X-dim?
 

I have a fairly inclined mechanical mind. Can anyone simply, or refer an x-dim diagram. I have beginner - moderate experience with larger, moderate "speed" boats. The last one I drove "regularly" was a Tiara 38' convertible twin 502's cruising speed 30 knots. I want to get into single engine fast boats. I want to start at square one. Explain, help, whatever, it will not be forgotten.

Xdim is generally referring to prop centerline below keel. Lower the prop more drag. High the prop less drag = more speed. The higher you get the prop you then start piercing the water surface and should have surface piercing props.

Others will chime in more I'm sure but that is the jist of it

That is actually propshaft height, x dimension is the height of the crankshaft centerline, or input shaft of the drive. Must be measured 90* from the running surface, not parallel with the transom since the transom is angled and will throw your measurement off.

Ok. Thanks red. Seems a lot on here refer to prop centerline as x dimension. So x dimension has to do with input shaft which in turn would raise prop centerline if you raised that.

Yessir.

lil red is correct. the X-dimension is the crankshaft center height from the bottom of the boat. for some reason everyone wants to use x-dim as the prop shaft center. as an example, if the x is 19 inches and the prop shaft is 7 in below the bottom and you then add a 1 in spacer the x is still 19 but the prop center is now 8 in below the bottom. it would be nice is everyone would use terms correctly so it does not get confusing. the only way to change the x dimension is raise or lower the engine and gimble. changing the prop center height can be done with spacers or shorty drives.

comp is right, but it is, in the end, all about where the prop is in relation to the keel. and most people wonder about this as they convert from either alpha or TRS to bravo or other set-up. so, the length of the drive from input shaft to output shaft is a major consideration...

X dimension is really just used to find the crank centerline and where to cut the transom once you figure where you want your propshaft height. Once you decide how far below the running surface you want the propshaft, you subtract that from the drive input to propshaft measurement, then you have your x dimension. Once it's cut and you use spacers or short lowers it doesn't change propshaft height does. A lot of thought has to go into it. Your engine compartment, exhaust, swim platform etc may only let you go so high, then -2 or -3 short drives may be needed to achieve what you want

good reading guys. i have been around speed boats my hole life and have two but that is what i love about this site and the people here. always learning new stuff and now with me having bigger motors and wanting to do all the work my self, its a big help