Flat bottom skiff in parallel waves

Discussion in 'Boat Design' started by wollybugger, Jun 22, 2017.

  1. wollybugger
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    wollybugger Junior Member

    Hi, I built bottom skiff over the winter and ran it for its sea trials a few days ago.

    While running parallel with another boat's wake, the boat caught and quickly turned inward. Gave me a scare. Do you know if this is common and if it can be corrected?

    LOA 15.5'
    Beam 4'10
    Power 15hp

    I have 2 bottom strakes 3/4" high amidship to stern. I'll try and get a pic up, but have been unsuccessful with google
     
    Last edited: Jun 22, 2017
  2. wollybugger
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    wollybugger Junior Member

  3. wollybugger
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    wollybugger Junior Member

  4. PAR
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    PAR Yacht Designer/Builder

    It's difficult to tell what actually happened, but you likely dug in the bow and she started to "root" around a bit in the waves passing under laterally. A reasonable amount of bottom "rocker" helps solve this issues, but if your bottom is dead bang straight, you'll need to trim the boat with the bow slightly clear of the water, to avoid most rooting issues. Which design was this built to?
     
  5. wollybugger
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    wollybugger Junior Member

    Hey thanks for the response. I built it by eye, but taking common traits I found on other skiffs around the Chesapeake.

    I did have to ballast the bow to minimize the Porpoising. There are others in the area waters that do the same. It planes with forward half out of the water. I'll try and get a video up.
     
  6. tom28571
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    tom28571 Senior Member

    What happened to your boat is what I would expect for most small shallow draft boats. Did you really mean that you were running parallel to the other boat's wake or parallel to the other boat's course? If you were running parallel to another boat, that boat's wake would have hit your boat on the stern quarter, pushed the stern to the side and caused your boat to broach in toward the wake which tried to roll you over.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    You seem to have done a pretty good job of the build. It does sound like a broaching action, when the bow buried into the wave ahead. It was probably accentuated by the fact your boat was not travelling at much different speed than the waves. Flat bottom boats don't much like water that is not similarly, flat !
    The solution may be to have more pronounced skeg(s). Of course, you will need them to be compatible with whatever trailer you are using, and be suitably shaped and placed. Anything less than two inches maximum depth would not be much use, in my opinion. You also have to consider the implications of skegs ahead of the prop, which can cause cavitation, unless carefully installed. Too much boat profile in the water forward, combined with too little aft, produces a situation where forces acting on the boat will cause a pivoting about the more deeply immersed end. If you placed exaggerated skegs aft, to counter that nasty broaching motion, on that boat, which as you say has the front half barely in the water underway, steering would be sluggish, and the bows easily pushed off course by quartering waves. You want a situation where the geometric centre of the submerged profile is not at the extreme ends skeg.png
     
    Last edited: Jun 22, 2017
  8. wollybugger
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    wollybugger Junior Member

    What a great set of responses. Thank you so much.

    So I'm just going to limit "the envelope" until I pull her in the fall (she'll sit in the water until Nov) then pull and get the skegs off. Then I'll revisit this thread so you guys can see the new skegs based on above suggestions.

    I admittedly was pretty lost on skeg profile ...

    Thanks again
     
  9. jorgepease
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    jorgepease Senior Member

    It's not that it's flat bottom that is the problem, it's that your bow terminates in a point. Square bow flat bottoms love to surf.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Love to slam, too. :( But would likely not have the same directional stability problems.
     
  11. wollybugger
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    wollybugger Junior Member

    Makes sense as I have a 17' Carolina Skiff and I never encountered this before (have encountered slamming).

    And it does take time to visualize this. The boat rides bow high most of the time. Short strakes/skegs/runners were put on each side with the thought to prevent skidding turns. My thought was since Carolina Skiff runners/strake were only 1" high, so I'd make this crabbing skiff "runners" 3/4" high (1 on each side)

    But in that wake (or any lateral wave) it brings the bow down, and immerses it in the water, creating a moment. I would assume the moment is around the hull center? But the short 3/4" runners don't have enough profile to counteract that moment. The 2" runner would help but wouldn't eliminate because they are close to geometric center?

    That problem doesn't exist on the Carolina Skiff because that lateral wave passes under the bow, or strikes the boat closer to geometric center (creating minimal moment)

    I might be interpreting wrong but this really is interesting
     
  12. Barry
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    Barry Senior Member

    I have had this happen on a 40 foot, 20 knots, 24 degree V, 34 foot Silverton with about a 26 degree V, various 21 foot jet boats with 12 degree V's, a 20 foot Mastercraft, deadrise
    unknown and even rigid hull inflatables ( occasionally)

    The right set of circumstances, where you are crossing at a fine angle into large and steep waves where the following wave picks up the back mainly on one side and stuffs the other side of the bow into the next wave can make this happen.

    The Silverton was the worst, at 17 knots 3 -4 foot seas, you could feel when the bow/chine was starting to begin to catch a bit, you could feel the bow begin to abruptly slow you down and turn the direction of the boat. If you did not correct the angle that you were crossing the waves, eventually, the bow would cut in hard and turn abruptly one way or the other. Either follow the crest/valley exactly, or slow down or turn more into the waves. The Silverton being a 17,000 pound slug was always high nose up due to the deadrise.
    I doubt that any minor changes in skegs will improve this much.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Broach-prone planing hulls are known to have the following characteristics:
    Deep forefoot combined with broad, much flatter stern sections. Boats with more heavily vee-d sections aft, especially with fuller sections forward, and with the shallower forefoot, are usually very broach resistant. Look at jet boats used in open water, the forefoot is not pronounced in the better ones, and even with moderate deadrise aft, track well enough despite having no keel or drive legs aft to counter broaching. There is no doubt that adding lateral plane aft will retard broaching tendencies, a boat presenting little profile to the water aft, but much more right forward, sets up a very strong turning couple. A boat's tendency to broach can be tested by running with a modestly sized wave train, and putting the helm over hard when the bow dips into the back of a wave, if it jerks around violently, it is likely going to give grief in following seas. The OP's boat appears to have no rocker, were it more like a surfboard right forward, there would be more dynamic lift to counter the dipping tendencies, the old "hang ten" manoeuvre is dependent on it ! But the slamming would be worse, too, probably.
     
    Last edited: Jun 24, 2017
  14. jorgepease
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    jorgepease Senior Member

    I also don't think those little strakes are going to help. Just watch where you point and keep weight aft and the bow up as mentioned.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Keeping weight aft is not really a solution, imo, the boat will squat, and moving COG back to raise the bows gives back what it gains by increasing the force available to broach the boat. Suitably arranged skeg(s) offers the best hope, imo, though a wide, upwardly curved spray knocker on the topsides at the bow could also assist.
     
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