Sea Sled madness. It’s in my brain.

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

  1. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    I thought you wanted to remain faithful to the original shape, DC, no looking at alternatives allowed !
     
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  2. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry I aim to misbehave.

    I am ashamed. (DC bows head humbly.)
     
  3. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    Y'know, I used to be pretty good at aerodynamics. I can calculate flat plate drag for the opening of the tunnel on a sled. From that I can get pressure rise over the opening. Apply that pressure to the underside of hull in the tunnel, and get a meaningful approximation of lift.
     
    Last edited: Nov 19, 2019
  4. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell "Whatever..."

    John,

    I read through most of what's going on with your SOR and build, yesterday.
    I agree with your idea of going a bit wider.
    You have lower power and may have more weight.
    I'd keep the width.

    Have you talked with Dave Gerr at all?
     
  5. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry I aim to misbehave.

    Thanks BlueBell. I've emailed Dave Gerr a few times. Nothing heard. I don't know how active he is these days, or if he has time. He may be crazy busy, or on a yoga retreat.
     
  6. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry I aim to misbehave.

    Aaand, in my head, while driving, gives around 700#, or 320 kg of lift, at 30 knots. I assumed Cd of 1.5, which is that of a hollow sphere, open into the air stream. And STP, of course. That’s actually enough to matter. The Cd may be low. A rectangular prism is around 2.1. An open box is higher still. Doesn’t matter. The rough value is still revealing.
     
  7. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell "Whatever..."

    Interesting.

    How many pounds of lift for 25 knots?
    And what's your margin of error in these numbers?

    STP, standard temperature and pressure?
     
  8. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry I aim to misbehave.

    Since this is a very rough calculation, the margin is high. Lift is just is just the pressure over the opening, applied to the dry bottom. The coefficient of drag I used is 1.5. It could easily be 2. But it could be lower than 1.5 because of interactions between the curvature of the bow, and the water. I’ll try and set up a simple rig, once it’s running, and measure the static pressure.

    Drag is
    D=Cd*v^2*p*a p is just density of fluid, or 1.2kg/m^3. And a is just the size of the opening.

    25 knots would be (25/30)^2 or about 70% of lift at 30 knots. And at the Interdictor’s 60 knots, 4 times higher. Or almost the entire weight of my boat. I begin to see how this might cushion the ride.

    Yes, STP, is Standard Temperature and Pressure. :)
     
  9. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    Would you consider a single outboard?
    I read in your literature, it's been done, successfully.
    Would bring down weight, cost, drag and maintenance.
    I know you don't need anyone interrupting your plan, but I had to ask.
     
  10. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    All the air and foam generated in the tunnel run out under the transom, in the middle. So anecdotally, the prop has a hard time making enough thrust to get the boat on the plane. Salvatore Ianotu’s first sleds failed for this reason, and they had to heavily modify the form, adding a pod in the centre to give the prop hard water. That being said, I’m certainly going to try. The outboard well will be so large I can install a pair of massive, widely spaced engines. Or a single one, in the middle. Just to see. Science!
     
  11. BlueBell
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    BlueBell "Whatever..."

    Excellent.
     
  12. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry I aim to misbehave.

    bulkheads 1 and 2, last night. Bulkhead 3 and the ring frame at 4 tomorrow.

    IMG_0315.jpg IMG_0313.jpg IMG_0312.jpg
     
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  13. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry I aim to misbehave.

    I'm worried about how slow I'm going. I think I need to stop wasting my time with nonsense like bathing, changing my clothes etc. Stop heating up food. Cold chili from the can... Or even better, a 50 pound sack of bachelor chow, a stainless steel blowl from the pet store, and the garden hose. It makes it's own gravy!
     
  14. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    IMG_0372.jpg
    Bulkhead 3. B4 is a ring frame. Then some osb temporary sections, and then it's strongback time.
     
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  15. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell "Whatever..."

    Everyone has fallen silent...

    I commend you on your progress and courage in moving forward with this ridiculous project...
    ... ridiculously exciting project!!!!

    I think your shop needs a used, cheap wood stove, with a temporary chimney.
    Sell it when your done to some poor sucker working in an unheated shed next winter!
    You could insulate your walls with some of that sawdust.

    This build is so exciting!
    What's next?
     
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