There is another side to the question, The YOU factor.
The winch may be able to handle the load, but can You handle the winch?
Assuming a 1000# boat for example....
Pulling onto a set of Lubed Bunks will produce about 25% friction. ~250# ... More if Dry.
A 15% boat ramp will add another 7% due to the slope. ~70# ... 320# total. 32% total weight.
So a winch rated at 50% of the load should be well within its rated capacity.
Winches are commonly geared at ~5 to1, the ratio of a full spool to the size of the crank handle is about 3 to 1.
This gives you an additional 3 to 1 advantage. A good thing! 3 x 5 = 15
This gives the winch about a 15 to mechanical advantage overall.
So producing the 320# of pull will require You to crank with 320/15 = ~21# of force on the handle.
Or about 2% of the load. If the boat weights 3000# you will need ~60# of grunt to keep it moving.
A 1500# winch will easily handle the load.
But cranking the winch will be like bench pressing 60#, 20 time in a minute.
Ever wonder why you nearly have a stroke cranking that winch? Now you know!
2 stage winches commonly have 5:1 and 15:1 gear sets.
x3 of the crank advantage yields a 15:1 and 45:1 overall advantage.
Once the standard 15:1 get too hard to crank, you down shift to 45:1 and keep going.
60# becomes ~20# again! ~7# per 1000, Or ~0.7% of the load
I have a Fulton-F2, 2 speed winch, that allows the 69 year old Admiral to recover an 18ft 3000# bow-rider with ease.
Her idea! She won't let anyone help!
We usually get an audience at the ramp once they notice the lady is cranking the winch.
She occasionally gets a round of applause!
No one ever complained that they bought too BIG a winch.
If you are considering an electric powered winch, there are more factor other than pulling power that you will want to consider.