Re: [Paddlewise] Bow lines and racks

The force would certainly have been generated alone the length of the
kayak, with perhaps slightly more force (pressure) at some places than
others.  The problem is one of differential stress (and thus strain), not
forces or lever arms independently.  I also suspect that the force would
have been toward the CENTER LINE of the highway (horizontal), and not so
much upward (ref Bernoulii's principle).  Lastly, I question how this
"force" was measured by the observers.
The bottom line (as you already know) is that snug (but not too tight) bow
& stern lines hold a kayak more securely to the roof rack without warping
it significantly.

Les


At 05:40 AM 6/21/98 -0700, Dave Kruger wrote:
>Jack_Martin_at_jtif.webfld.navy.mil wrote:
>
>[snip]
>>      Was speaking with a Yakima customer service rep yesterday about kayak
>>      racks.  He gave me what sounded like a standard "please always use bow
>>      and stern lines" disclaimer, which grew into a short discussion.
>>      Apparently Yak did some tests recently, and discovered that when a
>>      vehicle with a sea kayak on the roof passes a semi-truck heading the
>>      opposite direction on a two lane highway, the wind blast created
>>      generates a 200 pound instantaneous vertical lift on the boat's bow.
>>      That's a bunch of pounds now trying to lever your boat out of its
>>      straps or, as Chuck Sutherland pointed out this morning on his CPA
>>      response, the forward rack and boat off the roof!  He was not speaking
>>      hypothetically, apparently.
>[snip]
>
>Two hundred pounds of lift could be significant in affecting the
>rack-roof connection, all right.  I suspect this was measured at the
>point where the rack meets the roof, and *not* the tip of the nose.
>(Tougher to measure there.)
>
>Is that what he said, Joq?  I think an unopposed 200 lb force exerted
>*at the tip* of a typical FG yak, if the yak is in its usual position in
>the rack, would do damage to the gel coat (stress cracks), as a
>*minimum.*  I'll test this with *someone else's* yak, not mine, thank
>you!  (BTW, I have seen similar "stretch marks" in a kevlar/epoxy yak
>which was overtightened at the bow line while in a vehicle rack --
>probably produced by much less than 200 lbs of force at the bow.)
>
>If the 200 lbs was measured at the rack position, there is an easy way
>to approximately simulate that vertical force:  Most of us can push
>upwards with at least 50 lbs of force.  Just push up on the tip of the
>bow, and then the tip of the stern, with the effort needed to lift the
>average 5-year-old.  Because of the lever arm between the tip of the yak
>and the strap position, there should be at least 200 lbs of vertical
>force on the straps, I think.
>
>FWIW, I've watched the bow of my FG Wind Dancer "jump" to the side when
>meeting a semi at high speed on a two-lane road.  It moves a LOT! 
>Because the straps I use are rated for about 2000 lbs, and I have a
>truck with real rain gutters, so the rack is firmly attached to the
>truck, I have not used a bow or stern line.  Probably should, to be
>belt-and-suspenders safe, though!
>
>-- 
>Dave Kruger
>Astoria, OR
>
>

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