Small Inboard Electric Lake Boat

Discussion in 'Boat Design' started by jdray, Sep 16, 2022.

  1. jdray
    Joined: Jun 2005
    Posts: 47
    Likes: 9, Points: 8, Legacy Rep: 10
    Location: Oak Harbor, Washington, USA

    jdray Paddle Guy

    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.


  2. bajansailor
    Joined: Oct 2007
    Posts: 2,907
    Likes: 1,137, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    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?
  3. rodrimonti
    Joined: Sep 2022
    Posts: 2
    Likes: 2, Points: 3
    Location: Argentina

    rodrimonti New Member

    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
    jdray likes this.
  4. jdray
    Joined: Jun 2005
    Posts: 47
    Likes: 9, Points: 8, Legacy Rep: 10
    Location: Oak Harbor, Washington, USA

    jdray Paddle Guy

    Thank you! Can you work from that drawing set? What else do you need?
  5. messabout
    Joined: Jan 2006
    Posts: 3,249
    Likes: 393, Points: 83, Legacy Rep: 1279
    Location: Lakeland Fl USA

    messabout Senior Member

    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.
    bajansailor likes this.
  6. jdray
    Joined: Jun 2005
    Posts: 47
    Likes: 9, Points: 8, Legacy Rep: 10
    Location: Oak Harbor, Washington, USA

    jdray Paddle Guy

    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.



  7. jdray
    Joined: Jun 2005
    Posts: 47
    Likes: 9, Points: 8, Legacy Rep: 10
    Location: Oak Harbor, Washington, USA

    jdray Paddle Guy

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