Choosing a Power System

[chickenblender]

I'm currently working on my first electric boat, a 33" Dumas PT-109. I know a lot about choosing an appropriate power system for electric planes and helis, but I need some guidelines for doing the same thing for a boat. My plan is to use a brushless motor and an 3-cell lipo power system. I would like to know:


How do I figure out how much power I need to generate? Is there a rule of thumb based on boat size or weight? I'd like this boat to be pretty zippy -- faster than scale.
How do I figure out how to match a prop to a motor? I can choose pretty well for air props, but obviously a prop turning in water is a different animal.
I'm sure there's a trade-off between torque (big, slow prop) and speed (small, fast prop) just like with airplanes. But unlike airplanes, do I have to worry about prop behavior (cavitation?) at excessive speeds?

[Rex R]

hmm how many motors/props do you wish to run? for a single motor/single prop set-up I'd go with a feigao 540 10xl running a prop in the 40-50mm range(use a 36-40mm prop to 'break in' the drive shaft).

[chickenblender]

I'll be running two props. What I'm really interested in knowing is, what are the "basics" for matching a motor to a prop with boats? And is running high RPMs a problem?

[pompebled]

Hi Chickenb,

On a boat, a submerged running prop with 25000 rpm is considered high(ish), depending on the set-up it may even cavitate at those rpm's.

Your size hull will have sufficient power with two 28mm inrunner brushless motors that have 2000-2200KV (on 3S), the props will (have to) be relative small, so I'd check out what props you are going to use and choose the motors accordingly.

You can also consider outrunners, more torque, allowing bigger props (more efficiencient) with less pitch. If you don't run full throttle all the time you may even get away without watercooling them (or else watercooling the motor mounts will be sufficient).
It's a matter of selecting the right size.

Keep in mind, you'll need one ESC for each motor and the forward/ reverse ones tend to be somewhat expensive...

Regards, Jan.

[CG Bob]

Figuring out how much power you need is a fairly simple calculation. Here's what you need to know:
Scale of the model, you will need the cube of the scale (scale cubed)
HP rating of the full size prototype
Battery voltage you want to use

For your Dumas PT, the scale is listed as 1:30, the scale cubed is 27000.
The early PT (109) had about 1250 hp per engine - 3750 total hp; late war versions had 1500 hp per engine -4500 total hp.
Since you said you want twin screws, we'll use 2200 hp as a base to figure motor size.

The formula is:
Proto HP x 746 = Proto Watts
Proto Watts / scale cubed = Mdel Watts
Model Watts is a theoretical figure, since we can't scale down the water, so add 10% to Model Watts
Scale Model Watt = Model Watts +15%
Model Current = Scale Model Watts / Battery Volts

2200 x 746 =1,641,200 Watts
1,641,200 / 27,000 = 60.8 Model Watts
60.8 + 15% = 69.9 Watts (Looks like 2 Brushless motors rated at 70 Watts)
69.9 / 11.1 = 6.29 Amp (Looks like a 10 Amp ESC is the minimum needed for each motor)

[chickenblender]

Thanks to both you guys that helped me out here. That was exactly the kind of info I was looking for.

Cory