I am a structural engineer in Miami and design for wind and hydrostatic pressure on an everyday basis.
My personal opinion regarding hurricanes is as follows:
I live inland and my boat is in the yard on a trailer.
Category 1: I don’t do anything to prepare other than bring in anything loose from outside.
Category 2: I do the Category 1 stuff and put up my shutters.
Category 3: I do all the Category 1 & 2 stuff, stock up on food, supplies, batteries, water, etc and think about getting a generator.
Weak Category 4: All of Category 1,2,& 3. Make sure all my insurance bills are paid for the house, car, and boat. Fill 30 gallons water tank on boat, let some air out of trailer tires. (my boat is kept on the trailer in the yard). Remove the canvas top and secure all lines below. Watch the forecast to make sure it doesn’t become a strong Cat 4 or Cat 5. Hook up the truck to the boat trailer to help hold things down.
Strong Category 4 to Category 5: Prepare everything to be left behind, bring family, truck, & boat (along with essentials) north to escape the storm. Come back when it is safe to do so.
Now to your question (forgive me if I sound like a college professor....)
First, as suggested, you need to know how high of a storm surge you should expect. I would add another foot to the highest recorded surge to try and be sure you’ll be ok.
Then I would check to see if my lift can go high enough to lift the boat completely above that elevation. If not, forget about using the lift. The boat will float and probably damage or severely damage your lift if the waves start pushing the boat as the boat tries to float off, regardless of how well it is strapped down. The straps will start pulling up and buoyancy will win the war somewhere, whether it’s the strap, the attachment, the member attached to, even the piles can lift off the seabed if your boat’s buoyancy exceeds the tensile capacity of the piles (although most likely something else will break before that happens)
Assuming your lift is high enough, you need to be sure it can resist the lateral wind forces on the boat when lifted up high. Yes, wider supports are more stable than narrower ones. Assuming the lift can take the lateral and uplift loads (our boats have a lot of windage), then it becomes a matter of adequately strapping the boat down. Most likely the lift is not designed to resist uplift, but only gravity. You may want to check with the lift manufacturer for recommendations during storms.
For the purpose of armchair engineering, I would suggest using a wind load of about 75 psf on the boat lateral, and an uplift of about 60 psf. You can reduce the uplift by taking into account the weight of the boat. These loads would act simultaneously in the lateral and vertical direction, to account for winds, and are based on loads calculated for south Florida using modern building codes (which should be applicable to the lift).
This is all easier said than done. You want to check every component of the load path, especially the connections of the members. The load path would be roughly from wind to boat, boat to lift, through the lift members (beams/girders/posts/etc) to foundation. Keep in mind the boat will try to overturn, and to slide. You need to resist both. I would want a safety factor of at least 1.2 for all components if it was my boat. Building Code would require a safety factor of 1.6 for wind, which is a lot more, for buildings.
As stated before, I wouldn’t worry too much about CAT 1 or 2 storms, as long as you don’t get a surge. Strap it as best you can and hope for the best. CAT 3 is where I start to worry, and forget about strong CAT 4 or 5. All bets are off when you get there.
If I had a boat on a lift, I would try and find a safe place for it in CAT 3 storm or above.
That’s the opinion of your friendly neighborhood structural engineer, anyway. Good luck....let’s hope we never find ourselves driving up toward GA on I-95 or I-75 to escape one of these things.
