If the drives are to deep will it cause me not to be able to spin 4 blade props? 23 mirage 5100. 23 rev4 4100. Any ideas.

since you are adding a blade you need to go down in pitch to a 21. how far below the bottom are the propshafts?

About 10 1/2 directly below the drive. ( twins)

how far below the bottom of the boat? what kind of drives are they?

Bravo 1 they are about 2 1/2 to 3 " below the point also the outside trim cylinder on both drive is right where the water comes off the back/ bottom of the boat makes a small spray.

that doesn't make any sense the propshafts are 8" below the cavitation plates and the trim rams are above the cav plate. how can the shafts be 3 inches below the boat bottom and the trim rams be kicking up spray? Maybe you can post a picture.

regardless it wont stop you from spinning 4 blades as long as they are the right pitch.

Post some good pics of the rear. also drives level with a rule from bottom of boat

Prop shafts are 4 1/2 inches below the bottom-most point of the boat. Prop shafts are 10 1/2 inches below the bottom of the boat at the center of the drive. I will try to post pictures.

We're these the original drives? You have transom water PU's so it seems likely the drives were originally TRS...correct? What boat is this? Length? BBC or SBC power? If the drives are too low, the fix is likely big $$$...either shorter lowers or a total re-rig. If you can get on plane with 3-blades, on most boats you will trade off top end MPH with 4-blades.

4 blade props require more trim,,trim th drives out just until you hear the props brake loose a tiny bit then ease into it andyou may have to drop the drives a tad ..try it and get back to us