Help with keel design on my mini jet mod-VP project?

Discussion in 'Boat Design' started by DSR, Jul 22, 2017.

  1. DSR
    Joined: Mar 2017
    Posts: 70
    Likes: 8, Points: 18, Legacy Rep: 10
    Location: Michigan

    DSR Junior Member

    Hi everyone,

    I posted this question in the Jet Drive section a while back but haven't received any feedback so I'm hoping that re-posting it here might work better?.....

    I'm finishing up the design details on a PWC-powered mini mod-VP style tunnel hull and I want to add a "drop keel" to lower the jet pump intake and keep the pump loaded at higher speeds, but I'm having a lot of trouble obtaining any info on the correct design for this. This is the last piece of the puzzle and I was hoping that the wealth of knowledge here may be able give some insight on this?
    I've previously contacted a couple companies that build similar types of designs but that's been basically an exercise in frustation. I'm ready to start building as soon as I can nail down the correct design for this final detail and any and all help / feedback would be greatly appreciated!!

    I'm not sure what information is required for feedback but I can provide any specs needed.
    To start off, the hull length is 12' 6" from bow to transom with the center pod extended 12" aft of the transom to form a drive extension for the jet drive and the center pod is 12" wide, The keel centerline is 9/16" below the bottom edges of the sponsons at the transom with a very shallow deadrise of 2 deg. at the transom. The bottom design is based off a low-deadrise warped bottom design and the deadrise increases going forward from the transom. Going forward from the transom, the keel stays level for 74" and the chines are angled so height from the keel to the bottom edges of the sponsons also increases forward of the transom. I've attached the Linesplan to show the relationship of these components in the design along with some views of the model to hopefully help.
    The intake for the pump is flat with a large mount flange around the intake opening so I'm kinda relegated to a flat keel, which have been proven to work on the full-size jet tunnels. The intake opening is 15" from the lead-in to the center of the front edge of the shoe assembly and 6.5" wide and tapering as it goes back to the shoe. The lead-in is also 21" forward of the rear edge of the drive extension. I'm also planning to incorporate "intake fins" on both sides of the intake.

    I did create a couple different designs for the drop keel and I'll include them also to see if I could gather any insight as to whether either design would be viable for my hull?

    DSR mini jet mod-VP linesplan.jpg
    DSR Jet TNT white3.jpg


    This is the first design that I started with: 11" wide by 5' 6" long with a 3/4" drop below the keel centerline. The bottom is level and flat from the rear edge to 39" forward with vertical chines and then gently tapers and slopes to a point at the original keel centerline in what resembles a "NACA" shape with the chines tipping at a ~45 deg. angle to soften the entry. I had some help on this version from my father, who is a retired mechanical engineer and a pretty smart cookie, but it's pretty unconventional from what I've seen being done......

    DSR Jet TNT drop keel1.jpg
    DSR Jet TNT drop keel2.jpg
    DSR Jet TNT drop keel5.jpg

    The second design that I came up with: 6' long with the same 3/4" drop and the chines are 11" wide where they attach to the bottom of the hull and angle in to 9" wide on the bottom, or pad of the drop keel. Level and flat for 39" forward and taper and slope to a point again, but with a much less aggressive, more rounded entry shape.....

    DSR Jet TNT drop keel V2 1.jpg
    DSR Jet TNT drop keel V2 2.jpg
    DSR Jet TNT drop keel V2 4.jpg

    At this moment, I'm leaning more towards this second design. Maybe also incorporate the sharp chines from design #1 in the rear portion for better directional stability? I've spent so much time and effort on this project I want to make sure that the design is as right as can be and the hull reacts as predictably and as safely as I can make it.

    Again, any help or feedback on this would be great.

    Thanks,
    Dave
     
    Last edited: Jul 22, 2017
  2. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    How do you plan to use the boat?
     
  3. DSR
    Joined: Mar 2017
    Posts: 70
    Likes: 8, Points: 18, Legacy Rep: 10
    Location: Michigan

    DSR Junior Member

    Hi Ondarvr,

    The primary focus of the design is what the performance boat community calls a "lake racer". High speeds in straight runs on protected inland waters (basically a drag race boat with more cruise range). Cruising speeds of 25-45 mph.

    Thanks
    Dave
     
  4. JSL
    Joined: Nov 2012
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    Location: Delta BC

    JSL Senior Member

    Haven't seen a general arrangement but this sounds like a PWC. Take a look at some of these- they work, fast, and safe.

    Your hull looks like it could? entrap air and jets don't like that.
     
    Last edited: Jul 23, 2017
  5. DSR
    Joined: Mar 2017
    Posts: 70
    Likes: 8, Points: 18, Legacy Rep: 10
    Location: Michigan

    DSR Junior Member

    Hi JSL,

    The boat actually will be PWC-powered with engine and pump from a 2004 Honda R12X Aquatrax that I procured for the project.
    I have looked at various well-performing PWCs, but with the PWCs being deep-V hulls with 20-24° of deadrise and either radiused or Delta pad keels, they're completely different animals from my air-entrapment tunnel hull using a 12" wide center pod with 2° deadrise.
    On this type of hull, the drop keel is used to correct the exact concern that you're asking about.
    The premise of the drop keel is to create an area on the keel that's to be the lowest area on the hull to 1) localize the pressure of the water lift on the hull to the area at, and just in front of the pump intake at speed to maintain solid water for the pump to load from, and 2) is shaped to help keep the cavitated boundary layer water from entering the pump, as I understand it.
    So, hence my question as to what shape and dimensions would be correct, or at least a good starting point, to make this happen, is essentially the reason for my posting here. With the amount of knowledge and experience involved on this forum, even a pointer to an individual, company or even software, that could provide direction on this would be a tremendous help.

    Thanks
    Dave
     
  6. ondarvr
    Joined: Dec 2005
    Posts: 1,963
    Likes: 142, Points: 63, Legacy Rep: 506
    Location: Monroe WA

    ondarvr Senior Member

    I'd just take a close look at PWC's and other jet powered boats, then simulate the design that appears to best suite your hull.

    It doesn't look like you'll need anything special.
     

  7. DSR
    Joined: Mar 2017
    Posts: 70
    Likes: 8, Points: 18, Legacy Rep: 10
    Location: Michigan

    DSR Junior Member

    And that's the problem I'm having and the reason for the post ondarvr.
    There's a lot of talk in the performance jet boat circles here on the net regarding the use of drop keels on these types of hulls but, after a ton of searching, I have yet to actually see any actual pics or diagrams of how these are actually dimensioned and shaped. I've even contacted a couple companies that do these boats (the full-sized 18-20 versions), and unfortunately, they weren't very helpful.
    If I was expecting 50-60 mph with this project I'd be all set, but when I did a performance analysis on it, this boat is capable of 80+ mph as designed with stock power. That makes me stop and want to be sure this bugger is as correct as I can make it when it's built, not just for performance, but more importantly, stability and safety also.

    Thanks ondarvr,
    Dave
     
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