Filler choice to make thicker fiberglass layup?

I have cut out my transom center from the inside and replaced with new 11 ply plywood. The fiberglass on the inside was real thick. I have 4 layers of cloth and four layers of mat glassed over the wood. I need to be 3/16' thicker. Can I use a layer of glass reinforced filler the build up the thickness and then 2 layers of glass cloth over that or do I have to just keep adding layers of cloth or mat?

 

Welcome to the forum.

The smart a$$ in me wants to say the only things you have to do are breath and pay taxes. Some people even consider them optional, if you're willing to accept the penalties.

I'm missing some information to advise you better.
Type of boat?
Thickness of removed plywood?
Thickness of plywood replacement?
Original glass thickness in center of panel and along edges?
Type of fabric originally used? What fabric did you start with?

 

Here you go.
1987 Bayliner Cobra 2250.
Transom is fiberglass and the center section has a plywood core about 24" wide where the I/O is mounted. Core was about 1 5/16 plywood. I removed the core from the inside and replaced it with 1 1/8 eleven ply MDO.
The exterior glass is about 5/16 on the center of the hull. The glass on the inside was about 1/8 or a little thinner.
The rot had just begun to reach the end of the stringers, I cut out the rot and fit the new plywood in place with a resin paste made with sawdust to thicken it and all surfaces of the core were painted with resin. The core was clamped in place very tightly and left for 24 hours. The cut ends of the stringers were treated with thinned resin and the missing 1/2" of stringer was filled in with the long strand resin filler that has the glass fiber in it.
To make up the space left by the thinner core I laid up 4 layers of cloth and 4 layers of glass mat. I still need to make up about 1/8 inch in depth and then will glass over the core area and feather out into the existing interior glass.
Is there a better way to make up the 1/8 inch or just keep on glassing? I thought of using a piece of FRP (fiberglass reinforced panel) to make up the missing space and then glassing over it as the panel is the right thickness or very close to it. I would then glass over it and tie it all in.
I want to get back to the original thickness so my alignments don't change when I drop the motor back in.
Suggestions?

 

Thanks. I have a much better understanding of your situation.

I know you said FG cloth, but which type of weave and how heavy? Ie 6 oz twill vs 20 oz roving vs 18oz bi-axial ect. Both the original and your lay up schedules.

As you suspect, the plywood core's thickness should have been corrected before encasing it with fiberglass. But that ship has sailed.

If the cloth used on the inner skin is comparable to the original; dry fit the Outdrive. It might be able to adjust for different transom thicknesses. What is the model of Outdrives?

I wouldn't mount the drive on top of fairing compound, even fiber reinforced.

You didn't mention it but I hope you are extending the tabbing on to the surrounding hull and stringers

Good luck

 

What weight of cloth did you use? A small difference in thickness is not going to affect the boat much.

 

The only cloth I could find locally is Bondo #20128 which says it is 20 mm cloth and comes 8 square feet to a package. It is woven and the strands are at right angles to each other.
The final layers ( I figure 4) will lap well over onto the original glass in all directions after the original is ground and well cleaned.
When drill the mounting holes I ma going to drill over size, plug and the re-drill the original size.

 

Oh, and the drive is OMC Cobra.

 

I do FG repair and don't do any engine/drive. The mechanical gurus here should know the tolerance of your drive. They will need to know the specific model and year of your OMC Cobra.

That Bondo packaged cloth is 5 oz twill. It is a relatively weak fabric. I wouldn't trust 4 layers of it to tab in transom or stringers. I would use 8 layers with a thin layer of CSM between (skip CSM if using epoxy). The transom-stringers-hull joint is critical. I like to see tabbing at least as thick as the inner transom skin. Feather the edge of the tabbing by having each layer extend 1 inch less than the previous (10,9,8,7,6,5,4,3,11).

IMO the drive will crush into faring compound if it is used to make up the difference in thickness.

3 layers of glass covering oversized hole plugs.

Best of luck

 

I agree. The 5 oz twill is a finish fabric. Mat will provide better adhesion and build up bulk. The outdrive will not have problem with less than 1/4" difference. The connection between the drive and the engine is with a spline, which gives you about an inch of tolerance.

 

Thanks for all the advice. I have an automotive paint store that says he has glass. If not I will use mat. The outer skin of the transom is not compromised and has no stress cracks. The Stringers are tied into the bottom hull and to the transom on 2 of the 4 sides. I will be sure to have plenty overlap and strength, no filler.