12' Low-Power Electric Planing Skiff – Design Review

[alan craig]

"It's more of a learning and building exercise and conversion to electric interests me."
I strongly recommend getting a wattmeter if you haven't already; you learn so much - and quickly - with it. In my case, a 14ft rowing boat which doesn't plane: 50W moves the boat passably on smooth water, 100W keeps up with rowers and 250W beats any of them in a similar boat. 1500W makes great big waves with the bow in the air. Oh, 2500W means the prop is full of weed!
If you want to experiment with a bigger prop and you have access to machining you can have a shaft extension to put the prop further back.

 

[Waterwitch]

This guy has a 3000 watt motor as you specified in your original post in his build.

 

[kmcsharry]

After full disassembly of the lower unit, a few design iterations and spec'ing out parts, etc. I have decided to move back to my original direct drive concept. I will keep working on the lower outboard unit with the intention of isolating the drive shaft, gear box and prop shaft with the intention of using only those parts and building the rest.

Current plan with rough sketches i did last night:

Single-Motor Electric Outboard Design Recap​

Boat
12 ft plywood skiff, DIY based on "ideal" low power planing design.

Motor
Flipsky 65161 IP68 inrunner, 100KV, direct-drive underwater motor. Rated about 3 kW continuous / 6 kW max, with published torque around 9 N·m max. Motor weight is about 6.6 lb.

Propeller
Start with the Flipsky/Yamaha-style 7¼ × 5 in white 3-blade boat prop on the compatible round-shaft version. Other 7¼ in Yamaha-style props can be tested later for pitch changes.

ESC
Flipsky 75200 Pro V2.0, one controller. Use FOC and programmable battery-current, motor-current, RPM and throttle limits.

Battery
48V nominal battery system, 50Ah, with approximately 100A continuous discharge capability and 150A absolute peak. Battery energy is about 2.4 kWh nominal. Current motor testing should stay well below the battery limits, initially around 50–60A battery current, with later testing around 60–70A if temperatures and prop loading are acceptable.

Expected electrical operating range
Normal development range about 2.4–3.4 kW electrical input. This keeps the motor near its useful continuous range while maintaining battery reserve.

Motor leg / steering unit
Custom laminated plywood airfoil leg, approximately 28 in long, roughly 4–4½ in fore-aft chord and about 1⅛–1¼ in maximum thickness. Estimated finished plywood leg weight about 3 lb, or roughly 3½–4 lb including reinforcement, hardware and motor mounting plate.

The lower motor mount uses an integrated aluminum face plate extending into/supporting the plywood leg. The motor bolts directly to this face plate at its rear end.

Steering arrangement
Permanent transom bracket with a fixed 1 in aluminum vertical kingpin. The plywood motor/leg block rotates around this fixed rod using upper and lower polymer bushings fitted directly into the wood steering block. The motor unit remains removable from the fixed transom bracket.

Motor depth
Nominal prop-shaft centerline about 7½–8 in below the hull bottom, adjustable roughly 6–10 in below the bottom during testing. With the 7¼ in prop, this places the top blade tip about 4 in below the hull bottom at the nominal setting.

Estimated propulsion-unit weight
Motor: ~6.6 lb
Finished plywood leg/mount assembly: ~3.5–4 lb
Controller: roughly 2–3 lb class, depending on final cooling/mounting setup
Prop/hardware/wiring allowance: ~1–2 lb

Expected complete removable motor/leg propulsion unit, excluding battery and fixed transom bracket: roughly 13–16 lb.

 

[kmcsharry]

This guy has a 3000 watt motor as you specified in your original post in his build.

Cool Boat!!

 

[alan craig]

Did you notice the no load power consumption in that video? 750w to 800w! Probably due to the angle grinder gearbox, shaft misalignment etc.