Repair of Fiberglass Centerboard Trunk

[Davey Gee]

Hey Everyone,

I’m looking for some guidance on how to approach a fiberglass repair. I have some idea of how to perform the repair, from reading online, but I have never done fiberglass work before. This may not be a beginner's repair.

I was forced to cut a hole in the fiberglass trunk of my Aquarius 21, to free-up a frozen centerboard. I can go into details about that, but I think it’s kind of moot for this discussion. Suffice it to say, this was the absolute last resort.

The hole is about 2” above the waterline and is positioned directly under the support post for the mast, which transfers the load of the mast into the trunk. The trunk is made of ⅜” fiberglass and has an overall width of 2” (see attached top-down view). I think there is enough remaining material in the trunk to support the mast, but I’d like to avoid removing more if possible. I can only access the hole from one side.

My goal with this repair is to keep water from entering through the hole, since it’s close to the waterline. I believe this repair will be under some load from the mast, but I wouldn’t consider it a “structural” repair (i.e. if it cracks, the mast isn’t coming down). My understanding is that a one-sided repair is not ideal in this situation, but I don’t see another option.

Here’s the approach I’m considering from online resources:

1. Scarf sand the edges of the hole to a 12:1 taper from the outside (the greater the taper, the stronger the bond).
   • Mark perimeter before sanding.
2. Clean the surface with acetone.
3. Install a backing plate with a string (as described in Hole Repair With No Deck Access? | Boating Forum - iboats Boating Forums).
   • I have very little clearance behind the hole, so I’m considering a thin plastic sheet for this purpose.
4. Pre-cut each ply for the taper.
5. Weigh the reinforcement schedule of 50:50 fiber to resin by weight.
   • Ideally use the same fiberglass, resin, and weave orientation as the primary structure, to avoid creating new stress points. Not sure how I’d determine this though.
   • I’m considering epoxy resin for strength.
6. Mix hardener per instructors from resin manufacturer.
7. Pre-wet the entire bonding area with resin (including the backing plate).
8. Saturate each ply and apply to the repair area.
   • Smallest ply first and progress to larger plies to follow the taper.
9. Stop every 3-5 layers to compact the repair with a roller and get out air bubbles.
10. Let sit and harden.

Problem: The hole is up against the rounded end of the trunk, so I don’t think I can get the full 12:1 taper on that side (see attached pictures). I believe this will create a weaker bond, which may be susceptible to cracking from the stress of the mast. What are my options in this situation?

Since my goal is to keep out water, is there another option besides patching the fiberglass that I should consider? Any suggestions for my approach?

Summary

• ⅜” thick fiberglass
• 2” above the waterline
• One accessible from one side
• Under stress, but not “structural”
• Not enough space for 12:1 taper

I assume this is the right forum for this question, but please advise if it should be moved elsewhere.

 

[BAD ORCA]

There is no limit to how much glass you can laminate over the repair. Once you get the basic hole repaired, you could easily laminate many more larger pieces of glass over the repair site. It even looks like you might be able to partially wrap some big pieces around that trunk section. Its a small boat though. I cant imagine the loads would be so severe they would overcome a decent repair. If you do, sand that tan gelcoat well first before doing this.

I agree. Personally, I might not want to grind too much into that curved area just to get 12:1 either. It looks like your cutout is already starting into the curve. Trying to get the perfect taper might be grinding out too much strength in that section. Maybe save the large taper for the other three sides and taper much less on that leading radius, or a larger taper but much shallower in that area Like I said, you can laminate more glass over the entire thing till your satisfied. Maybe trying to wrap some bigger pieces over the finished fix concentrating on trying to overlap the radius.

A slower setting laminating epoxy might be ideal for this. They remain more flexible after cured and are better at bonding because of this. Not that it really matter a whole lot. Its not a stealth fighter but...it wont cost more than a fast set and will give you plenty of time to work. You will want to work on a nice warm day though or warm up the epoxy first.

Rolling out bubbles may not be viable. The roller will want to push the soft laminate through the hole. Maybe wet out a few layers at a time on a plastic sheet on a table, roll them out there and then apply to the hole without trying to roll them out again in place unless you keep to the solid edges of the taper and avoid trying to roll out the area over the hole. You could also try to apply a single patch of a few layers, let it cure solid and then sand it and finish your layup with a more solid backing. This would allow you to possibly use the roller over the repair area. I think rolling out the cloth a few layers at a time on the mixing table will be much easier and plenty strong.

Im decent at fiberglass work but no pro so maybe someone else might have more advice. Your plan sounds good to me so far though. Its not that big of a hole.

I would add, I would put the same amount of effort and attention into that smaller hole as well because it lines up horizontally with the top of the larger cutout.

 

[shemandr]

3/8 * 12=4.5. I agree that you don't want to engage the curve of the centerboard trunk that much. And you don't need to. The 1:12 recommendation would apply to a hole in the hull such as closing a cutout for a thru hull or scarfing a joint.
An alternative to using the backing with the string might be as follows: After the prep of the site you could template the smallest patch to cover the hole. And shape some cardboard (To be later saturated with epoxy), acetate or what have you to the template. Or, on a non-stick surface lay up a few layers of tape to the dimensions of the template. Let it cure. Then trim as necessary and epoxy to the hole preparation. Epoxy that is kicked but not fully hardened trims nicely. Then layer up tape until you have 3/8 or even more.
The string thing is more if you need a smooth inner surface. You just need for the patch to not encroach on the centerboard movement. Don't let perfect be the enemy of good.

 

[colemj]

1. Weigh the reinforcement schedule of 50:50 fiber to resin by weight.
   • Ideally use the same fiberglass, resin, and weave orientation as the primary structure, to avoid creating new stress points. Not sure how I’d determine this though.
   • I’m considering epoxy resin for strength.

Problem: The hole is up against the rounded end of the trunk, so I don’t think I can get the full 12:1 taper on that side (see attached pictures). I believe this will create a weaker bond, which may be susceptible to cracking from the stress of the mast. What are my options in this situation?

Your plan is good. Do you have access to the rounded portion of the trunk by removing whatever is in front of it?

50:50 is a good goal, but you are unlikely to make it if you don't have a lot of experience with fiberglass. Don't worry about it. A more resin rich layup isn't going to hurt you here, and the tiny extra weight is no problem. You don't want to mix resin based on achieving this ratio, or you will run out and need to mix more in the middle of the job.

There is no intentional fiber orientation in that layup. It was done with CSM and woven cloth. The entire trunk is in compression, so if you want to pay attention to fiber orientation, it would be in that direction.

You would want to either use the same woven cloth as original, or stitched biaxial or triaxial. 3/8" is a lot of glass, so watch the heat and tendency to pull off. 3/8" is ~10 layers of 18oz woven roving or 1708 biax. You might need to do this in two layups.

Your choice on resin. People have strong opinions that epoxy is necessary here, but it is not. Polyester is much better for multiple layup sessions, and the only resin you should use if you decide to use CSM in the layup. CSM will add bulk quicker, and wrap around the trunk curve better (you can see they used it for that reason in that area). If you do choose epoxy, do yourself a huge favor and get some peel ply to use between layups. This will save you a ton of work washing off the amine blush and grinding new tooth between layups.

If you can get access to the front curved portion of the trunk, then do grind back around it, but don't make it part of the 12:1 bevel. Do the 12:1 bevel where you have the space for it, do as much bevel as you can to the edge of the curve, and just grind to good clean glass around the curve. Then do your layup in the flat part like you describe small to large, but cut the last layer or two of glass large enough to wrap around the curve.

If you don't have access to the curve, don't worry about it. Just grind the 12:1 bevel where you can, and whatever bevel you can achieve to the edge of the curve you can access. This won't be symmetrical, and the bevel to the edge of the curve won't be much because you cut there, so you will need to cut your glass pieces and plan the layup to accommodate this asymmetry.

Thinking about it more, this could probably be done in all CSM because there really isn't much structural loading here, nor any dynamic forces on such a small part of the trunk. CSM would be polyester resin only.

Before you glass this back up, do yourself a favor and redo those two wire crimps with heat shrink.

Mark