Lateral Hunting/Wandering on plane

Discussion in 'Hydrodynamics and Aerodynamics' started by useless33, Mar 27, 2021.

  1. useless33
    Joined: Mar 2021
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    useless33 Junior Member

    I'm new to the forum, but have a question the experts here may be able to answer. I built a bateau FS14 (Mertens design) a few years ago and use it regularly. The boat is a 14' skiff with very little transom deadrise (5 degrees maybe) and a shallow forefoot as well. I run it WOT at 21 knots with a 20HP motor and cruise at 18. In very calm water it starts to dart laterally in very small movements. I recently added a skeg, which may have exacerbated this very minimal tendency early on, as it is now quite noticeable. The skeg is about 3' long, and starts at the planing waterline entry and terminates about 24" forward of the transom, which is another 6" or so forward of the prop.
    The motion is yaw, not roll, and it seems to pivot about a point much closer to the transom than the bow. The motor is steered with a tiller extension from the standing position. The helmsman has a T-post to brace himself. I can't see any irregularities in the water stream around the transom or the after sections of the hull sides.

    Eager for any ideas you all may have. It doesn't prevent me from using the boat, it's only a mild nuisance and is completely absent once even the lightest chop appears. Winds over 7kts or so will stir up enough ripple to ameliorate the condition.
     
  2. fallguy
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    fallguy Senior Member

    Your motor is up too high. If you are deeper in the water it is almost impossible to chine walk under that low horspower.

    My hunch is you built the transom a half inch too high. Get a manual jack plate that allows movement either direction and play with it. Or, if not all the way down, just move to a lower engine position and get back to me.

    Or, many vee hulls will walk some; how much is the only issue.

    My 16' Lund skiff walks a bit, but it is so minor it doesn't matter and could never be considered dangerous.
     
  3. useless33
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    useless33 Junior Member

    Hm. For my education, I thought chine walk implied roll motion back and forth between the two opposing chines. I run in very skinny water, so I may just live with it rather than lower the prop.
     
  4. gonzo
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    gonzo Senior Member

    Is there a difference in behavior when you are in deep water?
     
  5. baeckmo
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    baeckmo Hydrodynamics

    Looking at the design in plan view, it is narrowing going aft. What happens is that in calm weather there is a flow reattachement to one side aft, causing a yaw. But it is unstable and keeps moving from side to side as you compensate by helm. When there are small waves the attachement "suction" is broken and the outboard rig regains steering authority. This happens with low deadrise designs. As an experiment, tie a 10 mm line tight around the bottom from railing to railing a foot forward of the transom. It may take some experimenting to find the right position. Right there you fit small vertical ribs to the sides of the boat.
     
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  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Why did you add this skeg, that presumably wasn't in the original plan ?
     
  7. useless33
    Joined: Mar 2021
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    useless33 Junior Member

    Thanks for all the replies.
    Gonzo: No difference in deep water. The only variable affecting the issue is water smoothness. I've moved fore and aft weight distribution with little effect, though this boat doesn't change planing angle much because you have to stand in that spot to drive.
    Baeckmo: This seems like what is happening. Thanks for the advice. I'll give it a shot on the next calm day. So is that line meant to disturb the boundary layer and cause some turbulent flow in order to preclude the sporadic attachment?
    Mr E: I added the skeg because it would get sporty in a pretty small following sea because no appendage slowed a broach-induced yaw. Jacque mentions a skeg, so I put one on after owning the boat several years. It is still susceptible to broach (I know all boats are, but this one is a little more so). Overall, I'm not convinced that the skeg does much. Even without it, it would still wag its tail a bit on dead calm mornings.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    This re-attachment business has me intrigued, but not convinced, it certainly does not surprise me that the boat would skate around a bit, it is light and there is very little profile in the water, maybe experiment with the pin position on the outboard trim to see what difference you can get. If boats "weave", it usually involves waves, even barely perceptible ones, it makes no sense to me that a symmetrical hull on dead flat water would do anything other than run straight, unless there is something happening with an oscillation in the steering input. Is there some slack in that steering set-up, or does it answer with the slightest pressure on the tiller ?
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I notice that the illustration of this boat on the Bateau website shows a somewhat longer skeg than mentioned above.
    f5474f1a1a724906573428994297296d.jpg
     
  10. BlueBell
    Joined: May 2017
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    BlueBell . . . _ _ _ . . . _ _ _

    I couldn't understand how the induced yaw could be articulating about the stern/transom.
    Your description works for me.
    The proof will be in the pudding (the test will reveal the answer).
     
  11. useless33
    Joined: Mar 2021
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    useless33 Junior Member

    Thanks all.
    Mr. E: the skeg on the site is indeed longer, I was just concerned about any yaw input forward of the CG, as I was really trying to limit the broaching tendency in a following sea.
    I've tried the motor trim position, but have settled on a point where the water flow is generally perpendicular to the prop in most weight conditions. The boat is so small that load variations have really big impact on percentage of gross weight. 2 guys and 2 kids is a huge difference from 1 and 2. I love the boat not just because I built it, but because it is a jeep. I've got a suzuki 4 stroke 2o on it (4th motor) and it sips gas like a little sewing machine and will take me places very few other boats can go. I initially thought it was some sort of feedback loop with the tiller extension and me at the helm, but even under very tight steering friction (basically no tiller movement) it still does it on calm days.
    I've enclosed a pic.
    The designed transom was for a normal shaft engine. Upon completion I realized it would likely ventilate, so I went with a long shaft. I subsequently modded the transom with an oak plug in the central scallop area to lengthen the transom height and let me run a long shaft with the ventilation plate just above the bottom due to the significant setback. I ran a few tests and she ventilates in an unloaded turn, but I wanted to bias toward skinny water and I am usually going 20+miles when I go fishing, so I didn't care much about turning.
     

    Attached Files:

  12. fallguy
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    fallguy Senior Member

    I started to think this thing through a bit more.

    If one were to put a shorter and shorter skeg on a hull bottom to it becoming only a point or say a four inch long skeg; it would behave exactly as described. The point or short skeg would grab and the boat would make shifty movements with any changes to wind or tiller or weight shift.
     
  13. baeckmo
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    baeckmo Hydrodynamics

    ......and looking at your picture there is probably two more issues that will make things a bit worse. If this bottom shape is operated with too low angle of attack (ie the whole family up front), it will A/ have its center of lateral area forward of (or very close to) its center of mass (=backwards arrow), and B/ have its convex bow wetted, which causes a bow-down force. Both phenomena will reinforce the yaw effect from the asymmetrical side-wetting. And the long skeg is making things worse.

    So, when/if testing with the rope, take care to operate with a positive angle of attack of about 3 to 4 degrees, since the rope is functioning as a poor mans interceptor.
     
  14. fallguy
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    fallguy Senior Member

    This business of a rope is, quite frankly, confusing, at least for me. I am bold enough to say so; many are not.

    Now, I appreciate about 99% of your posts, so I am expecting the understanding shortfall to be my own.

    In all my life, I have never seen a boat with a 'vertical' strake. Water would spray all over the place and you would induce drag.

    Can you explain this better? Or explain what I am misunderstanding?
     

  15. fallguy
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    fallguy Senior Member

    I still think you ought to try lowering the motor as a test. Many guys around here running big Rangers want best of both worlds to fly and to get skinny, and so they use hydraulic jackplates to pull the engine up in the skinny stuff, then down running at or close to wot.
     
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