Black art?

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

  1. Ad Hoc
    Joined: Oct 2008
    Posts: 6,227
    Likes: 417, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Oh dear... things are serious now!!! :D

    Indeed..hence my Qs about the displacement at said/noted rpms etc.

    Since this is such a low length-displacement ratio fat hull form, the hump resistance is significant:
    upload_2020-1-3_8-34-58.png

    Even a small change in displacement at such low LD ratios, has a major effect. Look at the difference between ratios of 4 -5, massive change!
    Which also leads to - major squatting at such speeds...exacerbated by the hump resistance. A double whammy...

    Concur.
     

    Attached Files:

  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,034
    Likes: 360, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    The engine is rev-limited, which masks cavitation/ventilation issues to some extent. Checking the drive height would be an easy first step. Also, are you deploying tabs in an effort to assist the boat to plane ? Over-application can actually be counter-productive, and create excessive drag, and not much lift, at sub-planing speeds. That is probably more true if the tabs are not that generously sized. The boat has put on weight with the diesel, which must be a significant factor as well.
     
    Last edited: Jan 2, 2020
  3. gonzo
    Joined: Aug 2002
    Posts: 13,786
    Likes: 436, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    * Can you post how you did a cavitation check? I am not aware of you being in the USA, but you may.
    Can you explain, in engineering terms what "walzing" means and how you quantify it?
    There is no data anywhere supporting your claim that water accelerates in front of a propeller or that is creates a downpull; whether it is single or dual.
    Increased fluid velocity, not acceleration, creates lower pressure.

    Please show data or references to any study that supports your claims.
     
  4. TANSL
    Joined: Sep 2011
    Posts: 5,864
    Likes: 220, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    It is difficult to accelerate a body if an acceleration is not applied to it. Maybe you are talking about the same thing without realizing it.
     
  5. baeckmo
    Joined: Jun 2009
    Posts: 1,134
    Likes: 122, Points: 63, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Gonzo, I rechecked my preliminary calc.s and find that I owe you an apology; you are probably right about the gear ratio!

    That said, it is not sure the trouble is completely solved with a transmission rebuild, because this hull is fatter and heavier that normal, leading to high hump resistance. Again, this may well lead to a ventilation situation with the B III unit, but not with the B II. This is why I still think it is wise to check any installation issue that has an impact on ventilation.

    On the comments you raise in post no 33, I'll be back on those later.
     
  6. scotch&water
    Joined: Jul 2009
    Posts: 70
    Likes: 2, Points: 8, Legacy Rep: 51
    Location: Great Falls Mt. USA

    scotch&water Junior Member

    Gentlemen, I am reading and absorbing, forming no conclusions , waiting for the discussions to evolve, thanks. In the interim period I am rebuilding my exhaust to reduce back pressure. No scotch need to see clearly.
     
  7. gonzo
    Joined: Aug 2002
    Posts: 13,786
    Likes: 436, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    No scotch!!? I guess bourbon may do in a pinch.
     
  8. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,034
    Likes: 360, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    scotch, why is this boat not running Mercury props ?
     
  9. scotch&water
    Joined: Jul 2009
    Posts: 70
    Likes: 2, Points: 8, Legacy Rep: 51
    Location: Great Falls Mt. USA

    scotch&water Junior Member

    IMAG0045.jpg IMAG0038.jpg Since we are getting in to hull discussions a picture might help? Mercury Props, My thought was the Hill Marine are 4 blade perhaps more grab, so far not.
     
  10. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,034
    Likes: 360, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    You have had this thing over a weighbridge, and it is 11,000 lbs ? That is a lot of weight for a boat that length that is trailable width. Is the diesel a particularly heavy unit ? Do you have a pic of the boat in the water in its current configuration?
     
  11. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,034
    Likes: 360, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    The blurb about duoprop set-ups speaks of improved time to plane, as well as the other advantages, but in this case, that has gone on holiday.
     
  12. scotch&water
    Joined: Jul 2009
    Posts: 70
    Likes: 2, Points: 8, Legacy Rep: 51
    Location: Great Falls Mt. USA

    scotch&water Junior Member

    Holiday? no AWOL is more likely, Net Wt. was 10660 I have visited with other owners of the 2859 the early units where all heavy, then the been counter went to work and late Mod. came down to 8000 empty. Boat is 9Foot 7 inches wide, with the pods 30 foot long diesel Eng. listed at 1000Lbs. No pictures in the water with current set up. will post pic. running with 350 and Bravo II. Time for Scotch.
     

    Attached Files:

  13. scotch&water
    Joined: Jul 2009
    Posts: 70
    Likes: 2, Points: 8, Legacy Rep: 51
    Location: Great Falls Mt. USA

    scotch&water Junior Member

    One more
     

    Attached Files:

  14. Ad Hoc
    Joined: Oct 2008
    Posts: 6,227
    Likes: 417, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Based on those numbers, she has a low length displacement ratio....and as such, see the graph I posted above, a large prismatic hump.
    And even small changes in displacement, at such low LD ratios, can have major influence on the hump resistance. This adds to the drag and increases the squat with the lower pressure aft..hence the boats 'sink's into her own hole aft.

    She's struggling to get onto the plan basically ... any measures you try are all worth while exploring.
    But fundamentally - the boat has issues!!
     

  15. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,034
    Likes: 360, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    OK, wider than I thought from looking at the pictures, that fella standing on the swim platform must take size 14 boots ! Are you getting a lot of nose-up attitude, much more than before, with the previous engine ? Have you been applying the tabs heavily, or experimenting with the degree of application ? I don't know what is going on, if you have a 2.2 to 1 box, and a relatively low pitch prop set-up, you should be on-plane. What would be regarded as the effective pitch of those two props, in tandem ? If you can get a figure for that, you should be able to work out what slippage you have, using the other data of engine PRM and boat speed, a lot of slippage would increase the risk of cavitation/ventilation if there is something about the geometry of the drive that is pre-disposing to it. Looking at the pix of the attitude of the boat in the old config, it looks pretty good.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.