RC Boat

[BBray_T1296]

So my friends and I are planning and designing a fairly large scale (25ish lbs) R/C boat really just for fun.
We have opted to design for a single propeller, and in particular, this one, because it is metal and it is hard to find propellers near that size anywhere else conveniently.
Unfortunately, the few specs you see on the page are the only specs available, lacking pitch and max speed/torque figures.
We have little knowledge of designing nautical propulsion, and without the propeller in hand, it is hard to do anything but guess as to what will work best

Please tell me what you think of our proposed setup, which has been provided in the attachments below.

We do not want to do anything very exotic, as we are keeping it relatively cheap and want the construction to be simple, with preferably no difficult parts.

I feel like 30 RPS seems reasonable for how shallow the propeller looks and its diameter. The small ratio lets us put the propeller further down in the hull than with a larger spur gear. using the VexPro 11t pinion is also an option, though I question its durability. The boat is not intended to be particularly fast, we just want it to move at a reasonable clip of maybe 5'/s top speed
I estimate the pitch of the blades to travel ~4" per revolution, assuming perfect bite, that equates to 10'/s, but there will obviously be plenty of slip, so I believe 5'/s is reasonable. There is also, at normal load, 9lb-in of torque which with 2" blades, is reasonable to me. Of course there are inefficiencies in the power train as well.

Again, we really don't know what the actual load on the motor will be, because we don't know the load on the propeller without buying it. We are using the CIM's "normal load" spec as a best case.

The electronics will be a combination of an aircraft receiver, and an arduino. The signal outs on the reciever will enter the arduino which will interpret several inputs for other various functions the boat will do, and pass the throttle to the CIM by means of a motor controller. We are likely to use just a 2 way relay like a spike, for either full forwards or reverse, but may use a motor-controller, though we want to keep cost down on devices that might short out. The main battery is a 12v sealed lead acid one similar to the FRC ones, but only 9Ah (~6lbs), and some AAs for running the electronics

We feel that a CIM with some added silicon sealant will be capable of surviving in a potentially damp environment, especially compared to other motors

Any suggestions/questions/comments/concerns are more than welcome.

CIM info here

[IndySam]

There are lots of places online to by RC boat parts and many hobby shops carry them. You might check into buying a used gas boat on ebay or from a local shop and modifying it with your motors and control.

BTW you can not use airplane radio frequencies for surface use so you will have to change crystals.

[EricH]

Originally Posted by IndySam

BTW you can not use airplane radio frequencies for surface use so you will have to change crystals.

Showin' yer age, mate. If it's a 2.4 GHz system, like most of the newfangled control devices these days, you're probably OK. Just double-check the restrictions before you get the system.

(And yes, I do know how aircraft frequency control is supposed to work at a given flying field. I have also heard about what happens when a boat is using an aircraft frequency near a flying field, many years ago and very ugly the incident was.)

[Josh Drake]

Your propeller choice looks similar to something we use at work to agitate molten aluminum.

[DonRotolo]

That propeller is a mixing propeller. Yes, it will work, but not nearly as well as a propulson propeller. R/C boat props are easy to find if you look in an R/C boat store/web page

I think you will find that 30 rev/sec will cavitate badly. But try it anyway: With the prop chucked into a drill see what happens in a bucket of water.

5'/sec for a 2' boat is about 75'/sec for a 30' boat, which is (scaled) racing speed. Maybe a little slower should be considered. A 2:1 slip is very high, in practice much lower is reasonable.

Your hull design needs to have a way for water to get to the 'pull' side of the prop, leaving a 1" vertical space is not enough. If you look at a real boat you'll get the idea.

When building your boat be sure you consider the center of gravity (and center of flotation) very, very carefully, or your boat will flip over as soon as you try to turn. Real boats use steel or lead in the keel for weight.

An alternative: Consider building a 'swamp boat' which uses an air fan for propulsion.

Regardless looks like a fun project. Good luck, take pictures and post them here!