Choosing Motors / ESC's with Little Additional Information

[WarEagle2024]

Hey everyone,

I'm very new to the world of RC boating, so please bear with me on this one. I am currently working with a team of my peers to compete in a small electric boating competition next May, and have been assigned the task of designing the electrical propulsion system (motors and ESC's) for it. So far in the process we have decided that we are going to use a 72in deep-v hull due to the requirement of needing to be able to hold a removable 60lb payload, and use a 2-brushless motor design. As far as propellors go, we are choosing either 2-3in diameter ones, but that's as far as my knowledge goes. With a 55-volt battery limit in mind, I had went ahead and selected this motor https://www.offshoreelectrics.com/pr...rod=tp-5670-v1 and this ESC https://www.offshoreelectrics.com/pr...tw-300-7300410, but have no clue if they'd actually work for my application. Any tips or tricks as to how to select the right ones? Thanks!

[fweasel]

Whats the speed goal? How long does the boat need to run for? Is there a battery weight limit or a total mAh capacity limit? We need this info to recommend an RPM, which determines your motor kV and then will in turn tell you were you need to be in terms of prop selection. The 60lb payload takes the majority of this forums personal experience of running boats out of the equation. The hull is going to run wet (deep in the water) which will dramatically increase drag and amp draw when compared to how we runs much lighter hulls on the surface of the water at high speeds.

The other challenge that immediately comes to mind with such a heavy, and potentially slower setup, is water cooling. You might not be reaching the necessary speeds to pick-up water passively with conventional rudder or through hull water pick-ups. You may need a water pump to actively circulate water through the motors and ESC's.

[WarEagle2024]

Our goal is to reach at least 30mph, and it must run a 2-mile loop course with a turn at each end (hopefully we can get a 30ft or less turn radius but that might not be possible). The only limit on batteries is the overall maximum voltage of 55V, so we can play around with other parameters as we please. We are planning upon using two water pumps (one for each motor / ESC) to supplement any cooling loop as well. In terms of prop selection, we are also free to design and fabricate our own, so worst case scenario we can design one using CFD simulations that meets our criteria.

[Peter A]

I would go two of these motors https://www.offshoreelectrics.com/pr...s-56123-v1-500. You want a low kv so you can be more efficient. Are you running surface drive or submerged props?

With your speed goals submerged props will be fine, the added benefit is much better, tighter turning.

How long do you have to run for? This will determine how much battery capacity you will need.

As far as props go, you are better off purchasing some cnc props, something like this that you can get in left and right rotation https://www.offshoreelectrics.com/pr...od=cnc-5214250

You will not need water pumps if you use submerged props and put water pick ups behind the props.

[WarEagle2024]

We will be running submerged props, so glad to hear that will be our best bet. If we are running at 30mph for 2 miles, we will most likely be completing the race within 4 minutes hopefully (if not faster). Is there any good way to analytically determine the amp draw the motors will have, or is that only found experimentally? If we can estimate it now we can find the Ah capacity we will need. Additionally, is there a best way to determine the best prop diameter for our boat? Finally, would you know of any good calculators or methods online to determine the torque created by a motor / prop combo? Thanks for all of the help so far!

[Bande1]

you dont need 2 motors. that just complicates everything. you need a super low KV single motor and a high voltage ESC (I have one for sale for half the price of that ZTW). your amp draw will likely be 100-120 amps or 4000w-4500w @ 30mph on 12s. that is what you should run for simplicity. 2 6s packs in series around 8000mah will be plenty.

the TP motor KV options are all far too high. you need 150kv-400kv. the lower the better as it allows for a bigger prop.

[fweasel]

I disagree that only one motor is needed. There's a 60lb payload on board, plus electronics and batteries.

[Bande1]

what do 2 motors provide that one large one doesnt? more weight, cost, complexity?

[fweasel]

what do 2 motors provide that one large one doesnt? more weight, cost, complexity?

Twice the thrust. The additional electronics weight is inconsequential when compared to the payload weight. Then again, this is all conjecture as I'd venture to say no one here has built and tested a boat this big & heavy.

[Peter A]

We will be running submerged props, so glad to hear that will be our best bet. If we are running at 30mph for 2 miles, we will most likely be completing the race within 4 minutes hopefully (if not faster). Is there any good way to analytically determine the amp draw the motors will have, or is that only found experimentally? If we can estimate it now we can find the Ah capacity we will need. Additionally, is there a best way to determine the best prop diameter for our boat? Finally, would you know of any good calculators or methods online to determine the torque created by a motor / prop combo? Thanks for all of the help so far!

Unfortunately you will have to go with trial and error on this one. With two counter rotating props torque should not create any issues. What I suggested should get you close to where you want to be. Allow for bigger prop sizes in setting up, but 3 blade over 2 blade may increase speed with the cost of more power. You will have to play with props perhaps, but they are not the expensive part here if you use cnc ones. Note: use low pitch props 1.4 pitch or less.
12s 10,000 mah per motor should get you there easy on a 4 min run, but again you will just have to test.

[WarEagle2024]

After lots of discussion with the group, we have indeed decided to go with a single-motor design (not my decision unfortunately) that is driving two counter-rotating coaxial large-diameter propellors (Try saying that 5 times fast!). We are probably going to stick with the SSS 56123 motor in the 700kv configuration (we are limited to 55v of battery voltage for the competition so 14s is the max we can give it, and we want max power from the motor) and according the the spec sheet that would result in it drawing up to 248 Amps. Since we are only doing a single motor now, would the aforementioned one still work, or should we shoot for a larger one? We have also considered doing dual motors reduced into one shaft with a gearbox, but are shying away due to concerns related to our budget and power losses from yet another gearbox. Any help or knowledge is greatly appreciated, and please don't hesitate to correct me on any of my assumptions. Thanks!

[Bande1]

I got a 240A 3s-16s esc if you are interested. 700kv is more for 12s and 12s is a lot easier to configure.

TP makes 100mm motors

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as well they now have 120mm motors.

a trolling motor for a 14' jon boat generally pulls about 50A at full throttle if I remember right. that is on 12v.