Boat Design Proj

[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

[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! )

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