Boat Design Proj

[designfreak]

Hi, I'm a student and I really hope that I can get some opinions on this.
We have a project that requires us to design and build a scaled boat that can carry cargo. Basically, we are simulating the boat to carry cargo and moving from one port to another. This will be done at a swimming pool.

Some background information.
The only "power" source we get is compressed air (50psi) ..either one 2L bottle or more than one... haven't confirmed yet. We get a micro-controller in which we can program it to control the rudder or whatever we use to steer the boat. There is a size restriction...2m long. But we dont' wanna build too big of a boat 'cause we wanna minimize the volume possible of the boat sinec anything sticking out from the boat (rudder, propeller) will count as part of the "volume"

This boat needs to travel a straight line (with and without guidance wire) for 18m. It will also need to do U-Turns after specific distance travelled. In addition, it will need to do S-Turns as well.

So, my question is.......

how should i go about it????
hull shape...we pretty much decided
but we don't know how we can get the boat to move such a long distance.. and also be able to do u-turns to return back to shore with the compressed air we got.
What's the advantage and disadvantage of having two propellers...
Is there any other way we can use for propulsion? thoughts of using air motor but anythign better?

those are all the questions i can think of so far...

[Skippy]

How do these school projects work? Are you supposed to be asking for advice on the internet? We've had a couple students come & go very quickly, I always wondered if they got in trouble.

[DaveB]

what's the cargo? Is it defined by wt, volume, area or length?

[designfreak]

well, with school projects, we were never restricted to not be able to research ideas and look for ideas...
so no, i don't believe we will get into trouble. what gets us into trouble is when we write documents that's word for word...plagarism that gets us into super big trouble.

but newayz

the cargo is going to be 1.5" x 1.5" x 8" fir blocks
density of block is still not confirmed yet but is expected to be 545 kg/m^3 (12% moisture content)
objective is to maximize cargo (we can take as many blocks as we want) providing our boat will travel to the other end of the pool and in the shortest time!

we can't strap the cargo down or anything

[yipster]

only "power" source we get is compressed air (50psi) ..either one 2L bottle or more than one... haven't confirmed yet. We get a micro-controller in which we can program

you can think of a pulse jet a la http://www.newscientist.com/data/ima...99993321F1.JPG without having steam you may be able to blow air-water-air-water fases low above the surface, any ideas yourself already? anyway keep me posted.

on second thought; airpump and prop will do better...

[Skippy]

designfreak:
There is a size restriction...2m long. ... minimize the volume possible of the boat
This boat needs to travel a straight line ... U-Turns ... S-Turns ...
objective is to maximize cargo ... travel to the other end of the pool and in the shortest time!
we can't strap the cargo down or anything

As DaveB asked, it would help to specify the problem a little more precisely. Are other hull dimensions restricted, or only length? You can maximize cargo for a fixed transit time, or minimize transit time for a given cargo. What is the criterion for grading performance?

[designfreak]

some ideas for propulsion is to use air motor and props.
don't know if 2 props would work better or just 1 prop.
i doubt i can find 2 props with 1 that spins the other way (for keeping boat straight) ...

basically, the race...
we want to

• maximize the # of cargo units transported
• minimize the amount of fuel (compressed air) used
• minimize transit time
• minimize amount of cargo lost overboard

basically, we have 4 rounds.
1st round : straight path where boat has a taught line
2nd round: straight path w/out taught line
3rd round: u-turn manoeuvre
4th round: s-turn manoeuvre

for all rounds, the boat must reach the other side of the basin. in the case of round 3, go back to where we started..

basically, the boat restriction ...
max dimension is 2.08m for length, 0.32m for beam, and 0.18m for draft
HOWEVER, for scoring wise...
we're doing a revenue, expense scoring...
successfully carry cargo in least amount of transit time will allow use to get more points.
expense would be....the size of the boat.
SO, a smaller size boat will cost less; therefore losing less points.

[yipster]

a design like this cuts wavedrag and wont loose his cargo with a hatch on top,
wont be fast cornering though...

i was too quik replying, with 18 cm draft u can not use the design above

[dionysis]

• maximize the # of cargo units transported
• minimize the amount of fuel (compressed air) used
• minimize transit time
• minimize amount of cargo lost overboard

1. Monohull
2. Beam to length ~ 7 , displacement to length ratio ~ 350
3. Place cannard keel forward 1/3 rd distance from bow, rudder 1/3 rd distance from stern
4. Wood usually floats, so say place some lead ballast deep down - preferably in the keel and rudder

something to think about - hope it helps

[Andrew Mason]

Trimaran, main hull maximum length with beam and draft scaled relative to the displacement implied by full load with fairly circular sections i.e. similar in shape to a racing kayak.

Outer hulls about 1/2 the length of main hulls and very low volume, purpose is to stabilise main hull and provide load carrying deck area. Keep as light as possible.

For propulsion, I would assume that an air motor for a model aircraft connected to a conventional model boat shaft and propeller would be the most efficient, something like - http://www.mutr.co.uk/products.aspx?catID=52

[yipster]

thats 2 thumbs up here

[DaveB]

I like Andrew's idea... It'd lilkely be the easiest to push and load... If you have any drawings I'd be happy to help you with preliminary stability stuff... (send me a private message) It could be good to have a look at before you build it...


Might want to think about two rudders, one forward and one aft, or perhaps having the props spread out so that you could reverse one to turn... don't know how difficult that is with compressed air... The other thing you could do is use some kind of anchor to turn... likely not what they were hoping for when the project was laid out, but unfortunately most yachting races come down to beating the rules... If the basin's shallow enough you might be able to drop an anchor off of the bow causing the boat to spin 180 degrees n' then release it and proceed... Sounds kinda rediculous, but I'm suspicious that power conservation will be critical... trying to control the speed of the props let alone reverse them could be tricky... Regarding control systems, I'm not a controls guy, but when we did it at my school the most successful system was three photocells... A light would be set up at the target and there'd be three photocells on the boat, one in the centre so when things were lined up it'd go forward, when they couldn't see it it'd spin, and there was also one on each side for fine tuning (off course corrections). I think we used a rugged giant pic...

What school do you go to (where are you in Canada)?

Cheers,

Dave

[Skippy]

One advantage you can take of the scaling is that the multihulls will be lighter and stronger than they would be at full size. A cat will have trouble turning, so the tri makes sense. If you're not allowed to tie down the cargo, you might want to make the amas larger and use them as additional storage.

If you really wanted to sneak past the U-turn rule, you could just make the boat a reversible double-ender, maybe even a cat.

[Tim B]

now a double ender... must it change direction, or complete a turn through 180 degrees.

Since you're using a microprocesser, you should be able to control the speed/direction of the boat fairly easily, you can also work out the position by a myriad of methods. for rudder control, you can use a small model aircraft servo, connected to the microcontroller (directly) there are loads of web-pages about that (remember to reference circuit diagrams though)

From a manouevring point of view a monohull might be adequate. It may not give you the ultimate capacity, but it would definitely get there. Really, the design of the hull depends on what sort of speed you're operating at. My best advice is to search the libraries for books on boat design and cargo ship design.

Also, you might want to look for a hobby shop near you which would sell model boat bits and pieces. a UK distributor is www.ripmax.com there might be a few ideas on there (if only for the control/propulsion side). Are you limited on the amount of gas you can use? if you can use several bottles, I suggest you put 2 forward, 2 aft on outriggers (one either side). That gives you 4 bottles of gas to play with, so you should be able to make the target range at almost any speed.

Hope some of this ramble helps,

Tim B.

[Andrew Mason]

If it was within the rules I would suggest the following -

Trimaran with minimum volume amas and large deck area for as much cargo capacity as possible. Make the amas circular in cross section below the water, but flare them a lot above the water so that they will cope with small amounts of heel witout going under. This may be important in the maneouvering part of the contest.

Absolute minimum hull weight required. If you can use carbon fibre pre-preg try to do so, the weight and strength advantage will be worth it. Every ounce you save is an ounce more cargo you can carry..

Single engine driving single prop for minimum weight and complexity. It will be difficult to derive accurate resistance, or engine power and torque figures for the engine you are using, so you will need to get several props of different diameter and pitch and determine the optimum by experiment using different cargo loads in a swimming pool.

If it is allowable, have a straight line mode with single rudder and a skeg at the back for directional stability. Also have a maneouvering mode with no skeg and an additional forward rudder that can be plugged into an existing shaft for more maneouverability.

The maneouvering parts of the contest will take more elapsed time so you will not be able to carry as much cargo on these portions of the contest, they will probably also require a propeller change.

If it was just the U-turn requirement I would say that a double ender with an engine and prop at each end might be feasible, but I think the S-turn requirement probably rules that out. Best to go simple, light and reliable. Usually these types of contests are won by simple, elegant and reliable designs, any complexity usually causes failures on the day with catastrophic effects on the scoring.

Finally, get it built early and optimise speed, endurance, load carrying and maneouverability under real conditions in the water, don't try to do it all on paper and turn up to the contest with an untested design. Remember, in theory, there is no difference between theory and practice. In practice, there is.