Re: cartopping long boats

Posted by Laszlo on May 9, 2007

Here's an interesting thought about cartopping - The forces experienced by a kayak moving at 70 mph through air are actually less than what it experiences moving through water at 12 mph. This is because both air & water are incompressible fluids, but water is much denser than air. Nature even demonstrates this by having sharks streamlined like supersonic fighters, even though a biplane can outrun them. It's all in the density of the medium.

So what's the problem with cartopping? It's the thousands of pounds of ballast tied to the bottom of the boat. In particular, it's how the ballast is tied to the boat. If the combined boat/car structure is flexible, it will start oscillating. If it hits a resonant frequency for the boat/car structure, the oscillations will build up to the point where the structure may come apart. Low frequency oscillations are the more common while driving, so low frequency resonances should be avoided.

There are three ways to do this. First, increase the resonant frequency of the system by spreading the distance between the points which support the boat. This has the effect of breaking the boat up into 3 high frequency resonators instead of 2 low frequency resonators.

Next, reduce the transfer of low frequency energy into the boat by using the stiffest possible racks.

Third, raise the boat's resonant frequency by stiffening it. You can do this either with bow/stern lines or the frame described by RGR.

Once you get to a sufficiently stiff system with a high oscillation frequency, you can stop adding tiedowns. It's all combination of boat length and rack/car characteristics.

Be safe,


In Response to: Re: cartopping long boats by RGR on May 9, 2007



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