Afternoon my blue-blooded friends.

this quarter we have an engineering project, we have to build a boat of limited size to carry an unspecified payload approximately 50 yards. The paylaod will be somewhere in the are of 5-20 lbs. and the boat must be no bigger than 18x18x18 inches. Cannot contain combustion power, and must remain afloat :P

I ma thinking of using an eletric RC boat or plane motor, with one servo for use as a rudder, or one servo controlling duel rudders.

But I dont know alot about RC engines, and how effecient they are. I am looking at an overall weight of approximately 15 lbs, and do not have a hull designed yet. I dont know though if I want to use a prop positioned above the water and run it like a swamp boat, or put in an underwater prop. I also dont know how large of an engine would be best for this.

I also have a spending limit of 50 bucks for it, with the exception of any donations made to my project (hint hint)

If any RC gurus out there could help me out with a decent engine size and to use an above or below water prop it would be greatly appreciated. the final test on this project will be to race the boats built by the other engineering groups in my class, so speed is also an issue, thankyou, JD

 

When you say combustion power, does that include any form of fuel?
Can you use glow plug engines out of RC's?
Electricity looks like your only option.

 

If it's gonna weigh 15 lbs it's going to take a lot of electric motors to get that thing going! Using a propellor in the air will be horribly inefficient unless you plan on getting this thing to plane out, which isn't likely. The absolute maximum bouyancy that can be provided by an 18" cube of water is about 210 lbs, so carrying the weight you want shouldn't be too difficult, but keeping it stable might be challenging. I would consider using compressed air in some form to make it move. Possibly use an airtank as the main hull of the vessel and use outriggers to keep it upright. This should keep you from needing much directional control as long as the wind is light. Air pushing on air won't give you much thrust, but if you get the discharge nozzle under water it should scoot along pretty well while putting on a good show. Another possibility, and probably a better idea, would be to have a water reservoir in the boat and pressurize the water reservoir with compressed air. This would make the stored water the thing producing the forward thrust. Since water is more dense than air by a few hundred times you will get a few hundred times more thrust. If you have air at 100 psi pushing a water jet into a pool of water through a 1/4 sq. in. orifice you will get about 20 lbs of forward thrust. The water will also discharge more slowly reducing the volume of tank needed to complete the course. Two liter soda bottles would probably make pretty good pontoons.

After all the time I spent going to college to get my degree I realized that the only thing they taught me was some fancy math and gave me a well defined box to think inside of. The key to succes in engineering, life, and most other things is to take the box they taught you to think in and smash it all to hell.

 

Jimmy Dean,
I belong to a radio control model aircraft club with about 200 members. We ALWAYS have used stuff for sale CHEAP! I would look up a local club in your area. Most the hobbie shops know who they are, and when they have their monthly meetings. Pop into one of their meetings and introduce yourself. Let them know what you got go'n on. My guess is, you'll leave the meeting with more telephone numbers offering-up help than you'll know what to do with.

Good luck.

 

Ok, so your suggesting two sealed 2L bottles spread as far apart as possible (for balance) connected by a plat on top, under which suspends a tank filled with water with a 1/4" hole in the back center, and a hose conencted to the front that is hooked up to air tank at about 100 PSI? about what volume would the air and water tanks have to be to make it at least 50 yards? Should I run two tanks of each and have two outlets on either side to try and ensure a propulsion balance? I still plan on working in a servo conencted to a rudder to maintain straigh stability. I imagine that if it where in the least unbalanced with that type of pressure pushing it, it might end up traveling in circles.


And I had jsut finished designing in Solid Edge a 17x16 flat bottom boat with two 6 inch props sitting about 6.5" above the deck of the boat..but that was before my comp crashed. You dont think that would be enough to speed up 15 lbs or so?

If anyone has Solid Edge on their computer and is interested in giving suggestions, PM me with your emails so I can send over some deigns as we work on them


I have also thought about building a flat bottom hull and attatching two of the jet ski type RC motors to the bottom to us as both propulsion and steering.

 

I'm thinking something like a paint ball gun CO2 tank with some kind of compression restricker to release the presure at 50 yards per tank etc....


For the boat shell, I'd probably buy a can or 2 of foam spray etc.. and fashion some type of mold....


I just might try this my self, sounds fun....


Big MIck

 

I like the idea of the foam-in-a-can for making the hull. I would pick up some sign-maker's vinyl at an art supply shop to skin the hull to make it smoother.

For propulsion consider good old baking soda & vinegar. A small vessel with a vent piped off the top down to an underwater jet might provide a lightweight source of propulsion.

Good luck with your project. I always found those engineering projects a lot of fun.

Cheers,
Eric

 

That sounds right. The 1/4" hole was a figure I just pulled out of the sky for demonstration purposes. The orifice size you need will probably be different.

You have to do at least part of the work yourself. I would start by deciding what thrust to weight ratio you want and work backwards from there to size everything. The less pressure you run, the longer the tanks will last. I was thinking of a paintball canister with a regulator like the guy above. If you regulate a C02 canister down to 15-20 psi it should last a pretty long time.

No. You will never achieve a perfect balance and if something goes wrong with one side there is no amount of steering that will correct it because of the low speeds and small size of the craft.

If you just run one discharge nozzle you could actually built it so that the servo turns the nozzle. This will eliminate the drag of a rudder and will provide more steering authority. Stability will also be better if the nozzle is located at the front of the boat instead of the rear.

It will move, but you want to win. My RC boat runs a single 10x6 prop on an Enya .40. It weighs about 3-4 lbs and drafts about 8 square inches. It will flat move out, but your boat is going to weigh about 8-10 times as much and might draft about 30-35 square inches. A small electric boat probably weighs about 2-3 lbs and moves very well, but it has a planing hull and you will have to use a displacement hull. The small boat also uses a water prop that is much more effective than an air prop.

That would also work, but I would still just use one to make it easier to control.

 

how does anybody expect you to complete a successful design with limited dimensions and unknown weight of the payload? with a max dim inside an 18" cube, i would suspect your design might well include a styrofoam design (hull) with a centerboard counterweight and electric motor & control system. the counterweight will increase stability and you can have a potentially streamline hull shape (as much as you care to, anyway) to incorporate your cargohold as low as possible (along with the batteries, power and control components. at least; IMHO, that is how i would approach this assignment. still, my question would be to nail down a payload weight/bulk.

 

Twohands...
JUST about to post the same thing....
What are the complete requirements ?
I don't think we have enough info to "simply" solve it...
most solutions are more complex than the question in my experience !!!
Yes, the full blown work for 1000 years and handle ALL scenarios are elegant and beautiful...
but real world do you NEED 10000 lines of code to print one word?
*****************************
does it need to follow a definite course and be steerable ?

 

Many engineering projects aren't about the result as much as they are about the path taken to get there. One of mine was to design a brake that would stop a huge flywheel spinning at ungodly rpm (don't remember the exact numbers) in less than one revolution. My design report basically stated that it couldn't be done because the flywheel would explode at about half the rpm it was supposed to turn. Even if it would spin those rpm the brake would explode when applied. I then offered several design alternatives to a brake. Did pretty well on that one.

 

Oops, forgot to ask my question. Does the payload have to stay dry?

 

Well put.

Sometimes as engineers, we tend to over-think things a bit. My thoughts on this…..

Back in college, we too had a design competition. Our competition required that you use a mechanical device to launch a golf ball the greatest distance. The size of the device could not exceed a 36” cube, and a cost of $100 (we could also have donated stuff).

Our design (along with many others) was an overly sophisticated air cannon. We lost.

The winner’s design: A very simple medieval catapult.

My point is, Twohands is correct, simple is usually better. My guess is, that when the day is finished, the guys with the enhanced simple flat-bottom “Barge” design, propelled by an electric motor-driven prop, are going to be the victors.

Good luck.

 

Aye, I find the limits a bit ridiculous, as it will draft approx 486 in^3 if it weighs in at 15 lbs with payload, i.e. if I build the hull a flat bottom 18x18 it will still sit an inch and a half in the water the only extra info I have been able to get from the proffesor is that the weight will simply be a iron or steel plate, between 5-10 lbs, dimensions unknown at this time. Which means that on something no longer than a foot and half, there will b e a TON of drag I have to work with, which is the only reason I was thinking of using an eletric motor to drive it, because if the water/air pressure where to run out, my boat would become a sitting duck so-to-speak, but if it where to be going slightly slower with the eletric motor, at least it would make it across.

 

Does it have to be steerable or just carry unknown cargo 50 yards? I would probably shape my hull from styrofoam and coat it with resin.
Dono