RCU Forums - View Single Post - Help sizing props to motor, voltage, amps, ect.
Old 01-19-2021, 05:56 PM
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.