Shallow Water Tunnel Design

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

  1. YK GEO
    Joined: Oct 2009
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    Location: Marsh Lake

    YK GEO Junior Member

    I'm planning to build a plywood river boat that will typically have to navigate 6-12 inch deep water, with short stretches of bumping along the rocky bottom in shallower spots.

    Economy of build and operation is desirable, since this boat is intended for limited use mainly to make a 300 mile round trip to a friend's trapline once a year.

    I have in mind a flat bottom design, either based on MacKenzie River (Northwest Territories) river scows or the modified flatiron skiffs with rockered bows known as Alaska skiffs or dories, among other names. Length 18-20 ft, beam 5-6 ft. Payload will be usually two people, camping gear, and 3-400 lb of fuel.

    Hundreds of hours driving the MR scows (all with 4 ft wide bottoms) from 15 to 30 ft in length revealed the longer ones to be efficient cargo haulers in semi-planing mode, or planing if light, using single 20 to 30 hp outboards, at speeds of 15 to 25 mph. They don't squat no matter what speed they run at. Shallow water operation isn't a consideration there, since that river is deep, so regular outboards work fine.

    The obvious answer is an inboard jet drive, but does anybody make one in the 40-50 hp range? Downside is noise and fuel consumption.

    Another obvious answer is an outboard jet, however these are vulnerable since they have to hang below the bottom a bit to scoop water. My friend cracked his impeller housing on a rock first time up the river.

    I'm aware that some boats have a shallow tunnel intended to provide a sheet of water a bit above the boat bottom to feed the impeller. Does anyone know how well this approach works, and what shape and dimensions would be needed for, say, a 40 - 50 hp jet outboard?

    Another obvious answer is an outboard mud motor using a surface piercing prop. My son has one (Mud Buddy 30 hp) that I've driven briefly in deep water. He's used it to slide over gravel bars, with the final drive bouncing up. These things still need about 8-10 inches of water to get the bottom half of the prop and the skeg immersed.

    An airboat is out, due to fuel consumption, noise and discomfort.

    And the last idea is a tunnel drive, where the whole propellor is tucked up in the tunnel, with possibly 2 or 3 longitudinal protective bars along the bottom of the tunnel, to prevent the boat from lodging on a boulder, etc. stuck in the tunnel (ie - so it could slide over).

    I've browsed the DIY tunnel thread, and thanks to baeckmo's diagrams helping CDK debug his boat, have learned a bit about tunnel design, though only for a tunnel with half the prop in the tunnel and half below.

    Also looked at Atkin designs of Seabright power boats such as Rescue Minor, which has a lot of appeal, but doesn't offer much propellor protection in rocky river bottom conditions.

    I'd appreciate help with the following:

    1. Contact info re inboard jet drive sources in the low end power range I'm looking at.

    2. Design specs for the tunnel to feed water to an outboard jet.

    3. Design specs for an inboard tunnel that wholly shields the prop. Would a protective grating seriously degrade performance?

    My target performance is semi-planing to planing operation in the 15-25 mph range.

    Thanks in advance to anyone with info.
     
  2. WestVanHan
    Joined: Aug 2009
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    Location: Vancouver

    WestVanHan Not a Senior Member

    I dont think your plywood boat will last long "bumping along the rocky bottom in shallower spots"

    Sounds like you need to build a hovercraft.
    Built one with a friend 10 years ago-quite simple and effective.

    There's lots of resources online,and I'd guess a few forums.
     
  3. YK GEO
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    Location: Marsh Lake

    YK GEO Junior Member

    Thanks for the comment.

    Bottom protection comes from sheathing with an epoxy bonded abrasion protective cloth available from Defender Industries (can't remember the name of the stuff, but I can look it up if anyone is interested), plus a troweled on coating of powdered graphite mixed with epoxy. If the bottom still suffers too much abuse, the fallback is attaching a sheet of UHMW poly, which is the approach a lot of the jet boat mfg use.

    Building the boat isn't an issue - I've built enough of them to know how to do that - it's the points listed above I'd appreciate advice about.

    A homemade hovercraft could be a lot of fun for playing around with, but what I need is reliable and reasonably economical transportation into a remote area. I've chartered a plane in, but that's $3,000 per trip which does put a dent in the old pension.

    Cheers.
     
  4. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Sounds like you need a weeled amphibian vehicle..... . But if you want to stick to something "boat-ish", I can see three variants. All needing some water to push around. Without designing the hull for you, I would guess the laden draft to be 8-10 inches, including draft of propulsion unit. So if you say 6", you probably have to get the Wellies on and do the pushing yourself until there is more water again. Here the options:

    A/ Ordinary, submerged propeller in a full depth tunnel. Straight shaft and inboard engine. With the data at hand, a reasonable preliminary propeller diameter might be ~12", requiring ~2700 rpm for 50 hp. The tunnel roof has a "sine-wave shape" longitudinally, and the prop must be positioned accurately in the correct position. Rudder(s) overhung aft of transom in tunnel outlet.

    Pro: Simple, inexpensive, reliable, good efficiency.
    Con: Rudders (and prop?) possibly vulnerable, reverse thrust may be low due to air pocketing in top of tunnel.


    B/ Fixed surface piercing propeller operating in the outlet flow from a tunnel shaped aft bottom. Preliminary estimate of prop: ~14" @ 2100 rpm. for 50 hp. Inverted U-rudder "Levi-style" around propeller.

    Pro: Even simpler than "A", reliable, good efficiency, slightly better margin against damage from sea-bed.
    Con: Practically no reversing, since propeller disc is continuously ventilated, sensitive to operating loads, propeller to be specifically modified from standard type.

    C/ Waterjet, transom mounted, driven by inboard. Preliminary estimate of impeller inlet dia ~200 mm (~8"), optimum flow ~0,25 m3/s. Power should be increased to 55 hp for equivalent performance.

    Pro: Good protection, excellent maneuvrability, no gearing required, steering included in unit, quiet if correctly designed/installed.
    Con: Lower efficiency, heavier, may inhale gravel or marine growth (=damage risk) due to higher inlet velocity.

    Pick your choice, and beware the bumps!
     
  5. fjlegend
    Joined: Apr 2008
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    Location: HotLanta

    fjlegend Junior Member

    Like this?

    I can run the jack plate up 7" and the only item below the transom is a bit of the skeg.

    [​IMG]
     
  6. YK GEO
    Joined: Oct 2009
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    Location: Marsh Lake

    YK GEO Junior Member

    Thanks for the information. Much appreciated.

    I'll do my static buoyancy and planing area calculations to size the boat as necessary for the river conditions.

    I just narrowed my options down to one, since the local dealer enticed me with a good price on a new 4 stroke Merc 40/25 jet outboard, so now I'm designing a boat for a specific motor, and looking at a shallow tunnel (outlet 4" above the boat bottom) to channel water to the jet intake, in order to keep the trailing edge of the intake above the boat bottom.

    I realize that some folks will consider this engine small, but I have no interest in blasting along at warp speed.

    I'd appreciate info from anyone with experience with tunnels for jet outboards as follows:

    - cross section shape at tunnel outlet. I'm assuming a flat topped rectangle, with generously rounded upper corners.

    - width in relation to the jet intake width

    - lengthwise profile. I'm assuming the tunnel top should form a long intake ramp of perhaps 10 degrees from the boat bottom feeding into a smooth curve down to an outlet ramp of 3 degrees or so downward to the outlet. Or should the outlet just be parallel to the boat bottom?

    - tunnel shape in plan view. Parallel sides, or should they be wider at the beginning and narrower at the stern?

    - cross section shape at "feeding section". 90 degree sides, or should they slope, wider at bottom and narrower at top? Would that configuration fill the tunnel better? I expect all corners should be smoothly rounded, since water doesn't like to flow over or along sharp corners.

    - When calculating planing area, can I expect any support from a shallow tunnel, or should I just ignore the tunnel area in my calculations?

    - I've seen pictures of boats with the outboard set forward, and the sides of the hull extended rearward (sort of like sponsons), allowing clearance for the jet stream when turning. Also add-on "pods", presumably to help boats that have trouble getting on step. This seems like a useful idea, since there would both be static buoyancy and planing surface right under the 250 lb. outboard. This is easy enough to build into the boat, and would give me a couple of sealed flotation chambers right where needed. Am I on track with this idea?

    As always, I appreciate any helpful comments.
     
  7. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    I don’t see a 40/25 being able to plane an 18 to 20’ boat loaded like you described, outboard jets are very load sensitive and 25hp at the pump is almost nothing. A 40/25 would have a difficult time planing a small aluminum jon boat with two people in it, add a few feet of hull, gear, plus fuel and you'd be at displacement speeds. With a long narrow hull your speed may be a bit a little higher, but your draft would be greater, so you won’t be able to travel through the shallow water like you planned.

    Not many jet boats use tunnels, they’re very difficult design properly, but it can be done, Wooldrigde boats does a good job


    http://www.wooldridgeboats.com/
     
  8. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Unfortunately, I must agree with "ondarvr"; the outboard jet is a risky undertaking. In my note (#4) I mentioned the jet inlet dia as a dimensioning factor. If you select a pump with less area, it will be designed for low flow / high velocity increase. This combo is suitable for high speed/low thrust, and is the opposite of what you need.

    If you go for that solution, make shure you have a plan B!
     
  9. cor
    Joined: May 2008
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    Location: Alaska

    cor Senior Member

    An outboard jet with a tunnel is the way to go. People run them in shallow rocky rivers here in Alaska all the time.

    Most boats are alum. The tunnel is normally constructed out of flat sheet material, no complicated curves to make. The sides of the tunnel sometimes taper in plan view, wider in the front, narrow in the back. I believe this keeps the water flow to the jet compressed and helps keep the pump loaded.

    http://forums.outdoorsdirectory.com/forumdisplay.php?f=21

    Check out this forum, there are lots of people there with experience in the type of rivers that you are talking about.
     
  10. yipster
    Joined: Oct 2002
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    yipster designer

    before planing no extra squad in shallows?
     
  11. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    Nice job on the armor plating.

    BTW Army Beach is nice.

    A hovercraft isn't just "a lot of fun for playing around with" and certainly is reliable-ten years on with about 3000 miles a year,he rebuilt the drive engine and replaced the lifter even though they were both fine-with no problems at all.

    Remote home in the Northern Prairies-rivers,lakes,and swamps.
    A 1300 lb payload, 25 hp lifter, and he opens it up to 100 kmh-with 70hp- on the lakes.
    It'll handle over 3000 pounds (that's when we chickened out when testing it)if floating on water with no power on.

    Two feet of chop doesn't even slow him down.

    If it's good enough for the US Marines....
     
  12. YK GEO
    Joined: Oct 2009
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    YK GEO Junior Member

    Thanks for all the feedback, everyone.

    Cor - it's good to hear from my neighbors.

    The boat/engine/load performance will all be thoroughly tried and debugged in local water before heading into remote areas.

    Cheers.
     
  13. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

  14. YK GEO
    Joined: Oct 2009
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    Location: Marsh Lake

    YK GEO Junior Member

    Thanks for the tip about the Rescue Minor. It is one of the designs I considered for my river boat, but the wide reverse deadrise tunnel wouldn't offer much protection to the prop over a shallow, irregular river bottom. The mileage Robb White reported in his Woodenboat article was approx 20 mpg. Renn Tolman, who retired from designing and producing his Alaska skiffs, also built his take on the Rescue Minor and has a bit of information on his website.

    I'm building two boats: one very light and minimal draft for running up a shallow river to my friend's trapline, and I started this thread to get advice from others, which was provided and I'm thankful for.

    The other boat is a small 19 ft motor sailer based on Atkin's Surprise design, adapted for plywood construction. This one is for explorations on our extensive lake systems, where water depth isn't an issue, but waves are. This design is also a take on the Seabright skiff type. Atkin designed it for motoring speeds up to 20 mph, with reasonable sailing performance. Draft is a reasonably shallow 1 ft. We have reliable, strong winds blowing up the lake valleys from the south, and the road access to the lakes is mostly on the north ends, so motoring the boat south and sailing north is the general plan. Small cabin for accomodations, and we'll have a fine, economical two person and a dog cruiser.

    I really have no use for the typical short, wide heavy fibreglass boats that need buckets of power just to get on plane, and can't handle any rough water. 5 mpg in an unsafe boat just isn't my cuppa tea, no matter how fast it can go on flat water.

    PS - I've got enough grey hair to justify being as opinionated as I want to be!

    Cheers
     

  15. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    A little background on my opinion.

    I live on a river and normally have several jet boats at my place at all times, most are OBs, these boats are from 13' to 20' with various size motors.

    Boat 1. 13' Alaskan with 40/60 Evinrude, works well with two people, three is the limit and there's not much room for gear, so it doesn't get loaded with much extra weight, but doesn't handle it well.

    #2. 16' G3 flat bottom with 40/60 Yamaha, good with two people, OK with three, but can't handle much more. More than two people and you can't run it as shallow.

    #3. 15.5' with a 5 degree hull, Custom made and then rebuilt by me later, with a 60/90 Evinrude. This boat is heavier and wider than the others. Good with three people, four is the limit and becomes sluggish. I've used it for camping on rivers and it can easily be overloaded and not plane, resulting in multiple trips to set up camp.

    #4 16 or 17' Valco flat bottom with 60/90 Johnson. This boat is very light and can handle a load much better. I recently used it to shuttle the gear for a group of twenty men (multiple trips) and it did very well, I was surprised at how much I could load in the boat and still plane easily.

    As the boats get bigger than this the HP jumps to 150 or higher.

    We’ve used some 35hp (prop rated) 2 strokes on similar hulls and couldn’t get the boats on plane, but I do know a few people with very light jon boats that use 25/40s and smaller. They do get on plane, but even on these small light hulls speeds are low (20 or less) and even adding one more person can affect speed and performance a great deal.

    Four strokes when used with a pump tend to have a poor hole shot, so performance may be even worse when loaded.
     
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