Camber or plunging keel effect

Discussion in 'Boat Design' started by mitch184, Aug 9, 2024.

  1. mitch184
    Joined: Nov 2021
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    Location: Washington

    mitch184 Junior Member

    Just a general question. Looking at the various hull shapes we have up here in the Northwest. I was curious how impact a boat with a camber on it's keel line has versus one with a flat or flatter keel? By camber, I mean when looking at the keel from the side, it's lowest in the bow then rises slightly and gently aft but then drops again slightly at the stern. Similarly, what effect is a plunging keel have where the keel is just deeper in the bow. I think the technical term may be deeper forefoot?

    By impact, thinking about lift. Would camber create bow lift?
     
  2. kapnD
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    kapnD Senior Member

    Boats with keels are usually displacement hulls, not capable of generating much dynamic lift.
    I don’t think it’s going to be an issue.
     
  3. jehardiman
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    jehardiman Senior Member

    I don't think a sailing keelboats is what mitch184 had in mind. I think he is asking about typical inshore/offshore PNW deep V fishing runabouts, some of which exhibit the features described at centerline (i.e. the keel) when looking at them in profile.

    Ok, there are several things to unpack here that all roll into a hull form with the shape you describe. First, the profile shape you describe is a function of the change in deadrise (i.e. V angle) along the length of the hull and the profile has very, very little to do with hull lift (and therefore it is not "camber" in hydrofoil sense). The reason the forefoot is deeper is the narrowing forward traded off against the need for buoyancy forward while not planing. This means to get the the low speed buoyancy forward the hull needs to be deeper forward than aft for a V-bottom hull form. For a V-bottom hull form this has two advantageous effects and one disadvantageous one. The deeper V forward and deeper forefoot reduce slamming loads and increase sea speed, but the deeper forefoot could cause bow steer. The hook at the stern is an effect of using a deep V hull form optimised for the typical PNW chop. Instead of going for a very low V angle at the stern more typical of things like east coast or ski boats, these boats carry the deep V all the way aft in order to give smoother riding. In order to get the proper running angle, a hook is sometimes introduced to force the bow down; which, while using more power, again helps with slamming by keeping the sharp forefoot just skimming the wavetop. Notice that this is very different than a deep V cigarette hull which is totally for speed and can have severe pounding issues in the PNW.
     
  4. mitch184
    Joined: Nov 2021
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    mitch184 Junior Member

    Hi, yes not a sailboat. Planning offshore.
    Thanks for the explanation. I hadn't given thought to the increased bouyancyat rest by the deeper forefoot. So if you had 2 hulls that had nearly identical shapes in terms of narrowness the bow. The hull with the deeper forefoot would have more buoyancy at rest in the bow and reduced slamming loads in a head sea.

    So next... with an aluminum hull getting a deeper forefoot is somewhat limited by how much the aluminum can be shaped. However, with fiberglass the options are almost unlimited. So if this hull uses a gentle camber to get a deeper forefoot, what is the impact of camber on the keel line versus a straight shot, so to speak? Would camber create a slight low pressure area?
     
  5. jehardiman
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    jehardiman Senior Member

    I don't think you have wrapped you mind's eye around the 3D shape. There is no "camber" on the keel line. Both halves of the planing surfaces forward are convex and the keel line is just the effect of the intersection of the two planing surfaces. The keel line shape has no affects on the planing surfaces.
     

  6. mitch184
    Joined: Nov 2021
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    mitch184 Junior Member

    I apologize for likely not using the correct terminology. This is a glass boat. So when you look at the boat, on the trailer, from the side. If you look at the keel, there is camber. Meaning it's not flat. Similar, if you took a 20' long straight edge and set it on the keel. It would touch on the last 5', and then again in the bow/transition area. In between there is 1" at the most.

    I'm just curious what effect that would be having on the hull.
     
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