Small electric cat - hydrofoil

Discussion in 'Boat Design' started by Eciton, Sep 23, 2018.

  1. Eciton
    Joined: May 2017
    Posts: 18
    Likes: 0, Points: 1
    Location: Virginia

    Eciton Junior Member

    was watching youtube and seeing a lot of small jetboards using electric motors to push a surfboard to the speed required to "fly" using a hydrofoil system. I was tossing the idea around of something a little larger, perhaps a 12' cat with folding foils that can fold up into the space between the hulls.

    seems there are a number of sailing cats with foils, and human powered cats with foils - so the speed cannot be so much that a modern electric motor and batt system couldnt do the same.

    question: as I am not an engineer, just a tinkerer. ideally I would want no more than 2 wings under the boat and ideally the ability to swing them up between the hulls to beach or trailer.

    1.) how do you size a hydrofoil element? I assume its a combo of power available, surface area of the wing, wing design, and boat size but I'm not clear on the details

    DISCLAIMER: I have not done any research on the design other than a quick google to see that I have not seen a similar creature out there.
  2. Doug Lord
    Joined: May 2009
    Posts: 16,679
    Likes: 346, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    This may help you to think it out: Lift Foils - Home of the eFoil electric hydrofoil surfboard, now shipping
    Note-price is $12,000!
    You could use a strut system like the efoil-perhaps a bit larger to reflect greater weight- and have the whole assembly retract between the hulls using a trunk in the crossarm structure.
    If you are really interested get "Design Build Fly" by Ray Vellinga which is, in essence, a design/build manual for small hydrofoil boats.

  3. Eciton
    Joined: May 2017
    Posts: 18
    Likes: 0, Points: 1
    Location: Virginia

    Eciton Junior Member

    yes, that is the youtube video I saw that got my brain working on this idea to begin with.

    $12K is loco, there is a good amount of info on their site though thanks for the link.

    anyone else thats interested, theres a fairly active DIY efoil site

    Electric Hydrofoil Builders

    seems like the biggest issue is getting a motor strong enough with a battery system light enough.
  4. vinotinto
    Joined: Jun 2017
    Posts: 2
    Likes: 0, Points: 1
    Location: Earth

    vinotinto New Member

    Search for solar boat challenge, few teams have competed with a foiling cat, most successful boats have been a trimaran design. these boats are of course optimized around rules and fairly sheltered dutch canals, interesting design features nonetheless..
  5. Eciton
    Joined: May 2017
    Posts: 18
    Likes: 0, Points: 1
    Location: Virginia

    Eciton Junior Member

  6. portacruise
    Joined: Jun 2009
    Posts: 1,337
    Likes: 125, Points: 63, Legacy Rep: 218
    Location: USA

    portacruise Senior Member

    These really look cool, but curious about what the limitations are with foils? How long does it take to master the skills to use the surfboard type? What is the maximum allowed for wave heights, draft, and weed plus debris issues for being able to use them? Seems like the cat design might be a little more versatile, especially if the foils can be designed to be pulled up high enough to clear the water. The cat design might used at slow speeds under unfavorable conditions as a backup or limp home mode.
  7. Eciton
    Joined: May 2017
    Posts: 18
    Likes: 0, Points: 1
    Location: Virginia

    Eciton Junior Member

    After a long pause in this project I've been doing done more research in preparation to begin a prototype for fun. I came across the UFO sailing catamaran and think that this might be a simple conversion to electric from sail. I can't find much info on the tech specs of the foils though. As mentioned before I'm not an engineer and while I can copy something, design of the foil is above my pay grade. :)

    If anyone has any info it would be greatly appreciated!
  8. Dejay
    Joined: Mar 2018
    Posts: 721
    Likes: 138, Points: 43
    Location: Europe

    Dejay Senior Newbie

    I think for a small efficient to build and propel craft a small power trimaran would be better and easier to build. Just some small outrigger training wheels to stabilize. You only have one large structural sound hull and can attach the hydrofoils to that. I think in that size range trimarans also offer more space than a catamaran because you can utilize a slightly longer and wider central trimaran hull better than two thinner catamaran hull.

    I'm looking for a "my first boat" project to build so I've been thinking of an electric trimaran. Not planning on building hydrofoils though, but I'd like to experiment with them in the future as an addon.

    My ideal would be a displacement power trimaran and something with a solar roof that can be raised and lowered to close the boat and slowly recharge batteries. And is a bimini roof for a tent enclosure so you can camp inside the boat. Ideally also something that can be used as a tender but that is probably not practical. Just something to get some experience with and go spelunking in small rivers.

    I do think hydrofoils offer a lot of untapped potential to save power for an electric boat that goes fast. You don't need a super long and heavy boat. Honestly I don't know why speedboats aren't all hydrofoils by now! Most sailboat foilers have a kind of excess of sail power and a lot of their design choices has got to be about stability. So a power boat especially if you can build an electric control system for the foils should be easier and more efficient. But there seems to be a disconnect between what is possible and what development the market can afford.

    Have you read the book "Hydrofoils" by Ray Vellinga? I haven't finished it myself but it's pretty good.

    I'd design the hydrofoils so you can easily swap them out.

    Usual disclaimer, I have absolutely zero experience building boats so this is just my thoughts after researching similar things.
  9. Doug Lord
    Joined: May 2009
    Posts: 16,679
    Likes: 346, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    You can check this thread: Steve and Dave Clarks UFO and the much longer thread in "Dinghy anarchy" on SA.
    Also, Steve Clark is a member here-- you could send him a PM. Steve Clark click on "start a conversation"
    DA-Steve and Dave Clarks Unidentified Foiling Object

    Good Luck!

    PS get this book:
    Last edited: May 21, 2019

  10. tspeer
    Joined: Feb 2002
    Posts: 2,319
    Likes: 301, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    You first need to figure out how you will stabilize the craft in roll and pitch, and maintain a constant flying height. Figure out how you are going to stabilize and control it first, and then work on optimizing the performance.

    The principal sources of drag when foilborne are parasite drag, which increases with the square of the speed and depends mostly on the wetted area; and lift-induced drag, which decreases with the square of the speed and is proportional to the square of the span. So you want to minimize the area and maximize the span, subject to constraints such as structural strength.

    To size the foil, start by estimating the all-up gross weight and make a stab at what you want the takeoff speed, cruising speed, and maximum speed to be. Pick a symmetrical section to begin with and replace it with a cambered section later once you have a better idea of what the section requirements are. You probably don't want to plan on a lift coefficient much greater than 1.0 at the slowest speed, and this will set the foil area.

    Make the span as large as you think you can stand. This determines the chord. Structural considerations will set the physical thickness, and dividing by the chord will give you the thickness ratio. The easiest planform shape to build is rectangular with a constant chord. You can reduce the lift-induced drag some by using a tapered planform. There's no need to consider an elliptical planform. A planform with a constant chord middle segment and tapered tips can come so close to the elliptical ideal that the extra effort to build the curved planform is not worth it.

    Create a spreadsheet to estimate the total performance as a function of speed. This will serve as your thrust-drag bookkeeping and the means of optimizing the design.

    A program like AVL can be used to do more detailed analysis of the lift, drag, and stability of your foil configuration. Once you know the local flow conditions, you can use a program like Xfoil to design section shapes that are tailored to the local requirements along the span. The new sections can be inserted into the design by keeping the zero lift lines the same. This will ensure your previous estimates of lift and stability are not affected.
    Doug Lord and Dejay like this.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.