Why does the stern of my rowing shell squat in shallow water

Discussion in 'Boat Design' started by Squidly-Diddly, Jun 29, 2008.

  1. Squidly-Diddly
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    Squidly-Diddly Senior Member

    directly related to speed?

    I can tell the water is getting shallow because the stern starts to squat.

    Could it be the bow is getting pushed up?

    This occurs with about 2-3" of water under the rear skeg, and about 6-8" of water under the hull on a 21' x 21" Maas scull when rowing at 'medium' recreational speeds.

    Seems to be about 20-30lbs of force at work if measured at the extreme stern.
  2. TeddyDiver
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    TeddyDiver Gollywobbler

    It's becouse the water flow speeds up btw the hull and bottom and thou creates a "lift" , in this case a down force, like they have with some sport cars.
    The phenomen is quite obvious in rivers when rowing over reefs..
  3. eponodyne
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    eponodyne Senior Member

    It's a common and inescapable phenomenon, TeddyDiver had it in a nutshell. There's no getting around it, and the faster you go, the more your stern will suck down. Also the shallower the water gets, the worse the bottom suction is.
  4. Meanz Beanz
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    Meanz Beanz Boom Doom Gloom Boom

    It happens to my cat with less than about 2 foot under her. If I get a squirt on the boat gets sucked down. I always assumed it was similar to the "ground effect" talked about in F1 racing years ago.
  5. johnhazel
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    johnhazel Senior Member

    This squatting problem gets to its worst at a speed and depth relationship where the "critical" depth Froud number = 1. To calculate at what speed in m/s this occurs in a given depth you can plug into this formula:

    Critical speed = square root of 9.8*Depth

    The squatting actually becomes much smaller at speeds above and below the critical speed.

    I race 5.6m canoes mostly in depth varying from 0.1 to 1.5m. We see this critical depth Froud number effect quite often in the shallower water. A very noticable peak in drag occurs at the critical speed and then the drag actually is lower at higher speeds. The stronger-smarter teams will take advantage of this and do a brief sprint before going into shallow water so they are sure to be above the critical speed. This allows them to cross the shallow water at much higher speeds than paddlers who do not anticipate the shallow water. These weaker and less thoughtful paddlers are left way behind in such situations as they battle against the steep increase of resistance on the low speed side of the drag peak.

    Its also intresting to see that the divergent waves from the boats get to nearly 90 degrees at the critical speed.

  6. eponodyne
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    eponodyne Senior Member

    Another thing to note is that with some sorts of propulsion--I guess I'm thinking here of the twin-screw riverine tugs I work on-- this becomes very noticeable even when the boat's standing still. The stern just keeps suckin' down, the seagulls swoop in, the air fills with rank miasmas and meanwhile, I'm worried about the rigging piled on the stern just sliding off never to be seen again. "Might wanna ease off the juice for a sec, Cap," I say.

    They never listen.
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