I guess its time for an Update.

New Transom Glassed in - (2) layers of 3/4" plywood Epoxied together, and epoxied to the existing hull transom.
Then used Vinylester Resin and 1708 to glass in the outside of the transom (splashwell cutout).
Then Glassed in the inside of the transom.

Then built a mold out of Melamine, and glassed the pod. Pod is demolded, and now time to build the structure. Stringers will penetrate through transom and tie into existing stringers. Pod transom will get (2) layers of 3/4" plywood.
 

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I guess its time for an Update.

New Transom Glassed in - (2) layers of 3/4" plywood Epoxied together, and epoxied to the existing hull transom.
Then used Vinylester Resin and 1708 to glass in the outside of the transom (splashwell cutout).
Then Glassed in the inside of the transom.

Then built a mold out of Melamine, and glassed the pod. Pod is demolded, and now time to build the structure. Stringers will penetrate through transom and tie into existing stringers. Pod transom will get (2) layers of 3/4" plywood.
That brings back memories. I did the same thing to my whaler years ago.

If you care about gel coat cracks you're going to want to put a really big radius on all the joints between the pod and the hull. All four corners of mine cracked, despite there being over a half inch of 1708 tabbing. Not structural cracks, just cosmetic gel coat cracks. They don't bother me so never bothered to do anything about them. By really big I mean like 1.5" or 2" radius. I have around a 5/8" radius.
 

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It's nice to finally see some fiberglass hull extensions.

There is nothing wrong with a 2 piece bolt on either @Sttr
 
That brings back memories. I did the same thing to my whaler years ago.

If you care about gel coat cracks you're going to want to put a really big radius on all the joints between the pod and the hull. All four corners of mine cracked, despite there being over a half inch of 1708 tabbing. Not structural cracks, just cosmetic gel coat cracks. They don't bother me so never bothered to do anything about them. By really big I mean like 1.5" or 2" radius. I have around a 5/8" radius.
Yours turned out really good! Thanks for the tip! I will be sure to do that.

What core material did you use?
 
Yours turned out really good! Thanks for the tip! I will be sure to do that.

What core material did you use?
I used coosa board in both the original and pod transom. Bulkheads inside and the top of the pod are cored with whatever the regular light weight foam core stuff is. The sides and bottom of the pod are solid.

No guarantees the larger radius will prevent cracks but it should help. Make sure you run the tabbing over the top of the radius. Don't build the radius out of filler without a lot of glass over top.

One thing I regretted was gel coating the mold then trying to only gel coat all the holes and tabs to match the gel from the mold. I found it would have been way easier to gel coat it all at once, a continuous coat instead of trying to essentially blend a bunch of gel coat repair into the gel from the mold. Blending all the edges took longer than just sanding the whole thing at once.
 

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I used coosa board in both the original and pod transom. Bulkheads inside and the top of the pod are cored with whatever the regular light weight foam core stuff is. The sides and bottom of the pod are solid.

No guarantees the larger radius will prevent cracks but it should help. Make sure you run the tabbing over the top of the radius. Don't build the radius out of filler without a lot of glass over top.

One thing I regretted was gel coating the mold then trying to only gel coat all the holes and tabs to match the gel from the mold. I found it would have been way easier to gel coat it all at once, a continuous coat instead of trying to essentially blend a bunch of gel coat repair into the gel from the mold. Blending all the edges took longer than just sanding the whole thing at once.

Sweet, I plan to use Coosa for the stringers since I have enough left over from a different project, but not enough to do the transom. The width of the pod it ~57", if it were 48", then I could have done the transom in Coosa.

The gelcoat on my mold didn't turn out the best (first time doing it), and got some alligatoring in spots, so I plan to re-gel the whole thing once everything is glassed. The Swim step I plan to gelcoat the topside in the mold though. Then do gelcoat non-skid after its glassed on.

One problem I had when gel coating the mold is it wouldn't cure because the styrene off-gassing settled in the bottom of the mold, the sides cured up quick like they should. So then I put a fan on it to blow the styrene off-gassing out of the mold and it then it started to kick on the bottom of the mold.

Lesson learned. When I started glassing it, I would put a fan on it after I got everything laid up, and had no problems once I figured that out.
 
Question on the compatibility of fibreglassing on top of epoxy.. It is my understanding that polyester resin will not bond well to any surface that has epoxy on it, but epoxy will bond and cure fine on top of polyester resin. Is vinylester resin more compatible with epoxy?
 
Question on the compatibility of fibreglassing on top of epoxy.. It is my understanding that polyester resin will not bond well to any surface that has epoxy on it, but epoxy will bond and cure fine on top of polyester resin. Is vinylester resin more compatible with epoxy?
On your question, yes you are correct. Poly and vinylester don’t bond to cured epoxy that well. But epoxy will bond to anything.

I believe vinylester has better adhesive properties than polyester, so I do believe it bonds better to epoxy than polyester. But I don’t believe it is recommended as best practice.

To clarify, I used the epoxy to bond the plywood to the fiberglass hull, and for laminating the 2 pieces of plywood together, but I left the inside face uncoated in epoxy so that I could laminate with Vinylester resin.

Ultimately there are some areas (mostly at the fillets) where the vinylester had to bond to the epoxy, but I just made sure to wash off amine blush and sand.
 
Got the transom core done on the pod this weekend. 2 3/4” marine ply laminated with 1.5 oz csm + some thickened vinylester.

First layer of ply was bonded to the pod after I laid up 2 layers of 1708 and 1.5 oz CSM and wet set it with thickened vinylester. Screwed and clamped both layers. (Removed screws after cure)

Total layup on transom skin is 6 layers of 1708 and 3 layers of CSM. Inner skin is 4 layers of 1708 + tabbing which will increase with the tabbing for the stringers. The pod held my weight (230 lbs) without the transom core in. Total layup for the pod is currently at 4 layers + tabbing. In the end I’m planning for 6 layers 1708. Corners are up near 1/2” thick solid glass due to tabbing overlaps.
 

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I have interest in a project like this myself. Is all of the tabbing to existing done completely on the inside of the extension? What do you do at join between old and new on the outside? Fill what ever little gap with Sika flex or similar?
Alan
 
So far is it just tabbed to the hull on the inside of the pod. And I just cut the two slots for the stringers to go through. As is, it holds my weight 230 lbs no problem.

But the plan on the outside of the pod is as follows:

Sides - will be ground back 5” on pod and boat transom and tabbed to the transom with 4-5 layers of fiberglass then re gelcoated

Bottom- plan to grind out a mild V shape grove along the seam (10” wide v shape taper) then glass the bottom of the pod to the bottom of the hull, then re-gelcoat.

I looked all over the internet on a proper layup schedule but didn’t find anything definite so I could be overbuilding or maybe under building but with similar projects for larger motors I saw online, I feel pretty safe with what my plan is. Always second guessing though.
 
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A little update. Slow December pecking away at removing the bottom paint (wow that sucked) and slow start in January but I am getting back in the game.

As expected the port stringer is mush. I’ve got one side of the stringer cut out and wood removed.

Plan is to put a new core in a glass it back in without removing the floor. It will be difficult to access in the bow, but the floor is solid and wood is dry and the moulded non skid is worth saving.

I need to check out the starboard side a bit more. I think it’s damp but still solid. Either way I would plan to replace one at a time. (Boat is blocked above trailer & level)

My only downfall with my plan is the core of the secondary stringer is mush (port side). I would have to cut up the floor to replace. I personally feel the residual strength of the fiberglass for the secondary stringer is good enough for its job. I would be interested to hear your opinions on that.

After stringers are sound, I will tie in the stringers to the pod, and then build the swim platform.
 

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Looks like you've done nice work especially for the first time. I like how you followed the shape of the hull on your extension/pod.I do wonder about the width of the pod.I hope it doesn't give you more floatation than you need or want.How did you decide on the width and what power will be on it?
 
Nice work
The age old question “where do you stop”
You are so far in, I wouldn’t leave any rotten stringers. But if you do leave them nobody will know except you and the boat won’t fall apart…
Tough call
If you have the time and money to dump into it, cut the floor and do it right, it’s a small boat, shouldn’t take you that long and it should make it easier to replace all the stringers.
Just my 2 cents
 
I know you won't like what I have to say but you've gone so fa already, now do it right. Rip up the floor and replace both stringers. Wood is organic and will rot eventually. That's the problem with 30+ year old wood-component boats; they are due for major rebuilts at some point. If you decide to keep such a boat alive then do it properly. Then you also have a decent resale value.
 
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