Must be a obstruction in your pickup. Backflow your system with water. Lifts, putting the boat on a trailer and draining should not be causing impeller failure. It is running dry because it is not getting the proper flow. Check the hoses, make sure there are no kinks. Also check the valves and the pickups. Maybe there is some seaweed, or mud. Flush it. Should get 2 seasons out of an impeller.

Clay,

I keep my Bullet on a lift and I have not had that problem. Once the boat is lowered in the water, the seawater pump is close to water level anyway (very little head difference). I would check where the water hose connects to the transom addembly (behind the angle fitting). I had to replace both my transom assemblies due to heavy corrosion that collapsed the water line that runs through the assembly. Like someone else said, water should come out like a fire hose when disconnected from the pump.

That's where I was leaning. When I did the repower on my old Formula I had the same issue. The hose that runs from the outdrive to the transom assembly can get crushed by corrosion of the aluminum around it.

I had a similar problem last year with one of my pumps.
Turns out that after sitting on the trailer for a week, pump would loose its prime due to the fact that the pump housing was scored and would let it bleed off.
I changed quite a few impellers before I figured out what was causing the problem.
Changed the housing and problem is cured

I had an issue with my pump not priming (trailered boat) and it was a worn housing. Was not really scored so not super obvious the housing was bad, but once I had a new one next to it was for sure. Replaced and problem solved.

OK. I will replace the housing and see if that corrects the problem!

What's your water psi when the impellers are new? Before you spending $$$ on a housing check to see if you have a restriction at the gimbal housing as doraymond said. Very common problem and poor design on mercs behalf. Seriously doubt if its your housing. As others posted water should be exiting like a fire hose when you disconnected it from the pump.

I agree

I have seen many times over that the build up in the transom is the problem. If you can get to the fitting that bolts to the transom connecting the reinforced hose, you will probably see that the impeller is trying to pull through a straw sized hole. I am suprised that you have not seen high temps.

Most likely, the electric condom dispenser you installed crimped your outside water supply.