RPMs then you should get a little better top speed, but will be slower out of the hole
     
  6. That's not always true. You need to understand the Slip and how it can effect the speed variables. Also, these smaller low pitch props vary in performance between props of the same model due to manufacturing differences. It just requires tinkering.

    It takes Pitch, Prop RPM and Slip to estimate speed.
    12P @5500 rpm w/10% Slip = 26-27mph
    11P @6000 rpm w/10% Slip = 26 mph
    10P @6000 rpm w/10% Slip = 24 mph

    Now if you can decrease the Slip then the speed goes up. That's the holy grail in prop design is to get as close to zero Slip as possible. I've seen props on my skiffs calc to 4-5% before. There's lots of variables involved.
     
    Last edited: Apr 12, 2017
  7. What kind of speed are you getting with the SRA3 12P?
    How high is the motor mounted on the transom? Raising it will increase rpm too. And that would actually increase the speed with the same prop, but by the numbers hitting 30mph would be the top end and that's at 6000rpm.

    If you have 10% Slip then you should be hitting 26mph wot @5500RPM

    And yes I'd suggest trying the sra4 in 11p; it was mentioned for that motor on the CG forum when I looked awhile ago. Yes it's my used prop, but CG guys mentioned it specifically over there and that's why I bought it originally. I have a 15 Mitzi and it was just too much prop for my boat which is heavier and much wider than a LT.
     
    Last edited: Apr 12, 2017
  8. jfboothe

    jfboothe I Love microskiff.com!

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    Maybe I am not thinking of this correctly at all but here goes… Let’s say I have an outboard that creates max HP and Torque at around 5750 rpm. I have a 13” pitch prop on there and I am getting 5450 rpm and 26mph. It would seem that if you put a 12” or even a 11” pitch that you will obviously increase the RPM but you would increase speed since you are putting the RPMs much more in to motors optimum powerband. I would think that when you get the RPMs into the sweet spot of the motor that a 1” decrease in pitch will increase the RPMs more significantly (with a resulting increase in speed) than if you decrease the pitch when the motor is already topped out or is struggling to drive the prop since the engine has the HP and TQ to actually drive the prop efficiently.
     
  9. That's not exactly the way it works. Think of a prop as a screw with a specific pitch.

    Decreasing pitch decreases the distant the screw/prop will travel axially. Prop pitch actually is a measurement of the distance the prop will travel in 1 rotation with zero slippage. A 12 pitch will travel 12" forward; a 13" pitch will travel 13" forward.

    Rpm is the # of rotations the prop makes, multiply by pitch and gear ratio to get distance/minute. That can be converted to mph.

    Then slip is the difference between actual speed and the theoretical pitch*RPM calc.

    If you look a couple post back and you can see where I posted numbers showing example calcs.
     
  10. jfboothe

    jfboothe I Love microskiff.com!

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    Ok. I understand that. Just in my mind it takes XX amount of horsepower to push the boat thru the water at a certain prop pitch. The closer you match the pitch of the prop (RPM) to highest power produced by the motor the more efficient the prop will be. (less slip) I could put a 14" pitch prop on my 25hp outboard but it doesn't have the balls to push the boat thru the water 14" for every revolution. It will be bogged down to a low RPM where the motor doesn't make anywhere near it's capable hp.
     
  11. Well you could push a 14" pitch at the correct rpm if the prop was made for it..,maybe a smaller diameter or different blade design/profile. Even the Rake of the prop is highly important in the outcome. Low rake vs high rake can change things a lot. It just gets even more complicated the more you learn about it. But I've usually found the Pitch*Ratio*RPM calc to be useful and predict a good starting point and provides some realistic expectations for what's really achievable.

    Now if you go with a 4 blade; I usually calc it as if it was 1" of pitch higher to get the speed since you've increased the blade area by 1/3 more than a 3 blade. But that's just based on my experiences and real personal data I've gathered over the years.

    The smaller and lower pitch props are much harder to calculate accurately. Like I mentioned before you have to tinker to get there. Usually I find the prop that is closest to meeting my goals and then have it worked by a good prop shop to fine tune it.
     
  12. jfboothe

    jfboothe I Love microskiff.com!

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    Well I guess I should have just put a 5hp motor with a custom 14" pitch prop on my boat and I could be doing 30 mph...o_O
     
  13. Sublime

    Sublime Well-Known Member

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    30 etec here. I have a HEAVILY cupped PT SRA3 in 11 pitch and a stock SRA4 in 13 pitch. Those two props would run within 1 mph of each other.

    I just got the 13 pitch back from having it cupped by Crossroads, but I haven't run it yet.

    The calculators are great but there are factors they don't take into consideration? Like the three blade even though it is an 11 pitch runs just about as fast as the 13 pitch. Why I think is because I can run it higher on the jack plate without slipping thus reducing drag etc.
     
  14. jfboothe

    jfboothe I Love microskiff.com!

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    Hey Sublime, what RPMs are you getting at WOT on your 30HP ETEC with an 11 pitch? And is there much difference in diameter on the 3 blade 11 and the 4 blade 13? Most of the 4 blades I looked at seemed to be slight smaller diameter (maybe 1/2 inch) which I would think help them spin a littler faster all things being equal.

    Disregard the diameter question. Just looked it up and both are 10"
     
  15. Sublime

    Sublime Well-Known Member

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    I can get 6,000 out of both. Sounds crazy I know but we're talking the 3 blade has mucho cupping. I'll let you know what the 4 blade does when I get a chance to run it.
     
  16. Realize that the calc is based of the real test run numbers for a certain prop. You can take the wisdom and experience I'm sharing or leave it. But I do actually have a little professional experience with the fluid dynamics and engineering stuff too...;)
     
  17. Your dead on here, there are many different variables. I've run the exact same brand, model, and pitch w/o any alterations and got different performance numbers before. The smaller low pitch props have more Manufacturing variations/differences.

    One thing that changes when you get into extra cupping is that you are actually changing the diameter, pitch, and/or rake in the process too The effect depends on where it's cupped . Mercury has a pretty good write up on cupping....
    http://www.mercuryracing.com/prop-school-part-4-blade-cup/
     
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