Charles,
I have 4 jack stands that sit on the ground. I use them for bottom painting my boat and servicing the centerboard. The screw jacks look similar to the one in your photo. The lifting force available at each jack is not enough to jack the boat up like you are proposing. You might be able to increase the lifting force by using a piece of pipe on the collar as a “cheater”. But, that might damage the collar &/or the threads. They are not designed to be used like that. Considering the amount of weight involved, I wouldn’t want to try it.
I jack up the boat and trailer using hydraulic bottle jacks and wooden blocks. Then, I support the boat using the jack stands. You need to be certain to chain the jack stands to each other so they can’t slide out when the load is applied. I also put plywood under the jack stands so they can’t sink into the ground. Once the jack stands are in place and secured, I lower the trailer out from under the boat to create enough clearance to bottom paint. You can pull the wheels off the trailer to enable it to be lowered closer to the ground and give you a little more space under the boat.
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