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Help sizing props to motor, voltage, amps, ect.

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Help sizing props to motor, voltage, amps, ect.

Old 01-19-2021, 03:42 PM
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Snowdrift
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Default Help sizing props to motor, voltage, amps, ect.

Howdy. I'm in the early stages of attempting to build an electric kayak, so this isn't really a question about RC stuff, but I will be using RC components and I want to go a lot faster than what a trolling motor will push me to, so I figured you guys might be able to help me get a baseline on the setup.

I've yet to build the hull, but it will be a planing vee hull with an OAL of 8' and a beam of 3'. I'm not sure what it's going to weigh, but the bare hull should be under 30 lbs. Call it an even 30 for the sake of argument. I weigh 180 lbs. and have yet to determine how much battery I'm going to want or need, but I think the all up weight will be around 250- 275 lbs.

Due to the limited number of props out there for something of this scale, my choices are limited, but have settled on 100mm props with a PD ratio of 1.6. I can't post a link yet, but if you go to aliexpress and search for "100mm pitch 1.6 prop" you'll see what I bought. I got a counter rotating pair and I won't lie... they're much stiffer and much higher quality than I thought they'd be. The blade form has all the right elements- reverse rake and sweep at the entry of the blades leading edge, decent radial cupping to keep the water on the blade, and a generous amount of cup on the trailing edge. Chucking a shaft into a drill and spinning the prop at 2000ish RPM in the water makes a considerable amount of thrust and I feel that two of these will be enough prop for what I want to do.

Where I'm stuck is which motors to use. Before I build the hull, I'd like to get the propulsion sorted out so there will be more experimenting, but I'd like to be able to spin these things up to 10,000 RPM. Purely guessing based on no experience whatsoever, I tend to think that a direct drive 360KV 56104 motor at 100v and 95A would get me close but before I drop the money, I wanted to run that by you guys. If they don't hold up to 10K RPM, I'll need to look for metal alternatives and will cross that road when I get to it.

I did do some searching before signing up to post and didn't see anything that would help me, but if I missed it and this sort of thing has been covered, just tell me to search and I'll go dig deeper. I appreciate it and if anyone wants me to, I'll make a build thread as I get closer to beginning work.
Old 01-19-2021, 05:56 PM
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Hydro Junkie
 
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Before I start, I want to be clear that I'm not an expert on electric drives either but I do know a bit about building boats and some of the issues.
With that said, let's get a few things clarified:
  1. the boat is going to be a 8X3ft kayak
  2. you are looking at making it a planing hull
  3. you are planning on 250 to 275 AUW
  4. you are planning on twin drives
  5. You are looking at 100VDC and 95 amps
Okay let's take each one above individually and go from there:
  1. Is the kayak your planning going to be a "sit in" or "sit on"? I have an Ascend H-10 "sit in" fishing kayak and it's only got 6-8" of freeboard with a depth of around 12". A "sit on" generally has less freeboard and most have drains to let water out to prevent them from filling up and sinking. My H-10 is 10X2.5 so it's longer and narrower than your planned on build. Either way, you will need to come up with a way to keep water out of the hull
  2. Kayaks are normally displacement hulls with a very efficient hydrodynamic shape. This shape is good for speed but not for load carrying. What are you planning on doing to get the hull to plane as many kayaks will try to submarine when a powered drive is installed. In fact, the most successful style of prop driven kayaks are equipped with a center mounted drive system that is either peddle or small electric motor powered to keep from overpowering the hull
  3. 250-275 is a lot of weight to put on an 8ft boat. I know the scale hydroplane club I belong to has a pair of Livingston catamaran style boats we use as chase boats and, with a deep cycle battery, trolling motor and me in one, I have to be careful to keep my 275 body away from the stern or I will swamp it in seconds. Your kayak has much less flotation than the Livingstons so my question is have you considered where you are going to locate your motors, batteries and speed controllers? To keep the boat stable, the weight will need to be spread out or all placed in the center of the boat, stressing that part of the hull with that much concentrated weight. My kayak is rated at 400lbs so I have to pick and choose what I'm going to put in it and balance it carefully to keep it stable in the water
  4. Twin drives has me wondering what kind of twin drives, outboard or inboard? Outboard drives would be easier to install, one on each side for balance and also don't have the concerns of leaking stuffing tubes or cooling air/water to the motors. On the other hand, inboards don't have to be removed when traveling or putting the boat in and out of the water, that is unless you're going ot get a custom trailer so you can "wet launch" it
  5. Now comes the big one, batteries, motors and controllers. You said you want 100VDC at 95amps. How long do you want to run at that power level, 9500 watts is a lot of power. In fact, that is the equivalent of a 13HP engine or, if you convert it to 12VDC, it's just short of 800 amps which, coincidentally, is the amperage needed to start the 5.3 liter V-8 in my Chevy Tahoe. Now that I've given you a real world example of how much power you're looking at, we can get back to the question of run tine. To get to 100VDC, you will need eight 12 volt batteries, wired in series to get close, that being 96 volts. Now comes the interesting part, if you put in 25amp-hour batteries, you would get no more than 20 minutes power on time and that depends on how much load the props put on the motors and they on the batteries.
I know I've thrown a lot of info out there but I'm really curious as to how much experience you have with kayaks and power systems. What I've put out in this post is all basic stuff and not the proverbial "rocket science'.

Last edited by Hydro Junkie; 01-19-2021 at 06:00 PM.
Old 01-19-2021, 06:57 PM
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Hi and thanks for the quick reply. For clarification, I'll be designing and building the hull because no kayak has the proper hull shape for the speeds I want. I looked at a Lifetime Hydros 85 and it was close, but still too much rocker and taper. Truthfully, what I have sketched up in Fusion360 shares more design language with a microskiff than a kayak but since my question revolved more around the power requirement for the prop in question, I didn't think I needed to extrapolate beyond it being a kayak. When I said 250- 275 pounds, I meant total weight including me, the hull, propulsion, battery, steering gear, fittings, ect. I'm about 180 on my own. To reference the Hydros again, that has a weight capacity of 225 lbs (on top of the 37 lb. kayak weight), is roughly the same beam and LOA, and due to the tunnel hull has quite a bit less volume than what I have drawn so far.

I meant twin straight shaft inboard drives. Again, my apologies for not being more clear on that point. As for the power, when I said 100V/95A, that's the upper limit of the 360KV 56104 motor I mentioned and I was really only using that as an example. However, I think I may have misstated my question. What I was asking was how much motor I need to spin these props up to 10K RPM in the water to even see if they'll stay together. I really have no concept of how much power a prop this small will absorb. 10K would give a theoretical speed through the water of 60 MPH with zero slip and 48 MPH with 20% slip, so I'm using that as my ultimate RPM limit. It may very well come to pass that I only have to ask for 5000-7000 RPM to get what I want. If the props can't stay together at 10K I'll go with jets, and I'd rather cross that bridge now before the hull is built. Oh yeah- I'll be running the props in pockets so converting from shaft to jet would be more than just cutting a few holes out. I love building, but I hate chasing ghosts.

To answer your curiosity about my experience, I've been around boats for almost 40 years, got home schooled on boat design by a retired naval architect and have designed and built four boats between eight and 14 feet, have extensive experience rigging all sorts of full scale propulsion systems, was an electrician on military aircraft in a previous life, and can turn a Detroit 12V71 inside out with my eyes closed. What I don't know is how much power this little prop will need to turn 10k and figured someone here with more experience with model boats would have a rough WAG ballpark idea of where to start.

Last edited by Snowdrift; 01-19-2021 at 07:06 PM.
Old 01-19-2021, 07:35 PM
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That really clarifies things a bit.
When I bought my H-10, I was looking for a boat I could take easily that could handle my weight, be stable and be able to also handle the weight of a scale hydroplane full of water(RTR, my boats weight between 7 and 14 lbs and flooded, almost triple in some cases), thus, the H-10
Since this is now sounding more like a prop experiment first and a boat build second, let's look at your prop and how to test it.
  • The first thing to remember is that you can't test a prop in a test tank as it will dig itself a hole and will only need to hold the water out, giving a false result.
  • The second thing to remember is you will need to have some way to measure the RPM, current, voltage and thrust needed to get real world results.
To bad you're not in my area(Just north of Seattle) as we could probably come up with something and a place for testing. I don't know of any fresh water sites in your area that would be large enough to test on and yes, I've been to Ogden many times as I used to live in Idaho Falls and have family in Layton.
Getting back to the subject at hand, there is a shop in my area that might be able to help you out:
Prop Shop, LTD | Seattle's leading Impeller and Propeller Repair
They may be able to answer your questions or know how to do the testing your self. That is where I would start, with someone that knows props and this shop does
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wnewbury (01-20-2021)
Old 01-20-2021, 12:10 PM
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Snowdrift
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We have a boat and spend time on the various fresh water lakes around here- Pineview and Willard are a half hour away and bigger puddles like Bear Lake or Strawberry Reservoir are under two hours away. As I typed that, I just remembered that the previous owner of our boat had mounted a trolling motor to the cavitation plate on the outdrive, so I could make a bracket that bolts up to the holes left behind. If I got a 500KV motor, the dual batteries on the boat in series would provide enough voltage for 12K RPM unloaded. I only need to run it for a couple minutes at the most to load up the prop and see what happens, so I'm not too worried about damaging the deep cycle lead acid batteries.

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