Black art?

Discussion in 'Sterndrives' started by scotch&water, Dec 30, 2019.

  1. scotch&water
    Joined: Jul 2009
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    Location: Great Falls Mt. USA

    scotch&water Junior Member

    I am up the creek and lost my go. I own a 2859 Bayliner that I repowered from a 350 gas Eng. to 6.5 Peninsular diesel, I at the same time changed the drive to a Bravo III X 220 ratio, I started with a 20x20 prop set under propped to a 24x24 set still under propped went to a 28x28 OVER propped now have a 26x26 set but the boat will not get on plane- all props have been 4 blade from Hill Maine. Additional info. 1993, 2859 Bayliner wt. 12000 loaded 11000 testing> 6.5 Peninsular - AMG diesel 310HP 400 Torque preferred RPM 2800to 3000 Red line 3600, Bravo III X drive all new. Things I have done to bust a bit more power this winter ( we are in Montana) increasing air duct from 3 inch ( turbo inlet to 4 inch, pulled the muffler to decrees back pressure? It also has been suggested that I change top gear ratio to a 181 to 1 set up but I am hesitating. S what are the wise men of this Forum recommending. Fritz
     

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  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Hello Fritz,
    Could the boat get up on the plane previously despite being under propped?
    Or do none of the props mentioned allow her to get up on the plane?
    Re the current 26 x 26 propeller, can you reach maximum revs with this, or are you still over propped like with the 28 x 28?
    Re if you are over or under propped, is this using the red line max revs of 3,600?
     
  3. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    What is this 28x28 business ? These things don't take 28" diameter props or anything near it. Or are we talking duoprop with two 28" pitch props ?
     
  4. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    If it is a case of twin 24" inch pitch being "under-propped", I assume it red-lined, or at least revving out easily to the top end of the recommended RPM range, that being the case, and the 26" not planing the boat, seems a bit incongruous, but tells you that 24" is nearer the mark. Maybe you think the boat too slow with the lower pitch.
     
  5. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Well..there is a lot to consider.

    Firstly...what is the displacement at which you run the vessel.
    What rpms can you achieve?...in
    Can your boat achieve the maximum rated RPMs at Wide Open Throttle (WOT)?
    If so, do the RPM's exceed the maximum ie can it over-rev or does it fall short of the maximum??
    What is the rated power of your engine, and at what RPM?..and is this a 'sales' figure or have you had it on a dyno and tested?
    What ratio box do you have, 2.0 or 2.2 as is common?

    The figures you posted above..what speeds did you get with the changing prop size/pitch? ....as that is also a guide.

    Without knowing much about your boat, and not knowing answers to the above, a quick cursory glance indicates around 19/20 x 20 anyway.
    Thus before rushing off like a bull in a china shop...need to establish some basic facts first.

    Since the basic hp/tonne suggests a max speed around 23-25knots at best anyway.
     
  6. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    Put away the Scotch and get yourself a good scientific calculator. Then get a copy of this book.

    Propeller Handbook by Dave Gerr https://www.thriftbooks.com/w/propeller-handbook_dave-gerr/1486390/item/20002546/?mkwid=%7cdc&pcrid=395931190724&pkw=&pmt=&slid=&plc=&pgrid=83211845752&ptaid=pla-838337054151&gclid=Cj0KCQiAgKzwBRCjARIsABBbFujVMS72veG9mU4z9BVGxNZzMkdPPBj6dudG4lH9qxkQBcAiwJl0P1EaApOFEALw_wcB#isbn=0071573232&idiq=20002546

    One very important specification is the actual weight or displacement of your boat.

    Gerr's book, used properly, is some fairly complex engineering. You'll need that calculator to solve engineering formulas that use fractional or decimal exponents. Gerr provides the formulas and even walks you through what decimal exponents are. The most difficult part of using his formulas will probably be figuring out how to enter your formulas into the calculator, which buttons to press.

    It's not black art but you don't have to guess either. You may be surprised at what you find after reading and using Gerr's book. Now's as good a time as any to hunker down with a good book and do some engineering, especially in Montana!

    Good Luck and Happy New Year.
     
  7. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The Bravo II uses 18 1/2" propellers. There are small variations in aftermarket products. To start with, the ratio is too high. That engine generates maximum power at 3600 RPM instead of 4600 RPM. You should have reduced the gear ratio by about 30% to keep the same pitch. 2.20:1 ratio is the highest available and applied to the lowest HP engines. The problem is likely your turbo, which is not spooling up. What is the boost pressure?
     
  8. scotch&water
    Joined: Jul 2009
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    Location: Great Falls Mt. USA

    scotch&water Junior Member

    1. bajan sailor, none of the props worked to this point, the 20x20 under propped 24x24 under propped the 28 set over propped 26 set got to 3600 RPM but did not get the boat on plain, at the upper RPM The Eng. was pumping black, removed the rubber 90 from the turbo air intake ( feeding moor air) and she cleaned up , that is why I am going to 4inch intake. 2. Mr. Efficiency Bravo III = duo prop, the 20 set and the 24 set over reve. the 26 set struggling, but will get there with more air. 3 Ad Hoc, Boat Wt. Net. 10,660Lb. 1/3 tank fuel, no water, not loaded. Grain terminal scale, trailer and truck Wt. deducted. I can get over the 3600 RPM with the 20 and 24 set. Hp. 310 Torque 400 sales # not dyno. Gear Ratio 220 was recommended by manufacture. so don't know what is off. Speed has not been reached 20 to 25 is fine with me, when things are sorted out, I will be back in Ketchikan AK. fishing and with logs up there reduced speed I the best. Book ordered.
     
  9. scotch&water
    Joined: Jul 2009
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    scotch&water Junior Member

    Thank you to missing in action, and gonzo, I think all the information I have supplied might work, to me it is still black art, till I get the Gerr book, at that point more head scratching, ( no hair left to pull out).
     
  10. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    You should be in the ballpark with the 2.2:1 ratio, the problem lies elsewhere. Is the exhaust going throu' the Bravo leg or through the transom? What is the installation height (antiventilation plate vs keel line)? Is your tacho correctly calibrated? Where does the (too small) inlet pipe start, and is there any kind of filter?
     
  11. scotch&water
    Joined: Jul 2009
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    Location: Great Falls Mt. USA

    scotch&water Junior Member

    I am thinking that you are correct, that is why I have been hesitant to pull the drive and change gears, parts would be $1500. - $1800. plus grunt time. So I will detail air intake first. air dumps in from side louvers in by way of a square 4"x4" tube to a 12''x12'x12" collection box ( fresh air syst.) from there 3' tube to the turbo, Pictures---. Exhaust out of the turbo 90 and up 12" and with a big bend back down to the water injection Elbow, from there 5" to the muffler and out of the side above the waterline. Muffler now removed and will replace with 24" of strait pipe F.G.
     

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  12. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    The engine red-lined, but the boat did not get on plane ? There is something wrong with the numbers here. Maybe the tacho is on the blink. I can't see what else explains those figures.
     
    Last edited: Dec 31, 2019
  13. scotch&water
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    Location: Great Falls Mt. USA

    scotch&water Junior Member

    I have 2 tach's. They're close, so don't think that is the problem, loss of power could be, but I am addressing that, so what if the prop RPM is increased?
     
  14. baeckmo
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    Location: Sweden

    baeckmo Hydrodynamics

    My first guess (not knowing what speed you got when "red lining" though) is that your props ventilate when trying to pass the hump speed. Hence my question re installation height. Could you post picture showing the 20 or 24 inch propellers? I'm interested in the blade shape close to the trailing edge; is there any cup to be seen?
     

  15. scotch&water
    Joined: Jul 2009
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    scotch&water Junior Member

    Speed on this hull will only clime to 12Kt. at that point if power is available she will start the climb to go over the hump, none of the props have gotten over that point, the 20 and the 24 pitch would just go to red line, with throttle left over. So if I could turn 4600 RPM it might work, bur red line is red line. You question as to height still in original location, the difference from the BravoII (removed ) and the BravoIII is the later is a duoprop, the transom shield is the same. Pictures of both props front and back. My thought is the difference in RPM, the old Eng. 350 Chevy. had to turn 4000 RPM to start over the hump and would run at 4400 RPM 22 MPH and reach 25 at 4600RPM if my memory is correct the ratio on that drive was 165x1. Happy New Year.
     

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