planning a 30'(ish) flat bottom landing craft

...and this is why he needs a flat bottom barge/jon, because it offers maximum displacement for his shallow water need

 

Sounds right to me, but he needs the shallow water drive capability of the (tidal) "flats" boats.

 

ok messabout, thank you very much. without disparaging any of the other posts, your post finally brought me some clarity. a couple guys had been close, but hadnt phrased it in a way that made sense to me. basically the "pontoon" bottom i had been thinking of wont help because the water is displaced the same way whether its sealed or not, so its a matter of surface area to increase buoyancy. im not dumb....but i can get myopic lol.oh and yes i had been planning ribs between the sheets.

now help me with hull shape if you guys can please. several have been saying flat bottom, and im ok with that but is it better than a tunnel type hull? it seems like the tunnel hull would help keeping the props in the water while maintaining a very shallow draft. im not certain how much speed loss there would be using this type of hull vs a typical flat bottom, but if this boat could maintain 30-40 knots it would be about all i could ever need. i could only do those speeds in the main rivers anyway im guessing. my idea on it is around halfway back begin to tunnel the hull toward the rear in almost a conical shape. im not sure about the effects of cavitation, turbulence etc so this might not be viable but in my mind it seems like it would buy me some room.

again, to all of you posting, thanks for the help.

 

So a 3 - 4 ton (gross), flat bottom, 40 knot (max), ~30', aluminum landing barge.
That's going to require at least a couple of 650 pound four-stroke 250's (HP), and fuel for how many days...?
Good thing you've got 4 - 5 years to raise the cash and finalize your design.
How long have you allowed for building, fitting-out and sea trials?
And, sorry, what's the budget?
And, does it have to be insurable?

 

20 knot cruise is realistic and not going to require huge power.

 

None of us want to be naysayers or to rain on your parade. However, your hoped for 30 to 40 knots is probably unrealistic. Both Blue Bell and Mr. Efficiency are right. The speed of the boat is a matter of how much thrust you can generate. At 20 knots the boat will need some serious horsepower for a boat of that size. At 40 knots it would need four or more times the power of the slower boat. A graph of speed vs horsepower is not a straight line, it is more nearly a parabolic line. That boat is going to have a lot of frontal area and at speeds in the 30 to 40 range, aerodynamic drag will be a major factor.

I mentioned before that a flat bottom at speed will pound badly and it will be unbearable at 40 knots in even a small chop. When planing the boat that needed 6 inches of draft at rest will take less draft because there is a substantial dynamic pressure component on the bottom which will tend to lift the whole thing.

You could use the pontoon bottom configuration but if you intend to use two motors there would be no shallow water advantage unless both of them were side by side within the width of the tunnel. You'd need jack plates to make this work. There would be some complications with that layout too. Consider that both pontoons will make their own wake or spray field. The two opposing wake or spray patterns will converge at some point between the two pontoons. That will make a bigger convergent wave which can and will interfere with the bite of the props and at certain speeds would throw up a wall of water behind the boat and in some circumstances send a small but annoying volume of water forward into the boat.

If may seem that I am claiming that there is no way to make your project work, that is not the intent. You can make the boat work but your are going to have to make some compromises. Probably the first concession would be operating speed. Thirty to forty knots is unrealistic for a shallow water boat. Consider fuel consumption if you use 500 horsepower. That would come to something like 35 plus gallons per hour which would weigh approximately 250 pounds. Figure the weight of fuel into your calculations. Enough fuel for a four hour high speed trip will be near a thousand pounds. When tinkering with the fuel consumption estimates you can ball park the consumption of a modern four cycle engine as 0.50 pounds of fuel per horsepower per hour. That is an engine assessment figure called brake specific fuel consumption (BSFC). Gasoline weighs about 6.5 pounds per gallon. You must figure that into your displacement calculations.

Do not give up. You can have the boat that you need if you will recognize some of the constraints and be willing to forego some of the things that you wish for.

 

There are a number of issues with the speed wishes.

First of all, a larger engine bears significant weight, risk, fuel consumption. Risk to lower unit in a river and try replacing a 350 hp lu. $$$

A houseboat barge styled rig can get along pretty well in the neighborhood of 100hp, but not what you had hoped for..

 

As another data point, the flat-bottom rig I've operated will run about 33 knots when empty, and around 27 knots when loaded with 3000lbs cargo. This is with a single 150hp Mercury four-stroke. There is a jackplate on the engine, but no tunnel on this rig. The jackplate adds six inches of setback, thus allowing the motor to run a couple inches higher while on plane.

If you decide on a tunnel, know that there are compromises. You will need a highly-cupped prop to deal with aerated water, and probably a four-blade design. You will lose most of your reverse thrust. Operation is a bit of a learning curve, determining what position to have the motor in and for what loads and conditions. Also, you will lose 3-4 knots top speed with the tunnel unless you design in a vent at the leading edge of the tunnel. The vent should be able to be closed. Run open for higher speeds, deeper prop, run closed for lower speeds and elevated prop in shoal waters.

Tunnel design is a complicated science. A cone will be very inefficient. Look at the tunnel design of Rescue Minor to get some ideas, as well as modern flats boats on the Texas gulf coast.

 

How is he gonna pull off a gravel bar with three four wheelers without a LOT of reverse?

 

I would try to locate a Muskrat River boat, used, some in Grande Prairie Alberta, note the UHMW on the bottom so it slides over rocks etc
Buy a used Muskrat, you could install a prop outboard, or just change the leg, maybe make some mods, waaay cheaper than building from scratch
If you build, use a 5086 alloy instead of the 5052 and the appropriate weld filler
Flat bottom is fine but it will slip sideways like the dickens unless you put in some directional strakes to the bottom.

 

Air boat!
Put an air prop on it with big directional fins!
Bottom out where ever you like, beach it to unload!
Noisey as hell!

 

Exactly! That is why i mentioned it in the first place.

It can be done, but takes planning and muscle....such as pull offshore a bit before loading, then push the boat out with the last ATV to make sure you can still get out before loading the last unit.

On that note, make sure the bow is square, not pointy. Then, by swinging the stern of the boat back and forth, the bow will walk itself off of the shoreline....it just takes a little time and some fuel.

 

I think the tunnel idea is trouble.

But for one thing, you can probably stay off the shore a bit if you extend the ramp. But this is harder to do than say. A houseboat tie up comes to mind. You line upstream and tie off. And first or at the right moment, post the upriver stern with a harpoon. Drop the ramp into the river. The post would be like two rings on the side at the stern for your sharp post to drive into the river as a positive stop. Some adjusting of the bank line or a double post system may also work...not alone.

Leaving.

Arriving is easier.

 

We use power poles at the stern on ours for temporary anchor just offshore. Also, two manual spud bars at the bow if parking for a while, or in heavier current.

 

a lot of good replies since i was away, so thanks guys. from what im seeing here its probably just best to use a normal flat bottom. i was concerned about the turbulence it would cause, but it was an idea.

fall guy....as to the landing issues do you think a set of maneuvering jets in the front would be feasible? i was considering them anyway with a boat this big, once im of main river i thought they might be very useful.

no can do on the fan boat, i would love one lol just not for this project.

barry i will definitely look up the muskrat, thanks. whats the benefit to using 5086 alloy instead? i thought 5052 was kinda standard.