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As for adding weight, don't forget that other people make great temporary weight. Have your buddies sit on the tailgate as you pull up the ramp. Heck, ask the cute girl at the dock who's waiting for her boyfriend....you never know. Ask her bigger mama and you'll get the double bonus! I've sat on the hood of many stranger's Buicks and Oldsmobiles as they tried to pull small boats out of ramps with their front wheel drive cars.
I'm all for helping, but sitting on a vehicle of a complete stranger is something I will NEVER do, nor ask anyone else to do. Very low odds of something going bad, but when they do, you are going to get run over. You don't know how they load, or if they are going to mash the gas halfway up the ramp.
I've hooked onto the front bumper of more than one small 2wd truck at the ramp, and pulled them out with my subaru sports car! Still substantially safer than getting on the tail gate of a strangers vehicle.