I have a fiberglass core craft 17 foot canoe from the mid-to late 70’s that has been passed down the family. The thing is heavy. I am just trying to figure out its weight so I can make sure the hoist system we have is adequate. Does anyone have a good guess what a large, fiberglass canoe of that era weighs?
Rather than guessing, you can figure it out pretty easily.
Method 1: For each person lifting the canoe, the lifter should be on a bathroom scale. Compare the weight reading on the scale while holding the canoe vs. not holding it.
Method 2: Buy a spring balance. Suspend the canoe in the air with a rope tied to a solid object. Place the spring balance in-line between the anchor point and the canoe.
Some margin of safety should be built into the system for several reasons: strength ratings of ropes, etc. could be incorrect, rope might deteriorate over time, and more stress is put on the system when lowering/raising/moving the canoe than when it's at rest.
Strictly a guess…
70 pounds ?
A modification to make this easier
You can do all this with one scale, used twice.
Method A: Two people lift the canoe from the ends. One stands on bathroom scale and records the weight. Then the two people, still holding the canoe, switch postions (or a bystander moves the scale) and the second person steps on the scale and records the weight. From the total of those two weights, subtract the weight of both people.
Method B: By whatever means you wish, do the same as above with a single spring scale lifting one end of the canoe at a time. Just make sure that for both measurements, the location where the scale is attached and the location of the support at the other end are the same, relative to the respective ends of the boat. Also make sure the boat is level for both weighings. Add the two spring-scale readings. If the support points are chosen accurately, the weight that’s determined will be spot-on. Chances are, you will get an accurate enough weight from a single measurement if both support points are the same distance from the respective ends, and you simply double the spring-scale reading.
Margin of Safety
This is a good point, though actually it's during lifting that the tension in the rope will be far higher than that generated only by the weight of the boat. At least, this is true of there are any pulleys in the system (if no pulleys, it won't matter much).
A commercially-made hoist will probably take this into account. If the hoist is home-built, pulleys and anchor points that are "just good enough" to tolerate the weight of the boat will be tiny, and common sense would suggest not using them, but it's good to mention anyway.
I use ropes on my hoists that are far stronger than necessary, but the stress of constant tension and repeated lifts eventually wears them out and they must be replaced.
The rope you use for your hoist system should be rated for significantly more than the weight of your boat. Knots reduce a line's strength by as much as 50%, and friction losses combined with angle of pull will increase the force on the line while you are raising the boat.
For the hoist I built, I first tried a big box store bicycle hoist kit. Returned it unused - way too cheaply made for my tastes. Then I rigged up a 3 to 1 system using pulleys and 1/4" nylon rope. Despite the rope being rated for 300 lbs, it still made me nervous. The rope was obviously being stressed, and the narrow diameter did not roll smoothly through the pulleys. 3/8 diameter big box store nylon rope cost almost as much as climbing rope, so I just went with 8 mm accessory cord from a climbing supplier. That stuff rolls really nicely through the pulleys, takes knots really well and is not even close to being stressed by the weight of a canoe.
Even a tank would be under 80 pounds.
Put it on a trailer or on top of a truck and use a truck scale.
Figure 100 Lbs.
Build the hoist for 100Lbs and you will be safe.