Opti-Step Drag Reduction

Discussion in 'Powerboats' started by Grant Nelson, Feb 16, 2010.

  1. Grant Nelson
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    Grant Nelson Senior Member

  2. mark775

    mark775 Guest

    I didn't read the whole thing, Grant, but if they don't even understand why the steps work they have little chance of getting them right... "The boat pushes down the water, so that when the water comes back up it lifts the boat - kenetic lift"- is that the gist? One step is "unstable" two are "inherently stable" - what, every boat out there with one step is like a teeter-totter or something?
    They are right in the need for "advanced engineering"... Steps are for people who do tank testing and are generally for a fairly narrow range of trim. Much sales hype here.
     
  3. daiquiri
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    daiquiri Engineering and Design

    There is a lots of "homemade" hydrodynamics with some fancy new terminology on that site ("potential kinetic lift" - what is that?).
    As Mark775 has pointed out, it is not really clear if they understand how the steps work.
     
  4. jehardiman
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    jehardiman Senior Member

    I wonder if Midnight boats has heard of D'Alembert's paradox?
     
  5. Easy Rider
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    Easy Rider Senior Member

    One thing I liked about it was the fact that the steps were set at an angle so as to to be at (roughly) 90 degrees to the water flow. Most stepped bottoms nowdays are on deep V types and the water flow is (to a very significant degree) from keel to chine. Otherwise it seems a bunch of bull roar.

    Easy
     
  6. u4ea32
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    u4ea32 Senior Member

    I agree with Easy on this.

    They really don't get it, they are just talking. Blah blah blah. Nothing is correct in their explanation.

    A planing surface is a supercavitating foil, where no lift is provided by low pressure flow over the "top" (like on a typical airplane wing), instead all the lift is provided by the high pressure flow beneath. This is also how high speed propellors work -- you basically can't keep flow attached at over about 50 to 70 knots, as cavitation occurs (the pressure drops lower than the vapor pressure in the water, so "air" appears in the low pressure side).

    All foils provide best lift/drag at some angle of attack. A planing surface generates the best lift over drag at about 5 degrees angle of attack. However, running at best angle of attack also leads to a very rough ride. Since the shape of the L/D curve is fairly flat, you don't lose much L/D by going to 3 degrees, yet the ride is far better (and control is better).

    Since the bottom is a foil, and the higher aspect ratio the better L/D, you really want a wide, short planing surface for best L/D. That's why stepped bottoms work -- instead of one long low aspect ratio foil, you have two with twice the aspect ratio, or more.

    In fact, the fastest way is to shingle the bottom, with lots and lots of steps. Think a lapstrake bottom, with the boards athwartship.

    So why don't we see this kind of bottom? Well, we do sometimes, and they porpoise like crazy!

    Hence having just one step (or two close together) at the CG. The best handing stepped boats have always had this layout, because it ALLOWs the boat to pivot about the CG, instead of PREVENTING it. If the boat can't rock, it jumps: it porpoises.

    Now the reason Midnight is not so horrible that they stop building boats is because in fact their steps are quite small, so they don't have much effect.

    There are many pictures of high speed powerboats on the net. Download some that give you a good side shot, then use Illustrator to draw the horizontal line and compute the angle of attack. The world speed record boats always show an angle of attack built into the bottom (from transom to step) of about 1.5 degrees, and the boat running at another 1.5 degrees, for a total angle of attack of 3 degrees -- right where the L/D curve gets pretty flat, while still providing sufficiently smooth ride to maintain control.

    You can confirm the CG is at the step by looking at the boat on the trailer.
     
  7. TollyWally
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    TollyWally Senior Member

    David,
    Would you mind posting a picture that has been altered with Illustratator in the fashion you mentioned? I found your explanation about half of the attack angle being in the hull and half in the trim fascinating. Simple stuff I suppose but I had never quite looked at it like that before.
     
  8. dand0_4
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    dand0_4 Junior Member

    I have personally seen these boats in action down here at South Padre Island, Tx. The border patrol and Homeland security recently acquired some. I was driving 60 mph across the causeway to the Island and the BP was chasing a smaller coastie boat and easily passed me up. Both boats then began making hairpin turns, the BP boat having to do it while jumping the coasties wake. The larger boat was able to hang with the smaller boat very well. These boats also, have to chase drug smugglers in the middle of the night in very choppy, confused 4-6 ft. seas at speeds up to 70 mph. I dont think these people would be risking there lives in a boat that wasnt well engineered. Maybe the steps dont do much but it was an impressive display.
     
  9. Grant Nelson
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    Grant Nelson Senior Member

    Thanks for all your great replies everybody... most of what is posted makes sense to me, and matches my expectations. Nothing wrong with steps, it may still be more an art than a science on how best to apply them, but there are some general excepted aspects, like not to wide a gap, only one, maybe two, near the CG, keep the edge perpendicular to the water flow, etc.
     

  10. marshmat
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    marshmat Senior Member

    I've read some pretty favourable reviews of the Midnight Express line. Never seen one around here, though. US border patrol seems to like them for chasing smugglers.

    The site has some of the worst, most ill-informed marketing copy I've read in a while. But the designers do seem to know what they're doing, and the stepped hull does seem to work well, even if the marketing team has no idea how or why.
     
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