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Thread: Help with prop

  1. #1
    Member c6 batmobile's Avatar
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    Default Help with prop

    I have a 30 horse honda. 9.9 X 11 prop (16 foot v bottom boat). I took it out the other day and the boat was a little heavier than I would like to see. The boat refused to get on step. The rpm sounded fine and the motor didnt seem to be "bogging" but it surely did not want to push the boat. Without the weight it seemed OK, it wasnt going to set any speed records but it did plane out.

    Im thinking I need to adjust the trim first of all (it is manual so I didnt mess with it out on the water). But I also believe I would benefit from a new prop.

    What would the advantage be of going from a 3 to a 4 blade in the same pitch? Which way should I consider going with the prop to get better performance? The lowest number available for my engine is 9.9 by 9 and the highest 9.9 by 13.

    Ive tried to do some research on my own but I dont really understand the dynamics very well especially when numbers are involved.

    Thanks in advance.
    Makin fur fins and feathers fly.

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    Member cormit's Avatar
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    Quote Originally Posted by c6 batmobile View Post
    I have a 30 horse honda. 9.9 X 11 prop (16 foot v bottom boat). I took it out the other day and the boat was a little heavier than I would like to see. The boat refused to get on step. The rpm sounded fine and the motor didnt seem to be "bogging" but it surely did not want to push the boat. Without the weight it seemed OK, it wasnt going to set any speed records but it did plane out.

    Im thinking I need to adjust the trim first of all (it is manual so I didnt mess with it out on the water). But I also believe I would benefit from a new prop.

    What would the advantage be of going from a 3 to a 4 blade in the same pitch? Which way should I consider going with the prop to get better performance? The lowest number available for my engine is 9.9 by 9 and the highest 9.9 by 13.

    Ive tried to do some research on my own but I dont really understand the dynamics very well especially when numbers are involved.

    Thanks in advance.
    First number is the diameter. Second number is the pitch. 9.9 x13 means the prop is 9.9 inches in diameter and one revolution of the prop would theoretically move forward 13 inches. Three blade or four blade ..... one revolution is still 13 inches. The 11 pitch prop you're using now is pretty flat ....... plus sounds like you're getting the rpm's ...... good chance the 13 would be what you need. If the 13 won't get you going and it ends up you have a "too much weight" problem ..... then the fourth blade (staying with the 11 pitch) will let you grab more water.

  3. #3
    Member c6 batmobile's Avatar
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    Quote Originally Posted by cormit View Post
    First number is the diameter. Second number is the pitch. 9.9 x13 means the prop is 9.9 inches in diameter and one revolution of the prop would theoretically move forward 13 inches. Three blade or four blade ..... one revolution is still 13 inches. The 11 pitch prop you're using now is pretty flat ....... plus sounds like you're getting the rpm's ...... good chance the 13 would be what you need. If the 13 won't get you going and it ends up you have a "too much weight" problem ..... then the fourth blade (staying with the 11 pitch) will let you grab more water.

    Ok Im kinda pickin up what your putting down. Now would going with a 13 pitch make me get out of the hole slower or quicker? Reduce top end or improve it? What if I went with a 4 blade prop with a 13 pitch?

    Props are a hundred bucks a pop so I wanna get it right.
    Makin fur fins and feathers fly.

  4. #4

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    The 13 would improve the hole-shot and reduce top speed by hitting max rpm sooner. With that, I ran a 16' and 40 4-stroke and had the same problem until I went with an adjustable pitch 4-blade prop. The Propulse prop made a huge difference and let me tune for load in about 3 minutes. As a matter of fact I run one on my RIB/115 combo and was able to get exactly the max rpm I wanted and the speed I wanted...usually you give up one or the other.

    http://propulse.se/english/


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    Member cormit's Avatar
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    Quote Originally Posted by c6 batmobile View Post
    Ok Im kinda pickin up what your putting down. Now would going with a 13 pitch make me get out of the hole slower or quicker? Reduce top end or improve it? What if I went with a 4 blade prop with a 13 pitch?

    Props are a hundred bucks a pop so I wanna get it right.
    It is not totally clear if you have plenty of power. You seem to think you do ....... and if you do ..... then yes, a 13 pitch will both get you out of the hole quicker and give you more top end. I'm guessing you don't have a tach on your 30 HP Honda ...... so, knowing what rpm's you're getting with you 11p prop is not known. The correct prop will put your rpm's where Honda wants them to be at full throttle (WOT). If your motor is rated at 30 hp at 5200 rpm's ........ and you knew you were only getting 4500 at wide open throttle with your 11p prop ...... then you would have to go down a pitch or two ..... maybe to a 9p prop. If you are actually getting 5800 to 6000 rpm's right now with your 11p prop ..... then you have the power needed to go up in pitch ...... to maybe a 13 pitch prop. I'm just using hypothetical rpm ranges ..... don't know what yours actually is.

    A four bladed prop is for moving big loads and towing ....... and there should be no reason for you to have to have one. The extra blade will actually rob you of top end because it has more drag. You didn't say if your boat was aluminum or fiberglass or wood but, 30 hp on a 16 foot boat (if not overloaded) should zip along just fine. Maybe you can see if someone on the forum has spare prop for you to try ...... just to get zeroed in on the correct size before you buy.

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    Quote Originally Posted by AK2AZ View Post
    The 13 would improve the hole-shot and reduce top speed by hitting max rpm sooner. With that, I ran a 16' and 40 4-stroke and had the same problem until I went with an adjustable pitch 4-blade prop. The Propulse prop made a huge difference and let me tune for load in about 3 minutes. As a matter of fact I run one on my RIB/115 combo and was able to get exactly the max rpm I wanted and the speed I wanted...usually you give up one or the other.

    http://propulse.se/english/
    I think you meant to say improve top end? 13p will have more top end than 11p.

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    Member c6 batmobile's Avatar
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    Boat is aluminum. I seems to move pretty decent with a lighter load and I had a 35 2 stroke on it before the repower and it moved it great. I know what your thinking.... unload the **** boat or go back to the 2 stroke. Unfortunately not always an option.

    Having someone with a prop I could try would be awesome but I personally havent seen a lot of hondas running around and I dont know of anyone with one. Especially someone that would let me borrow their prop.

    It sounds like a 13 pitch would do the trick from how you explain it. I know you say a 4 blade shouldn't be necessary but Im wondering how much top end would I really lose? Would it be to the point I regretted it?
    Makin fur fins and feathers fly.

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    Determining the right prop is not an exact science ...... hard to get it right with out trying them.

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    Member c6 batmobile's Avatar
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    Quote Originally Posted by cormit View Post
    Determining the right prop is not an exact science ...... hard to get it right with out trying them.
    Yeah I know. Im just trying to get as close as I can before I buy one. I appreciate your input. You've been a lot more helpful than me reading or trying to understand others.
    Makin fur fins and feathers fly.

  10. #10

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    You gotta know your rpm's, if no tach, Cabelas or Amazon sells Tiny Tachs. Simple to use & set up.
    then work from there with other props, and trim angle. When doing the same thing on a 16' jon I had with 25 4 stroke yamaha, I was surprised to find that I was over-revving the motor, wouldn't have known that otherwise, so adjusted trim angle and swapped props.
    The tiny tach was 10 bucks, nothing compared to the costly damage to the motor if always running it over-revved.

  11. #11
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    since you think your getting top end rpms might the prop hub be spun ??? call some of the prop shops and see if they have any rebuilt or used props, tell them what your trying to do and they should let you try a few before you buy one, just be up front with them and either pay for the prop or leave a good deposit. I know garret at Ak Frontier Fabricators does prop work so that might be a place to start, he should be able to repitch your prop also.

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    Quote Originally Posted by potbuilder View Post
    since you think your getting top end rpms might the prop hub be spun ??? call some of the prop shops and see if they have any rebuilt or used props, tell them what your trying to do and they should let you try a few before you buy one, just be up front with them and either pay for the prop or leave a good deposit. I know garret at Ak Frontier Fabricators does prop work so that might be a place to start, he should be able to repitch your prop also.
    Might work on this.

    I dont believe the hub is spun. Its a fairly new lower unit and seems pretty tight but I cant tell for sure.
    Makin fur fins and feathers fly.

  13. #13

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    C6, potbuilder is referring to the prop hub being "spun", doesn't have anything to do with lower unit itself. Basically the rubber/metal bushing in the hub slips.

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    Quote Originally Posted by Big Jim House View Post
    C6, potbuilder is referring to the prop hub being "spun", doesn't have anything to do with lower unit itself. Basically the rubber/metal bushing in the hub slips.
    Doesn't that attach inside the lower unit? How would you check this without a shop? Put it in gear and try to move it by hand?
    Makin fur fins and feathers fly.

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    Quote Originally Posted by c6 batmobile View Post
    Doesn't that attach inside the lower unit? How would you check this without a shop? Put it in gear and try to move it by hand?
    It is not inside the lower unit ....... it is inside the propellor hub. It is a rubber donut that is in between the splines and the outer hub. Sometimes a separation in the rubber can be seen ..... sometimes it can't. The motor can be turned over pretty easy by turning the prop by hand ..... so that might not be a good way to test it.

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    Quote Originally Posted by cormit View Post
    It is not inside the lower unit ....... it is inside the propellor hub. It is a rubber donut that is in between the splines and the outer hub. Sometimes a separation in the rubber can be seen ..... sometimes it can't. The motor can be turned over pretty easy by turning the prop by hand ..... so that might not be a good way to test it.
    Fan freaking tastic. Well I guess Ill take a peak at that and try to get it figured out. I would love to just dump tons of money into it taking it to the **** repair shop but thats just not practical for me. Hopefully I can get this crap figured out.

    I have a tach on order hopefully it works. I could only find one that said it worked for 3 cylinders. Maybe Ill just drive it into the bay and collect the little bit of insurance on it? This is what I get for working on the **** thing all winter.
    Makin fur fins and feathers fly.

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    You mentioned you took this boat out and it was heavier.....you did not say whether or not you ran this motor on this boat before......?, but you did say you repowered..........so assume you have not run this boat with this motor, hard to help with limited info and if you are not familiar with this setup....you need a tach......no one on here can tell you if you are achieving the proper RPM........! That is key to sizing a prop.........If you had run this motor for years and could tell by the sound/feel then you might have a chance........ you can either buy a tach or buy props and swap them until you maybe get it right....
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    I just purchased a tach today. I have a wire harness that goes with it but the instructions might as well be in greek. Im going to try to google my answer and hopefully find what I am looking for.
    Makin fur fins and feathers fly.

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    Does anyone know about wiring harnesses? Honda sold me a harness and it looked like a plug and play type deal but I dont have any spare connectors inside the cowl and I dont clearly see how to install this thing. I may take it back and just try to find a different tach and wiring harness if I cant figure it out tonight.
    Makin fur fins and feathers fly.

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    C6 are you close in Anchorage area?? I am pretty handy with OB's and mechanicals in general. Willing to give you a hand.

    Took me a year to convince the local Honda dealers that there really was another gearset for changing a prop OB over to a jet drive, even had part numbers. Once I had them in hand they said they would not fit. So it's possible that you either got the wrong harness or the connector you are looking for is tucked away under the left side(?) under the main harness.

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