Selph-Inflicted

I am working on a project in which I need some pointers on. I know this is the airplane section, but my application is for a boat. The information I'm looking for has to do with electric motor set ups with air props.

I will tell you a bit about the project before I ask my questions. The boat will be 26-30" long X 13"-15" across the bottom. When completed may between 5-7 lbs and will be equipped with a GPS, depth finder, and an onboard CPU. The boat will not be very fast but will have to navigate a course and take depth readings at pre-selected GPS coordinates. The boat will be propelled by an air prop. I will need about 20 minutes of run time (not continuous). I'm not looking for speed (max of 6 mph permitted) but looking for torque.

Now my questions.

What size motor do you guys recommend (preferrable brushed, due to budget)?
Should I use a gear reduction to allow swinging a larger prop?
With recommended motor and gear reduction, what size prop can I swing?
With recommended set up will 2 4200 Mah 6 C cell battery packs work with a micro switch to switch from dead battery to charged battery w/o returning to shore?
Any other recommendations, thoughts, or opinions?

Thanks in advance for your time!

ozrcboy

Hi Selph-inflicted,

That is an interesting one...

In electric plane world we talk about volts, amps, thrust, and pitch speed. However, none of our standard rules are an exact fit for what you are after - for example we say you need 150w/lb for unlimited vertical - well what does that mean for a 6lb boat?

So with respect to your questions:
1. brushless will be more effecient. If running time is important to you I would go brushless. I have no idea how much power or thrust you need though. Perhaps you could hook a fish scale up to a line and then use it to drag your boat through the water at about the speed you want to go..., read off the fish scale and come back and let us know what it reads. Alternatively, someone here may be clever enough to figure out what sort of thrust you need. However, my gut feel is that for a 7lb boat you are going to want at least a 1000g of thrust (about 2 and a half pounds) - but I really am just guessing - there's an old rule of thumb that says each input watt gives 4g of thrust, therefore you need 250watts of power..
2. If you use a brushed motor then yes - use a gearbox to increase prop size. Larger props generate thrust more efficiently than smaller ones. If you go brushless you can either buy an outrunner designed to turn a large prop directly, or an inrunner which you will need to bolt into a gearbox (like a brushed motor).
3. you can swing as big a prop as you like within the design limits of your craft. For example a 7.2v with a 1500kv (revs per volt) outrunner could spin an 8x6 drawing about 12-15A for somewhere in the order of 100 watts and 400g of thrust. You could also spin a 900kv motor and something more like an 11x7 assuming you have the room.
4. Rather than putting a switch between the packs you connect them in parrallel - this makes the equiavlent of one 6c 8400mAh pack, and removes the complications of the switch.
5. This sounds like a really interesting project - but we really need to know how much thrust you need to achieve the speed you want to. Also some idea of what size prop you can spin within the limitations of the boat, and a budget would be good.

Cheers,
oz.

Selph-Inflicted

Ozrcboy,

Thanks for your response and info!

I'm building this boat for a client and told him that I was not responsible for the power plant engineering due to the lack of my experience with electrics. I prefer nitro personally. Since there will be the need for no thrust at certain times, they need to use electric power. I was looking for some info so I could point them in a certain direction. Since I'm not responsible for the power plant, I'm going to take your info and pass it on to him.

When the project is completed, I'm going to ask the college team to take video of it when they use it in competition. When (and if) I get it, I'll share it with you guys in this section.

Thanks again for your info and time to post it! Greatly appreciated!