Stepped hulls

Discussion in 'Boat Design' started by Vibtor, May 1, 2005.

  1. Jimboat
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    Jimboat Senior Member

  2. cyclops
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    cyclops Senior Member

    Fuel and oil consumption increased. The acceleration dropped off too much to be cost effective. The acceleration bit seems contrary to the first part.--------------------------------- Any naval hull people available to explain the seeming error?
     
  3. Frans X L
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    Frans X L Junior Member

    the reason for a step in a seaplane float is to control the position of the centre of pressure of the float relative to the centre of lift of the wing.
    in an unstepped float the centre of pressure of the float will be behind the centre of lift of the wing and the plane will not be able to nose up and take off.

    the use of steps in a racing boat is best used to promote pitch stability, using 1 large step. in a catamaran think of the boat running on four water skis. look at the us unlimited hydroplanes which run on the tow front sponsons and the rear of the centre hull. they are running on 3 points.

    I have also witnessed towing circulating tunnel test on a hull with one step and air injection and the drag reduction is quite spectacular. only problem is you need fences on the sides to prevent the bubbles from escaping. so this is strictly a smooth water item.

    the other very real issue is safety. having pipes pumping air through holes in the bottom behind the steps can cause the boat to sink if the pipes fracture.

    the small multi steps used on monohulls and cats will be tricky to get to work. as far as I can see if the hull has any vee in the bottom any entrained air will get swept out sideways and not travel very far down the hull. the small steps cant aid pitch stability.
     
  4. cyclops
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    cyclops Senior Member

    About the pumped air holes. I know of 1 case where the exhaust gases are run into the same area. No feedback on results.
     
  5. Cary
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    Cary Junior Member

    By the way steps can be considered as waves, most of those computer programs, calculate in smooth water, I honestly don't see much of an advantage
     
  6. EPClement
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    EPClement Junior Member

    The very interesting case cited by Cyclops of significantly increasing the speed of a planing boat by adding steps illustrates the potential that exists for improving the performance of that type of boat. The resistance of a conventional planing boat is very high in comparison to what is achievable. The chief reason for this is that the amount of the wetted bottom area of a conventional boat when planing is several times as large as the area necessary to support the weight of the boat. Accordingly the frictional resistance is several times as large as it need be, and the speed attainable is restricted by the unnecessarily high resistance. Adding steps (or a step) to the bottom can significantly reduce bottom wetted area and frictional resistance and therefore make it possible to attain a significantly higher speed with the same amount of power.
    The interesting topic of reducing planing boat resistance by incorporating steps has been extensively explored at the Navy’s David Taylor Model Basin. Both single-step and multiple-step models were tested (a model having seven steps was tested). A conclusion from those tests was that larger reductions in wetted bottom area, and therefore in resistance, could be achieved with a single-step hull form than with a multiple-step hull form. An example of the result that can be achieved with a single step can be seen on the web site: www.dynaplaneboat.com
    A comparison is shown there of the wetted areas when planing, for a conventional boat, and for an efficient single-step boat. The amount of the wetted bottom area for the stepped design is approximately 30% as much as it is for the conventional design. Accordingly the stepped design will have only 30% as much frictional resistance when running at planing speed. For the cases shown total resistance for the stepped design is approximately one half as much as it is for the conventional design.
     
  7. yipster
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    yipster designer

  8. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    All people are speaking about resistance vs speed, but what about propulsion?

    Are you sure that the best dynamic trim angle is 5 deg.?

    My own experience (most in military craft) is that with a hull shape ASD (Arrow System Design) you have an increase in wetted surface due to the 2 annexes in the after part, a very small trim angle over 40 knots, and the speed.....? Less speed...?

    The propellers are working in a parallel fluid and even if I have more resistance the gain in total efficiency is greater.

    Shortly, the most famous Naval Architect (for me) tested one of my boat (Star Naja ASD), "very interesting, but you never will reach the contract speed of 40 knots in tropical condition.."

    All 12 boats 14 m length, 2 x 435 Cat, Trimax, Rolla screws and 11 t displacement reached 40/41 knots in trop. cond.

    All this only to say that resistance is not perhaps the most important parameter for high speed boat and stepped boat pictures show hull flying in air.... or jumping in water....
     
  9. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    This is the 40 knots boat named "Star Naja ASD"
     

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  10. wdnboatbuilder
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    wdnboatbuilder Senior Member

    You know guys THis is my third day on this site and alot of these thoughts have rolled around in my head. I have attended boat school know some about design ( how to draw and figure calcs, what to do with most of them i'm still learning) but I really have enjoyed these discussions on this site.
    Now a comment on the step, I agree that steps in a hull works, but I was watching a show on history or discovery channels and they were talking about ships sinking if you get a gases erupting from the bottom of the ocean. A they were in a test tank and yes introducing the amount of air / gases the model sank. well if this is true then when you introduce air from a step would it not make that water, air mixture soft and that would allow the transom to sink instead of lifting or am I really out there?
     
  11. cyclops
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    cyclops Senior Member

    I am sorry everone, I forgot to give the type of boat that had the 6 steps added. It was a WW II PTB, A ELCO 80'. 1944 vintage. John F. Kennedy type.
     
  12. marshmat
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    marshmat Senior Member

    Wdn, the gas bubble phenomenon you mention is accurate for displacement hulls. Gas displacing the water under and around the boat reduces the effective buoyancy force on the boat in the area near the bubble.
    For planing hulls, and virtually all stepped hulls are planing types, the boat is held up by forces resulting from its forward motion. At rest it floats like any other, but at speed (where the step comes into play) it is not displacing water so much as sliding overtop of it. So when you let air underneath, the air part of the air/water mixture is simply compressed a bit, and the boat continues to ride on the same dynamic lift that would hold it up without the air.
     
  13. cyclops
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    cyclops Senior Member

    And the extreme examples include 3 point racing hydroplanes . All the air trapped in one big step.
     
  14. Jimboat
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    Jimboat Senior Member

    Stepped Hull Design

    wdnboatbuilder - the gas/water mixture occurance does, indeed, work out as you suggest to make the mixture "soft". So there is really no advantage to air injection - and possibly a detirment. The hydrodynamic force generated by the planing motion of a performance hull is influenced by the density/viscosity of the water. (even salt vs. fresh water is different). The dynamic Lift forces generated are lesser if air is mixed with water, but unfortunately the drag is not reduced similarly. Overall result is degraded hydrodynamic lift. This is even (much) more enhanced if bouyancy is a contributor to overall lift. But stepped design for planing hulls can still generate performance improvements - for other reasons.

    Re: steps in general, performance can be enhanced, however step design is particularly tricky to make work at all velocity of hull's intended speed range.

    Check out this posting in response to question on Stepped Hull Design on other forum.
     

  15. wdnboatbuilder
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    wdnboatbuilder Senior Member

    I really hate the home building business. I got to get back into building boats. I really miss building boats.
     
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