View Full Version : Need x dimension measurement!!!!!

chime walker
11-16-2012, 05:30 PM
I am ready to cut the new hole in the transom of my '72 sweet 16..... Was a sbf/volvo penta set up, now will be sbc/merc alpha gen 2 (mercruiser 260 out of a '86 Donzi Minx) was hoping to get the measurement from beneath the drain plug to the bottom of the drive on a merc powered '16. My friend has an '18 2+3 ready to paint and I'm not sure the transom to gunwale would be the same dimension......

11-17-2012, 04:37 AM
I would be real leary about taking the X off the drain plug they aren't always in the same spot. Merc has a procedure for finding the center line of the boat then taking the x measurement off the bottom of the boat with a framing / Sheetrock square. They also have a range on what the X should be Maybe one of the merc guys here would know the range I think the range was 14.5 to 17 something but it was a few years ago

I would be cautious on how high you go with the X as hatch clearance is an issue on the 16 with a v8 plus ride and performance could be affected

The instructions for the X come with the transom jig for the out drive there was a thread on the ultimate when the transom was cut I'll see if I can find it

11-17-2012, 04:59 AM

chime walker
11-17-2012, 05:53 AM
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We have the center line and appreciate the responses, we have the jig as well...

11-17-2012, 10:11 AM
Wasn't sure if you had all you needed already. Matty's advice and the thread he gave both looked like superb info.

I just went through this a few months back, so I'll vomit out some stuff I learned along the way.

Lemme start by saying I'm not sure what the ALPHA's recommended x-dimension is. The standard BRAVO x-dimension, using the 90-degree method (where transom angle does not matter) is 13 and 9/16 inches. I imagine the standard for an ALPHA could be the same OR could be significantly different.

I only skimmed this doc http://www.boatfix.com/merc/Install/gas/86017211.pdf but doing so makes me *think* the stock x for an Alpha is the same as it is for the Bravo. But it may be different and I missed it.

It also discusses both lowered and raised x-dimensions. (Lowered, with the drive deeper in the water, is apparently only for heavy duty and sub-25 mph applications. For a Donzi, only raised x should apply.) Page 14 says the absolute maximum you can raise a drive is:
Alpha 2.5 inches
Bravo One 3 inches
Bravo Three 1 inch

I also didn't look closely at the doc to see if whether "Alpha" means Gen 1 or Gen 2 or both, though I'd GUESS it meant Gen 2 or both. But I bet with some reading you can sort it out and get whatever you need. I also didn't check dates to see when this was printed-it could in theory have been superceded by newer info or testing by Merc.

And again, those are MAX raised-x numbers. The book says use the stock x dimension and only raise it after you test (hah). You might be able to survey the field to see what people have done and with what results. If it were mine, I doubt I'd even consider raising it much more than an inch.

With all that said, I'd then go back to Matty's point about measuring clearances in the motor box, as well as any other mounting considerations. I just went through all this with my install this year (different boat and drive, sorry) and highly recommend measuring 287 times and cutting once. :)

Also, Merc's gotten VERY picky about some other stuff (because of alignment issues I think). Once upon a time they said transom thickness could be anywhere from 2 to 2.25 inches, or something like that. Now I think they say it HAS to be uniformly flat and exactly 2 and 1/8 inches. (Pretty sure that's the number, but check, esp for ALPHA--all the stuff in my head is for Bravo.) Coming from a very old Volvo Penta setup you might have something different. If so, I'd consider taking the time to build up or grind down and reglass some stuff so as to avoid perpetual alignment issues that are hard to diagnose and only lead back here again. Far easier just to make it right now.

Especially since the other thing that you've probably noticed is that boats aren't shaped right. Meaning, they're not perfect, not always symmetrical, etc. Good to use a ton of blue tape and a sharpie, to measure and mark all over your transom and draw your centerline and such about 10 different ways. Hopefully the answers agree but often they won't. But doing it a bunch of ways should sort out what's actually right. The temptation is to measure between a couple pairs of points (like chines, etc) that SHOULD be equidistant from center, and run with it. But that may not be right. (Even the factory makes mistakes. Brian Orlandi, who did my repower, recently diagnosed and then fixed a Donzi 22 that had never behaved properly. Turned out the cutout was both crooked and WAY off-center. Like 3/4" or more off.) If you don't already have a monster T-square with an 8-foot tail, might be worth buying one. :)

Regards, hope this helps,


11-17-2012, 11:43 AM
Just a sideline note. I believe the maximum X dimensions are due in a large part to the water intakes needing to be sufficiently in the water so they don't suck air. If you use external water pickups, you can raise the X. Bill

11-17-2012, 03:12 PM
IMO wait for someone to get a proper 16 measurement, the 16's compartment is tight. Believe me you do not want to be any deeper than you need to be, the 16 porpoises pretty bad as. Any deeper than need be and it will be worse. My 16 had about 1/8" between the risers and the hatch.


Edit a side note to Bill's comment about the water pickups, my 22's X is just a hair under 17" with plenty of drive depth.

11-17-2012, 08:48 PM
I wouldn't raise an ALPHA drive more than 2.0-2.25 inches..