During a weekend while my wife was away, I decided to clean out our garage and relocate her 6 meter long canoe. Straddling the garage wall next to my car, the canoe had for years forced me to clamber across the passenger seat when entering and exiting the vehicle.
Several factors combined to make this project challenging:
To support the canoe from both ends would require a beam just under the door track. But the door handle sticks out and would have to clear the beam. So there would be little vertical room for tackles.
The center (of gravity) of the canoe could hang a few centimeters ahead of the maximum extent of the door. Suspending it from this point seemed an elegant solution.
My motto for woodworking projects is "throw the first one away"; and that seemed appropriate for this effort as well. My son and I lifted the (overturned) canoe with a plywood board; and it seemed stable enough. We put up a pulley and tried lifting the canoe with the board slung from a four-point harness.
The canoe seemed much heavier than its 35.kg. It was so tipsy, we could barely manage to lift both ends off of the ground. Once hoisted, the canoe twisted and swung back and forth and was difficult to still.
Its tipsiness is not hard to explain. In order to be stable, rotation away from level orientation should raise the canoe's center of gravity. The base of the triangle formed by the board and harness was too narrow. Lengthening the board would make it difficult to remove once the canoe was lowered onto the car. My son realized that there was no reason to have the board stretch from gunwale to gunwale. So we replaced it with two boards; one under each gunwale. Lifting one side of the canoe would allow the board to easily slide out from under it.
Our second try performed better. It was possible to hoist the canoe, although it still required both of us. The twisting was largely eliminated. The canoe's swinging was restricted to the direction of the pulley axes.
The two boards under the gunwales were not stable. When bumped, they tended to slide out at one of the ends. I had thought that supporting the boards from their inside corners would avoid this problem. And that would have been correct if the gunwales were flat; but they are cupped.
So we nailed blocks to the boards to make contact with the widest part of the gunwales. To further ease hoisting, and to suppress swinging in all directions, I changed to a four pulley design where the pulleys were not collinear. With this arrangement, any lateral movement must cause rope to move; and the rope and pulleys have substantial friction.
I can now hoist the canoe single-handedly. Bumping the canoe does not cause the boards to shift. If the garage door is open and a stiff breeze is blowing, the canoe will turn somewhat; but not enough to cause a problem.
How is that for white-knuckle engineering? It amazes me every time the door opens.
Copyright © 2000 Aubrey Jaffer
I am a guest and not a member of the MIT Computer Science and Artificial Intelligence Laboratory.
My actions and comments do not reflect in any way on MIT.|
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