12' Low-Power Electric Planing Skiff – Design Review

[kmcsharry]

Hi All,

My name is Kevin and this is my first attempt at designing and building a hull/boat. I am looking for feedback and thoughts about my current design.

I’m developing a purpose-built 12' plywood electric skiff with the primary goal of achieving stable planing performance on approximately 2-3 kW.

The current hull is 12' LOA, with a 41" maximum bottom width and 40" transom bottom. The aft hull is essentially flat with zero aft rocker, a long straight planing run, and a progressive V forward. Target loaded displacement is approximately 300 lb, with 330 lb as the upper design limit.

Construction is lightweight stitch-and-glue plywood: 9 mm bottom, 6 mm sides, 18 mm transom, with three transverse structural frames and twin longitudinal plywood girders. The objective is a light, stiff monocoque structure with a finished bare-hull target around 75–85 lb. The hull will be built from 1 transom, 2 sides and 2 bottom panels with bracing at S3, S6 and S9, 2 longitudinal bottom stiffeners and knee brackets tied into the transom, gunwale.

CURRENT ELECTRIC OUTBOARD – DESIGN NOTES
Motor:Waterproof BLDC inrunner, 100KV, IP68. Rated 3,000W / 4.0 hp continuous, 6,000W / 8.0 hp maximum, approximately 6.6 lb. 12mm shaft. 25.2–84V operating range.
Electrical System: 36V nominal battery system, controlled by programmable VESC/ESC. Intended normal operating power approximately 2–3kW / 2.7–4.0 hp.
Motor RPM: The 100KV motor theoretically turns approximately 100 RPM per volt unloaded.
36.0V nominal = ~3,600 RPM no-load
Actual RPM with the propeller loaded will be lower. A reasonable preliminary design/test range is approximately 3,000–3,500 loaded RPM, but actual RPM must be measured during testing rather than assumed from KV alone.
Propeller: Direct drive from the submerged motor shaft. Current target approximately 7–8.5" diameter × ~7" pitch, with final diameter and pitch selected through RPM, watts/current, GPS speed and motor-temperature testing. Flipsky has historically paired the 65161 with a smaller 7.25 × 5" prop.
Outboard Construction: Motor is completely submerged and clamped horizontally to the bottom of a laminated marine-plywood/fiberglass strut. Motor wiring runs up the strut to the ESC and batteries inside the boat.
Mounting/Steering: Removable outboard assembly using a vertical stainless steering/removal pin and transom receiver. The complete tiller, strut and motor assembly lifts off while the transom bracket remains attached. A separate horizontal pivot provides tilt/trim.
Primary Advantages: Very lightweight, direct drive, no gearbox, no belt, no hull penetration, minimal mechanical losses and completely removable.

Preliminary performance estimates at approximately 300 lb loaded displacement:
250 watts: approximately 4.7 to 5.0 mph
500 watts: approximately 6.5 to 6.7 mph
750 watts: approximately 7.6 to 7.8 mph
1,000 watts: approximately 8.5 to 9.0 mph
1,250 watts: approximately 9.7 to 10.1 mph
1,500 watts: approximately 11.0 to 11.3 mph
1,750 watts: approximately 12.0 to 12.2 mph
2,000 watts: approximately 13.0 to 13.1 mph

I can offer more details if anyone is has questions.

Thanks!

 

[BlueBell]

300 pounds total? Battery, controller, pilot, motor, emergency equipment.
So, I have to ask, how much do you weigh?
And what is the intended use?

 

[SolGato]

The preliminary performance estimates look a little optimistic based on my real world experience, but you are making a wise choice going with Flipsky and VESC as you will be able to tune and dial things in.

Assuming the figures are based on consumption not power at the prop after losses?

In the end you may find the need to up the voltage a bit to get over the hump and swing a more aggressive pitch prop to achieve your performance goals.

Looking forward to seeing/hearing how things progress.

 

[kmcsharry]

300 pounds total? Battery, controller, pilot, motor, emergency equipment.
So, I have to ask, how much do you weigh?
And what is the intended use?

165# agree 300 is optimistic hopefully land 300-350. Testing will be on flat water lake. Will definitely run in protected salt water on nice days.

 

[kmcsharry]

The preliminary performance estimates look a little optimistic based on my real world experience, but you are making a wise choice going with Flipsky and VESC as you will be able to tune and dial things in.

Assuming the figures are based on consumption not power at the prop after losses?

In the end you may find the need to up the voltage a bit to get over the hump and swing a more aggressive pitch prop to achieve your performance goals.

Looking forward to seeing/hearing how things progress.

Agree these are optimistic but somewhere to start. 36v was selected to try to get rpm’s down because we did not have a reduction gearbox however I am looking at other options that would get rpm’s at prop down to more typical rpm’s for traditional prop designs. 48v and running reduction gears will open up more/better motor choices that will hopefully help decrease slippage and get us a motor with higher max power in case 2kw proves insufficient.

 

[Skip Johnson]

Interesting project, hope it goes well for you.

Can't comment about the power end of things other than mention that direct drive and smaller props puts a limit on overall efficiency.

Proposed hull construction seems reasonable, you might do OK with a 12mm transom panel; find the lightest ply available.

 

[philSweet]

Your weight is what is going to bite you here. If you just nail the wood together with no glass at all and paint the thing, you will be at more than 150 pounds bare hull. A normal adult with a cooler and life jacket is going to weigh 250-300 pounds. The motor and mount and controls will weigh about 45 pounds. You will need about 1 pound of battery (LFP) per minute of operation at 2-3kVA - lets just say 100 pounds. So 550 pounds is about the starting point for one person, 700+ for two people. Can you plane on that? Well almost, but not quite. My 8.5' cat dink will shift 600 pounds total with an old 4hp 2 stroke.

Given the brief, build it out of 4mm and 6mm marine fir and glass foam reinforcing strips into it for rigidity The transom can be 6mm with reinforcing where needed. Integrate the seat benches as stiffeners, and use 1x2 on the sheer. And make it about 10' long. That gets you about 30 pounds of hull weight saved. You'll want a motor and controller rated for more power - 5 kw at least, because you won't be able to perfectly match the prop and drive to the motor specs, and you will want to be able to deliver at least 3.5 kW. Prop really wants to be 8" or more for this. But now you need about 1.25 pounds of battery for each minute of duration, so lets say 125 pounds target. A draw of 15A at 36V is 5.4kW, <edit, math error handled in next couple of posts> so not an issue <edit, it is an issue>. Reputable brands run $25 - $30/pound. There are some cheapies out there now, though. So 3 150Ah 12V LFP batteries will weigh about 125 pounds with battery boxes, cables, and fuses.

 

[The Wing Guy]

This looks like fun! I think your power calculations are about right. Having built a durable 17 foot rowboat at 65 lb, I think that your empty weight (135#, including motor and battery) is a little optimistic, but not crazy. However, finding a 36V battery (say 50 AH) that supports more than 1C discharge might be challenging. (A bigger battery would easily fix that, but adds weight.) Curved panels can be far stiffer than flat panels for a given weight (the eggshell effect).

I am building a 10' foil-borne boat with similar power requirements. We can race! I'm planning to power mine first with a gasoline engine, then an electric motor.

 

[The Wing Guy]

A draw of 15A at 36V is 5.4kW,

By more conventional math, 15A x 36V yields 540 watts, far less than the required power.

 

[philSweet]

Good catch .
So now we need 150A at 36V. That's an issue. The max continuous discharge for a typical 150Ah LFP is only 100 Amps. So the solution is to bump up to 48 volts and make sure your batteries can deliver per their specs, or pony up for a high-spec 150 Ah 150 amp continuous discharge battery. The latter are expensive - like $1000+ each.

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[kmcsharry]

Thanks Guys, Wing Guy do you have a link to the 10' build that sounds awesome....

WIP....

Changing things up a bit on propulsion, 48v system and 5kw motor the motor will now be above the water per typical outboard system and will be using old outboard take-off shaft, reduction gear (2:1) prop shaft and prop. Will size up motor now that we have many more choices at 48v and dry location. I do realize the project is optimistic but I have no practical experience so its all good as I will be learning a lot throughout the process. If the goal is not met I'm OK with that as it will still be a cool little boat to kick around in. I agree the battery system weight is a big factor right now I will probably go with a high end 48v 50ah off the self battery with bms and blue tooth connection for testing, battery can be moved around to help with pitch.

Other motor system under consideration is 2 smaller motors with belt drive system to shaft, benefits of this would be running motors at lower operating condition, ability to further adjust max rpms with belt rearing and have ability to run on 1 motor if a motor fails. Next step is getting plywood and building hull, then source motors, esc, battery, etc.

Weight Estimates:
Estimated V2.6 bare hull weight by major component group:
Hull shell — bottom panels, side panels, transom: approximately 54–58 lb
Internal structure — S3/S6/S9 frames, twin longitudinal girders, transom knees, gunwales: approximately 14–18 lb
Fiberglass and epoxy — bottom glass, seam tape, fillets, sealing coats: approximately 11–14 lb
Finish and miscellaneous permanent hardware — fairing, primer, paint, drain plug, small fasteners: approximately 4–6 lb
Estimated completed bare hull weight: approximately 83–96 lb
Realistic target for a careful lightweight build: approximately 86–90 lb
For planning, I would use about 88 lb as the current expected bare hull weight.

Revised realistic loaded weight:
Completed V2.6 hull: approximately 86 lb
Operator: 170 lb
48 V 50 Ah battery: approximately 50–60 lb
Custom direct-drive electric outboard: approximately 20–30 lb
Controller, throttle, cables and electrical hardware: approximately 5–8 lb
Basic safety equipment / small gear: approximately 5–10 lb
Estimated total operating displacement: approximately 336–364 lb
A good working design number is therefore about 350 lb loaded, rather than the earlier 300 lb target.

Keep the thoughts and ideas coming I really apprciate it!

 

[SolGato]

Just build your own battery if you want something other than “consumer” spec.

Most “marine grade” LiFePo4 are not.

If you build your own you can use high quality components suited specifically to your use and have more confidence that it was built right and not a ticking Batttleborn time bomb!

Auto grade Prismatic cells have no problem with 1.5-2C discharge.

Cell monitoring and cell balancing when doing high capacity rapid discharge and charging are key to being able to use full capacity and long life.

Again the beauty of Flipsky’s products and VESC is that you can use a motor capable of operating at multiple voltages.

So you could start out running it on 12S then go to 16S if need be.

Their products have good reputation in the E-foiling community and when you see what one of their small fast spinning motor can do coupled to gear reduction, it’s impressive.

And they are also well designed to handle the marine environment.

Using the lower end of an ICE outboard is also a great way to go.

You get trim and depth and a proven drive system with reduction.

Keep your eye out for an older 10HP 2-stroke leg which were some of the lightest.

The challenge to this type of conversion is always keeping the ESC cool and happy since it won’t be living underwater.

Definitely consider a closed loop cooling system and an ESC with a large heat sink.

 

[The Wing Guy]

Wing Guy do you have a link to the 10' build that sounds awesome....

Yes:

T

Post in thread 'Micro Foiler Build'

Aug 3, 2026

Back to your thread, most of my wingsail work has been based on Dr. Elkaim's "Atlantis" for aerodynamics and Peter Worsley's circular cam sail control along with some guidance from the homebuilt aircraft crowd for tail volume coefficient. I will probably build a replacement for the original Bionic Broomstick once a couple of other projects are done. The magic that comes from a proa accelerating that's not experienced in other craft is addictive for sure.

When I was developing the Windrocket (which went just short of three times wind speed... unlike my current...

• The Wing Guy

• 

I have a 5100 watt outboard from Servovision (Thailand) on a 31' catamaran. It uses a 6hp Yamaha frame. Cooling has been problematic, with passages too-frequently clogging. I think I'll rig up a filter just before the inlet to the controller (although one clog point seems to be in the adapter plate between the Yamaha and the bldc motor -- and getting an accessible filter before that point would be troublesome). My creek is shallow, so my pump is picking up muck and sand and shrimp and who knows what else.

I agree with SolGato re the desirability of a 10hp lower leg. My 5100 watt motor would benefit from a larger prop, but the 6hp leg does not have any room for one.
I also have, sitting around, a 24 volt trolling motor that I have used to push the catamaran, and it did surprisingly well. It is geared and turns a large and moderately high pitch prop (roughly 11" x 9", if I remember) at 1100 rpm max. Its controller is in the pod with the motor, so there are no cooling issues. I fret about the resistance from dragging a motor, gearbox, and and controller through the water, but there is something to be said for simplicity.

A nice thing about using the chassis from an outboard is that the kick up feature works well, being free when in forward, but locked down when in reverse. The are some electric outboards that lack this feature, and if set to kick up in forward will also kick up from reversing when trying to back down.

 

[wet feet]

I'm looking at a different aspect of the proposed design,the totally flat bottom aft.I think it will harm performance as there is no possibility of reducing wetted surface with speed. I would expect that with 3 to 5 degrees of deadrise at the transom and more as you move closer to the bow,there is at least a possibility of losing some skin friction as lift is developed.Was the flatness in recognition of limited building skills?

 

[kmcsharry]

I'm looking at a different aspect of the proposed design,the totally flat bottom aft.I think it will harm performance as there is no possibility of reducing wetted surface with speed. I would expect that with 3 to 5 degrees of deadrise at the transom and more as you move closer to the bow,there is at least a possibility of losing some skin friction as lift is developed.Was the flatness in recognition of limited building skills?

Building the boat is not an issue I have very good woodworking skills and a decent shop with all required tools. My issue is that I don’t have any real experience with boats. I looked at stock plans and read as much as I could and used Chat GPT for working through several design ideas. Admittedly AI is very good for engineering work but I am not sure it’s great for designing boats

The completely flat bottom aft is a combination of what I have read and what GPT calculated for an “optimal” lightweight planing hull. My understanding is that the larger surface area in contact with the water will get the most lift but you are probably correct that once on plane this becomes less desirable.

I can certainly add some dead rise if would make the boat better overall.

As I continue to work on propulsion the hull can change because originally I was just trying to get on plane with a lightweight hull but now that I am moving to a more powerful motor I can change hull design accordingly. I like the idea of designing everything from “scratch” but I do understand that this might not get me the best finished boat.

Most of the stock plans I have found are either row boats that can run small outboards or power skiffs that require higher power. I think the “electric skiff” has not really been considered by most boat designers because it is a newer concept and really power with commercial electric outboards is cost prohibitive for most people that want a small DIY boat…..open to suggestions for hull design.