Stiffening flange repair - seeking advice

[Dan]

The way this boat is made, there are stiffening flanges molded into the inner mold and the liner. These flanges stiffen the structure and also provide a place for the bulkheads to screw to.
I have a crack in the flange where the bulkhead, that has the chainplate bolted to it, is screwed. The chainplate is bolted to the bulkhead and the bulkhead is screwed to this flange with some #10 screws.
Anyway, This flange is cracked and I am seeking advice before I go nuts with the grinder.
I am puzzled as to how this cracked, but it needs to be repaird anyway. I'm looking for advice on how to access this to repair it.
You can sort of see what is going on here. Ugly crack.
http://home.wmis.net/~dhapp/crack/crack.html
I can add more pictures tomorrow if needed.
Dan

[tartan30cirrus]

Dan,

IMHO, the only way to do this right is to do whatever you can to stiffen the flange region. On my boat I had a flexy chainplate which caused flex in the deck = cracks = leaks = freeze thaw etc. M bulkhead was fine but after reglassing the deck around the chainplates and using large metal stiffening plates all my stress problems went away. Therefore, I would go with the repair that you think would provide the most stiffeness even if it is a LOT of work. This is an area of the boat we want really stiff and without leaks! Hopefully you get some real good specifics from others. I'll be interested.

[Tim]

I am puzzled as to how this cracked...

Well, frankly, it cracked because it's weak, and a bad way to construct boats. Please forgive me if that sounds mean-spirited, as it's not intended that way--but cracks like this are proof that the structure is weaker than it needs to be for that particular application. You have all the stresses of a major structural bulkhead--the one to which the chainplates attach--resting against a flimsy molded liner, with minimal reinforcement and no means of distributing the stresses over a larger area. Add in that screw that's driven into the fiberglass, shattering the nearby fibers, and you have a certain recipe for cracks. I am not a fan of mechanically-secured bulkheads stuck into slots or flanges in liners, but many inshore, high-production boats are built this way because it is quick, cheap, and easy.

In all honesty, this sort of construction works just fine for the boat's intended use, despite my dislike of the practice in the first place. Therefore, please evaluate carefully how these cracks truly affect the overall structure at hand; I feel, after looking at the photos, that this issue is far simpler and less critical than you might think. Not all boats need to be engineered to sail the blue waters.

Those cracks are nothing new: you can see how some previous owner tried to "repair" them by dabbing some white gelcoat on the surface of the cracks on the after side there. That suggests that the fine cracks have been there for some time. It's my guess that they began to show up almost immediately, back when the boat was new and started sailing. Like a fine crack in a windsheild, as stresses continue, cracks such as these continue spreading, slowly, inexorably.

During normal sailing, the boat wants to twist and contort. Bulkheads, coupled with the "top of the box" that is the deck, resist this motion and provide stiffness. When bulkheads are only mechanically fastened, their ability to properly resist this stress is minimal; fasteners will tend to elongate their holes, and the structure loses some rididity as a result. Add in the large stresses caused by rigging tension, and you can start to see why cracks like this might occur.

In particular, your boat's design has these stresses being placed on a lightweight interior liner. There's probably some form of additional reinforcement behind those flanges, but not much. It's equally possible that the builder did very little, relying instead upon the supposed box-beam "structure" of those little molded pieces to resist the stresses at hand. The liner is secured to the underside of the deck, usually with blobs of resin or putty here and there, but liners shouldn't be relied upon for any sort of structural application.

Cracks are indicative of movement to which the laminate is unable to properly adjust. They normally highlight an area where excessive movement occurs; fiberglass is quite forgiving, and flexes a large amount with no damage, but once that threshold is reached, cracks begin to form as the laminate is moved beyond its ultimate limits. This sounds worse than it usually is; most boats form cracks at one place or another, usually hard corners just like the one in your photos.

Your boat is what, 25-30 years old? That shows just how forgiving fiberglass actually is. Despite a pretty lightweight bulkhead-securing structure, all the boat has to show for this shortfall is these small cracks in an isolated area. The cracks formed there because of the hard corner, the flexing of hull and deck forcing (imperceptibly, perhaps) the bulkhead in and out of that corner all these years, hastily-driven screws at the time of manufacture, and related stresses from all these factors.

In my opinion, you'd be better off leaving the crack as is than you would be trying to repair it through a tiny slot, or, even worse, by opening the whole deck and cabin side as you propose. That's an exceedingly damaging and complex solution to a small problem.

If you truly want to reinforce the area, the bulkhead should be tabbed to hull and deck. This would stiffen the boat and structure more than you could imagine. I'd rather see you cut out the liner in that area and repair it that way, instead of trying to do a repair from the outside.

Without seeing it in person, and being able to witness any collateral damage or the whole circumstance, it's impossible for me to guess exactly how critical a real repair even is here. Boats act as a complete system, and the strength of your bulkhead installation is not dependent on this one area alone.

I don't condone band-aid repairs and the like, but there are definitely some cases where things like this are best left alone, and you come to terms with this particular limitation of the boat. Let's not forget: it's a Catalina 25, designed for inshore daysailing and such, and the boat is lightweight in construction everywhere. There's little sense in over-improving one area of the boat like this. From what I am seeing in the photos (admittedly I can only tell so much), this cracking is not going to represent the end of the world. It's been there for a long time.

Frankly, a successful repair in this instance might be as simple as applying a couple layers of glass over the outside of the crack, on the bulkhead side. If the visible cracks have any ramifications as they are now, it appears to be only that the flange may have cracked through the entire corner, in effect separating the upper, cabin-side section from the remainder. Tying this back in with some fiberglass would return the continuity to the structure. It doesn't need to be much, honestly. The boat is not southern ocean-bound, after all. That liner, and the flange, are probably built mostly of chopped mat in the first place. Perhaps there's a single layer of woven roving in there.

There's no need to reinvent the wheel here. Keep in mind how the boat is built as a whole, and how you intend to use it. Then, a sensible repair, as needed, can be tailored to the situation. I will be happy to look at more information and discuss the matter further, but my overall impression is that probably less is more here.