Why You Can’t Lift the Boat

It’s a common notion that paddlers can, through good paddling technique, actually “lift” the boat so that it sits higher on the water.  This feeds into the notion that the boat becomes “lighter” to the water than when it sits at a standstill, decreasing water drag, and increasing the potential for speed.

It’s complicated and there aren’t any studies that I’m aware of looking at this phenomenon with dragon boats specifically, but based on existing science of water craft physics, it doesn’t appear that paddlers can actually lift the boat when paddling as generally thought.  I’ve got some reasons for thinking this, but perhaps the Mythbusters can put this to the test.

Reason 1: Paddlers can’t directly exert an upward force on the boat by paddling.  When the paddle “anchors” in the water and the paddler pulls, they transfer this force to the hull through their butt and foot to propel the boat forwards.  In transferring this force, paddlers actually push the boat downwards into the water on the side they sit.  At practice, try having several rows paddle on one side of the boat.  The boat will dip to that side during the pull phase and rock to the opposite side during recovery.

Reason 2: Boat lift is generally attributed to either hydrostatic (buoyant) lift or hydrodynamic lift.  Hydrostatic lift is the phenomenon that allows boats to float because the hull displaces an equal mass in water volume as the craft and all its cargo weigh, which is why dragon boats and those like it are said to have displacement hulls.  At a standstill, boats float by hydrostatic lift.  Once the boat starts to move, some lift is gained by hydrodynamic lift where the water is pushing the boat upwards vs being pushed out of the way by the hull.  At a certain speed, the boat’s hydrodynamic lift will exceed the hydrostatic lift and the vessel begins to plane across the water.  This requires a very high amount of power to achieve.  Think of trying to walk on water.  Unless you’re especially holy, you’ll likely sink the moment you step foot on the surface.  Now, if you get tossed out of a speed boat going 200 mph, you’ll painfully skip and bounce off the surface of the water because hydrodynamic lift is keeping you from sinking.  It would take a very strong motor to get a dragon boat even close to planing speed.  IMO, paddlers can’t put out enough power to make hydrodynamic lift that effective.

Reason 3: Like I wrote above, paddlers will have a very hard time reaching planing speed because of the physics of displacement hull speed.  Displacement hulls are subject to something called wave making resistance which occurs when waves made from pushing water off the front combine with waves made in the wake.  This combination of the waves at either end cause a rapid climb in water drag.  This point is called hull speed.  It is a calculation based on the length of the water line of the hull as it sits in the water.  A fully loaded dragon boat has a a certain measurable length of water that contacts the hull which is measured as the water line length.  A larger waterline actually makes for a higher hull speed value!  Lifting the boat out of the water becomes less desirable in this regard.  Certain boats are designed to allow the athlete(s)/motors to exceed calculated hull speed without planing, which THEN causes a strong decrease in drag.  Essentially, water drag increases as hull speed is met, a boat with a larger water line length has a higher hull speed, lifting the boat decreases the water line length and decreases hull speed causing earlier rise in water drag as speed increases.


3 responses

  1. Pingback: To Read « backsixpush

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