Transom Questions

76SeaRay

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Ok, so let the PB be the filler instead of trying to layer mat on the hull and sanding that to get it completely parallel to the outside of the transom. If I use PB over the whole area you see in the earlier picture of the cleaned inside of the transom, is mat still required between the hull and the new transom plywood??

Clamping is a problem. In one of the first pictures you can see I have about a 4 inch wide gunwale over the top of the transom so it is not straight up like an outboard transom. That prevents running 2x4 clamps vertically The only way I see to clamp it is to clamp through the OMC hole, move up about halfway and clamp with holes drilled through the transom on each side, and then another clamping board near the top using the holes already there for the tie up fitting through bolts on each side of the transom.

For now, back to the transom to measure flatness horizontally across the outside of the transom.
 

76SeaRay

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Oh, an interesting note to add... Before I dove into doing this myself, I took it to a boat shop that works on everything from small to very large to ask about a quote to make the change.. The guy who was supposedly very experienced told me that I needed the engine mounted in the boat to determine where to locate the Mercruiser keyhole cutout. Based on my research, WRONG. I have the Mercruiser manual and it explains how to locate vertically the Alpha keyhole cutout based upon measurements from the bottom of the hull which then sets the center line for the Alpha drive. I also have the drawings on how to mount the 5.7L engine showing that it is located off this center line for the Alpha drive keyhole and setting up the motor mounts for proper alignment.

That is how/why I decided to do the transom myself....... I did not want to trust the guy to do the transom if he didn't know the correct procedure for setting and cutting the keyhole for the outdrive or was going to take a short cut to put it in.....
 

Scott Danforth

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many times, the only difference between a DIY amateur and a professional is the professional gets paid.
 

Woodonglass

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The OMC keyhole and the Alpha 1 drive Keyhole are different patterns. You'll need to fill in the old OMC keyhole first and then cut the new Merc keyhole in the patch transom glass. I'm NOT the I/O expert here on the fourm but I've seen this done a few times and this is the normal procedure. The I/O experts will be along to chime in. On I/O boats the clamping is most often done with 2x4's and bolts thru the keyhole.
 

76SeaRay

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Yup, I know the difference in the OMC and Alpha openings so prepared for that... I will put in the new transom plywood closing up the OMC completely so the OMC opening can be filled in backed by 1 1/2 inches of new transom plywood. I will probably have someone who knows hull repair fill in the 1/2 inch deep hull hole after the plywood is in. I won't actually cut the keyhole until the hull patch is installed.

I have figured out how to clamp it now too without drilling extra holes after reading your sticky notes and watching a few youtube videos.

I will outline the Alpha template on the stern and then drill a couple of bolt holes through the new plywood at the top and bottom of the future cutout. That way I when I cutout the keyhole, the clamping holes are removed with the cutout. Doing that and I will have one 2x4 clamp horizontally across the bottom of the keyhole and another 2x4 clamp at the top of the keyhole. I will also use one clamp across the top of the transom plywood bolted through where the bolt holes are for the tie off hooks located port and starboard on the stern. There are two bolt holes there on each side so I will use the top pairs has alignment hangers when I put the PB'd plywood in place (after dry fit). The hanger bolts will act as guide pins to keep the plywood from moving around when clamping everything. They will also free up hands to put the clamps on. I will use the bottom pair of those holes at that location to clamp all the way across with a 2x4 like the two clamps at the keyhole area.
 

Woodonglass

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filling in/ patching the old hole is NOT that hard. Something you can do with the advice of the guys here. If you want to give it a go, just let us know and we will walk you thru the process. Probably only a couple of hours and some Resin and glass with a bit of grinding and feathering. It would be done AFTER the new wood core has been installed.
 

76SeaRay

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So, I checked flatness of the transom today using an 8 foot level across the back at the OMC opening. It appears to be flat out to about 18 to 24 inches each side of the center line. After that there is a gap in the last foot or two between the level and the hull. Guessing there is a bit of bow out towards the end due to the plywood being removed from the inside or maybe it was designed that way.

I am going to pick up two sheets of marine plywood next weekend and cutout the new transom ply's. I am trying to decide if it is better to bond those two together on the bench or put one in at a time. I have a 4x8 flat solid table for a work surface so I can keep the two sheets flat if I bond them outside the boat.

Looking earlier in this string at the layers to build the new transom, I assume the CSM and PB both are applied to the hull and then the transom plywood wetted out with epoxy is put into position and clamped. Or is the CSM applied with epoxy to the hull and the PB applied to the plywood? How much working time do I have? Does this all have to be done before cure sets in if the CSM and epoxy is first applied to the hull?

Thanks.
 

chevymaher

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Attach the 2 pieces together outside the hull. Use PB or glue and clamp together.

PB is applied to both the hull and the transom then put it in and clamp.

You got about 20 minutes to get it done.

Now questions concerning covering the old hole and making the new one and what order that is in. You got to wait till someone who knows comes along.
 

sphelps

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If your using epoxy then forget about the CSM ... It’s not needed ..
 

76SeaRay

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Great, thanks for those answers... I will laminate the two 3/4 inch plywood on my table and then install as a single piece. I have a helper lined up for this.. I will put her to work spreading PB on the transom while I spread it on the inner hull.. I will hang the transom on the two bolts through the hull where the dock tie eyes are to go and then have the helper put the clamp boards on the outside while I put them on the inside. That should speed up the process. The hanger bolts at the top will make it easier to install the clamps and keep the transom plywood from moving around during clamping...

I am using epoxy so will just go with the PB between the hull and transom plywood.. I wasn't sure about the CSM since several of the notes call for CSM between those two layers...
 

sphelps

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I would suggest you wet out both sides of the plywood with epoxy resin so when you glue together . The pieces the ply does not soak up the resin to fast out of the pb mix ...
 

76SeaRay

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Ok, will do. I was thinking that the joint between the two layers of 3/4 plywood would be just epoxy resin to "glue" them together. But, based on your comment, it should be a thin layer of PB.
 

sphelps

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Yep epoxy can be a little runny. So thickening it a bit I think would give better adhesion .
I would use a 1/4” notched trowel or smaller to spread the glue .
 

Woodonglass

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The accepted method for gluing the wood to the glass is...
Thin coat of resin on the plywood flat and edges. Use Cabosil to thicken the epoxy to the consistency of Creamy Peanut butter (Thus the name) and then use a 3/8" notched trowel to spread it onto both the wood and the glass. Medium pressure on the clamps. Just enuf to get some squeeze out around all the edges. DO NOT CLAMP DOWN Hard. You want plenty of the PB between the wood and the glass.
 

Scott Danforth

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between the two pieces of wood, i have used PB, Epoxy, and Tite-Bond III. they all work
 

76SeaRay

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I picked up two sheets of 3/4 marine grade ply yesterday. I will be making a template this coming Saturday and then cutting the two pieces of ply to laminate. How much do you let the epoxy "soak" coat set up before putting on the PB and laying the two pieces together.
 

76SeaRay

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Another question on bonding the two layers of 3/4 plywood outside the boat. I have seen in some threads where people talk about adding "weep" holes to allow the epoxy to weep out under clamping. Is this necessary? If so, how big should the holes be and how many/how far apart?
 

76SeaRay

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So, I made the template this weekend. However, as I was cleaning up the surfaces for a good fit, I found that the fiberglass near the bottom of the transom has a gap between the surface and the transom outer hull. Hard to explain but it looks like a layer of fiberglass over the entire transom but at the bottom it is thicker and not attached to the hull. As I show in a picture earlier in this post, the fiberglass and hull is thicker at the bottom of the OMC hole. Here are some pictures:
 

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76SeaRay

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In addition to the above, it looks like the thickness starts increasing from the floor line down to the bottom of the hull. So, I am wondering what to do with this. My initial thought is to use a belt sander and grind down that lower portion so the thickness is consistent top to bottom, remove the lip and fill in with CSM extending about 2 to 3 inches from the bottom of the hull, and then smooth it out.
 

76SeaRay

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Here is a drawing of the problem, both the lip and thickness problems...
 

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