Weighing Your Boat

The following article was first published in the Log in April 2003. It is repeated here for the benefit of members who have joined us since then.

Here’s an interesting one. It is taken from an article by John Wright, which appeared in Practical Boat Owner No. 134, February 1978. It details a method of weighting your boat (on the trailer) without access to a public weighbridge. Equipment required is a tape measure and a spring balance or the bathroom scales. I’ll let John Wright explain:

“The trailer is used like the Roman steelyard, but with the unknown weight being moved instead of the known one. In the normal trailing position on level ground, I measured the downward pressure on the hitch, and also the distance from this to the trailer axle. “I then moved my boat back a few inches until the pressure on the hitch was upwards. I measured the upward pressure and the distance I had moved the boat. Provided that both

pressures are measured with the hitch the same distance above the ground, then the weight of the boat is equal to the sum of the two pressures multiplied by the length from hitch to axle divided by the distance the boat has been moved.

Mathematicians can easily work out this simple equation [W=(p1 + p2)(L/d)] and will realise that since the relationships are linear, various unwanted constants like the trailer moment are eliminated.