Cutting a Mono (dinghy) in half to make a cat

Discussion in 'Powerboats' started by sabahcat, Apr 30, 2012.

  1. sabahcat
    Joined: Dec 2008
    Posts: 792
    Likes: 28, Points: 0, Legacy Rep: 273
    Location: australia

    sabahcat Senior Member

    Thoughts on this please

    Taking this which I bought as the tender for my current build
    a 4 metre tinny with a 20hp 2stroke with a max speed around 20 knots.
    [​IMG]

    and doing this to it
    [​IMG]

    The cutting, modification and welding is no issue for me but I want to know if stability and handling will be improved.

    Its been a long time since I had anything to do with mono's and broaching when overtaking waves is one aspect I have to get used to again and am hoping will all but vanish if doing what I propose.
     
    1 person likes this.
  2. BobBill
    Joined: Oct 2009
    Posts: 826
    Likes: 19, Points: 18, Legacy Rep: 157
    Location: Minnesotan wakes up daily, in SE MN, a good start,

    BobBill Senior Member

    I think loa will affect your boat, but that does not mean much. I had a very short cat duck boat once, and it seemed to do okay, but had no comparison. Center hump might be a pain, but offset by other factors. Looks fun and likely safer hull...thought about foils? :]
     
  3. Frosty

    Frosty Previous Member

    Sounds like a good idea to me, loads of fun too. Hope it works.
     
  4. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,388
    Likes: 1,022, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Madness, unless you like making work for yourself. Sell it and buy a second-hand 14' Clark Abalone, you won't find a more seaworthy boat that size anywhere.
     
  5. TANSL
    Joined: Sep 2011
    Posts: 6,807
    Likes: 499, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    This is standard practice in some shipyards building FRP hull. With the same mold they can do monohulls and catamarans.
    You must bear in mind several things. Among them :
    - Main deck, joining both hulls, must be strong enough to withstand the torques produced by the two symmetrical hull.
    - Reinforcement of the transom to support the engine thrust.
     
  6. sabahcat
    Joined: Dec 2008
    Posts: 792
    Likes: 28, Points: 0, Legacy Rep: 273
    Location: australia

    sabahcat Senior Member

    2 issues.

    1) A 14 fter is only 1 ft longer than what I have so I fail to see how it will miraculously be a better boat.
    2) As a tender, a 14 fter will not fit in the space I have.
     
  7. philSweet
    Joined: May 2008
    Posts: 2,487
    Likes: 280, Points: 83, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    1. You need to put a skeg in front of the motor leg that seals as tight as possible and runs down to or below the leg's cav plate.

    2. Downwave broaching/handling issues would likely be more pronounced. Those hulls are very asymmetrical, if the waterline is different at the bows, it's going to dart off.

    3. It will be heavier with more wetted surface and the planing surface won't be as effective.

    4. suggestion- Make the bow so that a person can scramble aboard from deep water. Tenders should be able to handle your largest anchor and chain. Add stuff on the bow to let you rattle the chain over the bow.

    5. On the plus side, it is oh so nice to have a clean side and a dirty side in the tender. Groceries and laundry on one side, busted outboards and disassembled winches and oil waste and mucky anchor lines on the other.

    6. Oh, I just wanted to say I really like the color matched milk crate. Goes with the boat I take it. Doesn't get borrowed for other uses. Nice touch.
     
  8. sottorf
    Joined: Sep 2007
    Posts: 192
    Likes: 20, Points: 0, Legacy Rep: 202
    Location: South Africa

    sottorf member

    Phil is right, the problem with doing that is directional stability with the sharp bows. You need to make a semi-symmetrical bow section there to eliminate this. See the pic.

    An assymetrical hull like this makes it ideal to fit a HYSUCAT hydrofoil system to get more speed. You could add 20-40% more speed doing that albeit at some additional complexity to the boat.

    I dont recommend fitting a single engine to the boat unless you install a a pod in the tunnel upstream of the engine. If you dont have this the submergence of the OBM leg will continuously be changing with speed and just create uneccessary resistance.
     
  9. sottorf
    Joined: Sep 2007
    Posts: 192
    Likes: 20, Points: 0, Legacy Rep: 202
    Location: South Africa

    sottorf member

    here is the pic.
     

    Attached Files:

  10. sabahcat
    Joined: Dec 2008
    Posts: 792
    Likes: 28, Points: 0, Legacy Rep: 273
    Location: australia

    sabahcat Senior Member

    That was a given as seen on other similar vessels

    I felt this may have been the case as well but I thought I may have been wrong seeing as boats like the Prout Panther have these hulll forms

    [​IMG]

    I had always planned on building a nidaplast version of this but at this stage just cant bring myself to take on yet another project.
    Cut and shut on alloy would have been a weekend.
    [​IMG]
    [​IMG]
    http://www.websterstwinfisher.com.au/site/index.cfm?display=130892#
     
  11. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,388
    Likes: 1,022, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Okay , if it can only go 13 feet it has to be so. I suppose it depends on the intended usage for your tender as to what would be most suitable.
     
  12. beachcraft
    Joined: Nov 2011
    Posts: 49
    Likes: 2, Points: 0, Legacy Rep: 30
    Location: Marinette

    beachcraft Junior Member

    Neat idea. Following this thread to see how it turns out.
     
  13. sabahcat
    Joined: Dec 2008
    Posts: 792
    Likes: 28, Points: 0, Legacy Rep: 273
    Location: australia

    sabahcat Senior Member

    I have no real intention of chopping a good condition tinny if it aint going to work, I was hoping that the responses here would have been more positive.

    Here is another admittedly smaller dinghy with V hulls and flat inside faces.

    He did have a video of it out and about and it seemed to do nicely on flat water. Never saw a video of it in a chop.

    [​IMG]

    [​IMG]

    http://www.mahnamahna.com.au/February 2007.html
     
  14. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,388
    Likes: 1,022, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    There was a 16' glass hull made in Aus years ago called a "Sea Cat" or something similar, it was notorious for running off course stern quartering, there are other similar attempts that have exbited the same vice.
     

  15. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,388
    Likes: 1,022, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Not to be confused with U.S. brand of a similar name. It had a box tunnel with a single engine, I think the name was later changed to a "barleaper" or something, but it never captured any sort of market. If they jink around unpredictably it gets very tiresome.
     
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.