Small Prop pitch question

Discussion in 'Bubba's Outboards' started by Old Bill, Mar 17, 2008.

  1. Old Bill

    Old Bill New Member

    Messages:
    448
    State:
    Oklahoma City
    Bubba & Crew, (Crew being all you other smart people!)

    The Nissan 4 HP four stroke motor comes with a 7.8 X 7 pitch prop. The Nissan 5 HP and the 6 HP motors come with a 7.8 X 8 pitch prop. See Prop Chart below

    I have a 4 HP Nissan motor that I have boosted to 6 HP by installing the Nissan 6 HP carburetor on my motor. Motor starts and runs so much better now!

    Do you think that it would be worthwhile to spend $70 to buy a 8 pitch prop? I'm hoping to increase my boat's speed a little bit.

    One of the local propeller repair shops said that they doubted if I would notice any increase in boat speed with a 1 degree increase in pitch.

    I'm a little worried that the larger carburetor will allow me to over rev the motor with the smaller 7 pitch prop.

    What do you think?
     
  2. Wabash River Bear

    Wabash River Bear New Member

    Messages:
    3,019
    State:
    Indiana
    I dunno Bill. Are the 4 and 6 hp motors the same other than carburation and tuning? If they are, I would say you could prop it to the 6 hp recommended operating rpm. If not, I would keep it propped to the 4 hp rpm. Bubba will be able to tell ya for sure, I'de wait for his expert advice. Good luck.
     

  3. Old Bill

    Old Bill New Member

    Messages:
    448
    State:
    Oklahoma City
    I have been told that the carburators are the only major differences in the motors. The 4 HP does have an internal gas tank. I know of several people who have made the change, but they all wanted more power, and were not concerned about more speed.

    I'm looking forward to Bubba's no nonsense advice!:big_smile:
     
  4. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    As for speed Bill I don't think that the prop will give you that much more but here is an idea you can toy with. Go into the library and get the idea that Bobpaul used to make him a jack plate to get that motor back and up into cleaner water. But what I would recommend doing before all this is to go on the net and get you what is called tiny Tach and it will have a wire that wraps around the spark plug to read the RPM's because there is always that chanch of running it out of the frame or way under the frame



    Tinytach.com
     
  5. Old Bill

    Old Bill New Member

    Messages:
    448
    State:
    Oklahoma City
    Thanks Willard! The Tiny-Tach looks like it would be a good thing to have. I have sent them an email asking what would be the right model for my motor.

    Having a tach will sure help in knowing what the motor is doing, and if I really need to do anything at all.

    I will look at the article about building a jack plate that BobPaul wrote. Of course, that might lead to me having more questions for you to answer!:wink:
     
  6. Bubbakat

    Bubbakat New Member

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    4,532
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    Ask away bill, Ill either know it of make something up that some one will bleive.:big_smile::big_smile:
     
  7. Old Bill

    Old Bill New Member

    Messages:
    448
    State:
    Oklahoma City
    Okay!:big_smile:


    Willard, I read the article about making a jack plate, and naturally I do need some help understanding it all!:eek:oooh:

    My Nissan owners manual states the the anti-Ventilation Plate should be 1.2" to 2.0" below the bottom of the boat's hull. I measured my motor and hull awhile ago, and the Plate was 2 1/4" below the bottom of the hull. (see Photo below) (The lower unit of the motor is tilted forward in an earlier attempt to get the bow of the boat down. I now have added a 60 lbs sandbag to the bow and moved the fuel tank forward to help level the boat out.)

    I can easily raise the motor by placing a shim on top of the transom if that alone would help. Do you believe making a jack plate to move the motor away from the transom would be worthwhile?

    If so, what distant would be best? The further the better??

    Just think Willard...if you were just receiving a dollar for each answer you gave!:wink:
     

    Attached Files:

  8. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    Bill I would start out by making a jack plate that would exted backwards for at least 5 to six inches. I would raise the motor up to where the plate is even with the bottom of the boat ans start testing there. I would raise it one notch at a tim until I had blowout and the lower it untill I acheived good water hook up. Those tines on the bottom of that boat has that prop in what I call dirty water now. My boat is made the same on the bottom. It already has air bubbles in it when it raches the prop so this is creating more slip. In other words less forward movement. Afetr you get it set up in the right heigth and it stays withen the rpm's then you can take the prop and add a cup to it to give you more bite.
     
  9. bowhunter6925

    bowhunter6925 New Member

    Messages:
    21
    State:
    Tennessee
    Bill, Bubba has you on the right track. Once you get the prop in clean water, you will then see a big difference in how your outboard handles. As for the difference in prop, you will not get much of an increase in speed, but you will be able to tell that your motor likes one or the other better for your setup. The only real way to figure out which is better is to put them on and run them. I have a '58 model Johnson 18hp and it came factory with a 9.25 x 11. When I replaced the prop two years ago, it was going to take two months to get the correct prop:angry: and about twice as much money. Unfortunately, I settled for a 9 x 10. My motor does not like this smaller prop, but it was recommended by Johnson for my application. Again, nobody could tell that it was not going to work as well as it was supposed to. My suggestion (my $.02) is to get the new prop and see which is better. You will probably find that with the larger carbureator, your motor will be happier with the larger pitch prop. :wink: Good luck, and let us know.
     
  10. Old Bill

    Old Bill New Member

    Messages:
    448
    State:
    Oklahoma City
    Thanks Willard! I'm going to have to give some thought about what materials I can find that will move the motor back the 5 to 6 inches. BobPaul used aluminum 3 inch angle, but if I read the article right he moved his motor back only 3 inches.

    Any thoughts how I could make a jack plate to do the job?

    Thanks again,
     
  11. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    Bill those 3 to 4 inches will get you the same effects from your motor ain't a large h/p motor
     
  12. Old Bill

    Old Bill New Member

    Messages:
    448
    State:
    Oklahoma City
    Thanks Willard, I believe I can make a jack plate that will do that!

    Now I've got to find some 3" aluminum angle...at a price I can afford!:smile2:

    Bill
     
  13. Old Bill

    Old Bill New Member

    Messages:
    448
    State:
    Oklahoma City
    Gasp! Choke!! Sputter!!! :eek:oooh::eek:oooh:

    I just priced a four foot piece of 3-inch x 3/16-inch thick aluminum angle from a place that sells small quantities of metal.

    $30.00 plus tax! Egads...that's $7.50 a foot!:crazy:

    Ah! The quest for a lower price goes on....
     
  14. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    Now keep in mind bill after all this you will still have a small h/p motor and its only going to do so much.
     
  15. Old Bill

    Old Bill New Member

    Messages:
    448
    State:
    Oklahoma City
    Hummmmm? Perhaps if you would overhaul my carburator so it would run on 100% Nitro??:wink:

    Yeah, Willard, I know you are right, no matter what I do the motor is still going to be a small HP motor! But, my goal is only to increase the overall speed 2 to 4 MPH.:roll_eyes:

    Maybe I should just paddle faster?