bootleg electric boat project - I know my loads, what do I need for batteries etc.?

[0xkruzr]

hi all - inspired by this guy:

and these guys:

I've decided my spring project is gonna be building an electric (eventually solar-electric) boat for cruising around our local lakes. no gas allowed, so low-horsepower electric motors it is.

• motor: https://www.amazon.com/dp/B0BB996ZWJ/ref=syn_sd_onsite_desktop_0?ie=UTF8&pd_rd_plhdr=t&th=1 these usually have crappy resistive throttles that always chew up 100% of max draw while the motor is turned on, so I'd probably replace the head unit with a:
• PWM: https://www.amazon.com/dp/B0991XYV4...olid=1GM6TPCAZ7DF7&psc=1&ref_=lv_ov_lig_dp_it
• buck converter for 12V loads like lights, fans, etc.: https://www.amazon.com/dp/B07JZ2GQJ...olid=1GM6TPCAZ7DF7&psc=1&ref_=lv_ov_lig_dp_it

so, that motor will draw a max of 35A at 24V. the PWM can handle 40A of continuous current (60A peak), so that should be fine. the transformer shouldn't draw more than 3A or so at 12V (so 1.5A at 24V), so all in all that's well under 40A of max continuous current draw. I figure two 12V 100Ah batteries should be good to start with as energy storage, so that's 100Ah of supporting 24V load. great!

okay, so I'm not this kind of engineer -- I design and build HPC systems, not DC electric control systems -- but I know enough to know that I don't know anywhere near enough about how to proceed from here especially if I'm adding solar eventually, I feel like I need some kind of controller gear in the middle of all of this managing power input and output. does anyone who's worked on a project like this before have ideas about obvious things I'm overlooking?

TIA.

 

[Rednecktek]

Have you considered a rectifier and 3 phase motor?

 

[0xkruzr]

Have you considered a rectifier and 3 phase motor?

not even slightly, mostly because I don't know why I would! I'm all ears

 

[Rednecktek]

not even slightly, mostly because I don't know why I would! I'm all ears

My understanding is that with a rectified 3-phase electric motor your speed control would only put through the amperage needed for the motor rather than 100% power doing 20% work and 80% waste heat.

 

[0xkruzr]

My understanding is that with a rectified 3-phase electric motor your speed control would only put through the amperage needed for the motor rather than 100% power doing 20% work and 80% waste heat.

ah. I don't know that such a thing exists without me having to put a lot of work into a gearbox, waterproofing, etc., as opposed to just putting a PWM in front of an already-existing trolling motor to accomplish the same end.

 

[Rednecktek]

I've seen it done on electric motorcycle conversions that don't use gearboxes so I thought it might be an option worth looking into. ?

If you were mechanically up for a project you could take any gas outboard, split the engine off the top, and just put in an electric motor with the same shaft size. That way you could spec a motor that fits your window more exactly. But that would be a shop project.

I'm having a hard time understanding how they get an 800w motor to produce 3Hp though, the numbers there just don't work in my mind.

That PWM should work fine but it's going to be much more effective if you can pair it with a 48v motor so you can get the motor up to full speed. I worry that if it's voltage control like many DC controllers that if you try to crank it up to its 60v setting you might fry your 24v motor. It will also generate less heat if it has to supply less amperage, i.e. 18a @ 48v instead of 35a @ 24v.

 

[0xkruzr]

I've seen it done on electric motorcycle conversions that don't use gearboxes so I thought it might be an option worth looking into. ?

If you were mechanically up for a project you could take any gas outboard, split the engine off the top, and just put in an electric motor with the same shaft size. That way you could spec a motor that fits your window more exactly. But that would be a shop project.

I'm having a hard time understanding how they get an 800w motor to produce 3Hp though, the numbers there just don't work in my mind.

That PWM should work fine but it's going to be much more effective if you can pair it with a 48v motor so you can get the motor up to full speed. I worry that if it's voltage control like many DC controllers that if you try to crank it up to its 60v setting you might fry your 24v motor. It will also generate less heat if it has to supply less amperage, i.e. 18a @ 48v instead of 35a @ 24v.

oh, interesting stuff about the PWM. is there a different model I should pick that would be more suited to it? I don't mind spending a little more for a part that makes more sense. ETA: of course it might make way more sense to pick a motor more suited to it entirely; this one appears to have a PWM built in judging by the note about "continuous throttle." https://www.amazon.com/dp/B093ZKHNF...olid=1GM6TPCAZ7DF7&psc=1&ref_=lv_ov_lig_dp_it

 

[wholybee]

I'm having a hard time understanding how they get an 800w motor to produce 3Hp though, the numbers there just don't work in my mind.

It's a marketing trick. It is "equivalent" to a 3HP ICE because it develops full torque at 0RPM. Since an ICE doesn't produce max power at low RPMs, the "real-world" performance is equivalent. They will have similar acceleration from a dead stop. Or some such non-sense. Even the expensive high end electric motor manufacturers fudge numbers on the same theory.

 

[wholybee]

Have you considered a rectifier and 3 phase motor?

Brushless DC motor?