I have a 24' Sea ray with twin engines and alpha drives. Im tired of pulling apart the lower unit every spring to replace the raw water pump impeller. Im installing engine driven water pumps and want to have the water come in thru a bottom hull mounted water pick up if it is deemed that the pump should not pull water thru the outdrive.
My two options are
1: just remove the impeller in the drive and have the engine mounted raw water pump pull water thru the drive.
2: drill the bottom of the boat for clamshell style water pickup like these. If I go with the clamshell, where are the no no places to mount the clamshells (ie: not inline with the drive, how far from the drive, how far off center from the keel)

I already have one clamshell in the bottom for the A/C unit.

Not helpful i know, just a question, what is making you go thru an impeller every year ?

So you want to crawl into your engine compartment instead ? Unless you are , or have a close friend who is a double jointed midget , be happy !

I think it depends on the compartment. Some are easy to access the pump.

Spot on. This was my 21' Bryant.

I have about 15 inches of clearance in front of the engines. I can lay infront of the engines down in the bildge.

I’m sure some will argue, but on that boat, it won’t matter. Put them 16-24” off the centerline or anywhere close that’s convenient, in front of the engines.

Did i miss the answer to this ?

Not sure why. The starboard drive impeller is 3 years old. The port side was 2 years old Last year I went to replace it and found one of the vanes broken. put another Sierra one in and it took a set and wasnt pumping very well. I replaced it again and the water flow was better but I still get a 15 degree temp rise between idle and cruising on the port while the starboard stays rock steady.

Alpha pumps are perfectly good pumps. I definitely wouldn't change to an engine mounted pump.
You have pieces of impeller blades upstream of your pump. You should (with the outdrive lower removed) backflush ALL of the water flow path including the exhaust elbows, the check balls in the thermostat housing, the thermostat, back thru the oil and fuel coolers and back thru the transom assembly and out the feed pipe that normally sticks into the drive's water pump snout.