prop tunnel hull jon boat?

Discussion in 'Boat Design' started by Doober, Dec 25, 2006.

  1. Doober
    Joined: Dec 2006
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    Location: PA

    Doober New Member

    I recently bought a 17' Alumacraft tunnel hull with a 75 HP Evinrude E-Tec (not a jet). I bought it to run in rivers and shallow water. The motor sits about 7-8'' below the boat which seems pretty low. I was wondering if anyone has a boat like this and would like to know how shallow they will run and how to take advantage of the tunnel even if the motor sits pretty low. I took it out and ran into about 6" of water and banged up the prop already. any info on the boat will help. Thanks.
  2. tja
    Joined: Sep 2004
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    Location: canton oh

    tja Senior Member


    You could get a jack plate to raze the motor straight up. To get all you can out of the jack plate you will also want a low water pickup for the lower unit. Alot of flats boats are setup this way. Or you colud sell the outboard and buy a mud motor. There are several manufacturers Gator Tail, Prodrive, Mud Buddy. Each brand will tell you they are the best. I'm familiar with all of them but don't have any direct exsperience. Any of these will do what you want. Sincerely, Tom.
  3. stonebreaker
    Joined: May 2006
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    Location: Shiloh, IL

    stonebreaker Senior Member

    He's right, you get going with the prop below the stern, then when you're up to speed you raise it up so that the bottom of the prop is even with the bottom of the boat. You need a jack plate to do this.
  4. RAWRF
    Joined: Dec 2006
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    Location: Eagle, Alaska

    RAWRF Junior Member

    I have a lot of experience with running shallow water, and I also used to have a tunnel hull. they are only slightly better than a regular hull/outboard prop setup, they won't work very well at all with a jet unit. A transom jack does help in fine tuning where the prop is in relation with the bottom of the boat. My advice would be to get a standard hull jon boat with a jet unit and stay as far away from those "mud motors" as possible if you are running in hard bottom or rocky streams. Those things were made for muddy swampy conditions, they are horrible at rocky bottom creeks/small rivers because they just bounce out of the water when the propeller rake hits bottom, and the force of this collision is enough to knock you out of the boat sometimes. If that isn't enough, they are extremely hard to steer even when the prop isn't bouncing along the bottom. I used to own a Go Devil, and I have operated similar brands, and I can't say enough bad about them, they suck. A person in my town has a nice shallow setup--they took the entire motor/drivetrain/intake/jet unit out of a PWC and put it in a std hull jon boat. If you put stainless steel aerator screen inside the intake grate you won't suck up anything bigger than sand. But once again, you can't hardly beat an outboard jet, and they are easy to put on and take off.
  5. LinedTheBlind
    Joined: Jan 2007
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    Location: Northern Virginia

    LinedTheBlind Junior Member

    I just sold my 15ft alumacraft tunnel jon. i liked the tunnel aspect of it, sold it for other reasons. first thing you need to do is make sure you have the right length motor on your boat; short shaft vs long shaft. with the correct length motor shaft, the cavitation plate on the motor (the horizontal plate above the prop) should be just about even with the top of the tunnel on the boat. so the tunnel allows you to effectively raise the motor the height of the tunnel, no more. my boat took a short shaft motor and i still had about 6-8 inches of motor hanging in the water. i could get into shallow areas easier than a non-tunnel, but still had to watch out for underwater objects.
  6. Jimboat
    Joined: Feb 2002
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    Location: Canada

    Jimboat Senior Member

    Doober - you should be able to optimize your motor height, so that you get better performance, improved access to shallow waters, and while ensuring adequate water cooling still to engine. Give me a call or PM me, and with more details of your hull design and setup, I can give some options.

  7. Village_Idiot
    Joined: Oct 2007
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    Location: USA

    Village_Idiot Senior Member

    Six inches is probably about the minimum you can run with your current setup while on plane. You'll probably need twelve inches of water to get up on plane.

    There are several things you can do to improve performance. As mentioned earlier, start with your cavitation plate even with, or slightly above, the top of the tunnel. Electric-hydraulic jackplates are almost a must-have to get the most out of your tunnel and motor.

    A jet won't work with your prop tunnel. A jet tunnel should be no more than three inches deep, otherwise you'll get too much cavitation. You could maybe fill in your tunnel with bondo or something to get it to three inches in depth, then try the jet motor. Avoid jets if you run in vegetation or very sandy (sugar sand or powder sand) rivers - they work best in rocky rivers.

    Run a cupped prop. Many owners report best results with a double-cupped four-blade stainless steel prop. The cupping helps the prop get a better bite on the aerated water coming through the tunnel and provides more thrust. Merc makes the best factory props; since you have a 'rude, you might find a Michigan Match or Quicksilver that will let you mount a Merc-style prop on your engine. If you got a little more money, get a custom prop from a good custom shop, like Baumann - they can set you up right, and you will have a much better prop than OEM can provide.

    If you are looking for more performance, a lot of folks drill a hole at the leading edge of the tunnel and run a hose from that hole to the back of the boat and over the transom. The idea is to let air into the tunnel, which might seem counter-intuitive, but it breaks the vacuum in the tunnel and lets the boat jump up on plane quicker and run faster at WOT. However, if you do this, you will really need a cupped prop, maybe even a semi-cleaver surface-piercing prop. Do a search here for more info.

    Many folks (and more and more manufacturers) put floatation pods on the stern to offset the loss of buoyancy created by the tunnel. The pods also increase the effective running plane of the boat, thus improving efficiency. You might gain an inch or two advantage while running in shallow water.

    Trim tabs are also nice to have, but can be a maintenance issue if you run rivers with lots of obstructions. They work on little boats just as well as, or even better than, on larger boats.

    My experience with mud motors is that they don't provide the same performance as an equivalent HP outboard. For example, a 25hp outboard will give you better performance than a 25hp mud motor, especially if pushing a load. The mud motor props seem to wear quickly also, and decrease performance further. They are good for getting around in muddy swamps, that's about it.
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