how to get bow lift

Discussion in 'Powerboats' started by weehenry, Oct 6, 2008.

  1. tom28571
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    tom28571 Senior Member

    The Flexitab operates in the same way as the tabs I advised when this question was first asked. I am against adding weight to a planing boat to cure a design problem. Weight is the enemy of speed. Although probably expensive, the Flexitabs, when tilted upward will generate negative pressure to pull the stern down at speed, thereby lifting the bow at the same time.

    You can mold some tabs on your own. The only problem being that they are not adjustable and some experimentation is needed to get them right for the speeds you want to run. Nearly all of the offshore fishermen on the Carolina coast have some rocker built into the the aft hull bottom to do the same thing.

    You can't get around that shaft angle and its stern lift/bow down thrust.
     
  2. weehenry
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    weehenry Junior Member

    thanks guys for all your replys what size of plates would i need to add and roughly what angle
     
  3. tom28571
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    tom28571 Senior Member

    That is what the experiment is all about. The plate at the transom-bottom edge should be parallel to the bottom and then curving upward. How much upward? Experiment. The "Flexitab" is capable of a variable arc and maybe you could make yours to flex a bit also. As a guess, the width of the tabs could use up all of the available transom between the chine flats and the shaft and give the answer. Or you could try 12" - 14" wide by 10 or 12" long and curving up about 1/2" to 3/4" at the end That should tell you whether more or less is needed.

    It does not require a lot of hook or rocker to make a big difference in the trim angle of a boat.
     
  4. weehenry
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    weehenry Junior Member

    cheers tom i really appreciate the help
    when i got my boat it was modified to suit a jet hence the chines dont go all the way to the back i also thought about making the chines wider
    what you think
     
  5. marshmat
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    marshmat Senior Member

    It might be worth noting that, if I'm not mistaken, Brunello Acampora is the owner of Flexitab.
    That being the case, and recognizing that advertising a product without declaring your personal stake in it is generally considered bad forum etiquette.... the Flexitab is an interesting product and I can see how it could be an ideal solution for some situations.
    The experiment Tom suggests should prove very useful for this particular application. I'm looking forward to hearing how that goes.
     
  6. Volare
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    Volare Non lo so

  7. tom28571
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    tom28571 Senior Member

    They look like the same to me but the basic idea is well known. Any patent is probably not on the basic idea but on the particular execution of the individual device. The last photo on the Arneson site looks like they needed to add a vertical skirt on the chine side of the plate to prevent aeration and loss of suction. Note that the forward screw on the skirt appears to be in an elongated slot to allow vertical movement for flexing.

    Edited to say that I don't really understand what is meant by the chines going all the way back. Adding chine flats aft, if that is what you mean, is opposite of what you want since that would lift teh stern.
     
  8. Volare
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    Volare Non lo so

    From what I've read, Arneson used the fin as a turning fin or a skid fin

    None of these things will overcome the negative shaft angle though. Try try as you may, it will be far less expensive and time consuming (I've been reading about this boat for what seems like forever it seems) to simply do the right thing or live with it as it is.
     
  9. weehenry
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    weehenry Junior Member

    correct me if im wrong but when a arneson is trimed to where it should be the centre of the hub is lvel with the bottom of the boat
    i have my unit set as an arneson would be trimed at flat i thought the advantages of a arneson was the handeling an low and high speeds
    i actually lifted the prop about a half inch put it killed the speed

    however im gona try the tabs in the next few weeks and ill let ya all know how i get on
     
  10. tom28571
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    tom28571 Senior Member

    That is not the best place for a skid fin, which should be further forward to allow the stern to rotate about the fin in a turn. Of course, it could work as a skid fin there, but it's not the best place for it.

    On with the experiment. As it is, we're just talking.
     
  11. Willallison
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    Willallison Senior Member

    That's what I'd do. True, your average drive won't cope with the torque of a Yanmar 440, but I'm fairly sure that some of the drives from Konrad would be up to the task. They've been around for a number of years, so should be well sorted.
    Of course, it depends on how much you want to spend... The tabs would almost certainly be a less expensive option
     
  12. bruceb
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    bruceb Senior Member

    Lift strakes ?

    What happens if short lift strakes are added low and just forward of CG? I have observed many production powerboat hulls with various "add ons" to cure ill handing. I know they create drag, but that seems to be an acceptable trade.
     
  13. Willallison
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    Willallison Senior Member

    Spray, strakes, call them what you will, only increase drag if they are immersed. Their primary function is to separate the flow of water cleanly from the hull, thereby reducing wetted surface and drag. They will also produce a degree of lift and improve dynamic stability
     
  14. weehenry
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    weehenry Junior Member

    i was thinking of adding like a trim tab between the keelline and the chine where the hull starts to straighten out
    what you think
     

  15. Todd D
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    Todd D Junior Member

    A great way to add bow lift would be a prop mod. Adding tip cup and or rake will increase bow lift of your boat. What pitch and rotation are you currently running?
     
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