River Boat Design

[mrdancer]

I need a boat for working on sandbars in the river. The boat needs to be lightweight (so we can push it off of submerged sandbars), able to run in shallow water (so we can get close to sandbars) and able to carry at least five people. Here are some thoughts I have...

1. Overall design will be a jon boat due to shallow water capabilities and utility.

2. Long & skinny. I'm thinking 25 feet long and 5 feet wide. Length is good because we can get the bow up on a sandbar and still have the motor out there in deeper water. It also helps for pushing the boat off of a submerged sandbar, e.g. - shift the cargo to the deepwater end, etc. I'm going with a narrow design as that will help keep weight down.

3. Flat bottom for maximum shallow-water capability. I'd like somewhat of a modified-V bow for cutting waves. I know it will ride rough in a chop, but I plan to combat that with soft seats and cushy pads to stand on.

4. Tunnel-hull, for shallow-water capability. Tunnel would be about eight feet long for a 25-foot boat, about 14 inches wide and 6 inches deep. It will be either five-sided or round.

5. Engine: I'm used to 60hp motors, but I'm not sure if that's enough oomph to plane a 25'x5' boat easily, especially with five people aboard. Yamaha makes a 90hp 2-stroke that is relatively lightweight, so may go with that. Motor will be mounted on a hydraulic jack plate to maximize tunnel effectiveness. Will need a good cupped four-blade stainless to get a good bite on the tunnel water.

6. Boat will have an elevated center console, maybe a foot or two higher than a typical center console. This will provide maximum visibility for the captain and aid in avoiding those phantom submerged sandbars. Nothing quite as drastic as the tower boats that are running the flats, though. Console will also be mounted a little further forward to help the boat run flat at slow planing speeds.

7. Floatation pods at the stern. Help to keep the boat running flat.

8. Trim tabs. A lot of the best flats boats use them, and they swear by them, especially for getting up and running in shallow waters. Would help provide a drier ride in a chop, also.

9. Sides of boat will be around eighteen inches toward the stern, and twenty-four inches toward the bow (hoping to provide for a little drier ride this way, while keeping weight down).

That's most of what I can think of off the top of my head. Any glaring omissions, comments, etc.?

[icetreader]

mrdancer,

Looks like a nice plan.

Why 5 ft wide? (why not more and why not less?)
What are you planning to achieve with the tunnel?

[longliner45]

I used to own a 24 ft lapstreak great lakes riverboat it was designed by the sandusky boat works in sandusky ohio she was 12 ft wide and24 ft long she didnt have a screw in her all made with dowes. engine was 292 chevy;forget 5ft wide the first barge that goes by ya will sink ya ,,,,,,,what do you whan the boat for?

[tom28571]

A 25' by 5' lightweight flat bottom boat with five people moving around doesn't sound too good to me. Not real stable and there is no compelling reason to go flat. With 90 hp on the stern, I would not let anyone I particularly like go with you and three others in that boat. I'd go beamier at about 6.5' to 7' with some bow shape and the tunnel you like. 50 hp should be more than adequate to plane a decently designed boat to suit you. Even 25 hp might do it.

One boat that comes to mind is Atkins Rescue Minor, although it's a bit small at 19'. http://www.boat-links.com/Atkinco/Ut...scueMinor.html This is not a planing boat but should run in the high teens with not so much power. I see no reason why it could not be made larger with a little design work.

[mrdancer]

I chose five feet width as a compromise. Narrower could lead to stability problems and wider may unnecessarily increase weight. We currently have a jon boat that is 19 feet long and 54 inches wide. With a 40hp 2-stroke Evinrude, it planes easily and runs stable with five people aboard, as long as the water is smooth. With a little chop on the water, it becomes a wet ride (console mounted forward, 18-inch sides). It is a riveted single hull design, and is going on 15 years old, leaking a little and starting to fall apart in general. We'd like to upgrade to a little more capacity and a drier ride.

This is a large shallow river we are working on (upper Missouri River). Typically the river may be anywhere from a half-mile to a mile wide, and most of it less than a foot deep, consisting of shallow, braided channels with an occasional 20-foot-plus deep channel (thalweg). I say occasional, because in some places the thalweg disappears completely. No barge traffic on this river! Current is typically around 3mph in the main channels. Water is often turbid, so sandbars more than a few inches below the surface are often hard to spot, hence the ease of running up on one, then having to manhandle to boat off of it. This is a National Recreational River, so airboats are not allowed, nor are jet skis. Jet drive outboards have been tried in the past, but the fine suspended sands in the water eat up the impellers quickly and they become a maintenance nightmare.

I'd like to stick with a 60hp four-stroke if it will plane the boat easily with five people aboard. If that's not enough power, I think Yammie makes the lightest 90hp two-stroke outboard (only 24 lbs heavier than a 60hp four-stroke). We went with Yammies because recent Mercs we had would keep plugging up with sand.

The reason for the tunnel is for getting into the shallow water. I know the tunnel hulls don't handle very well and some buoyancy is lost; however, we are willing to give that up to get the extra inch or two of shallow-running capability. We would also rather have a 10-15 year life span single hull that weighs 500lbs than a 30 year life span double hull that weighs 900lbs.

[safewalrus]

Just one small problem as I can see! Freeboard needs to be as low as possible, it sounds as if your going to be getting in and out of the boat fairly regularly! to shove off etc! With that narrow beam each time you try to step over a high bulwark your going to tip it! As it tips it keeps going, and gone - three knots current that everything you've got disappearing down river quicker than you can run! Plus it 'only needs a cup of water to drown' in if somebody goes in awkward! other than that can't see much of a problem - go build your boat!

[tom28571]

You should reconsider the beam issue. Adding to the beam is the most effective way to increase stability, load carrying and easy planing with minimum hull weight. I built a cruising powerboat with a hull weight of about 1900 lbs, LOA is 24', beam is 8' and a hull draft of nominal 7 to 8 inches. With 6 adults aboard she planes easily from about 11mph up to a max speed of over 20 mph and is powered by a Yamaha T50. Don't understand the need for so much power.

Robb Whites version of the Rescue Minor can run in 6" of water. Look at the impressive photos in: http://www.robbwhite.com/rescue.minor.html

I don't think long and skinny is the best way to reach all your goals. No, that's not right. I'm positive that long and skinny is not the best way to go.

[tom kane]

As a life time shallow water boatie I believe that you must have the necessary amount of buoyancy to keep your total load on top of the water at all times,and a very shallow planing angle so the stern of the boat does not drop to much when starting to go over the hump or in shallow water.A narrow beam does not give a lot of buoyancy.A V hull with a 6 inch deadrise 20 feet by 6 ft 6 in beam should support five people luggage and a large outboard with a draught of 4 to 6 inches if you have not external keel (tripple keelson).Any tunnels added would reduce buoyancy in an important most area.A hull with a nose down attitude at rest that levels up with a few RPM on helps to navigate the shallows better,keeps the stern up.A flat bottom boat does not necessarily meam a shallow draught if there is not enough buoyancy to carry the weight.The outboard motor needs to be able to rise automatically to the surface by a pivoting mounting like an rise and fall outboard bracket.The motor will push it`s self up on striking the bottom and pivot back as thrust is lost.Photos in my photo album give some idea of shallow water boating with a pivoting drive inboard.

http://www.boatdesign.net/gallery/sh.../2/ppuser/1988

[mrdancer]

Thank you all for the responses. As I've said, we have a 19-foot jon with a 54-inch bottom width (around 60-inch top width), and it seems to be plenty stable for us, which is why I thought maybe I could add six more feet of length along with 6 inches width and still keep the stability. I will think more about going to a 72-inch width. Remember, we'll probably be pulling this thing off of submerged sandbars many times, so I want something that is relatively lightweight and flat-bottomed so it doesn't hang up or get sucked down in the silty sandy substrate. Our best boats right now have about a 6-degree deadrise at the bow tapering to a 0-degree deadrise about 1/4 to 1/3 of the way back to the stern, and they are single-hull riveted models (I understand, though, that a high-quality welded boat may weigh less than a riveted model). Fiberglass won't last long in the environment we run these boats in (sand, gravel, snags and other hazards).

Tom, I saw your pics of the pivotal drive, but no real good photos of how the drive actually works. How is the engine-cooling accomplished without plugging up the cooling intakes with mud/silt/sand? Is it a closed-loop cooling system with air-to-water heat exchanger? That would be too heavy/complicated for our purposes. We have used the GoDevil-type motors (air-cooled) and boats here on the river, but those boats are undersized and slow, and still have problems in the sand. We've also tried the jets, and keep going back to the outboard prop drive as the most reliable low-maintenance system, as long as we can keep silt/sand out of the cooling system. We typically run in 90-degree ambient temps with water temps approaching 80 degrees, so overheating can be a problem if the cooling system isn't up to par.

I've attached a few photos to give you an idea of what we are working in. Shallow water with a lot of submerged sandbars that shift around constantly, and water level varies quite a bit to add to the problem. Deeper water comes in the form of braided channels with an occasional thalweg, making navigation rather tricky, especially when the water is turbid or skies are cloudy.

Perhaps a boat such as the flats cat www.flatscat.com would serve our purposes just as well? I talked with the owner and he says that they build them in aluminum also.

[kach22i]

May I suggest a hovercraft?

[mrdancer]

Originally Posted by kach22i

May I suggest a hovercraft?

Poor capacity/power ratio, high maintenance. They are considered airboats and thus are not legal to run on this stretch of the river (considered a National Park).

[tom kane]

Like you we have tried all the propulsion methods,including cushion craft for water ways similar to yours.Outboard are easier for most people.but put the weight where it is not wanted.The trimable shaft fitted to a boat is as light as you can get with any type of motor,closed fluid cooled (quiet) or air cooled.There are many ways to set it up and the main advantage is that the skeg and shaft slide up with a cam like action and the operator does not need to control it as it follows the bottom and moves up and down depending on the depth of water.Good propeller protection can be added.

[lewisboats]

Sorta like this? (disregard the waterline, I didn't bother to adjust it)



Steve

[JEM]

You won't displace as much with a tunnel as you would with a flat bottom.

To get the shallow water ability, couldn't you just set the motor on a jack plate or something?

[mrdancer]

Originally Posted by tom kane

Like you we have tried all the propulsion methods,including cushion craft for water ways similar to yours.Outboard are easier for most people.but put the weight where it is not wanted.The trimable shaft fitted to a boat is as light as you can get with any type of motor,closed fluid cooled (quiet) or air cooled.There are many ways to set it up and the main advantage is that the skeg and shaft slide up with a cam like action and the operator does not need to control it as it follows the bottom and moves up and down depending on the depth of water.Good propeller protection can be added.

I'm still not sure what advantage your system has, Tom. I can see where it might protect the motor (and transom to a degree) if one should hit an underwater obstruction, but otherwise I don't see what advantage it has over a hydraulic jack plate for shallow water running. We are limited to running outboards, which have open cooling systems.