I am in the process of purchasing a new prop and need some info on X-dimensions. How do you measure this and what is normally considered a high and low x-dimension measurement. Need to see if I need a three or four blade. Any thing would be helpfull.

I have been told from the center of your prop shaft to the bottom of the boat= x dimension?

actually it the dimension is from the bottom of the boat to the center of the crank shaft.

actually it the dimension from the bottom of the boat to the center of the crank shaft. Sorry bout the typo on the first post.

If you give me the angle of your transom. I can give you a stock measurement and give you the lows and highs.
Best regard Phil

Kind of hard to measure. It is much easier to measure the propshaft height with the drive at neutral trim. Use a straight edge off the bottom and then measure how far the propshft is below the bottom.

What boat, size, year?????

X dimension IS from a line running back from bottom of hull(pad or centre of V on a single) to the drive shaft (centre of trim/tilt sender) the vertcal measurement.
On my 25 Outlaw its a (typical Baja) conservative 15 1/3 inches. My buddy´s ´93 272 is 16 inches and his boat responds better to a three blade where mine likes four. However it is all dependent on a number of other factors.........

IMPORTANT: The 34.5 cm (13‑9/16 in.) dimension should only be raised or lowered after proper
testing.
• ‑To lower drive unit ‑ Subtract from dimension "a".
• To raise drive unit ‑ Add to dimension "a".
2. Place the 90 degree tool along the boat bottom at the vertical center line.
3. Locate the point at which top of the 90 degree tool contacts the transom on the vertical center
line. This is the crankshaft horizontal center line or X‑dimension.
4. Draw a line perpendicular to the vertical center line at the crankshaft horizontal center line.

With all the different options of lower units and spacers, the 'X' dimension is a rather old measurement. You can have a standard lower, -1,-2,-3, and spacers 1/2"-3" every 1/2". So your X will stay the same no matter which lower you are using but the real dimension that you want is the 'propshaft to keel or bottom' of the boat.

Some might raise the propshaft until the 3 blade doesn't bite and then use a 4 blade and then raise the 4 blade until it doesn't bite and then use a 5 blade all the while measuring your performance, rpm, mph, slip, and see which has the best tradeoffs for your desires. Remember everything is a compramise.