Small Inboard Electric Lake Boat

Except for the appearance, are you set on a pointy bow? Otherwise, you can buy a jonboat and just add the motor.

 

Yes, in order to meet my “scalable design” goal (which may not be as attainable as I think), a pointy bow is necessary.

And I’m really more interested in making a working design than buying a boat and converting it. At the top end of the “scalable” … um… scale, I’m aiming for something along the lines of a Century Resorter at 5 m (16.4’) long, but with a more modern look to it.

 

In that case you would have to design the old style warped bottom style. It is obsolete though. Scaling up is not straightforward. Maybe up to 20% the outside dimensions can be scaled directly. However, the structure and weights do not scale directly. Also, if you look at those old designs, the sections forward are concave, not convex like you have. Also, they are narrow by modern standards. In short, you need to go back to the drawing board.

 

I haven’t given up on my efforts. I feel like the design is coming along nicely.

The details aren’t perfect yet, but certainly communicates what I want to build.


The battery and motor mount in the central bench. The motor tops out at 8 kW, hull speed of ~4.75 kn is achieved at just under 5 kW.

I need to figure out how thick the material for the infrastructure needs to be. The skin material is designed at 1.2 mm. With the ribs at 2.5 mm and the keel at 5 mm, deck plates at 3 mm, the weight of the hull, with other components, comes in at 110 kilos, which is… heavy. With a 90 kilo pilot aboard, total displacement comes in at 276 kilos. At that displacement, DWL is just about perfect, and is just below the chine. Not sure yet what adding two more 90 kilo passengers will do, but above the chine, every 25 mm (~1 inch) constitutes ~90 kilos of capacity. I feel good about the model, except for the weight.

Any recommendations for the infrastructure material are welcome.

J.D.

 

How long is this latest version of your boat please?

Re your hull shell at 1.2 mm - seriously? I presume that you are going to rivet it to the frames?

Rather than having all those 2.5 mm thick frames, and 3 mm thick deck, you might be better off forgetting about a deck, and putting this material into the hull shell instead.
Very few dinghies this size have a 'double bottom' (for want of a better name) like what you are proposing.
Let the hull bottom shell be 3 mm instead, and then you should be able to stand on it quite happily. If the topsides are 3 mm thick as well, you might just need one central frame for the seat, and to establish the shape, a bit like plywood 'stitch and glue' construction.

 

Overall length is 3 meters (10 feet). This is intended to be a specific-use boat. I looked at hull material on a lot of small runabouts (jon boats, etc.), and 1 mm appears to be common for a hull material even when it’s being stood on. The central frame is where all the battery, motor, and drive system components go. Total weight of the drive system, including prop shaft, prop, shaft seal, etc. is 77 kilos, which is only a little heavier than a 12 hp outboard and a five gallon fuel tank (full).

Lightweight aluminum boats (what my family calls a “tinner”; dunno if that’s a common term or not) in 10’ length weigh in around 35-40 kilos without engine, so much lighter than this design. But upscale RIBs, with integrated steering, flat decks, etc. are upward of 75 kilos without engine.

What I’m really asking here is what the appropriate thickness of the rib material would be for a boat designed like this, not whether or not I should change the design. But I appreciate your input. BTW, going with your suggestion of a 3 mm hull and essentially a couple benches inside, the weight exceeds 80 kilos. That’s a definite savings from 110, but by the time I add in the infrastructure for the motor and battery support, it gets closer to 110 kilos.

JD

 

You have continued to ignore the very real advantage of aft rocker. Are you determined to build your boat with lines that are only appropriate for planing boats while you may not furnish enough power to make it plane? There is the matter of amperage economy that is worth contemplation. If you figure 5 Kw at .....say 24 volts, amperage draw will exceed 200. Batteries will not last long at that rate unless they are giants. In such a giant case they will be plenty heavy, and use a lot of space, which takes us back to the power input that will be required for a given velocity.

 

I guess I don’t understand what an “aft rocker” is. Maybe I’m missing something critical here.

I’ve been working with an engineer who has been analyzing the hull shape in MaxSurf. Initial calculations were that it would plane at 5.075 knots and 6 kW of input power. As I’ve developed the design and found how much the as-designed hull would weigh, performance figures dropped. However, the motor (48V) will deliver 8 kW at 200A, which is the continuous output capacity of the (two) batteries, which total 10.2 kWh.

If this boat goes flat out at 8 kW and six knots, and can run for 75 minutes that way, then… well, it’s not going to go very far. But if it could go twice that long at 4.5 knots… wait, still not very far.

My goal with this boat isn’t about speed or range, but about having a small get-around boat that, so long as you charge it, needs no real maintenance. There are no oil changes, fuel to stabilize, spark plugs to clean, or anything of the sort. I saw a video recently of a guy going out for a run with an outboard on the back of his little aluminum boat. It wasn’t clamped on, and flew off the back, leaving he and his cameraman stranded in the lake. Somewhat ridiculous, and an uncommon occurrence, but still, it wouldn’t happen with this boat.

Also, I’m doing this to study boat design. It’s why I joined this forum, and something I have a strong interest in with virtually no training. Every day I learn something new about boat design, engineering, and building. It’s a slow process, but I’m progressing nonetheless.

 

jdray,

(I haven't read your thread)
I've driven a Duffy 22, commercially as a tour boat.
It meets most of your SOR.
-no planing
-too long
We could tour eight hours at 2/3 throttle which was 3-4 knots depending on load (2 - 13 passengers).
That was with a large, lead/acid, golf-cart battery.
Check out their website for more insight, especially their live-rudder:
Home https://duffyboats.com/

BB

 

I’ve studied Duffy boats, though have never been on one. I like them.

 

Did the live rudder interest you?
Did you learn anything design wise that would forward your goal?

 

That “live rudder” is definitely interesting, though I would think that a pod drive, particularly at the low speeds, would do the same without the integrated rudder. But again, I’m no naval engineer.

The Duffy hull is a full displacement design, and not what I want for this particular boat, but am very pleased to know that such a large boat moves so far on so little power.

 

They work well as little tugboats too- I remember one night at a restaurant in Newport Harbor a large Chris Craft was nosing it’s way to the dock to go have dinner, and 12 or so Duffies it was going through to get there decided this was bullying behavior, pushed the 50er around and um, guided it away. Torque! Most had been out all day, which said a lot about economy. Roaring mice….

 

jdray,

You won't get planing by what you describe, embrace displacement.
And consider going longer, it's faster.

But it sounds like you've got it figured out.
When's your paper due or are you going to build this one?

 

My pedal powered dinghy was built from 1.2 mm Al. I wouldn't try to weld anything as thin as this but in my case I didn't need to. How are you going to construct this boat? What will hold the 1.2 mm sheet to the structure?