I'm re rigging a old outboard hull to a alpha one drive. I have been looking through every stern drive manual to find what the intended X-Demention is. As a old school outboard guy I always consider the X-Demention the relationship between the centerline of the prop shaft to the very bottom of the boat or "pad" in outboard terms. All I could find in the manual is how to figure out the engine crank shaft height using one of too methods A) the 4ft 90° method or the B) tape measure method and figuring in your transum angle. Using the later method I came up with the X-Demention of the center of the prop shaft being 9inches below the pad of the boat, this sounds very deep to me. The Manuel also said that if speeds of over 50mph are expected the X-Demention could be raised but no more than 2 1/2" form the figured X-Demention, that would be 6 1/2" which to me still sounds deep seeing that this hull ran best with the V-6 Mercury cle cased outboard running the center line of the prop shaft 1inch below the pad or very bottom of the boat. I do intend on running a low water pickup nose cone on my gearcase.
Looking for any and all input from anyone who has experience and a good idea of what the limitations are

Bump, back to the top

https://zmm.ca/en/br/documents/installation-drawings/

I couldn"t add a pdf, so here is a link to the mercury installation drawings.
you'll have to scroll 1/2 way down until you see alpha gen 2 stern drive to get the out drives dimensions.

These drawings with dimensions will go a very long way in your conversion just click the closest engine your installing ie small block, v6, 4 cyl whatever and you got all your math.

I will look, hi and lo for the x dimension page but my money is on 15 1/2 inches.
even tho the mercury template says you can go up 3 inches depending on performance, don't bother, put it where mercury suggests based off your transom angle, or up no more than 1 inch regardless of what you hear about slippage and rooster tails.
it's an alpha not a speedmaster. No offense just being realistic.

You also need to make 10 b.s.posts about nothing so you can upload pictures of your project which is crucial if you want good help and advice. Don't forget how important engine alignment is or do some reading if don't know about it.

oh wow i guess the pdf did load, can't see it until i click post

Thanks for the information and the input, is it safe to assume that the alpha one gen one and gen two are the same on these details of gimbal housing placement? Thanks again for your post

The X dimension is the crankshaft centerline and thus also the centerline of the outdrive input shaft.

If you are trying to figure out where to set the X dim, it varies by boat make and model.
Most Bravo boats, depending on model, are set with the propshaft at 3-6" below the bottom of the boat. The distance from the propshaft to the input shaft is 21.75" on Bravo drives. Not sure what that distance is on Alphas.

Thanks for the post and the information

Thanks for the post

I get ya, x dimension is technically crankshaft centerline but alot of us think propshaft height.
simply put 21.75 minus 16.75 equals 5, as in propshaft centerline is 5 inches below bottom.
Still haven't found the mercury template but betcha x on an alpha will be 14 1/2

Still think you should do a post count so you can post pictures of your project.
what kind of boat ?
A v6 outboard at 1 inch may have worked well but an alpha w/ a low water nose cone at 1 inch may not work at all.
what engine are you installing?
what was outboard top speed?

Measure from the cavitation plate to the prop shaft centerline
Which engine, what hp ?
What boat? you said pad, how long pad? how long boat? what deadrise? What is your goal?pictures pictures and more pictures.
it's a total crapshoot without all the info and then with all the info it's the hardest decision you'll ever make, where to cut that hole ?

Thanks again, I know there is no magic formula to this, and I know no one has a crystal ball, I just thought I would see what others have learned, than pray that I get this as close to correct as possible. My fear is running to deep as I know will promote "chine walk" with this style of boat I have, and to high will have other issues like prop cavitation and excessive slip. The hull I'm using ,and it will be rebuilt to accommodate the stren drive as in new and added core, transum, transum angle braces that will be part of added stringers to support the inboard aluminum small block Chevy.
The hull that I am using is a modified pad-v with air entrapment 21ft . With roughly 240hp 2.5L Mercury 2-stroke V-6 with a cle 4 hole lower gearcase with 1.87 to 1 gearing, 26 et prop turning 6800 Engine RPM, it would run best at one inch prop shaft CL below the pad or very bottom of the boat. I known these are two different worlds, so does the added weight and its placement in the hull instead of hanging off the hull should warrant the CL of the prop shaft to be deeper?,,, by how much?, again I have learned with this style hull running to deep with the outboard would promote chine walking. And let's face it a rooster tail on a boat running at neutral trim angle and a surfacing prop is Cool 😎