by MookaCB » Mon Jul 18, 2016 1:14 pm
I am an engineer dealing with cranes, rigging, heavy lifts in oil refineries, construction, and maritime.
A word of warning about tow straps, ropes, etc. They are not designed or rated to be used for lifting. Generally, lifting gear is rated for a 5:1 safety factor. If it has a 1000 pound breaking strength, you can safely lift 200 pounds. This is to account for dynamic forces and the loss of the efficiency at the connections.
If you are doing a lot of lifting at a club, purchasing proper slings would be best and because they are pretty small, you can probably get them for a couple hundred dollars. I am used to ordering $5000 slings to only be used once. They also have adjustable ones! Many rigging lofts (industrial, not sailing) can even produce custom lengths.
All that being said, I have definitely lifted my boat with Costco ratchet straps...
Also, remember to account for the angle of the slings used. Assume the boat weighs 500 pounds, if 4 slings are used and are at a 45 degree angle, each slings will be lifting 177 pounds instead of the 125 pounds.
I would be happy to run quick numbers for anyone that is thinking about making gear. SAFETY FIRST.