Builders' Forum Archives
Re: cartopping long boats
Posted by Laszlo on May 10, 2007
Jon is right. Air is compressible - in a container or at supersonic speeds.
Camper's got the explanation for moving through uncontained air at subsonic speeds.
Jim makes some good points. In relatively smooth water the forces are definitely less than on top of a moving car. But if you put that boat into surf, breaking chop, large wakes, etc., you'll get forces that are at least as extreme.
The forces are definitely proportional to the square of the speed -in the same medium. In media of different densities it's possible to have the same forces at different speeds. The space shuttle, for example, is built to withstand no faster than 300 mph or so - at 1 atmosphere of pressure. It can fly hypersonically because that happens at an altitude where the air density is much lower than at sea level, so it experiences forces less than what it would experience at 300 mph at sea level.
Whether the flow is in line with the kayak's axis depends on too many real world variables to be able to definitely say one way or the other, but the confused flow on top of the car can certainly be similar to the confused flow in surf, breaking chop, ...
Besides the lift Jim mentions (which would be more extreme for the plane-shaped windsurfers than our toothpick kayaks), there's also the raised center of gravity (if your boat's big enough) and the possible misalignment of the centers of pressure and gravity which would change the car's handling characteristics. So he's absolutely right - reducing speed when carrying a boat is always a good idea for safety reasons. In addition, it gives the spectators a chance to admire your handiwork.
In Response to: Re: cartopping long boats by Laszlo on May 9, 2007