paddleboard and kayak teardrop shape

Discussion in 'Boat Design' started by Alwaysthinking., Sep 5, 2019.

  1. Blueknarr
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    Blueknarr Senior Member


    In this small of vessel, where you sit/stand will affect the trim far more than anything done on the design software. Design the shape you want then position your self in the optimal trim position.
     
  2. gonzo
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    gonzo Senior Member

    It is an advantage under 12 Meter rules. On any boat designed for maximum speed for a given length there will be little or no overhangs.
     
  3. gonzo
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    gonzo Senior Member

    The theory is wrong, like any textbook in hydrodynamics and/or boat design will show. Square wheel may be novel and nobody uses them, but not because everybody is stupid and refuses to use a better turning wheel.
     
  4. upchurchmr
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    upchurchmr Senior Member

    I always liked Gonzo's input.:D:D
     
  5. gonzo
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    gonzo Senior Member

    I think that any CAD technician student can model a simple teardrop shape. They will work for $15 and hour and probably take no more than two hours, including some modifications.
     
  6. Dolfiman
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    Dolfiman Senior Member

    Yes, the flatter the bottom, the better the upright stability, but at the cost of more beam (at equal displacement) and more wetted surface, so less speed potential, actually the usual trade off .... Speed itself is hazardous to estimate (human propulsion + ergonomic consideration, paddling efficiency, right coefficient for the residuary drag, … ) but at least the combination load and its position (sit, stand), waterline beam, wetted surface, GM (upright) et GZ curve can be investigated accurately with the knowledge of a hull geometry. To note that stability when standing and when sitting cannot be appreciated exactly with the same criteria : standing, that leads of course to a lot less stability but, after a little training phase, offers you more ability to move fast and accurately your center of gravity to restore an equilibrium. Sitting, you have a limited capacity for that. A bit like
    standing sailing (windsurf) versus sitting sailing
    (dinghy).
     
  7. johneck
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    johneck Senior Member

    Modern submarines do not have 50% reserve buoyancy when surfaced. Probably closer to 20%. The Collins looks like it has a lot of volume above surface, but mostly superstructure. The pressure hull is fairly well submerged. It is trimmed by the stern, which does help with surface resistance. But overall you are correct that the resistance of a sub on the surface is really, really high. Modern subs can probably go 2X faster when submerged with the same power.
     
  8. Alwaysthinking.
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    Alwaysthinking. Junior Member

  9. Chuck Losness
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    Chuck Losness Senior Member

    Nobody thought that the mini transat boats with a scow bow would sail worth a darn and would be slow to boot. Yet the naysayers were proven wrong. When I first read the title to this thread my first thought was a scow bowed kayak with a long flat run aft.

    The first shape that I would try would be patterned on the 4 digit foils that have been used on keels for decades for the waterline shape of the bow and for the profile of the bottom. These foils have an elliptical shaped leading edge and reach their maximum width at 30% of length before tapering to a point. I would not taper to a point at the stern. Rather I would extend the maximum width all the way to the stern while flattening the run.

    I don't know the length that you are thinking about. Assuming a 12' LOA and 0009 foil your boat would be 26" wide. I would pick a 0004 or 0005 for the bottom profile and tie the two together with elliptical sections. This would be a starting point. I use google sketch for drawing which isn't very good at rounded complex shapes. I'll try to do a drawing of what I am thinking about and will post it later.
     
  10. Chuck Losness
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    Chuck Losness Senior Member

    Here are some crude drawings of what I call a scow bow kayak. It was too hard for me to draw the very bow shapes but I think that you will get the idea.

    scow bow kayak profile.jpg
    scow bow kayak top view.jpg
    scow bow kayak bow.jpg
    scow bow kayak stern.jpg
     
  11. Chuck Losness
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    Chuck Losness Senior Member

    When I drew this late last night I left out the topside. Added it this morning.
    scow bow kayak.jpg
    I also thought about using the same shape at the bow and the stern.
    scow bow kayak 02.jpg
    I am just playing around here thinking off the top of my head.
     
  12. Waterwitch
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    Waterwitch Junior Member

  13. KJL38
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    KJL38 Junior Member

    Your teardrop design would be very directionally unstable similar to a slalom or polo kayak unless you add a skeg.
     
  14. gonzo
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    gonzo Senior Member

    Minitransat boats are specialized for downwind sailing. They are scows, and can go upwind fairly well in moderate winds. In either light or high winds, their performance upwind is poor compared to a conventional design. Anybody with knowledge of boats knew that scows would sail well in the restricted condition of the Minitransat race.
     
    philSweet likes this.

  15. sharpii2
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    sharpii2 Senior Member

    It is interesting to note the European small sailing working craft often had their maximum Beam forward of their half lengths and US craft of the same size class tended to have their maximum Beam aft their half lengths. Some might argue that the European boats were developed earlier than the US ones, so were designed to faulty traditional theory, where the US ones were designed to more scientific theory. My guess is that the European boats faced much different conditions, even though they sailed on the same ocean. The European boats typically faced windier conditions and had to be more directionaly stable in strong winds and big waves. The US boats faced more calms, more glassy swells, and often choppier conditions. The longer, more pointed bow was a far greater asset in these conditions. It also appears that the US boats were expected to sail upwind more often, so shoal keels quickly gave way to deeper center boards. Of course, this quick adaptation to centerboards could also be explained by the existence of far fewer natural harbors on the US side, so a lot of the US boats had to be hauled ashore after each outing (what a sucky way to end a long work day).

    I once briefly co-owned a sixteen foot sailboat which had its maximum Beam forward of its half-length. Haul out time had arrived and I had already taken the rig down. The boat had to be towed to the other end of the lake where a ramp was available. The lake was flat calm that day. My dad drove the tow boat, and I steered the sailboat. I advised my dad to go slow, around three knots, but he was soon doing nearly six. I noticed that there was next to no wake coming from the sailboat. I was quite perplexed. But I would never want to race that boat against one that had a longer bow upwind in any kind of a chop.

    As a general rule, I understand that a boat's hull design should reflect the S/L ratio it is expected to sail at. A really slow S/L ratio of say 0.75 or less can have chunkier ends. One that is to sail at say 1.0 to 1.2 should have finer ends. And one which is supposed to sail at 1.3 to higher should have fuller ones, but ones which are not chunky. The fuller a vessel's ends are, the greater the prismatic ratio.

    Examples of S/L ratio speeds for a boat with a 16 ft WL are: 0.75 S/L = 3 kts, 1.0 S/L = 4kts, 1.2 S/L = 4.8 kts, and 1.3 S/L = 5.2 kts. The important thing to note is that the resistance goes up with the square of the speed difference. For example, the boat of the same size and type, going at an S/L speed of 1.2 is going to have 1.44 times the amount of drag as the one traveling placidly along at an S/L of 1.0. This is why paddle powered boats tend to be so long. S/Ls beyond 1.0 tend to get most of their drag from wave making. Slimmer boats tend to make less waves, so can often reach speeds well beyond that of what the S/L ratio predicts for the same amount of power. This is why kayaks are as narrow as possible

    I see one advantage see of putting the maximum Beam forward on a kayak. This is that the average Beam can be increased without increasing the maximum Beam. It is the average Beam which gives you the initial stability. So, if your kayak had a nearly rectangular deck plan form aft the maximum Beam, it could have a smaller maximum Beam and have the same initial stability. Moving the maximum Bean forward will heighten this effect. The actual waterlines, while the craft is being paddled, can be quite fine as long as the maximum Beam is not too far forward. Once the kayak starts to tip, the waterlines on the low side will quickly widen providing extra stability.

    But one should not go nuts about putting the maximum Beam too far forward. John C. Hanna did this with several of his square stern boats, and they quickly developed a reputation for being quite slow--although surprising seaworthy. I would not put the maximum Beam any further forward than 1.5 times its number behind the start of the surface water line.
     
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