Need help or suggestion on correct prop.

Discussion in 'Bubba's Outboards' started by Big Dav, Jun 18, 2008.

  1. Big Dav

    Big Dav New Member

    Messages:
    1,016
    State:
    Southwest
    I have had this boat out a few times now and I need to get the correct prop for it. At WOT it turns 5100 rpm's 42 mph with only me in the boat and no gear. With gear and one more person in the boat it turns 5000 rpm's at 40 -41 mph. Here is the strange part, with all three live wells (approx 85-90 gallons of water / 680- 720 lbs of water), gear and one more person it will still push 38-39 mph and 4900 rpm's. With all the weight it still performs better than I ever imagined and still gets on plane faster than I would have ever would have thought.
    After the first couple times on the water I only had one complaint with the boat / motor. In order to stay on plane I had to maintain 26-28 mph depending on water conditions. This was with just myself in that boat with gear. I did some research and ordered a Turbo Lift fin for the motor and that brought the on plane speed down to 14-16 mph. I can live with that.:big_smile:
    I was told to install the fin and then purchase the correct prop for the package.

    The current prop is an aluminum 14 x 19 that came with the boat / motor when I bought it. The motor is a 2005 Johnson 115 two stroke and is on a Lowe 2070VTC 20.5 ft Jon with a 12 degree hull. I was suggested by a "local" shop to purchase a Viper stainless prop but the offered no suggestion on size or pitch.:confused2: They also said it would be three weeks before they could get me in to help figure it out.:eek:oooh::sad2:

    Any info on what I should look for or need in a prop would be much appreciated. I would also be up for a road trip if someone would be interested in helping me figure it out without having to go through several high $$ props to get what I need.:big_smile: Just as long as we can take the poles when were testing out the boat.:wink:

    Thanks in advance for any and all help and or info.
    David
     
  2. Wabash River Bear

    Wabash River Bear New Member

    Messages:
    3,019
    State:
    Indiana
    Howdy Dave. In my opinion your gonna have to find the h.p. specs for your motor, and try to pitch it to the rpm that makes the best hp. A prop shop should be able to give you a ballpark rpm to be gained or lost by going up or down in pitch increments.
    What I'm trying to say is if your motor makes max hp at 5500 rpm, and you are only pulling 5200 rpm.then you need to change pitch to gain 300 rpm to get the most out of it, or visa versa. Hope this helps. Later, WRB.
     

  3. Big Dav

    Big Dav New Member

    Messages:
    1,016
    State:
    Southwest
    Thanks Barry,
    This is some of the problem I seem to be having. I have asked three different Johnson dealer and gotten three different answers. 5000, 5500, and 6000 rpm's.:eek:oooh: I called Johnson (BRP) today and was told 5500 rpm's and to talk with the boat builder about the correct prop. I called Lowe and they could only offer info on Mercury motors with there boats.:angry::confused2:
    I ended up going to Ebay and buying a Stainless 13.88 x 17 Viper OMC prop at a good price. It hopefully will be here some time next week and I can see how close it gets me to the 5500 point. I still need to raise my motor a little with the turbo lift and that should (from what I was told) increase my RPM's just a little. :confused2: If not I will have a slightly used prop for sale and be looking all over again.:crazy:

    Thanks again for the reply and info.:0a31:
    David
     
  4. AwShucks

    AwShucks New Member

    Messages:
    4,532
    State:
    Guthrie, Oklaho
    I would think the manufacturer or the motor matched the prop to most general conditions. Myself, I'm not really seeing a problem with what you have. Course, My understanding is you have one prop for speed, another for water skiing. But any prop will work for drifting. LOL
     
  5. Wabash River Bear

    Wabash River Bear New Member

    Messages:
    3,019
    State:
    Indiana
    Dave, I found this article on the web. They say as a rule of thumb, 2" of prop pitch equals approx 400 rpm. Heres the rest of it.

    Choosing the Right Propeller
    To some, choosing the right boat propeller can seem like a job for a marine service center. Anything that has to do with the backend of the boat can be a scary thing. Despite popular belief, choosing the correct propeller for a particular boat and motor is actually a fairly simple thing to do, once you understand what a propeller really is.

    First think of propeller as a gear, whether it be on a car or even a bicycle. The lower the gear, faster hole shot, more power, but revolutions are much faster so at a point you can only turn that gear so fast until you over rev the engine (or you legs). Now think of your car stuck in only 5th or 6th gear, very slow out of the hole, little torque, but eventually it will catch up with itself so you can cruise to a good speed.

    The same basic concept applies to propellers, except with a prop you are committed to only one ?gear". So what to do now? Find the prop size that will give the best overall performance. Sound difficult? It's really not.

    Your engine has a recommended wide open throttle rpm range. Let take an example of an18' boat with a 125 hp engine with a rpm range of 4500-5500. In most cases you purchase a boat with some sort of propeller on it. Let's say this motor has a 21 pitch prop on it and at wide open throttle under a normal boat load, the engine turns 4900 rpms. Well it's in the range so it's the correct prop, right? Well yes and no. For the health and longevity of the engine it's okay, but for overall performance, you want to run a propeller near the top of the range. Well how do you change the size to get that?

    Remember this rule of thumb: For every 2" of propeller pitch, rpm's will change approximately 400 rpm's. As you drop in pitch, rpm's increase, and as you go up in pitch, rpm's decrease. So if we want to raise our rpm's we should choose a lower pitch prop. Following the rule, going down to a 19 pitch should raise our rpm's to about 5300. That would help the hole shot and also gain us some more speed.

    As you follow the rule of thumb, remember that sometimes rules are meant to be broken. This is not an iron clad rule, sometimes varying 2" of pitch will only change rpm's by 200. Switching prop brands or manufacturer, it can sometimes throw off the rule too.

    At this point the most important thing to understand is that less pitch means more rpm's, more pitch means less rpms. For your boat to perform at its peak, you need to run at the upper end of your engine's recommended rpm range. That's it. Once you're there and still want more speed, handling, lift, etc, that's when choosing the right propeller brand comes into play.

    Here's a simple adjustment you can make to improvement performance under varying boatloads: If you normally run with one other person on board, use that boatload as your measurement when determining your wide open throttle rpm's. But if you sometimes load the boat with 2-5 more people or try pulling a water skier or tube, get another prop 2" of pitch lower than your normal prop. It will compensate for added weight and drag and make a huge improvement on performance in these circumstances. But remember to keep an eye on you rpm's when running a lower pitch for these circumstances.

    Most props have the size marked somewhere on the prop, either on the barrel, on the blades, or in the front or back of the propeller. Use that pitch that you're currently running as your starting point. If it's already running correctly then that pitch is your pitch.
    (back to top)
     
  6. Big Dav

    Big Dav New Member

    Messages:
    1,016
    State:
    Southwest
    Thanks again Barry. I had actually found the prop wizard part of that web site but had not read/see the information you posted.:big_smile: Very good read and information that will be useful.


    I just didn't want to end up like times past, with me taking the boat to a "local" shop and ending up with a nice shinny stainless prop that turns about the same rpm's as the one its replacing with my whole purpose of changing the prop in the first place was to increase rpm's.:angry: I have went at this a couple of times before and went at it blindly. I wanted to be a little more educated this time around and do it once and be done.:big_smile:

    Thanks again for the help.:0a31:
    David
     
  7. Big Sam

    Big Sam Well-Known Member

    Messages:
    4,385
    State:
    Booneville AR
    Name:
    Sam
    Dave my boat is a grizzly 2072 with all the stuff on it like yours mine came with the same prop as yours:wink::eek:oooh: I broke it in with that prop then went to the prop man that is all he does is props:tounge_out: He hit the engine mfg web-site then matched it to the boat type then hit the chart for correct pitch:smile2: I got a 17 degree stainless pushing that big boy 43mph at 5250rpm thats enough it is rated for 5650 on the 115 optimax but 5250 is all i need. I forgot it will run 43 loaded unloaded live wells full or empty it is a performance prop all the way i love it......Well worth the 350.00 it set me back now i dont break an aluminum prop every time i "bump" something:wink::eek:oooh::smile2:
     
  8. SkipEye

    SkipEye Well-Known Member

    Messages:
    7,525
    State:
    Winfield, MO
    Name:
    Darryl
    Get out of that 'shaller' water and you'll quit 'bumping' stuff!:smile2::smile2::smile2:
     
  9. tofish

    tofish New Member

    Messages:
    3,923
    State:
    arizona
    when i was trying to figure my prop pitch out, i talked to the prop shop in havasu city, az. they "loaned" me different props to try until i found the one i was happy with. you might want to check with a prop shop and ask them if they have loaner props to check with. might have to put deposit on them, but they will start you close to right pitch you need.
    gary
     
  10. cantstopgrandma

    cantstopgrandma New Member

    Messages:
    955
    State:
    MD
    I'm nowhere near a prop expert, but isn't there some kinda rule of thumb about stainless props?? I think what i'm trying to say is, an aluminum and SS prop w/ the same pitch will rev different because the SS one flexes less. That's something else to think about if i'm correct about that. Hey, i've been wrong before, so check that out before you take my word on it.
     
  11. Big Sam

    Big Sam Well-Known Member

    Messages:
    4,385
    State:
    Booneville AR
    Name:
    Sam
    I can't help it Skip.....:tounge_out: when i hit something & Johnny goes flying out-the boat i want him to be able to stand up after a few minutes instead of chasing him down the currrent a couple of miles:wink::tounge_out::cool2: Save gas skip..save that gas!!!!
     
  12. Big Dav

    Big Dav New Member

    Messages:
    1,016
    State:
    Southwest
    My prop came in and I installed it on Wed but have not been able to get out on the water. Every time I try to leave for the lake another storm moves in and I not much for being on the water with lightning in the area.:eek:oooh: If the weather report is correct I should be leaving this evening to give the prop a good test.:big_smile:

    The prop that I won on Ebay was a 17 pitch OMC Viper stainless and was brand new in the box. (as listed) :big_smile: I hope it increases the RPM's enough so I don't need yet another prop.:eek:oooh::sad2:
    I will post how the prop worked out when I get home in a few days.
    Thanks to all that replied and helped.:0a31:

    David
     
  13. brad kilpatrick

    brad kilpatrick New Member

    Messages:
    2,666
    State:
    Kansas City
    wish i would have seen this sooner. could have saved you a bunch of hassles. call these guys they are the best http://propgods.com/content/index.aspx

    after answering a few simple questions these guys will suggest a prop and 99% of the time they are dead on