Amphibious Car Design - College Student Project

I'm a mechanical engineering student working on a design project.

The objective of the design project is to design and construct a single seat amphibious offroad vehicle. There are many guidelines for this project which are located here

The basic dimensions of the vehicle are:
Width = 1.524m (60in)
Length = 2.743m (108in)
weight including driver = 317kg (700lbs)
Engine Horsepower = 7.457kW (10hp)
Max Engine RPM = 3800RPM

My question is:
Using the model of a flat bottom boat with an outboard motor, what would the maximum velocity of the vehicle be using the dimensions given above? Please provide references to mathematical models used, or point me in the right direction.

here is what our car looked like last year

here is the car driving on the water #1

here is the car driving on the water #2

here is the car driving on the land

Thank You,
Mike Hoyer

 

Hydrofoils

I would take a serious look at retractable hydrofoils to get as much of the vehicle out of the water as possible.

 

Thanks for the suggestion. That would be a very elegant design, similar to the Rinspeed Splash.

However, we need to take baby steps. It's hard to tell from the pictures, but we are using deep vee chevron pattern tires as paddle wheels. I need to figure out if the added cost and weight of a propeller or impeller would be justified by the performance gain. Our current top speed is about 2knots.

Once we have justified the use of a propeller or impeller, then we'll work on adding some hydrofoils

PS - Just a note on forum etiquette. You don't have to quote an entire post if you're the first one to respond.

 

I wouldn't look at the added cost and weight of a propeller or impeller as an either/or situation. I'd look at it for both the displacement condition and the foilborne condition. For the speeds you'll be going even just wooden foils would probably work. If you stay with a displacement mode, I'd also look at ballasting the car to give it a better pitch attitude in the water.

Low speed hydrofoil links.

http://members.aol.com/jfreeent/hf.htm

http://lancet.mit.edu/decavitator/

 

I like the idea of using the displacement condition with an impeller. I'm currently looking at using an axial-flow pump from a kawasaki 550 jet ski with a 406.4mm (16in) impeller pitch and 124.5mm (4.9in) diameter

I need to find a Power vs. RPM curve for this pump now... anyone?

Maybe next year we'll use some hydrofoils to get this thing flying.


I'm trying to find the pressure drag coefficient
I am assuming the hull is a square prism.

I used the graph titled

"Variation of drag coefficient with aspect ratio for a flat plate of finite width normal to the flow with Reynolds # > 1000"

Hoerner, S.F., Fluid-Dynamic Drag, 2nd ed. Midland Park, NJ: Published by the author, 1965

and got a Coefficient of drag of 1.2 which equals a powerloss of ~1.056kW (1.417hp)

I know this isn't right because the graph is only valid for immeresed objects, and my model is between two fluids... help?

 

hyrdofoil

You got me thinking about hydrofoils now. Maybe I can add them to the front and rear control arms to get some of the car out of the water. I estimate the maximum power I'll be able to transfer to the water will be 5hp, and with the displacement condition I calculated a top speed of 5knots... whoo hoo That calculation is from using the immersed rectangular prism model. Maybe it isn't a bad model after all, the thing is a floating brick.

I'm learning xfoil, and reading up on foils.org. Hopefully I can get this damn thing going faster that 5knots! That would be awesome

 

I'm not an engineer and have not posted in this thread because of that. However my preconcieved impression is that with the HP/top speeds you are talking about you will not be planing the hull, using skis, or able to make use of foils/wings.............................gonna get stuck using good old displacement mode, cutting through the water, and not on top of the water.

I could be wrong, just putting it out there for consideration.

Weight and surface area will dictate much.

 

no, the car is not going to plane on only hydrofoils, but I think they will help lift the vehicle out of the water a little bit. That should increase the top speed of the car to maybe 5.5knots , and every little bit helps in a race. Thanks for the post

At the very least I'm hoping to be able to use them in the front to the trim the angle of the vehicle in the water. The front is very very light compared to the rear

 

Unless you can control the entry angle of attack and exit angle as well as you can the cruising angle, the foils may act as big drag collectors.

 

one note here, forward speed of about two knots.

If you add foils and succeed in lifting it, the transmitted power will drop.

I would seriously consider a jet-drive with a clutch (running them dry isn't good). There are loads around on the web and most have pretty good details, otherwise pester the manufacturers.

Good Project,

Tim B.

 

Maximum dimensions approx 64" x 108", 10hp B&S propulsion... I haven't read the whole spec, but with these dimensions, you should be able to get a good planing hull! Maybe 15+ mph in the water! Look at Bolger's "Skimmer" for a planing hull of this dimension - it has 2x2 skids on the bottom, to serve as planing surface.

Consider a "long tail" prop, hanging off the transom.

 

Thanks for your interest in my project.

I am assuming you mean transmitted power lost to drag drops, right?

Thanks! I was planning to run it dry. Good thing you told me that

Loads for the impeller? Yeah, I'm going to try to pester Kawasaki for a pump curve, but I'm pessimistic if I'm going to get it.

Thanks again,
Mike Hoyer

 

The car plus driver usually weighs about 317kg (700lbs), and the problem with the floatation is that it's a battle between ground clearance for driving on the land, and keeping the wheels out of the water as much as possible when on the water. Currently the ground clearance is winning that battle with 10" of ground clearance.

I looked up the Bolger skimmer. It planes out at 5hp, and it looks very light

 

If you want to go for a something completely different, you could try something like the Chrysler Marsh Screw. It could go on water, land, and everything in between.

 

that's awesome, but I don't think it's for me. Thanks for the picture though