How To Weigh Your Boat and Trailer Part 1 PDF Print E-mail
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Sunday, 09 August 2009 23:10

How to Weigh your Boat and Trailer Part I

 This article demonstrates an easy way to come up with an approximate weight for your boat and trailer using a bathroom scale, a metal beam, and a car jack.



   Most bathroom scales can only weigh up to about 300 pounds.   A speedboat will weigh between 1000 and 4000 pounds, which is likely to break any normal scale.   We will use the simply principle of leverage to weigh your boat without any special equipment.


  First of all, the weight of the boat is distributed among the two wheels and the tongue of the trailer.   A typical boat might have 1300 pounds pressing on both wheels, and 120 pounds on the tongue.   The total weight of the boat on the trailer would be 1300 + 1300 + 120 = 2720 pounds.     We are going to use the principle of leverage to measure the amount of weight on one trailer wheel,  then we will double that value, and measure the weight on the tongue directly with the scale.


 Balance of even forces equidistant from the fulcrum

  So if the fulcrum is in the middle, to lift a 500 pound thing, we need to press down on the other side with 500 pounds.


Balance with non-centered fulcrum

 But if the fulcrum is moved so it is closer to the thing that you are lifting, then you need much less force to lift.   If the thing is 1 foot away from the fulcrum, and your pushing force is 5 feet away from fulcrum, then you need to push down with 500 * 1 / 5 =  500/5 = 100 pounds.

  Now you aren't getting away with something for nothing.   Although you need to push with 5 times less force, the thing barely moves at all as you move your side down.  I.E., if you move your side of the beam down by 10 inches, the thing will only move up by 2 inches.   

  However, all we need to do is lift one of the trailer wheels off the ground by a tiny bit.  We will use the bathroom scale to measure how much force we needed to press down with make that wheel come up off the ground.  Then we measure our distances from the load to the fulcrum and from our pressure point to the fulcrum.


Things you need

   A bathroom scale, a hydraulic jack, some blocks such as concrete blocks or large wooden blocks,  a very sturdy beam such as a steel tube, and a tape measure.

Step 1 - Attach trailer to your car and set the parking brake


 This is a very important step.  For safety, we don't want any possibility the boat can roll while we are trying to weigh it.


Step 2 - Jack Up one Side

Jack Up One Side

  We jack up one side of the trailer and put a block under it.   Make sure it safely supported.   Try pushing and pulling on the side of your boat to make sure the blocks are stable.  Make sure to let the jack down so it is not still pressing up on the trailer.  This would ruin the measurement.

Safety Note:  At no point should you ever have your hand between the block and the trailer.  The jack could fail or topple at any time.  It is up to you to make sure that if at any time the trailer falls off the jack, or your blocks topple, that you and anyone else will not be harmed.  In general this is easy, just be careful where you put your hands or any other body part.


Step 3 - Building the Lever

Setting up the lever

 On the OTHER side of the trailer from Step 2, we will build a lever.   We want the point of contact to be the same as the point of contact of the blocks on the other side.  For the fulcrum I used a pressure treated 4x4 and a hardwood leg from a broken stool.  Since the boat is attached to a vehicle with the parking brake set, the trailer couldn't move forward or back, and the round fulcrum did not roll.   I put the fulcrum at the closest convenient point, which worked out fine for my boat.  If it had been much heavier, I would have probably needed to put the fulcrum closer to the point of contact so I got more leverage, or perhaps used a longer beam.


 Safety Note:  It is necessary to be sure the beam you are using can handle the force.  I used a 3" square steel tube with 1/8" walls.  This withstood the forces for my boat with no problem whatsoever.   I do not know if a wooden 4x4 would have been strong enough.  If the beam breaks suddenly, you could injure yourself, so be careful and use common sense.



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Last Updated on Monday, 10 August 2009 03:48

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