Seaplane Stepped Hull

Discussion in 'Boat Design' started by Eppler, Jan 21, 2008.

  1. Eppler
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    Eppler New Member


    I'm working on a student project which aim in the development of a new seaplane. As only few recent litterature is available I would like to gather your feedback on the following issue:

    1- Considering a stepped hull, what would be a good sternpost angle? Some papers from the 50s recommend an angle of 7 degrees or more. How comes none of the recent stepped boat have a sternpost angle? What is the advantage/disadvantage of a sternpost angle?

    2- One of the biggest issue with a seaplane is the need to put the engines way above the WTL, which in turn induces a high "nose-down" moment implying a long pre-planing period. How would you optimize the hull so as to tilt on the step much faster?

    3- Any general thoughts regarding seaplanes?

    Many thanks for your suggestions.

  2. diagram
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    diagram Junior Member

    You might look into information that SNAME may have on file. Also, I believe, Daniel Savitsky did a lot of testing on this subject at Stevens Institute. Their may be some info available. You could try calling one of the manufacturers and see if their techs might be in a talkative mood.
    Good luck.
  3. Eppler
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    Eppler New Member

    Thanks for the info,
    Had a look at SNAME website, is there a free section or am I suppose to pay the 300$ to access the papers?
    Regarding Savisky he did wrote alot about planning vessels but not really on seaplanes. To be honnest the only papers I found were the NACA series dated post world war II. Thats old!
    My concern is how to minimize pre-planning resistance in order to accelerate quickly and jump on my planning area. Once on the planning section of the craft resistance is almost secondary.
  4. Tim B
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    Tim B Senior Member

    There is probably a lot of info to be found in the NACA papers.

    There was quite a comprehensive book on seaplane float design, but it has long since gone out of print. It might be worth your while looking around second-hand or antequarian book shops.

    Tim B.
  5. Eppler
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    Eppler New Member

    NACA papers are interesting, no doubt about it. But I found the method relatively "light". Basically it all about water tank testing with a variation of afterbody length, step depth and another parameter I forgot.
    The paper give you a good basis on seaplanes design, if you look around you won't see any major evolution in seaplane design since the 50s, while speedboat have undoubtedly. The Alpha-Z for example: I understand the use of big strips all along the hull to provide additional lift and control @ high Froude #. But Im confused about sternpost angle which are close to 0 deg while its recommended to have a 7° angle for seaplanes (based on the NACA papers). Also the step cutout is not straight but rounded (looking below), what would be a possible reason?

    Thanks for your help!
  6. westlawn5554X
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    westlawn5554X STUDENT

    Yes, is there any blue print for old version of seaplane avaliable somewhere? Or a simple GEM plane that can carry dozen of person for village relief work? Thanks
  7. TollyWally
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    TollyWally Senior Member

    I used to fish up north in AK and several of my buddies had floats for thier supercubs. Being boat obsessed I use to really check out the float hulls. They were very interesting.

    The main body had quite a bit of rocker and was a moderate V. The aft section that was maybe 15 % of the length or so was set at an angle and when the boat was just floating ot taxiing stuck completely out of the water. The aft section was pretty flat and was stepped maybe 4 or 5 inches above the rockered V section. There were some little chine angle stuck on to keep water from riding up the float hull.

    When the plane was ready to take off they would be just putting along. When the pilot nails it the plane rotates back on the rocker and the aft flat part is now running pretty level. When the plane gets going fast enough of course it pops into the air and you're flying.

    Now my theory about this whole thing, and it is just a theory from observation, is that the main float has a lot of rocker to handle the varying loads and not have a tendency to dig in the bow or have excessive drag. When you nail it the stepped flat part that was just sticking in the air is now
    bearing much of the load and operating like a hydroplane to help minimize drag and help the plane take off.

    It was very interesting watching the planes land. When they first touch down the wake is like a little roostertail way way way in back of the plane. As the plane slows and tips forward off the stepped aft section the wake comes shooting forward to catch up with the plane. It looks like it is being chased by a high speed shark!

    Anyway I hope this helps. Incidently these floats really sort of looked like they were empirically designed. Form following function following the easy way the metal went together. Guys would swap floats or buy a set that sort of fitted and make it work. If you know what I mean. I will be interested to see what some of the more experienced guys here post.
    Last edited: Jan 21, 2008
  8. Tug
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    Tug Junior Member

    The step should be located slightly behind the center of gravity...
    Otherwise the aircraft trys to fly before it has enough speed...
  9. RatliffFranklin

    RatliffFranklin Previous Member

  10. Eppler
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    Eppler New Member


    Yes that could be an alternative... except that you have to land/TO from an "oil sea".
  11. FranklinRatliff

    FranklinRatliff Previous Member

  12. diagram
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    diagram Junior Member


    Sorry for late response..... I believe Savitsky did quite a bit of tank testing at Stevens Inst. for seaplanes. They may have results. I spoke with him once years ago and I believe that is what he told us -this woulld be circa WWII testing. As you noted, this information would be dated but does not preclude it's valididty, I would think. I have always thought that a lot his work on seaplanes was the basis of much of what we see for V-hulls and consiquent papers regarding such. Good luck with your search.

  13. Lin Olen
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    Lin Olen Junior Member

    Hello Eppler, My experiments point to a deep fine entry progressing to a moderate V at the tail, thn thik sea eagle with its normally large dihedral in flight. A flying wing using the wing as the floatig hull. Low drag, sea kindly, short take-off. Wish I was a bit younger. Lin Olen. Tuned hulls and rotary sails. Cheers.
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