# Sea Sled madness. It’s in my brain.

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

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1. Joined: May 2023
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### Geno67Clueless Member

The average of 24" and 30" is 27" (length range). Times 1.3889 (the proportion between 25 feet and 18 feet) is 37.5 inches or 0.95 meters if my very basic math is any good.

33.3" or 0.85m on the low end and 41.7" or 1.06m on the high end.

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### fallguySenior Member

I'm not a flow expert, but the easiest thing I think you would try is tipping an angle off the hull.

given a 12% transom, the angle would direct flow downward at 12 degrees and the stagnation might be less

of course @baeckmo ought to remark, but this is the simplest solution to test; it would push the bow down some is all, but it probably only needs to be 8-12" wide, whether it'll be enough is the question, but super easy try

my brain says 2" or so, but you can measure to get it best

and you could try it with wood even.. at zero cost in time or money

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### fallguySenior Member

The other thing, of course, is just tweaking the depth some...the prop at a full 5" lower has good water, but it looks really far too low, a manual jackplate is certainly not inexpensive ito time because you have to manage setback, but if you only needed to be 3" down, then some percentage of speed losses will be due to the depth, too.

And, messing with setback might affect any efforts on the boat, thus I feel the need to remark. Sorry, but it would blow to do all that work and decide you want to come up 2.5" and then for the setback to be 4-8" for that change and the other stuff be wrong..

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### Geno67Clueless Member

Do you know what vortex generators are?

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### DogCavalrySoy Soylent Green: I can't believe it's not people

Indeed. They were critical to the overspeed protection system on the wind turbine I invented.

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### DogCavalrySoy Soylent Green: I can't believe it's not people

So many elements to manage. I think a jackplate is in Serenity's future.

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7. Joined: May 2023
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### Geno67Clueless Member

Throw a bunch on the hull turned backward. Rip the surface tension loose. Would need to be in a random pattern. A consistent pattern would increase drag. They do seemingly impossible stuff on aircraft.

A jack plate would do wonders. Moving the engine back gets it farther from the yucked up flow and you could adjust down or up as needed. I'm going to have one on mine - just a simple manual model with a pretty good offset.

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### DogCavalrySoy Soylent Green: I can't believe it's not people

With the terrible condition of the transom/hull transitions, there's turbulence galore already.

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### DogCavalrySoy Soylent Green: I can't believe it's not people

So, configuration and dimensions of the diverter are the next questions.

Is there a minimum drag form? Or a form that has more drag, but reduces outboard drag more? An elongated tetrahedron form? Or a trapezoidal section? Maybe some sigmoid curves...
Flat bottom, or sharp bottom, etc.

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### baeckmoHydrodynamics

Yes, you copy the forward 25 % of the extension profile, stretch it to roughly 450 mm length at its top level, and 380 to 400 mm at bottom, so that you get a backslope at the leading edge (lets seaweed slide off).

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11. Joined: May 2023
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### Geno67Clueless Member

I would keep the elongated tetrahedron with straight edges from one end to the other as we know from previous builds that the form works for it's intended purpose. The rear top to bottom distance is fixed - it should equal the vertical distance from bottom rear of hull to AV plate.

12. Joined: May 2023
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### Geno67Clueless Member

I don't remember if your larger diameter prop is cupped or double cupped? That also plays a big role in ventilation.

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### baeckmoHydrodynamics

The tetrahedron has higher specific resistance than the NACA nose, and to "shadow" the leg extension it will have a considerably larger transverse area. All in all, it is not the best shape.

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### DogCavalrySoy Soylent Green: I can't believe it's not people

Got it. Roughly parabolic. Flat bottom, rather than solid of rotation.
Edit: that's NACA 0007. I can find an exact form for that online.

Loa as stated above. Diameter equal to max dia of extension. Depth equal to depth of AV plate below transom. That could easily be solid enough to support Serenity on hard ground.

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### DogCavalrySoy Soylent Green: I can't believe it's not people

Yikes. I think I'll just get a good image, blow it up and crop it in Paint, print it and trace it.

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