Designing and building a houseboat

Discussion in 'Boat Design' started by JohnMarc, Mar 13, 2018.

  1. JohnMarc
    Joined: Feb 2018
    Posts: 25
    Likes: 1, Points: 3
    Location: Knysna South Africa

    JohnMarc Junior Member

    I live on an Island within the Knysna lagoon. Having just finished designing and building my new home the next logical thing seems to be a houseboat. I would like to be able to take my wife and friends (four to six people) up the tidal salt water lagoon for overnight stays.
    I am not kidding myself thinking this will be a quick 'n easy build but I have the equipment and workshop for a build and I consider myself a quick learner (especially from my mistakes.....;) of which I made plenty)

    I have a few requirements of the houseboat:

    Needs to be get under a low bridge (maximum height at low tide 2.5 metres)
    Would like it to have a sundeck on the roof
    Does not need to be fast
    Needs to be able to handle shallow water (min 1 meter)
    Does not require to be trailerable
    Require a displacement (is that the right word...or capacity) of approx. 8,4 tonnes
    I would like to have a fibreglass coated wooden hull....or any other materials that are suitable but not aluminium or steel.
    I would like it to be approx 14 metres by 3,5 meters (11,4" x 45")

    So some questions I have are:
    Is a flat bottomed hull stable enough on the water, I am assuming I cannot go pontoon as its height above the water would not meet my 2,5 meter clearance requirement.
    What alternative hull shapes should I consider to make the floor, level or below the water level....(hope that makes sense) to give me the clearance below the bridge....
    Does the attached drawings as a start that I am thinking of make sense?

    Please forgive me for not using the correct terms or for my general ignorance, I feel I am asking really silly questions of a pool of vastly experienced folk, but then again how else does one start the learning experience.

    Many Thanks
    John Marc
     

    Attached Files:

  2. TANSL
    Joined: Sep 2011
    Posts: 5,349
    Likes: 136, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    It is necessary to know the dimensions of your pontoon to give a concrete answer. In a first approximation, it can be estimated that the draft, for a displacement of 8.4 tons, would be around 0.25-0.30 m. This means that out of the water there would be a pontoon height of, approximately, 0.50 m. Depending on where you place the deck, which seems to be below the highest point of the shell on the side, the freeboard could be somewhat lacking. But, as I say, none of this is final.
     
  3. JohnMarc
    Joined: Feb 2018
    Posts: 25
    Likes: 1, Points: 3
    Location: Knysna South Africa

    JohnMarc Junior Member

    Many thanks for your response, I have put the dimensions of my sketch. I trust this will help, an opinion if I may ask on the shape.... would this be more suitable than a flat bottomed hull as far as stability goes. Please excuse my ignorance but what does "freeboard" refer to.
     

    Attached Files:

  4. JohnMarc
    Joined: Feb 2018
    Posts: 25
    Likes: 1, Points: 3
    Location: Knysna South Africa

    JohnMarc Junior Member

    Just realised free board is the measurement from the water level to the highest point of the vessel.....thanks google :) so are you saying I might not get the 2,5m I require to get below the bridge ?
     
  5. Heimfried
    Joined: Apr 2015
    Posts: 258
    Likes: 18, Points: 18, Legacy Rep: 37
    Location: Berlin, Germany

    Heimfried Senior Member

    No, simply said freeboard is the heigth of the lowest part of the deck (where water could swap in) above water level.
     
  6. Heimfried
    Joined: Apr 2015
    Posts: 258
    Likes: 18, Points: 18, Legacy Rep: 37
    Location: Berlin, Germany

    Heimfried Senior Member

  7. TANSL
    Joined: Sep 2011
    Posts: 5,349
    Likes: 136, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    The initial stability is a function of I / V
    I = second moment of inertia of the water plane area
    V = volume displaced by the boat.
    Since the volume will be the same (8.4 m3), the greater the area of the waterplane, the greater the initial stability. Therefore, from this point of view, the catamaran is less stable than the monocoque of the same beam.

    Snap1.jpg
     
  8. TANSL
    Joined: Sep 2011
    Posts: 5,349
    Likes: 136, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    With his data I have calculated a draft of 287 mm. Taking into account the navigation in calm waters, without waves, add there a freeboard of about 300 mm and it will give you a depth (distance from the bottom to the deck) of, approx. 600 mm
    I would also take into account the fact that the area exposed to the wind will be very large so the boat should have a drift area (submerged side area) as large as possible. This will lead you to consider the convenience of a larger draft, for which you should resort to a catamaran type hull.
    What I want to tell you is that there are several aspects to consider, many of which are contradictory to each other, and that you should make an SOR as complete as possible in order to make the decisions that lead to the best design for you.
     
  9. JohnMarc
    Joined: Feb 2018
    Posts: 25
    Likes: 1, Points: 3
    Location: Knysna South Africa

    JohnMarc Junior Member

    Many thanks for your reply, being at the very beginning of the design phase I was hoping to get some leads to make the initial decisions, the most important being wether it be flat bottomed or catamaran style as far as stability goes. If I understand your advice appreciating the lack of info at your disposal, the flat bottom would be best .... or is that to bold a statement to make.
    I would like to have a negative freeboard (and I am sure that is the wrong way to put it) but my ignorant logic tells me the lower the deck (below the waterline if possible) to the waterline the lower my centre of buoyancy would be therefore the more stability ??
     
  10. Heimfried
    Joined: Apr 2015
    Posts: 258
    Likes: 18, Points: 18, Legacy Rep: 37
    Location: Berlin, Germany

    Heimfried Senior Member

  11. TANSL
    Joined: Sep 2011
    Posts: 5,349
    Likes: 136, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    The deck must ALWAYS be above the water. The contrary is prohibited. As the deck is above the waterline, it does not influence at all the position of the buoyancy center, which is the geometric center of the submerged volume. You can have a very low buoyancy center but, if the center of gravity is high, it can be a complete disaster.
    I think that, in fact, you are more interested in a catamaran, but that is a decision that must be taken, and it is one of the most important, after taking into account many other considerations. For example, do you have limitations on the draft? If so, it is likely that you should choose a monocoque.
    Talking about more or less stable does not make much sense. The ship must meet certain standards regarding stability and nothing else. Saying if they are fulfilled better or worse is not good for anything, you must fulfill them, even for the minimum, and nothing else is asked. Another thing would be that you would like, for example, to limit the balance period to make the boat more or less comfortable for the passage.
    My advice is that you create a list of minimum requirements, as complete as possible, for your boat and that you put your project are in the hands of a technician.
     
  12. SamSam
    Joined: Feb 2005
    Posts: 3,811
    Likes: 147, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    I would take your advice and avoid the pontoon hulls you have drawn. A flat or slightly v shaped hull has more stability and is a lot easier to build, it will also give you the lowest height above water and the shallowest draft. The lowest height above water will allow the widest time range for being able to get under the bridge, so you might be stuck on the wrong side for only 2 hours instead of 8, the shallower draft expands your choices of places to go.
    It could depend on what sort of sea conditions you are likely to experience.
     
  13. TANSL
    Joined: Sep 2011
    Posts: 5,349
    Likes: 136, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    A pontoon hull, parallelepiped, would need a draft of about 170 mm, which, in my opinion, is very little. Although, of course, it is very simple to build. Therefore, before choosing a type of hull or another, we must analyze many things that the OP has not yet told us. On the other hand, the shape of the bow that he has drawn would allow him to get close to shallow coasts, although the pontoon draft was greater.
     
  14. SamSam
    Joined: Feb 2005
    Posts: 3,811
    Likes: 147, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member


  15. JohnMarc
    Joined: Feb 2018
    Posts: 25
    Likes: 1, Points: 3
    Location: Knysna South Africa

    JohnMarc Junior Member

    Many thanks this is awesome..... learning something new is an amazing journey.
     
Loading...
Similar Threads
  1. dylantorquerol
    Replies:
    10
    Views:
    4,568
  2. Baywatch Towers
    Replies:
    4
    Views:
    4,214
  3. Sandy Ewen
    Replies:
    4
    Views:
    2,743
  4. richardmg9
    Replies:
    21
    Views:
    547
  5. Kkuzmicki14
    Replies:
    8
    Views:
    562
  6. Pammie
    Replies:
    89
    Views:
    3,402
  7. Mely
    Replies:
    26
    Views:
    1,008
  8. Peter Vella
    Replies:
    5
    Views:
    498
  9. useragentseven
    Replies:
    13
    Views:
    853
  10. Joe M
    Replies:
    12
    Views:
    1,029
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.