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 :P 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/images/ns/9999/99993321F1.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]

http://www.swath.com/History/Boericke.gif
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. :D

[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.

[designfreak]

thanks for all the ideas!!
hrm, don't know if this will work but before reading all the replies since i last was here....

thinkin of using air motor from toys and props...3 blades probably and 1 prop!
We can choose our compressed air...we can have 500mL, 1L, 1.5L or 2L and we can have as many as we want.

thinkin of using wood as material. but definitely waterproof wood and probably seal it with something else.

hull shape...flat bottom, flat sides...curved bow and stern
thinking of using 1 prop only cuz can't find any plastic ones of the desired size. any suggestions where i might be able to find and order them (need to have quick shipping!!!) thinkin of plastic 3 to 4"
3 blades.

thinkin of place it at the back but problem is, don't know where to put rudder!!!

oh yes, forgot to mention, no anchor 'cause this is not done in shallow water

[Andrew Mason]

A google search for model boat propellers gives this link among many others - http://www.hobby-lobby.com/grboatprop.htm

regarding the flat sides, flat bottom, curved bow and stern, you would actually be better with curved bottom and sides with a straight bow and stern. You are trying to minimise the wetted surface area, hence making the sections as round as possible, as well as maximising the length by using the full 2m waterline length with vertical stem and stern.

If you don't have access to advanced construction techniques and materials, strip planked balsa with a light layer of epoxy and glass over the top (or even just epoxy by itself) would be adequate.

Regarding rudder position, put it behind the prop.

I'm really surprised that there is no limit on the number of bottles, this turns it from an optimisation problem of carrying the most cargo for a fixed amount of fuel, to a trade off between carrying bottles for fuel or carrying cargo - if this is the case it will end up being a contest for who can build the boat with the largest total load carrying capacity. Maybe you would be better building something the shape of a small oil tanker.

[RWL]

Just a thought, but can you use the air containers as part of your hull? I am thinking along the lines of an aluminum party boat catamaran, with the load carrying hulls made of some very thin PVC tubing which you can get at the hardware store. You can probably buy a simple glue on plug for the "blunt" end, assuming an inside flange so there is no extra drag and heat up the "pointy" end of each tube with a good hot air gun and pinch it in a vise. Cut the bow angle you want and file and sand it so it is symetric. This would allow you to attach a deck between the two "hulls" and put your propulsion and rudder(s) in the tunnel between. How about a paddle wheel in the tunnel? Not sure if you can get the efficiency you need though.

just a thought.

ps. all of the project/competitions I have seen like this tend to be won by simple designs that are well built and "tuneable". I suggest you use your controller to run an electric motor driven threaded rod with a simple plastic threaded nut. the tiller end should be attached to the nut, so when you turn the threaded rod the nut moves and the tiller turns. An advantage to this is that your batteries will drain as the competition goes on. If you rely on electronic timing for steering a certain angle, they usually don't go to the same points in reality. Use simple electric trip switches at the maximum turning point you want, and then use your controller to sense when a switch is hit and pause, or reverse. The threaded rod also allows you to fine tune straight travel.

[Tim B]

I don't know what type of microcontroller/microprocessor you're using, but I'd go for the hobby-servo route. It's light (9 grams) and you can get an exact position each time. The microcontroller handles the pulses. They will run directly off a Basic Stamp. I have tried it (with great success). When the servo isn't moving it goes into a low-power mode and draws practically no current. The electronics needn't be heavy so design it for lowest power consumption possible then use the lightest batteries available (remember to include an on/off switch).

Incidentally, the Basic Stamp (from Parallax) is an absolute pain to upload software to. If at all possible use a Basic Stamp 2 in preference.

Cheers,

Tim B.

PS... RWL, batteries draining is an advantage????? They remain the same weight you know.

[RWL]

I probably didn't explain myself very well. I meant that as the battery capacity dropped, clock operations tend to change. Thinking back, and reading your comments, I am probably talking about older technologies. It appears as if newer ones have this under control. I still suggest that the design have a mechanical "tuning" ability for aligning rudders etc. to the electronic/electrical controls, because project models of this sort are rarely dead straight or symetric and you have to tweak them a bit usually.

RWL

[designfreak]

I don't think a 2m length boat would be such a good idea as the same boat must be used for all rounds. It'll be hard for a 2m boat to do a 180 degrees u-turn and as well as the S-turn. At least the turning radius has to be huge...

having a curved base would be good but we want to load our cargo efficiently that's why i chose the flat bottom. as well, it would be really difficult to make a curved base comparing to just a curved bow and stern.

and yeh, basically it really is a trade-off between carring lots of cargo and going slow.... with a big boat or a small boat, less cargo, be able to turn better

putting the rudder behind the prop, wouldn't the water be turbulent around there so rudder wouldn't have that big of an effect? I really don't know...

RWL: have thought of using a catamaran design but it will be hard to turn. As well less cargo can be stored on board


i'll keep you guys posted on my progress and definitely will post some pictures when it's finished. :)

[Tim B]

The rudder usually lives behind the prop, usually because it's convenient, and also because you get some thrust vectoring (admits... not exactly right, but good enough).
Anyway, I was thinking about this this-morning, and there are some other plausible options...

the kort nozzle - for increased efficiency
the schottel drive - includes rudder and prop in one
Voith Scneider drive - gives thrust in any direction but insanely difficult to build
azimuthing drive pod - Power and steering in one package, but fairly simple
bow thruster - for manouvreing only. basically a reversible pump, pumping water from one side of the boat to the other, hence change in momentum, thus force.

My preferred system would be to make the aft-end of the boat reasonably flat and use two azimuthing pods. Then put a bow-thruster in to swing a longer ship around the U and S turns. Make the bow thruster electric!!! see my next post for diagrams...

Tim B.

[yipster]

Tim, you forgot putting one prop in reverse :D

[Tim B]

yeah, good point.... as long as you have twin shafts.... anyway, here's a few sketches (hopefully they'll clarify a few points.

[Andrew Mason]

Tim

I think your idea is an excellent one as long as designfreak has the fabrication skills.

I would suggest an azimuthing pod at the bow and a skeg at the stern. In straight line mode the design would have excellent directional stability, while in S-turn and U-turn mode the maneouverability would also be good.

I think it is important in a contest like this to have low complexity, using a single engine in an azimuthing pod at the bow handles the functions of propulsor, rudder and bow thruster in one.

Now you need to come up with ways that an azimuthing pod could be designed and built with standard parts and minimum engineering.

[Tim B]

Andrew,

Small drainpipe? It's good stuff you know. The only hard parts to build is an adaptor to go from the motor to the prop (but that was required anyway) and the seal onto the bottom of the hull (for which I suggest a sliding fit of the central tube and a thrust-race top and bottom). As it is a school project, I suspect there are vac-forming tools, The pod itself not too hard a shape to make a plug for.

And on number of pods... depends how fast you want to go.

Tim B.

[designfreak]

for those of you who're interested in how my proj's coming along...

hull is done (used plywood)
for those who want to see a picture of what it looks like, PM me 'cause I don't wanna put on a huge pic here that slows down the loading of the thread.

Gonna go with 2 air motors (pulled them out from toys... you guys probably have heard of this toy...Air Hogs. AWESOME TOY! :cool: )

Props... gonna make my own 'cause dont wanna risk delay if I do order online
it's gonna be very tough... :(

rudder...made w/ sheet metal

gonna be using the servo and program the microcontroller to control the servo to control the rudder

[Stephen Ditmore]

When you're thinking scale model keep in mind that stability does not scale with displacement alone. Let's say you scale down a container ship by a factor of 100 (from 200 meters to 2 meters). Assume both ship and scale model are loaded such that they float with the same beam to draft ratio, and both have their center of gravity at the waterline. The displacement of the model will be 1/1,000,000 that of the ship and the metacentric height will be 1/100 that of the ship, so the model's stability will be 1/100,000,000 that of the ship! For the model's stability to be roughly equivalent relative to its displacement the beam/draft ratio must increace by (the scale factor)^(-1/3). If the model's displacement is 1/1,000,000 that of the ship its beam/depth would need to be 4.64 times that of the ship for it to have the same relative stability!

If you can't compensate for this effect on stability by changing the beam/draft ratio or by lowering the model's center of gravity, you may want to go to a multihull configuration, as was suggested earlier. For carrying payload I might favor a catamaran over a trimaran, as do builders of catamaran fast ferries. A catamaran gets more stable as its weight increases; a trimaran doesn't as appreciably.

[Tim B]

A note on props...

If you design/build your own propellers, you'd be well advised to make them fit a standard shaft. If all else fails then you can always swap it for a commercial one.

Personally, I'd look in a few modelling magazines, write down some phone numbers, and mail-order. Delivery takes about 3 days to a week, depending on who you go to.

Cheers,
Tim B.

[designfreak]

well guys! tomorrow is the day of the competition!!!!!!!!!!!!!

we made our own rudder, own prop
and yeh, makin own prop is such a hassel...
interesting fact tho... we made a few sizes and the larger size prop doesn't seem to work at all. it doesn't move the boat at all!!

did some tests here and there and it's looking pretty good tho we're still eiffy bout the turning part of the competition

[Tim B]

check the pitch on the largest prop. you need to accelerate some water to get any thrust.

Tim B.

[designfreak]

DONE!!!! :cool:
successful in all rounds 'cept having a hard time keeping boat straight because there were filters going on in the pool so there're whirlpools in some areas

u-turn ... didn't do it 'cause microcontroller wasn't set when started!!! :mad:
s-turn...good stuff!!!

however, it was really eiffy though since most of it was guess-timate.. .. and go with whatever feels right and program the microcontroller.

ah well, it's done. it went awesome.

thanks guys for all the feedbacks given for this thread!
i'll post some pictures of cool boat design later on
16 days to design and build boat...
need some sleep now