River Roller! Pontoon Boat that Rolls Across the Water!

On the model, are you going to put separate motors, drives, throttles etc for each toon? I'm thinking weight of the prototype might become an issue.

 

Fair enough. So for a similar hulled craft at 1/12 scale what would planing speed be? I need a target speed for motor design.

 

Thanx! I will use that for my design speed.

 

I would really like to know this also. I was surprised to see that buoyancy at a 1/12 scale is far from 1/12. That being said I am also not sure what power would equate to 100hp in a 1/12 scale. Come on people this is where I really need you. All assistance is appreciated.

 

Progress update. Well I decided to do a simple drag it through the water test on the first half of my prototype. I'm glad I did but it at the same time it has set me back. I calculate that I would want no more that 10% submersion for the light load test. This is the test that should work very well and not require much to get on "Plane". After reviewing my calculations I came up with a weight of 0.77 pounds for one half of the craft. I'm currently over 2 lbs without drivetrain or batteries. This is resulting in about 33% submersion. Way beyond my limits. This same calculation at full size would give me 2500lbs.

Considering all of the nonlinear relationships between a full and 1/12 scale model I'm beginning to question the validity of a scale model prototype. Any thoughts are appreciated. Let me take that back, any CONSTRUCTIVE thoughts are appreciated.

 

It's been several years past that I had bookmarks on another computer that covered scale sizing of models dimensions and power, look for articles by Bob Bocher (Astro Flight) and others, especially things related to electric powered airplanes. RC Forums might be the best place to start looking.

 

Scale models, both powered and unpowered (towed) are a standard tool in designing boats and ships. The scaling relationships are well known and available in many different references, both print on paper and electronic.

Weight/mass scaling: A vessel displaces it's weight in water. A 640 pound boat displaces 10 cubic feet of salt water. A 1/12 scale model with the waterline at the same relative position displaces 1/12 x 1/12 x 1/12 x 10 cubic feet = 1/1728 x 10 cubic feet = 0.0058 cubic feet which corresponds to a weight of 0.37 pounds. Weight goes as the cube of the scale factor.

 

At this stage you only need to test whether the concept will do as you think it will which is to 'roll across the water' really fast etc. You don't have to test how 2 rows of toons will steer it, you don't have to separately power each toon etc. you just have to see if the concept works. You don't even have to have weights and power proportional to what a full size boat would have.
5 two liter soda bottles weigh 8 1/2 oz, two small sticks along the sides would tie them together. Glue on some paddles. 4 small screw eyes in the ends of the sticks would guide the thing down 2 tightly stretched strings or wires across a swimming pool. The strings would supply enough rigidity to the thing and keep everything lined up good enough for testing. 4 rubber bands around the bottles would make them all turn together. The only other thing needed would be a motor. If you were clever enough, the two guide wires could be used as a 12 volt or whatever power source, one for power and the other for ground, just like an electric trolley car. That would eliminate the battery weight. As it would only be run for short periods of time, an x volt motor could be run on xx volts and it would develop a lot more power for it's weight. One time I accidentally hooked up a 110 shop vac to 220 volts and it ran crazy good until it imploded the dirt barrel part.

 

Saw this today and couldn't resist. Sorry.

 

Surprise, surprise You will also find that actual weight of the final engineered product is no relation to your model either.

You are going to have to go MUCH bigger ( like 1/8 minimum to 1/4) to get any realistic results you can actually use.

 

RWatson is absolutely right. You must build to a larger scale if you are going to be able to carry all the mechanisms and batteries for the model. As DCockey has shown, the scaled weights are a function of the cube of the scale. If you reckon that the full scale boat would weigh 650 pounds then a one eighth scale model would have a divisor of 8 cubed.....8 x 8 x 8 = 512 therefore 650 divided by 512 yields 1.26 pounds.......Not enough capacity for your model.....try 2 inches to the foot....that would be one sixth scale. so.....650/6 cubed or 650/216 = 3.00 pounds. Now we are getting close to an acceptable model displacement. still not quite enough. Try 3 inches to the foot...that would be one fourth scale....4 cubed = 64 and 650/64 = 10.15 pounds which is more realistic. Such a quarter scale model will be far more useful as a test project than a tiny one twelfth model. The larger model will be less affected by minor ripples or waves in the swimming pool or village pond. The rollers will not have to climb such steep relative inclines and several other reasons for using a larger scale.

You can get architects scales with divisions of 1.5 and 3 inches to the foot in your graphic arts supply store. Those scales make the model building project more convenient. You can also get 2 inch scales but that is an infrequently used proportion and the store might have to order the scale as a special item.

DO continue to work on the project. I am one of the naysayers but I am more than willing to be proven wrong. That is the way that we learn new concepts. Building a big model will be fun whether it works or not.

 

Have you thought about making the outside edges of each 'toon open and just mounting the fins on the open section. Leave the rest smooth and water tight.

That would allow some of the radial forces others have said paddle wheels generate. I know nothing about this subject. Just an idea that occurred to me as I was reading.

For your model I wouldn't bother with steering. I would just power all the 'toons off one motor and see if it goes forward.

 

It appears that Matt is assuming that if you pull his mechanism under external power, ie the paddles of his toons not providing thrust that he will "prove" his concept
Dragging a series of cylinders across a pond will only show that dragging a series of cylinders across a pond is possible.
In order for his concept to move, ie paddles providing forward thrust, the trailing surface of the paddles must produce (to use the term loosely) positive pressure.
If it takes 500 pounds of force to overcome total drag at a given speed, this force can only be generated by accelerating a given amount of water to a certainty exit velocity

If the paddle height is 4 inches, then this depth of water (times the toon length adjusted for some sector of immersion) be the control volume in question.
From an engineering perspective, in order for this to work, the cylinder will be or might be supported by buoyant forces and planing pressures through the control volume.

 

I don't understand this part.... "through the control volume." ?

 

There has bee quite a bit of talk about "planing speed" and getting "on step" on this thread. I am familiar with these terms and what they normally mean relative to a boat. I am very confused about what people mean when applied to this proposal by MLT. Normal definitions of planing don't seem to apply to such a vehicle. Can anyone using these terms explain what they mean?