Craigcat Cat, options for shallowest Draft

Discussion in 'Hydrodynamics and Aerodynamics' started by Steve340, Jun 20, 2018.

  1. Steve340
    Joined: Jun 2018
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    Location: Key West

    Steve340 Junior Member

    Hello, I appreciate the board. I'm an aircraft guy and I am trying to do the impossible. I have Craigcats and I live in the Florida Keys. A billion square miles of water and most of it is 15". So my impossible goal is for the boat is to draft 12". Here is the issue, it has Evinrude ETec 30 horse engines, by design the anti cavitation plate is 5 to 7 inches below the bottom plane of the pontoons this gives me 18" of draft and even when the pontoons are on plane, the engine plows the water creating much drag and down force on the bow.After much discussion with various boat professionals I feel this is to keep the boat slow, stable and safe. Sadly it conflicts with my need of a 12" draft. Although the boats are thought to go 30 Mph, I have only had them go 26 top speed. For the shallow version I want, speed is not the goal, so even 10-15 MPH would be fine. ( Backwater Tours type thing ) I have considered raising the engine to put the anti cavitation plate just below the bottom of the pontoon, this might give me a draft of 12 to 14" I have considered a smaller outboard, and modifying the mount plate to have the propeller as high as possible between the pontoons, obviously requiring a rudder system similar to a Sea Plane, however I could create it using the same steering arm the boat comes with. I have might give me 6 to 8"searched for an inexpensive surface drive prop system, which might give me a 6 to 8" draft. I have also considered adapting a Waverunner drivetrain/Jet Drive to the boat with would probably give me a 4 to 6 inch draft. The boat floats as is in 4 inches of water. The knowledge I lack is fluid dynamics and how the pontoons displace water, at what rate vs the requirements of the throughput of water volume to propel the boat. Anyhow I know of you guys are professionals and this probably sounds like idiocy to you. I have been an aircraft tech for 37 years and am also a structures/composites person, so I am not totally clueless. For the record, as sold these boats are terrific, every customer we have had LOVE them. I simply need to make one run as shallow as humanly possible. Thank you

    Steve
     
  2. Doug Lord
    Joined: May 2009
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    Location: Melbourne, Florida

    Doug Lord Flight Ready

    I've seen those boats around here-pretty cool! Could you outfit one with a scaled down airboat engine and prop? That would be minimum draft for sure-sort of a mini airboat?
     
  3. Steve340
    Joined: Jun 2018
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    Location: Key West

    Steve340 Junior Member

    Actually thought about that but being the keys Airboats are strictly prohibited in most areas due to the wildlife conservation. A jet drive wave runner drivetrain in a wave runner is prohibited, but in a boat, it would be working around the law. :)
     
  4. jorgepease
    Joined: Feb 2012
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    Location: Florida

    jorgepease Senior Member

    How much do those wave runner jet systems weigh... They are pretty powerful for what you want. Maybe just get a jet outboard and be done with it.
     
  5. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Forget the jet drives. They suck bottom in shallow water. Most jetski places tell you to shut down in 2' or less water and walk it in to avoid sucking debris. Suck a rock or shell and you can stop the impeller. Gravel ruins wear rings.

    You need lift and you need jackplates and and power trim and perhaps a different or modified hull (sorry).

    The flats stalker series of boats over on bateau.com specialize in running minimal draft. Adding strakes helps a bit as well.

    How much rocker is in that hull?

    Post over on bateau. All Florida guys.
     
  6. jorgepease
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    jorgepease Senior Member

    It's a lot of hard bottom in the keys, I just sold my house there... I think an outboard jet will work but you could use a fixed bracket to raise the motor then get your prop triple cupped by Baumman Marine in Houston. Don't cup it in the keys, I tried, they don't know what they are doing. You might find a way to pressurize the water around the prop but it won't be a small project. If you lift, you might also need to attach a transom pickup for your cooling water.
     
  7. Steve340
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    Location: Key West

    Steve340 Junior Member

    The smallest outboard jet weighs about 60 lbs more than the etec 30. It would put the CG too far back. That was actually the first thought I had because it was the simplest. Depending on the wave runner the drivetrain is approx 140 to 160 lbs, but I can to some extent move it to keep the CG. I have actually been doing a little research into surface drive props however most are assembly’s not an add on for an existing outboard. And yes a duct would be needed for the water intake. I’m trying not to get carried away but an electrical water pump with a pickup on the side might even be an option.

    I have really looked into a lot of options and I realize I by no means am the first to attempt this but 18” draft is really limiting me.

    D2B83543-0BA3-47CA-9372-90B354DD536B.jpeg E4B1F334-4213-4993-9740-E6F277D94B5F.jpeg DC68DE7E-1617-4043-8DFA-8B74CC714D62.jpeg
     
  8. Steve340
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    Location: Key West

    Steve340 Junior Member

    As you can see from the above photo #2. If I created a water rudder system, how high could I bring the prop to run shallow, understanding speed is not a consideration.
     
  9. Steve340
    Joined: Jun 2018
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    Location: Key West

    Steve340 Junior Member


    That’s why I was asking about the fluid dynamics, the volume of water needed for operation vs the displacement to center from the pontoons. I don’t need to run fast so to some degree it’s a secondary consideration as long as the engine stays cool.

    Thanks

    Steve
     
  10. philSweet
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    A friend and I have built one of those from pieces and it hauled ***. It would fly entirely airborne with just the prop in the water. The key is to tilt/tuck the motor way down under to keep the nose down. A carefully designed Bimini top also is an absolute necessary. If the Bimini isn't as low-drag as possible, it will pitch you back and add several inches to draft. Build an additional 5-7 degree wedge for the outboard mount (about 1 in 12). Weight of motor, fuel, passengers, and cargo needs to be under 600 lbs in order to scoot. Otherwise, you are stuck and run deep. An '89 Merc 25 2-stroke will save you 50 lbs, and they can be seriously tuned for more power. This was in Key Largo, by the way. I lived there for 10 years.

    In barge mode, there isn't much you can do. They just drag their butt around something awful.

    One last thing, you need to put a skeg in front of the ob to stop the leg from scooping massive amounts of water when it's kicked under 20 degrees or so. This needs to be about 60% as thick as the leg, and wedged at about 3 degrees. I tried several different ones, and a 3' 2 x 10 cut to a wedge worked best.

    Craig Cat https://www.boatdesign.net/threads/craig-cat.18736/#post-559091

    Oh, and buy some new nylon bolts. The big ones that hold the thing together. There are six if I remember correctly. They get tired after 10 years of wave pounding.
     
  11. Steve340
    Joined: Jun 2018
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    Steve340 Junior Member

    Great information. ! For the rental boats we don't want them faster lol. I have an opportunity to rent a version setup for shallow draft if it can be done. Although I wonder exactly how fast they go if set up like yours was. But stuffing a wave at 50 would probably be catastrophic. Lol

    Thanks man

    Steve
     
  12. philSweet
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Yes, running in Hawk channel in 3' seas on a poker run, We were able to hold our own against a little 16' Donzi. But we had to just clip the top of each wave and not stuff any. I stood it right up on it's nose a few times, but always came out right side up. There was another thread here with a great photo, but link rot got it. I think he had a 75 on it, and padded suspension seats. He'd customized the motor mount as well.
     
  13. jorgepease
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    jorgepease Senior Member

    since you aren't worried about speed, then get a very heavy cupped prop and just lift your outboard till the top blade of prop is just under water. You don't need an electric water pump check Bobs nose cones - Transom mounted water pickup. My flats boat the prop tucked into a tunnel completely above the surface of surrounding water, I could run through 2 inches of water. When you get that shallow you pressurize the water, the boat flattens out, lifts and heels like it hydroplanes. Your hull is not made to do that though so
     
  14. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Then there are the dragon tail propulsion units. One of the brand names that comes to mind is ; Mud Buddy. There are no doubt other brands. Those things will operate in almost no water at all. Whether your charter people could be trusted with such an evil device is up for discussion.

    I have sailed Florida Bay with a Bolger Black Skimmer. Great fun, skinny water, and a gazillion mangrove islands A nifty place to go if you like the near privacy of a lot of square miles of nothingness except thin water.
     

  15. RobtH
    Joined: Feb 2019
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    Location: Houston

    RobtH New Member

    I have a CraigCat, and was using in shallow water. They are fun driving forward, if a potentially little wet.

    Several issues with this hull. Originally designed for electric power and low weight on stern, forced placement of 30 hp outboard in toward center, also reduces steering moment to safer levels at high speed (I made 29 mph once, 26-28 more commonly before the hulls warped; this is not a big issue, as this speed is only desirable in calm water. chop and waves means slower speed for comfort). With current design, one can not reverse with much speed at all, hull is subject to tail diving. Clearing prop fouling requires getting off and getting wet, as combined weight of human pilot and outboard will dunk the stern, risking turnover (CraigCat manual also warns against both of these occurrences).

    Current outboard placement means can't steer effectively with engine tilted up; prop will chew into pontoon. Having engine tilted down means draft is as much as any outboard motor system, water cooling means having to suck water up to cool engine. Hull issues easily solved by adding flotation on top of hull on stern, but hull to outboard drive interference remains problem with engine tilted up or raised for bottom clearance, unless engine is moved further astern, compounding the weight distribution and stern free-board issues described earlier.

    Air cooled engine and half cavitated prop like Go-Devil (mud boat setup, other mud boat brands elsewhere mentioned in other posts) would theoretically solve these issues, but I suspect that drive setup is too heavy for CraigCat hull. Paddle wheel drive is also very efficient for low speed drive, and not very subject to damage with bottom contact, also relatively quiet, if the engine system is also quiet.

    Chewing into hull with prop causes long term issues; Skin (polyethylene) is pretty easy to repair with hot air welding, and closed cell foam center short term is not bad. Problem seems to be that the urethane foam compound is subject to water chemical breakdown, causing pontoons to eventually crumple up, since the foam is also structural element. If pontoon is never punctured, should be good for very long time.
     
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