Please Sanity Check My Catamaran Design

Discussion in 'Multihulls' started by Nautically Obsessed, Mar 4, 2018.

  1. gonzo
    Joined: Aug 2002
    Posts: 13,117
    Likes: 292, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    You will notice that all the short length catamarans are slab-sided with flat bottoms. None have narrow vee hulls.
     
  2. bjn
    Joined: Jul 2014
    Posts: 123
    Likes: 5, Points: 18, Legacy Rep: 16
    Location: Stockholm

    bjn Senior Member

    Are you talking about new designs now?
    The old Hobie 14 had narrow vee. I think flat bottom is better though. Less drag and more volume.
     
  3. bjn
    Joined: Jul 2014
    Posts: 123
    Likes: 5, Points: 18, Legacy Rep: 16
    Location: Stockholm

    bjn Senior Member

    I think that is a good design. But why does most modern catamarans have a flat stern?
     
  4. Nautically Obsessed
    Joined: Feb 2018
    Posts: 18
    Likes: 1, Points: 3
    Location: California

    Nautically Obsessed Junior Member

    Thanks for the recent posts. A couple of those little cats are very close to my original idea, although I always planned to disassemble the hulls from the deck for transport. I think I can say I will not build the Hobby Kat.
     
  5. bjn
    Joined: Jul 2014
    Posts: 123
    Likes: 5, Points: 18, Legacy Rep: 16
    Location: Stockholm

    bjn Senior Member

    Attached Files:

  6. Nautically Obsessed
    Joined: Feb 2018
    Posts: 18
    Likes: 1, Points: 3
    Location: California

    Nautically Obsessed Junior Member

    Thanks, that attachment is right on target for what I need to know, although most of it is over my head! For the flat transom, it sounds like it creates resistance only at very low speeds, but once you get any speed on, the resistance goes away.
     
  7. gonzo
    Joined: Aug 2002
    Posts: 13,117
    Likes: 292, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    No, the resistance increases overall as the speed increases. The center of pressure shifts though.
     
  8. bjn
    Joined: Jul 2014
    Posts: 123
    Likes: 5, Points: 18, Legacy Rep: 16
    Location: Stockholm

    bjn Senior Member

    Well I'm sure he was talking about the added resistance from the abrupt change in flow behind the transom, and not the overall drag.

    The speed is not really that low if the transom is submerged a few cm. For example in your first 2.5m design, looks like 10cm:
    V = 4.14*sqrt(9.81 * 0.1)
    V = 4 m/s = 8 kn.

    The hullspeed is about 2m/s, and its not likely that it will ever go faster than that since it will have a bad length to displacement ratio. So it will always have that water swirling around behind the transom, creating drag.
     
  9. Nautically Obsessed
    Joined: Feb 2018
    Posts: 18
    Likes: 1, Points: 3
    Location: California

    Nautically Obsessed Junior Member

    I've redesigned the hulls based on your feedback and my research. I am shifting toward 10 feet. The pictures show the new features. I hope its not too hard to build!
    upload_2018-3-17_13-8-52.png

    upload_2018-3-17_13-9-26.png

    upload_2018-3-17_13-10-4.png
     
  10. Nautically Obsessed
    Joined: Feb 2018
    Posts: 18
    Likes: 1, Points: 3
    Location: California

    Nautically Obsessed Junior Member

    Follow up: I ended up finding a 13 foot mono-hull sailboat (see attached). I bought it for about half the material cost it would have taken to build my 10 foot catamaran. Although it won't be as roomy, but it is already built (complete with rudder, board, mast, and sail)!

    Here are some thoughts I had related to my design / build project:
    • The "flare" of the hull may not be buildable, as it reverses the curvature of the hull coming from the stern. I planned to build the hull flan-shaped (like the side of a bowl)
    • I looked at some beach-cat hulls (e.g. the Hobie 16 - actually they were off a MacGregor Venture 15). Even with their length, those hulls just don't provide enough flotation aft for this project with it's motor and/or battery weight. Just watch some YouTube videos of them sailing with a couple of people aboard.
    • I looked at buying two 10 ft recreational kayaks - made of High-Density Polyethylene - to use for my hulls. With any kayak-type hull, it seems I would have to build on some additional flotation (like sponsons) for additional flotation at the aft end. When I went to Walmart and inspected them first hand, they seemed too flimsy to build on.
    • The structural requirements need to be researched, especially if an outboard motor of more than a few horsepower is to be used.
    • It would be smart to purchase plans of a hull that is similar to what is needed, to gain the benefit of an (hopefully) experienced designer who had tested their design and worked out any problems found. The Glen-L "L Gato" is a candidate. Also, Ken Simpson's "The Wedge" over at www.portableboatplans.com. He says it can be scaled up as needed.
     

    Attached Files:

    Manfred.pech likes this.
  11. redreuben
    Joined: Jan 2009
    Posts: 1,585
    Likes: 50, Points: 48, Legacy Rep: 349
    Location: Beaconsfield Western Australia

    redreuben redreuben

    A good outcome. :D
     
  12. upchurchmr
    Joined: Feb 2011
    Posts: 2,735
    Likes: 72, Points: 48, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    About post #35.
    Remember that small catamarans are very sensitive for the sailors weight position.
    Any experienced sailor knows to move his/her weight forward in light air, in order to get the square stern out of the water. Actually, right at the waterline is best for minimum drag.
    On a 20' catamaran this might mean moving both sailors weight 1-2' fwd of a normal position for medium air.
    As was shown in medium to heavy air you move your weight aft, since the water will come off the transom cleanly as shown above.
    In heavy air you move back farther, to make sure the bows do not go below water level. Causing a summersault. Which can be painful and shocking.

    So with normal sailing skills, the full stern offers better performance all around.
     
    Nautically Obsessed and rwatson like this.
  13. BigCat1950
    Joined: Nov 2018
    Posts: 13
    Likes: 1, Points: 3
    Location: Seattle

    BigCat1950 Junior Member

    Boat design is harder than it looks. If you want to design your own boat without years of study, you are making a mistake.
     
    rwatson likes this.
  14. rwatson
    Joined: Aug 2007
    Posts: 5,648
    Likes: 236, Points: 63, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Um, James Wharram will be happy to argue that.
    shop-tiki30.jpg
     

  15. fallguy
    Joined: Dec 2016
    Posts: 1,322
    Likes: 51, Points: 48, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Do those flatten at the stern though?
     
Loading...
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.