Bow digging during corners

Discussion in 'Boat Design' started by aaronhl, Apr 5, 2023.

  1. aaronhl
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    aaronhl Senior Member

    What would cause a hull doing offshore circle/lap racing to dig the bow in on the corners? It is fast on the straight aways but has increase lap time because it does not fly around the corners. The bow digs into the water slowing it down in the corners.
    The laps run "clockwise" so the turns have the starboard chine hitting the water, we will be testing with strake up front, to have a reverse chine only up front on that one side...what other ideas do you have or reasoning for the bow to lower and dig into the corners ??
     
  2. bajansailor
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    bajansailor Marine Surveyor

    What type of hull is it - do you have any photos that you can post?

    Could the direction of rotation of the propeller shaft have an effect on this? What size engine (I presume an outboard?) does the boat have?
     
  3. aaronhl
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    aaronhl Senior Member

    I knew someone would ask what type of boat - it is a model boat about 4 feet long - has a 3.5 to 1 beam ratio so it's basically a offshore cigarette racing style boat with an inboard engine
    Not all boat this size dig the bow in around the corners and we are looking at speed around 50 mph

    [​IMG]
     
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  4. aaronhl
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    aaronhl Senior Member

    Sorry i dont have a picture of the corners, hopefully that gives you an idea of the type of boat, best way i could describe it is the bow wants to "pivot" tighter around the corner than the rear of the hull, if that makes any sense to you
     
  5. jehardiman
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    jehardiman Senior Member

    If that is the hull, something is forcing the bow down. I'd first look at the CG, moving it aft slightly may help. Next I'd look at the rudder and shaft/thrust angle, as both force the bow down in turns. Finally, remember that boats turn from the stern, meaning that the bow is "inside" the turning circle (made by the CG) while the stern swings "out". When there is not enough leverage the hull "skids" until it digs in to find the necessary leverage. If just going one way roundy-round, consider a small off center skeg just fwd of the CG like a 3-point hydroplane.

    upload_2023-4-5_11-34-6.png

    Also see these threads.
    Basic questions about V hulls https://www.boatdesign.net/threads/basic-questions-about-v-hulls.41264/
    SWATH speed and stability opinions https://www.boatdesign.net/threads/swath-speed-and-stability-opinions.62633/page-2#post-858522
     
  6. bajansailor
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    bajansailor Marine Surveyor

    Further to JEH's excellent post above, all I can think of now to ask is have you tried doing anti-clockwise turns as well?
    If yes, does she exhibit the same behaviour in the turn?
     
  7. fallguy
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    fallguy Boat Builder

    First thought comes to mind is you are 'developing' hook in the turns, or rather, hook is in the hull line in the turn. This would be easy to check. You simply turn the boat over and look to see if any running surface is concave with a straightedge from the keel to the starboard chine forward, or if anywhere that is running on the water is concave; even an entire chine measured on the edge; not the bottom would tend to pull it down.

    Otherwise, you are lucky hardiman offered such a detailed reply, because it is probably not hook..but had to mention. At the speeds you are running, a small error might mean a lot
     
  8. TANSL
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    TANSL Senior Member

    Even though the race is clockwise, check what happens in a counterclockwise turn. It could be a weight/buoyancy distribution issue (move the CoG aft or CoB forward). It could also be due to the propeller rotation direction (try a propeller rotating in the opposite direction). Two contra-rotating propellers mounted on the same shaft could be a solution. Try, if possible, with two shafts and propellers rotating in opposite directions. Consider changing the motor/shaft/propeller combination to waterjet propulsion.
     
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  9. BlueBell
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    BlueBell . . . _ _ _ . . . _ _ _

    Good quality video would help in analyzing this problem immensely.
     
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  10. baeckmo
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    baeckmo Hydrodynamics

    At these speeds, your prop is ventilating. In this mode the screw is sensitive to operating depth and advance ratio. The thrust line is not along the shaft line, but has strong transverse and/or vertical vectors, all varying with depth and advance. In addition, the spray rails forward seem to follow a buttock line, which creates a convex sector of bow panel area. When the flow is forced to follow a convex curvature, there will be a low pressure zone; the bow is "sucked down".

    It might help if you could show the prop and rudder arrangement to us.
     
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  11. fallguy
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    fallguy Boat Builder

    So, this is very interesting to me, and of course, opposite of my assumption, however, hull curvature seems like the issue to me, but I lack the training to recognize convex is also bad..makes total sense as you explain and even as simple as see saw.

    I want to highlight my potential error to the OP because if he saw no concavity, but found convex running surfaces; he may have assumed he was okay.

    We commonly see hook in amateur s&g builds, and it really robs the boat in overall performance.
     
  12. aaronhl
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    aaronhl Senior Member

    It is very nice and helpful for everyone to explain what may be happening, I will try to get some pictures and move video
     
  13. aaronhl
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    aaronhl Senior Member

    Do you have a picture describing what you mean by "convex sector of bow panel area" ?
     
  14. baeckmo
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    baeckmo Hydrodynamics

    See your pic in post no 3. The bottom area between the spray rail and the keel over the first ~30 % of the total length is convex. If, for some reason the prop thrust line becomes more vertical (which may happen in the case when resistance increases), the bow comes down and becomes wetted. You may also find that the hull is heeling into the turn as a result of rudder force. In this heeled condition, the rudder force has a vertical composant that is increasing bow-down moment, and oops.....!
     
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  15. aaronhl
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    aaronhl Senior Member

    Is this what you are talking about? Would the "Concave" or "Convex" sections cause more wetting/bow down?
    upload_2023-4-13_21-22-5.png
     
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