Shallow Water Tunnel Design

[YK GEO]

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.

[WestVanHan]

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.

[YK GEO]

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.

[baeckmo]

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!

[fjlegend]

Like this?

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

http://farm3.static.flickr.com/2509/3971398061_b607e23914.jpg

[YK GEO]

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.

[ondarvr]

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/

[baeckmo]

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!

[cor]

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.

[yipster]

before planing no extra squad in shallows?

[WestVanHan]

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....

[YK GEO]

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.

[FAST FRED]

If you want more economy than an outboard can provide.

http://www.robbwhite.com/rescue.minor.machinery.html

The Rescue Minor was reputed to go over 30 miles gal with a much used converted diesel.

FF

[YK GEO]

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

[ondarvr]

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.

[YK GEO]

Ondarvr,

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.

[ondarvr]

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.

[YK GEO]

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.

[kach22i]

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

Some hovercraft can be quiet and fuel efficient.

FYI, Re-post from another thread:
http://www.boatdesign.net/forums/boat-design/need-fast-efficient-commuter-boat-4692-3.html
I recently spent three weeks between Salem and Portland, Oregon. I missed the Northwest Hover-In though. If interested at all in a hovercraft, I can put you in contact with some people out there.

If going up river is the main/most important part and plus saving fuel is critical, then you might want to consider a hovercraft - really.

http://www.ushovercraft.com/
http://www.ushovercraft.com/commercial-hovercraft.jpg

http://www.amphibiousmarine.com/

Plans:
http://members.aol.com/sevtec/sev/skmr.html


Another poster said:
http://www.boatdesign.net/forums/boat-design/need-fast-efficient-commuter-boat-4692-3.html
I would pick something like this:

http://www.hovercraft.com/content/index.php?main_page=index&cPath=34_48

http://www.hovercraft.com/content/media/video/14p_256k_lo.wmv

http://www.hovercraft.com/content/images/kits/14p/14p_y_3b.jpg

This is a very quiet hovercraft, has very good speed and can have 20 mpg if you are in good conditions.

Video link;
The Great North to Alaska Hovercraft Adventure, Bryan's take
http://www.youtube.com/watch?v=SvWhBkPjwSA

[WestVanHan]

Guess it can't hurt to reiterate,but I suggested this already and he's not interested in hover crafts.

[kach22i]

Guess it can't hurt to reiterate,but I suggested this already and he's not interested in hover crafts.
Yea, that happens but a little more exposure can't hurt.:)

[YK GEO]

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.

[FAST FRED]

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.

FF