With the boat now on her trailer with wheels but pretty much empty, now would be a good time to check her weight. We do have a local weighbridge but you need to make two trips, one with the boat on the trailer and one with it off. This is not really practical for fairly obvious reasons. Fortunately, there is an easier option and that is to use a simple method taking advantage of what the physicists would call moments of inertia summing to zero. Fancy name for the fact that the centre of mass of the boat times the distance from the centre of the wheels pressing down must equal the length of the trailer to the jockey wheel pushing up.
But what about the weight of the trailer and the boats centre of mass, to say nothing of the centre of mass of the trailer, I hear you ask, well if you do the measurement twice with the boat moved a bit on the trailer between the measurements, all the unknown bits cancel out (I’ll publish the maths in another post for those that don’t believe me).
So, here is the method.
Move the boat backwards on the trailer about 4″ or 100mm. The more the better but don’t overdo it so that the boat tips the trailer back. Make a pencil mark on the trailer somewhere using a plumb bob from the bow of the boat. Now measure the weight at the jockey wheel, call it M1. You can use bathroom scales if you like but I use a 350 kg scale suspended over the hitch. Now measure the distance from the centre of the wheels to the point at which you made the measurement and call that L. Be as accurate as you can. Now move the boat back to its normal position and make another pencil mark on the trailer using a plumb bob from the same place that you made the first mark. Carefully measure the distance between the two pencil marks and call that D. Measure the weight on the jockey wheel again and call that M2.
Now, the weight of the boat is the (M2 – M1) * L / D
That’s it.
The only hard part about it is moving the boat backwards and forwards, but at least you don’t have to move the boat off the trailer completely and then back on again.
But before I can do any of that, I need to adjust the trailer in two ways. Firstly, the swing arms on the new axle are 3″ shorter that those on the old axle and I now need to move the new axle forward at least 3″ to compensate. Secondly, all the boat supports on the trailer are down very low. Too low to be able to put a beam under the hull but over the trailer that is strong enough to hold the boat up off the trailer, so the trailer has to come out from under the boat so that I can raise the supports as high as they will go.