Sea Sled madness. It’s in my brain.

Discussion in 'Fiberglass and Composite Boat Building' started by DogCavalry, Nov 11, 2019.

  1. DogCavalry
    Joined: Sep 2019
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    IMG_20230525_200033048_HDR.jpg
    Garbage run as an award for clean thoughts IMG_20230525_181000934_HDR.jpg
    Bracket loosened up. Apparently always going slow didn't apply as much strain as going less slow. IMG_20230525_180514520_HDR.jpg IMG_20230525_175544506_HDR.jpg IMG_20230525_175740123_HDR.jpg
    OB at normal trim, and full down. The water coming out from under the transom ought to go around the streamlined section, but apparently straight up is better. IMG_20230525_165005843_HDR.jpg
    Not much bigger possible. Maaaaybe a 16" prop. IMG_20230525_164620455_HDR.jpg
    Checking gear case oil. A few tiny metal wiskers on the magnet. IMG_20230525_164053728_HDR.jpg IMG_20230525_160249546_HDR.jpg
    Water under back deck enters here.

    Large radius chine to transom. Some drag here. Hullside same. Bottom slightly better.
     
  2. Darkzillicon
    Joined: Jan 2023
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    Darkzillicon Senior Member

    You can also get a non ventilated propeller. Not sure how to make that work but you’d get more blade area…
     
  3. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    I'll give the new prop a workout, then I'll know better.
     
  4. baeckmo
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    baeckmo Hydrodynamics

    Can you explain what you are talking about and give any references to facts?
     
  5. baeckmo
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    baeckmo Hydrodynamics

    Yup, the profile leading edge is fairly blunt. The stagnation pressure head at 20 knots is ~5 mwc, roughly 0,5 bar (11 mwc at 30 knots...), and with just a few inches of water, without a horizontal boundary layer fence, there must be a diverging spray.
     
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  6. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Well, I can certainly manage a boundary layer fence. I have aluminum plate.
     
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  7. fallguy
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    fallguy Senior Member

    Isn't the cause of the water flow a combination of being too deep (which is desired by the aeration), the trim angle of the engine, also maybe desired for hull angle, and the lack of anything to stop it? So can you add a piece of metal to the engine as the red? I don't know why not.

    also contributing is the transom bottom not being sharp..

    or are you talking to the hull like a small interceptor?

    IMG_0515.jpeg

    Reminding me I need to make two flow interruptors for my boat as well..
     
  8. baeckmo
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    baeckmo Hydrodynamics

    Nope, it is just the flow from the stagnation line. Have you seen the downward cusp, or "channel" on the windward side of a stone after a windy snowfall? That's the effect of the flow accelerating in all directions from the stagnation point; up, down and to the sides to get around the obstacle.
     
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  9. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    So, remedies. A horizontal plate just above the streamline section of the leg, attached to that convenient horizontal surface? Or a quarter tube screwed to the lower edge of the transom to redirect that water backwards? Or something else?
    Next year it will be a pair of smaller outboards, but that's about $40000 in the future.
     
    Last edited: May 26, 2023
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  10. fallguy
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    fallguy Senior Member

    My uncle nicknamed me the Master Aquatic Butcher; Baeckmo deserves the Master of Water title, and this is a compliment.

    Based on my interpretation, a piece of aluminum angle attached in front of the engine on the back of the transom flush with what should be a squared line, will flow the water 12 degrees downward, but will it hit the veed portion of the engine? Also, though, will it direct aerated water to the prop? And what effect on trim if say 12" wide?

    And, as a builder, the bigger caution is a shortcut of not overboring the wood and filling the hole with thickened resin which would result in a transom failure..at some point due to likely ingress.
     
  11. fallguy
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    fallguy Senior Member

    Is that green stuff raw glass?
     
  12. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Algae on raw glass/epoxy.
     
  13. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    A diverging spray like a firehose. The back deck was ankle deep.
     
  14. fallguy
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    fallguy Senior Member

    The hardest part of squaring off the transom is making the no penetrations jig, but since you are working the area; you can just ship tape a nice board and screw it to the transom. If you choose a facebowed board; you only need two screws on the outside edges bow to transom. Square it off first if B tells you to add a piece there; it'll be much easier. With your trowel skills; it'll be simple with a board on the transom, and you pickup some efficiency, too.
     

  15. DogCavalry
    Joined: Sep 2019
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    So perhaps I might be better served with a sharp leading edge on the leg, instead of the NACA 024 section. That's good at low Reynolds numbers, but I'm trying to get to high Reynolds numbers. I'm fine with making penetrations in the aluminum leg extension.

    A sharp leg would stall at higher angles during steering, but that might not matter much, because that's strictly a low speed thing. At high speed, I'm pretty gentle on the steering. Not going to be inputting control forces that could stall flow around the leg.
     
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