Re: [Paddlewise] How much is Alex lifting? (was: Kayak Trailers)

Matt Broze wrote:
> Shawn Baker wrote:
> 
> <SNIP>>>>>>Assuming the kayak's weight is spread evenly along its length, if
> the
> bow is on the rack exactly at the boat's 'quarter point' (1/4 the
> length of the kayak back from the bow), you would be lifting half the
> weight of the kayak, or 28 lbs., if you're lifting at the stern quarter
> point.
> 
> If you're holding the extreme stern of the kayak, maybe your grab
> handle, and the bow is over the rack at the quarter point, then you'll
> be lifting with a force of 33.6 lbs, and the rack will be supporting
> 22.4 lbs.<<<<<<<<<<<<<<
> 
> I agree with the first paragraph but not with the second. Could you explain
> how you arrived at your result? As I see it the paddler will only be
> supporting one quarter of the weight, 14 pounds. 

Close.  Under the stated assumptions, i.e. level kayak with uniform 
weight distribution and being supported at a point at one end and 
another point 1/4 of the way from the other end, we have the following 
forces:
1) gravity effectively acting on the center of mass of the kayak pulling 
straight down with 56 lbs.;
2) a force, X, pushing straight up at the point 1/4 of the way from the 
bow; and
3) a force, Y pushing straight up at the stern.
Since the kayak is motionless (no acceleration), the forces have to 
balance, so X + Y = 56 lbs.  Also, since there is no rotation of the 
kayak about its center of mass, the torques from X and Y have to 
balance.  That means that X = 2 * Y, since the force Y is pushing on an 
effective lever arm with twice the length compared to the force X. 
Together this means that 3*Y = 56 or Y = 18 2/3 lbs. [The problem with 
your analysis that led to 14 lbs. is that the stern half of the kayak is 
closer to the lifter at the stern than it is to bow support point, so he 
has to lift 2/3 rather than half of this portion of the kayak.]

But Dave's point earlier about the angle of the boat is correct.  When 
you start to lift the stern the boat is tilted up and this will 
substantially increase the amount of force that needs to be exerted by 
the person at the stern.

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