Small Inboard Electric Lake Boat

I came here asking for advice or insight on a design I’ve sketched and found the thread “Electric Leisure Boat II”, which I read with great interest. It seems that the author of that thread and I have similar design goals. I’m also a rank amateur, though I’ve done some research, and hope not to annoy anyone with questions that can be answered with more research (which is not meant as an admonition toward any previous poster, just a promise that I’ll try not to annoy).

My design goals:

• Small (3m/10ft) lake-worthy boat that can be managed by one person for fishing, travel, or light cargo drayage.
• Low-maintenance design, yet designed for maintainability.
• 3-4 knot general operating speed, though it would be nice if the boat could get up on plane and run faster for brief periods.
• Lateral stability so that side-entry and -exit isn’t troublesome, and also in-boat movement doesn’t risk capsizing.
• Comfortable seating for three people, seaworthiness with five people (800 pounds) aboard. [note: I understand that speed or range may be an issue with five people on board]

I took inspiration from a couple of sources. I love the vertical bow, vertical freeboard of the X-Shore designs. I took the outline of the hull from an existing 3 m aluminum runabout. I used a 15 degree deadrise angle after doing some research and somehow deciding that 15 degrees was the right answer. I’m willing to hear otherwise. When researching rudders, a 35 degree swing was suggested as sufficient, and I sort of WAG’d the rudder profile based on a range of workable values and the desire to keep a shallow draft. The prop and rudder are full-keel skeg-protected, and I presume that skeg will help the boat go in a straight line without inordinately affecting its turning radius.

I’m a better 2D draftsman than 3D when it comes to things with curves, and the complex shape of the drive tunnel is beyond my ability to model correctly in the software I have available, which is SketchUp. I’m pretty handy with Visio, though, and so produced what you see below using that. I tried to think of everything, which means I probably only forgot a lot of things rather than everything important.

The waterline length is 3070 mm, presuming that the waterline is the perimeter of the rim of the boat (based on vertical freeboard), though it might be slightly less depending on the complexities of the shape around the bow and how she sits in the water. The difference between LWL and LOA is minimal, though (7 cm, though someone check my math), but I don’t think for my design requirement of 3-4 knots it matters.

I arbitrarily decided that a 7 1/2” (190 mm), three-bladed prop was the right answer (pitch TBD), though that’s kind of baked in at this point due to the 200 mm diameter of the drive tunnel. I designed based on a Dakota Lithium golf cart battery (5 kWh at 35 kg) figuring that as a maximum feasible size. Final battery will be determined by motor selection, controller and charger selection, and operational profile. I feel like two hours’ runtime at 3.5 knots with 350 pounds of passengers and cargo is a good starting point, but of course it would be great if faster/longer/heavier was an option.

Constructive feedback is welcome. Please don’t ask why I don’t just throw an electric outboard on the back of an existing boat. It’s not part of the design goals.

Questions I have are:

• What is a good prop size/pitch/blade count to meet my 3-4 knot speed goal?
• Will that same prop size get this boat up on plane with sufficient power applied?
• Waitaminute, will this boat even get up on plane? Not a requirement.
• How much torque does the motor need?
• How many RPM is required?
• How many kW is required?

I realize that the torque, RPM, and kW are all interdependent, which is why I asked the prop size question first. Using my 190 mm (7.5”) prop size, sticking with three blades, pick a pitch, and that should (?) answer the rest of the questions, right? That seems right, but I don’t know how to calculate it all, nor how to pick the right pitch.

Another, possibly less important set of questions are:

• Is 2 mm aluminum sheet a reasonable hull material?
• How do I figure weight for the hull (sans motor, battery, controller)?
• How do I figure out what the DWL of the boat is (again, not too concerned about small variances in speed; it’s a lake boat)?
• How seaworthy is this design?

Here’s a PDF of my current drawing set: Boat 3

Thanks for whatever constructive feedback and support you can offer, even if it’s to tell me that it’s an unworkable design.

Cheers.

J.D.

 

2 mm thick ally should be plenty strong enough - but it will be more difficult to work with / get fair / weld than say 3 mm.

How do you figure out the weight for the hull - you do some estimates.
Calculate the total surface area of the hull, and multiply this by the thickness and the density of aluminium (while making sure that all your units are compatible) and this will give you a weight for the ally shell plating, approximately.
You also need to have a weight allowance for framing.
And estimate the weight of the crew, the electric motor and drive system, the outfit and safety equipment - you have to add up everything.
You will probably be surprised by how quickly it adds up!
Will you have enough buoyancy to support all this weight, especially with 5 people on board?

How do you figure out the DWL - first of all you have to calculate the volume of displacement of the hull, ideally at different waterlines - does Sketch Up allow you to do this (I am not familiar w1ith it).
You can then plot a curve of displacement against draft - and using your estimated total weight from earlier, you can then read off the graph what the draft will be for this weight / displacement.

The above are all basic things you need to figure out first before getting on to finer details like power required, torque, prop size etc.
One formula for power is Power = 2 x 3.142 x N x Q
where N is the revs, and Q is the torque.
But there is more to it than that - this is a simplified formula.

Re if she will be 'seaworthy' - what is your definition of seaworthy?
What sort of conditions on the lake will you expect the boat to be able to cope with in 'normal' usage?

 

Hi J.D. i can help you with the 3d drawing and weight estimation I really like this kind of projects
a good idea to estimate the power would be to make the model in 3d, make a detail of the weights as they said before and make a resistance analysis using savitsky

 

Thank you! Can you work from that drawing set? What else do you need?

 

JDRay. You cannot expect a 10 foot boat to accommodate three people, much less five. A five person boat would more than likely be fifteen feet or more, preferably more.

You will save a lot of time, money, and disappointment if you simply buy the plans for a proven boat designed by an experienced individual who has a proven track record. Try searching Duck Works or Glen L for starters.

You have decided to use 15 degree deadrise but you still want to be able to use side entry into the boat. It will be much more stable if you use no deadrise at all, as in flat bottom. It will also accept a heavier load per inch of immersion with a flat bottom.

You want the drive mechanism to be inboard. The tiny ten footer will have to accommodate all that machinery including batteries and controls where passengers need the limited space. Best consider an outboard electric such as the Torqueedo. We already know that the Torqueedo works, it has the right prop, the right battery, the right controls. The outboard powered boat will need no separate rudder, no separate steering arrangement . The boat will also be more easily transportable. it will not be as heavy because it can have less internal structure, and the outboard can be easily detached to make the whole thing lighter and more manageable.

Please know that I have no wish to rain on your parade. I am only giving you advice based on experience and the experiences of countless others who have sought to design their own boat.

 

Interesting that you should say that, considering that I’ve spent plenty of time in ten foot boats with two other people. It’s cramped, but workable for fishing.

I don’t believe you have enough information about my goals to make such a statement, but I will look at those designers. Thank you.

If the DWL is at or above the chine, and the freeboard is vertical, then how will that change the load per inch of immersion?

I know all of these things, and yet I chose to design with an inboard. And I asked in my post for people to please not recommend I use an outboard. I have a goal not detailed in this brief because it’s not material to the design.

Cheers.

JD

 

Duckworks has a 10’ inboard dinghy, done in wood rather than aluminum, but that’s something:
Irreducible Plans PDF https://duckworks.com/irreducible-plans/

 

JDRay, it seems that you have not gotten additional response since Sept. 19. We can only speculate as to the apparent disinterest in your original posts. Among the less gentle reasons is that your boat, as drawn, is a pretty poor design that is smitten with problems. In addition, you appear to have your mind made up about several of the details.

You said that you would like shallow draft. Your boat will need 8 inches of immersion in order to displace somewhere near 700 pounds. You said that you want it to allow a person to step into the boat from the side. You have opted for 15 degree deadrise. At 8 inches draft, the centroid of the largest sections will be at about 5.3 inches above the keel and 2.7 inches below the waterline. The boat will be tippy. Entering from the side will require some caution. A flat or nearly flat bottom, of the boat that you have drawn, will support 700 pounds at about 3 inches draft.

You have shown a tunnel for the prop. If the tunnel drive is to function well, know that details of the tunnel are critical and almost mysterious. ............there is more to critique but these remarks will do for now.

I suggest that you get the book titled; Electric Boats, The handbook of clean, quiet boating. Author Douglas Little. ISBN 0-07-038104-6. That little book has a treasure chest of information about small electric boats. For example, there are graphs that show the relationship between required shaft horsepower and speed length ratio. And lots of other good info. When you are ready to select a prop, refer to the Propeller Handbook by Dave Gerr ISBN 0-07-138176-7

I wish you well with this project. I do urge you to do some careful research before you begin your build.

 

Thank you for the book references and the tips. Indeed I have some design ideas I’d like to maintain, but there’s very little I’m absolutely adhered to. Three meters’ length, inboard electric motor. The rest, as my original post said, are somewhat arbitrary.

About the only thing that’s on my not-required-but-hope-it-works list is the vertical freeboard.

This boat is simultaneously entirely different (carbon fiber, gunwale seating, wheels in an aft notch) and very similar (three meters, vertical freeboard). I can’t really discern the bottom shape, but I caught a glimpse of the boat in a video that showed some sort of vee-shaped hull. It has a load capacity of 783 pounds. It did seem kind of tippy when people stepped on the side.
https://octenders.co.nz/wp-content/uploads/2020/09/OC300-Specs.pdf

The profile view of the hull shows an overall height of 70 cm, and the spec sheet says 44 cm of “topside height”, which I take to mean freeboard. This suggests a 26 cm (just over 10”) draft, which seems reasonable to me. The deepest part of the hull seems to be very forward.

I’m not looking to make this a super-efficient hull shape that slices through the water, but rather a boat that’s sturdy and portable. And has an inboard electric motor. Most feedback is appreciated.

JD

 

Hi JD, seems like you have a better chance of completing your boat than I have, well, good luck.

I have some suggestions regarding the inboard set up: I like inboards as well, however no boating experience with them.

Could you use a shorter shaft, a long one may be prone to vibration. You can move the motor closer to the stern and either mount it vertically with some sort of gear mechanism. The length of the wires can be reduced by moving the battery and controller also to the rear, with mechanical cable links to the controls. I always planned two batteries for my boat, with one used on the way up, the other on the way back, for making sure I can get back when I eventually make it out there.

How do you get to the lake and back with your existing boat, do you trailer it? Anyway good luck, I will be completing my virtual design (all designs are virtual anyway) and sit tight waiting for some break - namely, a boating community of some sort - I think as with anything, connecting to a community and taking advice is the way.

 

Thank you for your words of encouragement. Part of my goal for this boat design is scalability. Not direct scalability, but conceptual. I have a background in software engineering (much more adaptable than physics-bound, fluid-dynamics-dependent boat hull design), and design patterns are key to that field. They are to boat design as well (displacement hull; planing hull; power boat; sailboat; etc.), and part of this effort to design this three meter boat is learning about those various patterns.

Back to that OC Tender boat I linked above, here’s a great video about how they’re manufactured. It was very instructive.

 

Five people in a 10 foot boat would be really crowded. Adding an inboard motor will make it worse. You will need a very beamy boat; particularly with a pointy bow. A scow or jonboat type is more adequate to carry a large load on a short hull length. Planing is out of the question. The example of boat you show has a gas outboard. It will have less weight than an electric motor, batteries, battery controls, etc. I think you should at least use an electric motor to free interior space.

 

I agree that five people in a ten foot boat would be unfortunately crowded. I’ve been working on a 3D model of this design, and made some changes after seeing it in 3D, choosing a tiller over a steering station. The tunnel and rudder are essentially as originally designed. I was able to verify that the area of water ingress ahead of the prop is at least 10% greater than the prop thrust area, which should be sufficient for a craft that isn’t expected to go fast (for a fast boat, things like tunnel turbulence would, I believe, be more of a factor).

I hear what you’re saying about the weight of the gas outboard. A 5 hp Honda four-stroke weights about 60 pounds, and five gallons of gas plus tankage another 40 pounds (ish), so a I’ve been using 100 pounds as my budget. A 5 kWh LiFePo battery (Dakota Lithium golf cart battery) weighs in at 77 pounds all by itself, but a small (4 kW) electric motor is only about 21 pounds, and a controller and charger only add another ten or so to that, so the total of 108 pounds isn’t TOO much over budget. Of course, you’re carrying that entire weight forever, whereas with the gas engine your weight slowly goes down by about 30 pounds as the gas burns, but that’s just a burden to bear I suppose.

 

If you are going to use fuel weight equivalents, then consider a 5HP outboard will burn 0.45 gal/hr. The built-in tank is usually about 0.3 gallons, so you would only need to carry a can with 1 quart extra for the same running time.

 

I'm making progress on the design. SketchUp Web's rendering is showing the ribs through the hull material, which is 2 mm thick, so ignore those lines. There are other, similar artifacts showing here. I mentioned before, I'm no professional, nor even a knowledgeable amateur. I am, however, interested in constructive feedback that will help me learn.

I'm estimating the weight of the hull, benches, etc. at around 50 kg. That estimate is based on the weight of the boat I used for inspiration, which is 45 kg. The motor and drive system should come in somewhere around another 50 kg including the battery. So the overall dry weight of the boat should be ~100 kg.

Displacement at the chine (as shown) is right at 200 liters, which is convenient in that it allows for the boat plus 100 kg of cargo. Oh, wait, 100 kg of cargo isn't much; something like one adult and one child. Every 25 mm (about an inch) of displacement above the chine adds 75 kg of cargo capacity (about one adult). I have no idea how to determine performance in a craft of any size, let alone one this small. Feedback is appreciated.

Based on feedback from @slboatdesing, I moved the battery installation forward (the forward-most bench), which will shorten cable lengths.

This design has a full-keel skeg. Looking at it in 3D, it sort of appears that it would interfere with navigation with any sort of cross current, so I'm trying to figure out how to pull it back. What sort of angle should I maintain on the front of the skeg to keep from ripping it off entirely if the boat runs into something (deadhead, etc.)?

Thanks for any (constructive) feedback.

JD