Shallow Water Tunnel Design

Discussion in 'Propulsion' started by YK GEO, Oct 5, 2009.

  1. YK GEO
    Joined: Oct 2009
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    YK GEO Junior Member


    Thanks for the info. I was poking around the Mercury website, and it turns out the salesman and I both misunderstood the specs of the engine he sold me, so we're undoing the sale and looking at a possible deal on a larger one. No deception on the dealer's part, just an honest mistake - too late in the day, I guess.

    So my options are open again.

    As to the hull, if I go with the OB Jet, it looks like a 5 degree constant deadrise, about 17 ft overall, bottom 4 ft wide, essentially a narrow jonboat or scow form, except I'll bring the bow in narrower, since the wide bow hanging over space is just dead weight; plywood/glass construction as mentioned above; hull weight would be around 350 lb. A rough calculation would be about 50 sq ft + of planing surface, allowing for a tunnel. I've built several boats, so I know where I'm going with the construction. The appeal of the OB jet is simplicity compared to an inboard setup.

    I found some excellent info on OB jet tunnel design from some fellows in AK, so that aspect is okay.

    This probably looks narrow to modern eyes, but I've driven a typical MR scow, heavily built, 19-20 ft by 4 ft bottom, loaded 4-6 people plus gear, planing well at about 20 mph, pushed by a 20 HP prop OMC or Merc 2 stroke. They use the wide overhanging bow as a boarding step - run the boat nose on to the beach mud, then step off onto the grass.

    However, as I said, my options are open again, and after thinking about Baekmo's recommendations, the inboard tunnel/surface piercing prop approach has a lot of appeal. It's hard to believe that a ventilated prop can develop thrust, but I was impressed when I drove one (mud motor style). I've noodled around how to do the installation, and it looks pretty straightforward. SP cupped props are available from the mud motor mfg, I can do the machining and welding, and I've got a long winter to work on it. If I go this route, the boat would be the same, but possibly flat bottom.

    Thanks again for the info. Much appreciated.
  2. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    At one time there was shroud on the market that encased the lower unit of a standard OB and turned it into a poor mans jet. You could raise the motor several inches for shallow running. I think the name was Riffle Runner.

    As a kid I worked at local marines on the salt, they rented 16 or 17' foot wood boats for fishing, these were powered by 5 to 10hp OBs and performed very well. The problem with OB jets is they don't have much trust at low speed, from the feel, a prop seems to have more than double the thrust. Plus when they say you lose about 30% of the power with pump I think that's a very optimistic figure. I've used a prop and pump on the same hull and the performance is almost cut in half as far as load carrying capacity goes.

    Speed is what gets the hull up out of the water for shallow running, so I think you may need more HP than you think for what you plan to do.
  3. YK GEO
    Joined: Oct 2009
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    YK GEO Junior Member

    Thanks, Ondarvr

    Your comments re OB jet performance are very helpful. The only jet I've driven was a 105/70 on an 18 ft alum garvey with 4 people and no cargo, so of course it performed well.
  4. kach22i
    Joined: Feb 2005
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    kach22i Architect

    Some hovercraft can be quiet and fuel efficient.

    FYI, Re-post from another thread:

    Another poster said:
    Video link;
    The Great North to Alaska Hovercraft Adventure, Bryan's take
  5. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    Guess it can't hurt to reiterate,but I suggested this already and he's not interested in hover crafts.
  6. kach22i
    Joined: Feb 2005
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    kach22i Architect

    Yea, that happens but a little more exposure can't hurt.:)
  7. YK GEO
    Joined: Oct 2009
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    YK GEO Junior Member

    Hi, fellows

    After pondering all the advice from all and sundry, I've undone the underpowered OB jet purchase.

    Here's where I'm headed now.

    Power Unit: 35 HP surface piercing outboard. My son has one, which he was using this fall on mountain rivers in NT. He sometimes got further than the fellows with OBJ's, and didn't have to spend time sharpening impellers after sand/rock ingestion. I drove his rig last fall, and found the engine lugged a bit until rpm's reached the engine's max torque, after which it accelerated ok.

    I had some chat with a couple of SPO mfg, and the folks at Mudbuddy are building a 35 with a higher than normal reduction ratio, and their largest prop. That should take care of low speed thrust when my boat is heavy. When running light, the prop might not be able to absorb all of the engine's power, but that's no problem, since top end speed isn't the goal. Just throttle back, enjoy the ride, and save gas. (Typically $1 - 1.50/l around here).

    The engine model I'm getting is balanced so there is little downward force on the prop, the point being that when I get a rock strike it won't take a lot of force to lift the skeg. Since I will use tiller steering on the river, if I get into one of those "?!!***?" situations I can easily lift the prop up in a couple of seconds and bounce over whatever.

    My son's motor has the mfg's rock guard, which is just a heavy duty skeg. I'll build one a bit stronger. The stainless steel prop is quite durable, and again, I checked one of my son's props that took a solid rock, with only a tiny dent to show for the encounter. The belt drive gives some shock cushioning for any prop strikes. So anyway, that's the power plant.

    Hull: The hull I'm designing is sort of a junior version of a flat bottomed Pacific or Oregon dory, but with a greater L/B ratio. This will provide roughly the same shallow water performance as the river boat hull type I'm familiar with, and pound less on lake waves. I may work some deadrise into the first few feet to help in this regard. The engine will be mounted in a notch, 12-18 inches ahead of the stern to provide some static boyancy and planing surface where it's needed most. A shallow tunnel (3-4 inches high) will feed water to the prop. Watertight flotation chambers in the bow and stern, but otherwise an open hull to keep weight down. Possibly a removeable cuddy for lake expeditions.

    Hull and engine should total 6-700 lb. At a maximum gross weight of 1500 lb, and planing area of about 50 ft sq, I'll have loading of 30 lb/ft sq and 43 lb/HP, so gross weight performance should be adequate, but not wonderful. If it's hesitant to plane when heavy on deep water I can just steer into the shallows to get it up. When going up rivers, I sneak along the shore to take advantage of slower water and back eddies. It's easy enough to read the water surface and judge the depth and obstructions. Kinda senseless to churn up the middle. With the lower fuel consumption of this engine vs a vis an OBJ, I may be able to keep the gross weight down to around 1300 lb. And of course, when light it will scoot along just fine.

    I'll post the hull lines when drawn.

    Thanks again to all for the advice.

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    FAST FRED Senior Member

    I think the power unit from a jet sky might be one answer.

    There available used , and you might simply glass or bolt the hull bottom with power package in to your boat hull.

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