The JSD is designed to keep the boat from being damaged by breaking waves. It allows the boat to move a bit, but doesn't hold it in place like a sea anchor does, which makes the boat a big target.
Spot on SailingDog!! If you get hit solidly by a breaking wave onto the top of your boat - think in terms of around 500 or so gallons of seawater at about 8 pounds per gallon, it's like dropping a Buick onto your cabin trunk.
s/v Paloma, Bristol 29.9, #141
Slipped in Bahia Marina, easy access to Corpus Christi Bay and the Gulf of Mexico.