Questions on keel design for jet drive tunnel hull?

Discussion in 'Jet Drives' started by DSR, Jul 4, 2017.

  1. DSR
    Joined: Mar 2017
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    Location: Michigan

    DSR Junior Member

    Sorry that I just caught up with your reply Jim.
    Jeff sounds like just the person I need to talk to about this little project of mine. Is he pretty approachable if I email him? And would you be okay if I dropped your name to him?

    Thanks Jim!
  2. jim lee
    Joined: Feb 2007
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    Location: Anacortes, WA

    jim lee Senior Member

    Sure, I don't know if he would remember me, I was just the data guy for the team. But I always thought he was top notch. He was great to work with.

    -jim lee
  3. Ike Carlson
    Joined: Dec 2017
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    Location: Wisconsin

    Ike Carlson New Member

    I am new here, but I just built a boat with the dropped center you are talking about. It works very well.
    It is not a fancy boat, in that it was built for fishing the rivers near me, and on a rather small budget.
    It is a 14’ aluminum Richline with a 1988 seadoo hull section 6’ long and a little over a foot wide used for the “pad”. The engine is from a 1992 seadoo xp @60 hp.
    The two hulls are perfectly parallel, imaging setting a small hull inside a bigger one and then lowering the small hull 1” in relation to the bigger hull. The part left inside the large hull would be removed.

    The performance is beyond expectation. The only time the jet unit veltilates is when the intake section of the hull comes out of the water. It does extremely well in choppy conditions, only requiring reduced throttle when in waves that lift the intake out of the water. It will easily do 40 mph, which is at least as fast as the jet ski went before being used for this project. It turns well and tracks beautifully. I have put over 100 miles on the boat since building it, and am still surprised at how well it performs. The boat rides mostly on the pad and this cusions the ride in rough water. I have a few pictures I took during the build if you want to see what it looks like.

    To the OP, I would suggest eliminating the tunnels, since they will reduce primary buoyancy. You could replace them with veltilation channels to provide a cusion of air under the boat on either side of the pad that would reduce friction with the water. This would increase your top speed by a pretty good margin.
  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    What is your objective...pure theory research..or practical..and to what end?

  5. DSR
    Joined: Mar 2017
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    Location: Michigan

    DSR Junior Member

    Hi Ike. Welcome to the forum and thanks for sharing your build.

    Your Ridgeline sounds like a nice conversion that turned out great. It sounds similar to how a lot of these conversions are done, but you did it as I would have by making the pad longer and narrower than I've seen most do, which I believe works better for loading the pump and for better overall hull performance. I'd love to see photos of the conversion and how you did it.

    As for the tunnels in my design, they actually will provide less wetted surface area and water drag at higher speeds than the ventilation channels you suggest. The tunnels are set into the hull to provide aerodynamic lift at speed using WIG (wing in ground effect). As designed, the hull starts "trapping" air and generating aerodynamic lift at 48-50 mph. By ~70 mph the tunnels are generating enough force to lift the outer sponsons clear of the surface and the hull is just riding on the last couple feet of the center pod and drive extension.

    I have run the math on buoyancy for the boat and the static waterline is less than 1" higher than the original design at the transom and with the inboard conversion using the ski powertrain moving the COG forward, it sets the hull much more level in the water than the original outboard design the boat was based on.

    Thanks Ike!
    Last edited: Dec 3, 2017

  6. DSR
    Joined: Mar 2017
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    Location: Michigan

    DSR Junior Member

    Hi Ad Hoc, thanks for stopping by.

    As previously stated on my post:

    "This boat is a design / build exercise to build a unique, lightweight, fast 1-2 person toy for playing in calm protected inland waters (what I would call a lake racer)."

    So yes, I will be building it.

    And I may be a bit dense, but I am still not exactly sure as to what you and Baeckmo are referring to as "backwards engineering"?

    In response to Baeckmos post I also stated:

    "I'm not sure what you mean by "backwards engineering"? The design is based off of an existing low-deadrise warped bottom design for outboard power that I modified by adding the tunnels to create the current mod-VP bottom. The hull is also lengthened and the drive extension pod added to the transom to accommodate the engine and jet pump from the Honda PWC that I've acquired for the build, and provide the best static and dynamic C of G within the hull when complete and loaded. And now I am currently working on the keel design to best utilize the Yamaha PWC cast aluminum intake duct that I'm adapting the Honda pump to and for the expected performance. Based on all the research I've done, this type of conversion is kind of a black hole as far as info is concerned.

    I've run simulations on the design and it shows planing before 20 mph, at 45 mph (before transition into aerodynamic lift), WL is 6.25' with total drag of 616 lbs. The aerodynamic lift transition zone starts at 48-50 mph and the design shows decent stability past 86 mph with the center pod WL at 2.1' (another reason to move the pump intake as far aft of the transom as I did) and total drag at 348 lbs. At this speed it also shows aerodynamic lift at 278 lbs., or 32% of total lift."

    If you could explain your view regarding that statement to me I would appreciate it.

    Thanks Ad Hoc,
    Last edited: Dec 7, 2017
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