Fitting outdrive to new transom

[skunkedskippy]

Hello all, I asked about this in a different forum but will try here as well. I have a 1977 Skipjack that needs a new transom. I’m doing this myself with no experience, and with a lot of help from this site and the people that post here. Old one is removed and new one is cut out and I’m currently trying to dry fit the bravo 2 before I glue it to the hull.

A little background info (to the best of my knowledge): the boat was built with an OMC drive. The last owner had a Mercruiser 5.7 and bravo 2 installed. The installer patched the OMC keyhole with some ply and then cut the merc/bravo hole. This looked like crap so I removed the patch and will reglass it after I glue the plys in.

Now to my (current) issue. I cut the keyhole and drilled the outdrive holes using the skin as a template , before I removed the repair patch. But something was off because the keyhole is not centered relative to the drain cutout. Also the lower right side holes look to be a bit lower than the left. The drive seems to fit ok and if I lay the skin template/patch on top, everything seems to match.

Do you guys think this could cause issues with engine alignment or am I good to go? It’s very possible that the drive was “tilted” from the previous repower but I wouldn’t have noticed because this is my first boat.

Hope this makes sense.

 

[Scott Danforth]

that is the most creative way of closing an OMC hole

make sure the X-dimension is correct and that the key-hole is centered

you know bravo 2's are for tug boats and trawlers, (big-slow moving boats) correct. most boats use a Bravo 1 or Bravo 3

 

[skunkedskippy]

Thanks Scott. I will keep that in mind if I ever have a chance to swap to a bravo 1 or 3.

“Centered” is a bit relative in my case due to my transom cut probably not being perfectly centered in the hull.

If I measure the transom length at the top, the center line falls almost perfectly in the center of the top of the keyhole. But if I measure from the top of the keyhole straight out to either side I’m a bit off. 38 3/4”ish on the left and 38 1/2” on the right. Again, this might be caused by me cutting/grinding a little more off the right side. Hopefully you can see this in the pics.

I’m assuming there would be some type of tolerance to being centered. But are talking +/-1/4” or +/-1/32” or less?

thanks for the quick reply.

 

[Rick Stephens]

The engine is centered, both side to side and up down at the rear, by virtue of bolting up to the inner transom plate. If the keyhole is off by a teensy amount so also will be the transom plate and the front motor mounts will be bolted in straight to that. I can't see a 1/8 to a 1/4 inch making a bit of difference to the way the boat handles. Could be wrong, but many boats aren't nearly straight enough to be within a 1/4 inch anyway. I think you're golden.

 

[skunkedskippy]

Rick, that’s about the best answer I could have hoped for. I agree with you and now I need to get my wood prepped and clamped in quick before someone else chimes in with a different repsonse. One that requires starting over!!

 

[Bondo]

If I measure the transom

Ayuh,.... Yer measurin' From all the wrong places,.....

Boat hulls, above the waterline are rarely even, to the Keel Line, 'n where the outer transom starts from the hull bottom,.....

To find the X-dimension put a straight edge along the keel line, under the boat, atleast 6' forward, 'n over-hangin' the transom enough to measure from,....
Next pull a string line from the bottom aft corner of the outer chines,.....
The Center of that string line should be plumb, 'n true to the keel line of the hull,...
For Everything driveline related is based off this spot,.....

The ole '73 stringer I yanked outa the hull I'm workin' on came through the transom 'bout 3" off-center,....
I figured that how it was supposed to be,.....
Makes the squared, 'n trued Merc hole look pretty stupid, til ya figure out why,.....

 

[Rick Stephens]

The ole '73 stringer I yanked outa the hull I'm workin' on came through the transom 'bout 3" off-center,....
I figured that how it was supposed to be,.....
Makes the squared, 'n trued Merc hole look pretty stupid, til ya figure out why,.....

Great post. Stringer holes look pretty silly anyway :^)

 

[skunkedskippy]

Wow that sounds complicated. Ok I’ll give it a try.

 

[skunkedskippy]

Bondo, I think I understand now. Basically find the center of the hull, not the wood, from the outside.

Makes sense, and if that’s the case wouldn’t it be better to glue the wood first and then cut the keyhole? In other words, I made my plywood a bit smaller than the actual inside hull dimensions to avoid the dreaded “hard spots”. But there’s no way that I cut it with a perfect 1/4” gap all the way around. And even if I did, I doubt that I’d be able to center the wood onto the skin. Hope that makes sense.

I’ll ask it another way: once I find the center of the hull, how does that translate to centering the keyhole in the wood, and then lining up the center of the keyhole to the center of the hull? Hope that makes sense as well.

Thanks for for everyone’s input . Very informative and thought provoking.

 

[Bondo]

Ayuh,..... The Atlantis is exactly opposite this off-set, to the starboard side,....

My transom was very solid original wood, just too thick,..... over 3" at the bottom, tapered to 'bout 2, 1/4" at the top,....
Had to mill out a 2" spot for the Merc to rest in/ on,....

I filled in the OMC hole with round pieces of wood to make the 2" for the Merc,....

To start the fillin' process, I tabbed in a Merc transom skin that had it's original merc hole to use as a guide for dillin', 'n sawzallin' the holes out again, after the fill,....
I'd cut it out of an ole junk hull, 'n ground it to just sit flush with the original transom skin,...

Before that, for the lay-out, 'n placement, I used the 2' x 2' chunk of ole transom skin, found the bolt hole centers, the keyhole centers, 'n the crankshaft center-line, marked out with a sharpie,....

Indexed it to my marks on the hull, 'n screwed it on,.... made it easy to put up, 'n take down exactly in the same place,....
Eventually, I marked out the OMC circle for the fill's skin,....

 

[skunkedskippy]

Bondo, do you have any pics of this process? Having trouble visualizing it. Just a friendly reminder that I’ve never done anything like this before. Whatever I know is theoretical from research over the last two years.

 

[Bondo]

Ayuh,.... Got abunch on my camera, but my main 'puter is dyin', so postin' 'em is outa reach at the moment,.....

But I'll see what I can do,......

Which part of which post needs further explanation,..??

 

[skunkedskippy]

Not sure what “round pieces of wood” means. Sheets of plywood cut to fit the hole?

Also could use some more details on how you found the centers of the bolt holes, keyhole, and crankshaft.

But if I understand right, you fixed the wood first, and then cut and drilled using a Mercruiser template from a scrapped hull?

 

[Bondo]

Not sure what “round pieces of wood” means. Sheets of plywood cut to fit the hole?
Correct,...

Also could use some more details on how you found the centers of the bolt holes, keyhole, and crankshaft.

But if I understand right, you fixed the wood first, and then cut and drilled using a Mercruiser template from a scrapped hull?

Ayuh,..... I started with a roughly 2' x 2' chuck of fiberglass cut outa a junk hull that had a Merc outdrive,....

On it, I drew lines, horizontally, 'n a center vertical line,....
Of the horizontal lines, the 1s through the bolt hole centers helped me establish the crankshaft centerline,....
The vertical center line, 'n the crankshaft center lines will allow you to determine the X-dimension necessary to mount the drive at the proper height above the keel line, centered, plumb, 'n true,.....

 

[skunkedskippy]

Ok that makes sense. I’m thinking I should have cut the keyhole and bolt holes after I installed the new transom wood.

Is the crankshaft centered vertically between the bolt holes?

Also, centering horizontally seems obvious. But how do you know what the vertical distance from keel should be?

 

[Rick Stephens]

Ok that makes sense. I’m thinking I should have cut the keyhole and bolt holes after I installed the new transom wood.

Most likely would be the best way to be straight and level. Seems like you can shift the wood around a little bit to correct for it and layer more glass in to tie it together.

Is the crankshaft centered vertically between the bolt holes?

Yes it is.

Also, centering horizontally seems obvious. But how do you know what the vertical distance from keel should be?

Best way is to follow the installation manual exactly.
http://www.boatfix.com/merc/Install/gas/86017211.pdf

Rick

 

[tank1949]

Hello all, I asked about this in a different forum but will try here as well. I have a 1977 Skipjack that needs a new transom. I’m doing this myself with no experience, and with a lot of help from this site and the people that post here. Old one is removed and new one is cut out and I’m currently trying to dry fit the bravo 2 before I glue it to the hull.

A little background info (to the best of my knowledge): the boat was built with an OMC drive. The last owner had a Mercruiser 5.7 and bravo 2 installed. The installer patched the OMC keyhole with some ply and then cut the merc/bravo hole. This looked like crap so I removed the patch and will reglass it after I glue the plys in.

Now to my (current) issue. I cut the keyhole and drilled the outdrive holes using the skin as a template , before I removed the repair patch. But something was off because the keyhole is not centered relative to the drain cutout. Also the lower right side holes look to be a bit lower than the left. The drive seems to fit ok and if I lay the skin template/patch on top, everything seems to match.

Do you guys think this could cause issues with engine alignment or am I good to go? It’s very possible that the drive was “tilted” from the previous repower but I wouldn’t have noticed because this is my first boat.

Hope this makes sense.

I have rebuilt a POS Proline transom before. Fortunately, I was able to use outer skin and transom assembly holes as a guide. If I were you, I would fill in OMC hole (reglass) and then go from there. Perhaps you can use the inside part of the MC transom assembly as a bolt hole templet on the outside. When glassing, make sure transom thickness with new cloth/plywood is uniform and proper tapering for strength. MC has a manual on this alignment process. Google it. It is a free download.

 

[skunkedskippy]

Thanks rick. I think I’ll go forward with what I have and align it to the best of my ability using bondos method. If it becomes obviously misaligned I’ll cut out the keyhole section, replace the wood and glass it all back up.

Tank, if were to do it over I’d definitely use the inner transom plate as a template. Have the manual. Thanks for the advice.

 

[skunkedskippy]

I stand corrected. I have the service manuals, not the installation manual. But I now have the installation manual bookmarked thanks to rick!!

 

[Rick Stephens]

I stand corrected. I have the service manuals, not the installation manual. But I now have the installation manual bookmarked thanks to rick!!

Aw shucks. No problemo.

Download it to your computer, then you will really have it. Priceless guide.