OMC to Mercruiser Swap (this time with pictures)

Questions about boat repairs with our resins and fiberglass: hull patches, transoms and stringers, foam, rot etc.
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mbasura
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OMC to Mercruiser Swap (this time with pictures)

Post by mbasura » Mon Mar 23, 2009 10:47 pm

I recently purchased a 1986 Invader with a blown engine. Since the engine had to come out, I figured I might as well swap the outdrive as well. The stringers where the engine mounts were are in bad shape as well and will need to repair or replaced.

I am going to start with plugging the hole in the transom. I figure I will remove a section about 24" x 24" of the inner skin and plywood and build up a patch of glass and ply. What lamination schedule do I need to plug the hole in the transom? The transom is 1 7/8" thick, with two layers of 3/4" plywood and outside layer of glass 3/8" thick. Does the outer skin need to beveled, or can I just fill the hole with glass and let the epoxy bonding the large plywood patch to the outer skin provide most of the strength?

In regards to the stringers, can I just remove the rotted sections and splice new wood to the old using either a 12 to 1 bevel (as recommended by the Gudgeon Bros) or butt blocks? I certainly admire the full scale restoration work that is displayed here, but I have to get this boat back on the water.

Any help is appreciated.

Image

Image

Image

Rgds,
Matt



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Re: OMC to Mercruiser Swap (this time with pictures)

Post by mbasura » Wed Apr 01, 2009 10:45 pm

I am going to start demo'ing this weekend.

Once the plywood surrounding the outdrive hole is out, I figure I will fill the hole in four steps.

1) fit a previously laminated double layer of 3/4" ply patch to the transom and through bolt to get good contact with existing outer glass.

2) glass the plywood patch from the inside with two overlapping layers of 17 oz biax

3) bevel the original outer skin back from the existing outdrive hole

4) fill the existing hole with 2 layers of biax (and mat if neccesary) to match the original thickness.

I am hestitant to do the patch completely from the inside of the boat a la Shine's Aquasport as I am afraid I of leaving voids between the new plywood and the existing outer skin.

Sound logical? Please give me some feedback.

Matt

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Re: OMC to Mercruiser Swap (this time with pictures)

Post by mbasura » Tue Apr 21, 2009 11:59 pm

I have spent about 10 hours grinding and chopping. The rotted wood around the outdrive hole is completely removed. I am shocked at how far gone the engine mounts are. The boat hasn't been in the water since January and underneath the glass is still dripping wet.

Luckily the mounts and stringers seem to terminate at a tranverse bulkhead and the rot appears to have stopped there. We'll see as the demo continues.

wet engine mount:

http://gallery.bateau2.com/albums/userp ... resize.jpg

Ground down transom:

http://gallery.bateau2.com/albums/userp ... resize.jpg

Cross section of rotted engine mount. Seems to built from three pieces of 2" doug fir in and upside down U shape, then covered with three layers of 3/8" plywood. It appears this is how it came from the factory and is not several layers of repairs. Needless to say water got inside the cavity and it rotted from the inside out.

http://gallery.bateau2.com/albums/userp ... resize.jpg

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Re: OMC to Mercruiser Swap (this time with pictures)

Post by mbasura » Mon May 04, 2009 4:25 pm

Time to order more materials. Can I get you to work up a lamination schedule so I don't get myself in the weeds? :help:

The original transom was 1 7/8" thick with the outer skin approx 3/8". I need to build up an inside skin and an outer skin where the outdrive hole is. I am real unclear on this. Are the transoms typically tabbed with tape AND covered with cloth?

The orginal stringers were 3/4" Douglas Fir lumber, not plywood. They taper in height from about 10" to about 7 1/2".

The engine mounts were 2" x 4"s screwed to the stringers and covered with fiberglass on the outside. What would be a lamination schedule for the outside of the mounts?

I have a roll of 12 oz. biax tape and some 17 oz biax cloth based on my guesstimations from looking at similar projects.

The engine will be a 4.3 Mercruiser and outdrive with approx 190 HP.

rgds,
Matt

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Re: OMC to Mercruiser Swap (this time with pictures)

Post by tech_support » Tue May 05, 2009 11:22 am

Matt,

How was the condition of the plywood around the area of the transom cutout?
Are the transoms typically tabbed with tape AND covered with cloth?
Ours are, but your not replacing the transom, your just putting in a plug. This gets back to my question above. If the rest of the transom is soft, you need to remove the whole thing.
The orginal stringers were 3/4" Douglas Fir lumber, not plywood. They taper in height from about 10" to about 7 1/2".
replace with same thickness marine plywood. 12/1 bevel is fine, but less(or no bevel) is OK also if you use more fiberglass. Your fiberglass over laps should extend about 12" past the splice. To give you a lamination schedule on a repair like this is a guess. I have no way of knowing the condition of the stringers on either side. My advice is to build up to the same thickness for the stringer, tab the stringer piece in with three layers of Biax tape on each side, then cover with 3 layers of 1708 biax overlapping to the hull by 6". Then you epoxy glue the engine mounts, same thickness as before, but rahter than screws your going to use epoxy putty, then cover with 3 more layers of 1708. Overkill, maybe, but I would be extra safe with this one.

You are correct about the glass covering your transome plug, it need to be beveled with the existing inside skin (same for out side). if you can bevel it back 3", that will be great. For the inside I would go with another layer that overlaps the plug all around by 3".

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Re: OMC to Mercruiser Swap (this time with pictures)

Post by mbasura » Fri May 08, 2009 12:15 am

Shine,

Thanks for the input. Plugging only the hole for the outdrive proved unrealistic. The wood around the cutout for the outdrive was fine, it was just that the rest of it was rotted. It is now completely removed.

Here is a pretty current shot of the situation. Transom wood removed and ground down. Stringers and engine mounts removed completely as far as the frame surrounding the gas tank.

Image

Rather than rebuilding the engine mounts, I think I would prefer to bolt metal brackets to the stringers as Jacque recommended to Nova 21 on his rebuild project.
Instead of lag bolts, use L brackets bolted inside of the stringers.
The engine mounts will be bolted on top of the L brackets.
This means that the stringer spacing will now be wider.
Is that a problem? Do you have part of the stringers left towards the bow?
If not, the space inside the stringers will be L between mount centers plus 2 times half width of the brackets. That means in your case 22.5 plus 4 = 26.5.
Check if that works with the tank(s).
You will build up fiberglass, space in between core = 27.
Stringers thickness depends on the material.
If you use Renicell, minimum 2".

What are you going to use? Renicell foam or laminated plywood plus engine beds?
The existing layout of the engine bay seems to lend itself to a solution like this. The stringers are in the proper position and would be full height. I can widen the stringers by adding layers to the outboard side without encroaching on the engine bay.

I'm sure I would have to build up the stringers considerably but think that through bolting an aluminum bracket hoizontally would be a much better soltion than screwing a bunch of lag bolts vertically into a 2 x 6. Here is an early photo for reference. The orignal mount can be seen under the toes of the beat up tennis shoes (I'll be sure to tell that guy to get out of the way of future pics).

Image

I would prefer to work with marine plywood. How strong would the new stringers have to be? 2 layers of 3/4" ply tabbed with three layers of 12 oz tape and covered with 3 layers of 17oz biax? How strong would the mounts have to be? I guess maybe 5/8" aluminum L brackets with 1/2" bolts and a backing plate? Did the old mounts serve any structural purpose other than to hold down the engine?

Thanks in advance,

Matt

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