Prop and jackplate questions

Discussion in 'Bubba's Outboards' started by ChannelHead, Jun 18, 2007.

  1. ChannelHead

    ChannelHead New Member

    Messages:
    12
    State:
    Georgia
    I hope this is the right forum to post this question, if not, please re-direct it.

    I have a 15.5' blazer bass boat, with a 70hp OB. I have not owned it very long (3 months) so i cant answer "historical questions" about it. My problem is getting the boat to run WOT at the desired RPM range which is 5500. There is a manual 6" jackplate installed. When i bought the boat, he had a 17"p prop on it, and the cavitation plate about 3.5 inches above bottom of the boat. With this setup, and ONLY me in the boat, i can reach 5500 RPMs using the trim, but cavitation (ventalation?) becomes a problem. Yesterday i was on the lake and did some tests. I removed the 17p prop, and put on a 15p prop. I couldn't get the boat over 3500rpms without cavitating. I came back in and lowered the motor/jackplate about 1.5 inches. With the 15p installed, and the motor lowered, i could only get 5100rpms out of the motor, and the nose of the boat wanted to dig in the water, making the ride fairly rough and wet.

    The original prop is a 14p which i am currently having rebuilt. I think my simplest solution is to remove the jackplate, put the 14p on, and set the motor down where the cavitation plate and bottom of the boat are fairly square. I need some suggestions from you guys, i am taking my family fishing for a week in the near future.


    thanks for reading, sorry for the long post.


    -craig
     
  2. jim

    jim New Member

    Messages:
    2,579
    State:
    Jacksonville NC
    I doubt if you are having cavitation problems which is high temp or boiling in front of the prop causing actual burns or erosion of the propeller.What you are suffering from is a common affliction known as, "get every last ounce of speed out of my bass boat!!!":big_smile:Why you would do all that with the rig you bought I dont know because it isn't going to outrun anyone and I would bet with all that extra cost it didnt go 2mph faster.In any case you are probably suffering from ventilation.Put the 17 p back on and try it at the lower position,that should get you close to the WOT/RPM range you are looking for.Go to this website I just discovered and see if you can dial up your combo www.turningpointpropellers.com .Mercurys website has a similiar feature.Your prop may be designed to run partially out of the water so wont be suitable in a fully submerged position.Set the jackplate so you achieve the position you have described then try all the props.If none get close to 5500 call the engine manufacturer and ask them for a recommendation,based on your boat and load.:big_smile:
     

  3. ChannelHead

    ChannelHead New Member

    Messages:
    12
    State:
    Georgia
    i agree with you about the speed thing, i know its not going to win a race. its just that the jackplate was on the boat when i bought it, and i'm having trouble reaching WOT.

    i'm not a pro, but if i could only get 5100 out of the 15p, how can i get 5500 out of the 17p?

    would i be wasting my time and effort to take the jackplate off and lower the motor back down?
     
  4. jim

    jim New Member

    Messages:
    2,579
    State:
    Jacksonville NC
    I wouldn't take the jackplate off.Jackplates usually have a positive effect.Go to the website I gave you and use it.I went there and just guessed you had a Yamaha engine and at the model number of the boat but the recommendation was 13 1/4 dia and 15-19 pitch.Thats with a normal prop.If you have one designed to run breaking the surface thats another issue.Lower the engine so the cavitation plate aligns with the bottom of the boat and try all 3 props.I know its a pain in the butt but its the only way.What kind of props do you have ?Aluminium or SS.Who made them? are they raked or cupped?Again if you go to the website with all that info you should get a good recommendation.:smile2:
     
  5. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    With that size motor it should not have any problem turning that 17. I would do a compression check also and a spark test. Sounds like its not fireing on all when needed . Jim is offering some solid advice.
     
  6. ChannelHead

    ChannelHead New Member

    Messages:
    12
    State:
    Georgia
    great information on that website. turns out that, if they are correct, my pitch recomendations are between 8 and 15, so i may still be on the high side, even with the 15p. I'm still waiting the original 14p to come back.
     
  7. ChannelHead

    ChannelHead New Member

    Messages:
    12
    State:
    Georgia
    JIm, thanks for your interest. I did check compression last week, I have 142 on 1 and 140 on 2. The motor is a 1989 70hp Tohatsu (2cylinder).

    It starts, idles, and runs like a champ. Is there still a possiblilty that i should have the carbs rebuilt or something?
     
  8. jim

    jim New Member

    Messages:
    2,579
    State:
    Jacksonville NC
    I would say it will depend on what it turns when you set the jackplate down and get a medium range pitch.Personally I dont think you can ever go wrong with a carb rebuild,waterpump kit and lower unit oil change and some new plugs wouldnt hurt.I would get a quality fuel stabilizer in the fuel ALL the time and a can of Ring Free or some other carbon reducer would be good also.If you use PRI-G stabilizer you get the carbon reducer built in.I would do those things for any engine I bought used and also change to a good quality TCW-3 oil.If the engine is running good with good compression then carb rebuild might be optional.The engine seems willing to pull its weight if you get it matched to a proper engine height and prop pitch combination.:smile2:
     
  9. ChannelHead

    ChannelHead New Member

    Messages:
    12
    State:
    Georgia
    what about PowerTune? I have some of that laying around. If the carbs are dirty, would that do any good? I really dont think i have a running issue, but i'll take any advice i can get.
     
  10. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    Power tune will work just fine. I personally like sea foam but If I already had the power tune then That is what I would use.
     
  11. ChannelHead

    ChannelHead New Member

    Messages:
    12
    State:
    Georgia
    i set the motor down to the third hole, and the jackplate down as far as it would go last night and i still have about 1 inch difference between the AV plate and the bottom of the boat.
     
  12. jim

    jim New Member

    Messages:
    2,579
    State:
    Jacksonville NC
    That will be fine and give you a starting point to assess each of your props.:smile2:
     
  13. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    With a jack plate, the engine is required to be set higher than normal, due to the set back from the transom. About an inch, maybe a little higher would be a good starting point with a 15" prop.

    That's an inch higher where the anti cav plate would line up with the bottom of the hull.
     
  14. benelli101

    benelli101 New Member

    Messages:
    27
    State:
    mississippi
    Will the set back of a jackplate ( say typically 5 inch.) help with cavitation if the motor stays set at the same heighth just further back?
     
  15. Bubbakat

    Bubbakat New Member

    Messages:
    4,532
    State:
    McMinnvill
    For up to every six inches of set back you need to raise that anticavation plate 1 inch above the keel