How can I determinate an angle for topside ?

Discussion in 'Boat Design' started by GersonPerezbr, Nov 16, 2023.

  1. GersonPerezbr
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    GersonPerezbr Junior Member

    I'm designing a 20-foot Carolina sportfishing boat, but I can't find a good angle of the topside. How can I determine a good angle for it? This image is just for illustration purposes! I already found a good deadrise and transom angle. Another question do you guys have a rule of thumb to find a good chine width? Sem-título-1.png
  2. TANSL
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    TANSL Senior Member

    Start by copying a hull of a similar boat, define some preliminary shapes and check/change the shapes until you achieve satisfactory compliance with everything that the boat must comply with. The angle of the topside may just be a matter of aesthetics or ease of construction.
    bajansailor likes this.
  3. GersonPerezbr
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    GersonPerezbr Junior Member

    Thank you for answering! do you know where can I get resources? Boat plans to study? I search on google, but only found paid plans!
  4. messabout
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    messabout Senior Member

    You can have a boat with plumb sides. You can have a boat with steeply angled sides. There is no firm rule that specifies flare (sometimes called flam).

    If you have an open boat with a lot of flare, then passengers or crew can move farther to the sides which can cause the boat to heel more than ideal. Boats with verical up and down sides prevent that possibility.

    Grand Banks Dories have excessive flare for a good reason. As the load of fish increases, the relative displacement increases out of proportion to the load. GB dories do not have passengers so there is no risk of capsizing from athwartship weight shift.

    If you explore the works of other designers you can find Phil Bolger. He designed many perfectly good small boats with straight up and down sides. His boats were more than adequate for their assigned job. Flare is regarded by the general public as an essential feature. That is entirely a matter of popular belief not a matter of stability calculations.

    Lots of flare usually means that the decks are wider which means that they are heavier and that weight is in a disadvantageous place. If you are "designing" a boat, those those are just a few of the many considerations that ought to be accounted for.
    bajansailor likes this.
  5. GersonPerezbr
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    GersonPerezbr Junior Member

    Do you know where can I find good resource of boat plans ?
  6. Barry
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    Barry Senior Member

    Others have said to look at similar boats for the side angle, which is good advice. A plumb side will not look very good and the higher the side, the worse it will look.

    The other angles that are critical is the transom angle and the cabin side angle with respect to vertical.

    I have seen home built boats where there is not any angle in the side of the cabin and they look terrible. Especially as boats rarely do not sit perfectly horizontal in the water so if you are a few degrees off horizontal it almost looks like the cabin sides are tilted outward.

    The other important angle is the transom angle compared to vertical. While boats can range between vertical up to say 16 degrees. The more vertical the transom the more chance of taking a wave into the motor well ( I am assuming that you are using outboards though applicable to other drives) Most boats that I am familiar with are around 12 degrees, tilted rearward at the top
    So before you start the design it is worth the time to look up the Installation Instructions to see what your engine manufacturer prefers. This will permit the tilt/tuck range of motion to be optimal and the engines to clear the water when completely tilted up for trailering.

    Notethat the other important measurement if using outboards is to ensure that the engines can tilt to full tilt and do not interfere with the transom if the outboard is mounted on a
    swim grid or bracket.
    bajansailor, GersonPerezbr and TANSL like this.
  7. GersonPerezbr
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    GersonPerezbr Junior Member

    I will use 12° at the bracket and 14° at the transom, thank you for answering!
  8. Barry
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    Barry Senior Member

    (I am assuming that you are actually designing a boat that someone will actually build as compared to a school project)

    That was a very quick decision so I thought that I would re state the most important part. While the angle that the motor mounts to wrt to the bottom of the boat has a range that it
    can be built at, it would be quite imperative that you match the angle to whatever the manufacturer recommends.
    If outboards, the mounting angles are pretty much the same. I would pick say Yamaha, Suzuki and Merc, find the installation manuals and pick the average.
    For Inboard Outboards, I would chose exactly what the manufacturer recommends so that if you are making as the through transom bracket does not have a range of mounting
    between vertical and 14 odd degrees. (there are two reasons for this, 1) the through transom bracket has small variance and 2) if you have to trim for optimal hull performance, you do not want to have the output shaft angle and the input shaft angle to the drive at a larger number as the u-joints could create vibration
    I do not believe, but perhaps Gonzo might know , that the U-joints at this interface are CV joints

    Straight shaft or Vdrive it would not matter
    Last edited: Nov 22, 2023
  9. kapnD
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    kapnD Senior Member

    There is so much written and pictured on the internet concerning Carolina sportfishing boats, I can’t imagine how you’d be lacking for ideas.
    A deeply rooted Carolina design will have practically no flat spots anywhere, graceful curves throughout, transitioning from extreme flare at the bows and sharp, deep entry to practically flat bottom and tumble home in the stern.
    Not sure which angle of the topside you’re looking for?
    Longitudinal deck line (sheer) will curve down gracefully at the back of the house from a high bow to a low cockpit rail.
    Any well thought out design must start with a SOR.
    bajansailor likes this.
  10. rangebowdrie
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    rangebowdrie Senior Member

    L. Francis Herreshoff said in one of his writings, (paraphrasing).
    "The most important attribute that a yacht designer can develop is a sense of proportion".
    With a chine boat it's not so hard to make a simple model from cardstock etc.
    View your model from all angles, see how the lines "flow", or not.
    Edit/addition, be careful up in the bow sections where the flare is highest.
    It's easy to get a set of lines that look good on paper in the 3 views, but you want to avoid having "Powderhorn" when viewpoints change to various angles.
    Last edited: Nov 29, 2023

  11. gonzo
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    gonzo Senior Member

    That is not even close to a Carolina Sportfisherman. I assume you posted the stern. The sides are flat instead of tumblehome, there are wide flat chines and the deadrise is way too much (14 degrees would be common). I don't understand what you mean by angle of the topside. The sheer should have a reverse turndown where the forward flare meets the tumblehome. That is called the "s".
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