newbie with a question

Discussion in 'Fiberglass and Composite Boat Building' started by wolter9, Jun 29, 2006.

  1. wolter9
    Joined: Jun 2006
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: texas

    wolter9 Junior Member

    hi folks,

    a newbie here with a question if you don't mind. I have worked with fibreglass before but never tackled a project like building a boat. But I'm convinced that i can make the boat out of fibreglass once i have a design, but that's exactly the problem... the designing aspect of it! How do i calculate where the waterline will be?

    If all went well, there should be a drawing attached to this post. If i wanted to make a boat where the waterline would be at 2 feet high like in the drawing, how would i go about doing that?

    In case newbies before me have asked this question many times before and the veterans on this board are getting a little tired of answering the same basic questions, references to previous posts/webpages on the subject would be greatly appreciated.
    Thanks in advance.
    Wolter
     

    Attached Files:

    Last edited: Jun 29, 2006
  2. wolter9
    Joined: Jun 2006
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: texas

    wolter9 Junior Member

    Anyone?
     
  3. Thunderhead19
    Joined: Sep 2003
    Posts: 506
    Likes: 3, Points: 0, Legacy Rep: 21
    Location: British Columbia, Canada

    Thunderhead19 Senior Member

    What you're asking is easily explained, but not quite so easily accomplished.

    Let's say your boat is perfectly rectangular. It's 10' long, 3' high and 4.5' wide (for our example). If you want your waterline to be 2' up from the bottom, you have to calculate the volume of the "hole" that your boat is creating ni the water. In our example the hole will be 4.5' wide, 10' long and 2' deep; thats 90 cubic feet. Now, in order for your boat to push that much water out of the way, it has to weight the same amount as that much water. Water is 64 lbs per cubic foot. This gives us a weight of 5760lbs. So, for our example boat, it has to weight 5760lbs including the hull weight, gear, motor, people etc to sink down that far. If you make your boat shorter than ten feet the same calculation will show that you need less weight. Your boat has a trapezoidal hull bottom (it flares out at the top) your calculation will be slightly different to get the hull volume, but its still elementary-school math. So as I said earlier, you probably either don't want, or can't make a three ton row boat. (that's imperial tons, NOT long tons). If you want to know how the boat will trim ( tip front to back) it is a little more complicated, but there are easy ways to cheat a calculation.

    Cheers, blah blah blah
     
  4. marshmat
    Joined: Apr 2005
    Posts: 4,127
    Likes: 149, Points: 63, Legacy Rep: 2043
    Location: Ontario

    marshmat Senior Member

    Hi Wolter,
    On the forum menu there is a "Search" button; there is indeed a lot of information on this forum and that tool will help you find it.
    Designing a boat is a bit more complicated than setting the waterline. There's also stability to consider, static forces, dynamic loads and pressures, centre of {mass, buoyancy, lateral area, etc}, and about a thousand other aspects that need to be calculated before you can be sure your boat will float without sinking or flipping. I would suggest checking your local library for books on boatbuilding and design, as a starting point. You will probably find that it's a lot easier, cheaper and safer to buy plans from a designer and build the boat yourself, than try to do it from scratch.
     
  5. wolter9
    Joined: Jun 2006
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: texas

    wolter9 Junior Member

    thanks for the replies.
    I thought that since i'd to use the engine out of my jetski in my boat, i doubt i'd find plans for that and that i would have the design something myself. But if designing a boat is really that complicated(even if it's a small one?)... then maybe i have look a little harder for a plan that uses jetski engines. Don't suppose anyone here knows of any plans that do?
     
  6. marshmat
    Joined: Apr 2005
    Posts: 4,127
    Likes: 149, Points: 63, Legacy Rep: 2043
    Location: Ontario

    marshmat Senior Member

    Many plans intended for outboard motors can be adapted to jet-ski pumps. There are a few issues with these motors that you should be aware of:
    - Conventional jet drives (the impeller type seen with V6 and bigger engines) are usually built as complete bolt-in units. A jet-ski's axial flow pump is not a complete unit; its intake and mounting structure is usually moulded into the hull shape. Jet-ski motor conversions usually involve either moulding a similar shape into the new hull, or cutting the mount and intake portion of the jet-ski hull out and laminating it into the new hull as a single unit.
    - Jet-ski axial pumps are not very efficient when compared to props or conventional jets. A 100hp jet-ski engine typically ends up delivering the same thrust as a 60hp outboard, by the time all the inefficiencies and losses are considered.
    - Not all jet-skis have reverse buckets or trim adjustment. A jet-ski can get by without these but a useful boat needs the reverse and will be a lot more versatile overall if it has the trim.
     

  7. wolter9
    Joined: Jun 2006
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: texas

    wolter9 Junior Member

    thanks Matt. Your input is very appreciated. The fact that the intake and mounting structure is moulded into the hull of the jetski was another reason why i thought i'd be better off designing a boat myself. Because i thought no matter what plan i find that closest resembles something i want to build, i'd still have to make modifications to the hull because it needs to have the same shape as that of the jetski. And if i still have to measure and calculate things myself, then what's the point in buying a plan? Might as well start from scratch. But that's how i thought "before" you briefly explained what designing a boat takes and before you mentioned that there are plans out there intended for outboard motors that can be adapted to jetski pumps.
    Now i just have to find me one of those plans :)
    Thanks again.
     
Loading...
Similar Threads
  1. freddagg
    Replies:
    15
    Views:
    4,531
  2. Greg Vasquez
    Replies:
    2
    Views:
    2,514
  3. JordieS
    Replies:
    2
    Views:
    1,759
  4. Downtown
    Replies:
    3
    Views:
    1,777
  5. mberry
    Replies:
    11
    Views:
    3,100
  6. Fabuglas74T
    Replies:
    3
    Views:
    2,809
  7. Mandelstam
    Replies:
    4
    Views:
    6,094
  8. gainey
    Replies:
    17
    Views:
    7,567
  9. maritimebilly
    Replies:
    2
    Views:
    1,750
  10. Joe0157
    Replies:
    24
    Views:
    935
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.