- using the bathroom scales

From time to time I have wondered what is the weight of my boat + trailer, mainly to check that it is within the legal limit for an unbraked trailer, i.e. less than half the kerbside weight of the car. Although one can take it to a weighbridge this may be inconvenient, especially as one may want to reweigh it under different conditions, e.g. without the boat, with boat fully loaded with gear etc or after modification to the boat or trailer. The system described here is a DIY approach which allows you to do this at home; it also provides amusement for the neighbours.

The principle of the method is shown in the illustration. B is a substantial beam, see below, resting on a strip of wood acting as a fulcrum at F Another strip of wood at W defines the length of the lever and distributes the load on the surface of the bathroom scales. The lever arrangement is first rigged so it is under one tyre of the trailer. The other tyre, and also the nose wheel/tow hitch, are placed on blocks to make the trailer level. The reading of the bathroom scales is taken and also L1 and L2 are measured. The ratio of 'weight exerted by the tyre' to 'weight on the scales' equals the ratio of L1 to L2. The weight under the tyre is thus calculated by the formula: 'weight on scales' x L1 divided by L2. The arrangement is then transferred to the other tyre and the process repeated. Finally the weight exerted through the nose wheel/tow hitch is measured directly by placing the scales under it. The three weights are added up to give the overall weight.

There are several practical points to observe:

1. Do the operation on a hard level surface. With a spirit level arrange packing so the beam B is horizontal during measurements. Small chocks placed fore and aft of the tyres would improve safety.

2. L1 should be reasonably large so that errors in measurement of length are of little importance: I chose 72". L2 should be chosen to give a reading in the middle/top part of the range of the scales. I used about 24". The values of L1 and/or L2 need not be identical for the measurements on the two tyres.

3. The tricky part is to arrange for the trailer to be in the same attitude, i.e. in both the transverse direction and the fore and aft direction, for all three measurements. The simplest way to do this is to use a spirit level on any convenient surface; axle beam for example. By use of the jack the lever arrangement is first placed under one tyre. Packing is then placed under the other tyre until the spirit level, oriented in the transverse direction, registers roughly level. Observation of the bathroom scales during this procedure will give you a feel for how the degree of packing affects the outcome and therefore how accurate - actually not very, the levelling need be. The nose wheel/tow hitch is now packed up to give rough fore and aft levelling and the reading on the bathroom scales can now be taken. The whole thing is repeated with the lever arrangement and the packing swapped over to opposite sides, although a different thickness of packing may be needed to level it this time. Finally the lever arrangement and the packing is removed from both wheels and the nose wheel/tow hitch measurement made. As this is only a small component of the overall weight this measurement can be done after a cursory levelling in the fore and aft plane only.

4. If you have a second set of scales you can duplicate the lever arrangement so one is under each wheel. The transverse levelling procedure is then not necessary as the two readings are taken without disturbing the transverse attitude of the trailer.

5. The beam itself exerts a certain dead weight on the scales before the tyre is placed on it. This dead weight should be subtracted from any subsequent reading before applying the formula.

6. The only difficult component to acquire is the beam. Although this must be strong enough not to break, bending of it is not important as long as it does not contact the ground. A short scaffolding board, cross section around 9" x 1.5", is ideal and could be borrowed, or often bought cheaply in places dealing in second-hand building materials. I have also used two lengths of 1.5" angle iron with a piece of plywood sitting between the two to make a platform for the tyre.

If you want the weight of the boat alone you will also need to weigh the trailer without the boat. The boat may be 'launched' off the trailer onto soft ground, old carpet, car tyres etc., if the tow hitch is raised up high. This can be done with some pushing and heaving, lots of groaning, and possibly with the aid of the winch line led back through a block attached to the middle of the trailer, so the trailer can be slid forward from under the boat. Recovery is the reverse procedure.

It sounds complicated but it has taken longer to describe it than to do it. How well does it work? For my West Wight Potter and heavy trailer I came up with a weight of 937 lbs. I took it to a weighbridge and it indicated 970 lbs!