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· Telstar 28
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I wouldn't just use marinetex. Ideally, you should re-glass in a repair for the section of the "oval" that you're not going to be using.

One way to do this is to cut a section of "deck core" material that is the size and shape of the oval in question.

Grind back the glass to a 12:1 taper on the outside and inside, just around the edges where the "replacement" is needed.

Tape the core material "oval" in place, using masking tape or duct tape... on the side where the nicro vent is going to go.

Fill the gap between the edge of the oval and the existing core with thickened epoxy, just on the side being replaced.

Now, layup new fiberglass over and under the area that needs to be "replaced, overlapping the cutout area for the new Nicro vent a bit.

Let it cure.

Now, use your hole saw or jig saw to re-cut the opening for the Nicro vent... through the fiberglassed edge of the hole only...

Take the tape off and remove the "hole" from the cabintop.

Now, you should have what resembles a hole through a normal cored cabintop, and can basically proceed as if you had just cut a hole... with a few small modifications.

Use a router to remove the core material from between the two new layers of laminate...and fill the space with thickened epoxy.

Fair and sand the patched area and either paint or gelcoat.

Drill and pot the fastener holes... Don't forget to countersink the hole for the vent and the fasteners, so the sealant has a place to form an "o-ring".

Then install your Nicro vent—bedding it with whatever is recommended, depending on whether it is plastic or metal.
 

· Telstar 28
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I wouldn't use teak. Adding a teak ring really doesn't do much to fix the structural problem of having a big gap in the area around the vent and adds additional maintenance, since the teak is higher maintenance than fiberglassing in a repair properly.

I also wouldn't recommend using just MarineTex. While it is excellent for smaller repairs, the size of this repair really requires reglassing the area. Also, marinetex would have a fairly weak bond, as repairs go, since the only area it would really be adhered would be along its perimeter... If someone accidentally stepped on the vent on the side with the MarineTex it might not hold.

You don't have to grind away too much fiberglass... a 12:1 taper for the thickness of the laminate skins is all that is required. If the skin on top is 1/4" thick, then you only need to bevel/grind a 3" radius away from the edge of the hole. Same thing on the underside.
 

· Telstar 28
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Setting a solid plug of MarineTex over an inch thick would probably be a problem, since thick layers of epoxy have issues with heat and curing properly. Also, it would be far heavier than a properly installed cored section would be. BTW, thickened epoxy is probably not as stiff as a properly cored laminate of the same thickness.

sd, if the typical cored deck is generally over an inch thick...you don't trust an epoxy plug that is bonded to something an inch thick?! I'd just bevel out some of the core if it was balsa (and leave it alone if it was Brunell ply) and trust it. And that's from seeing how similar epoxies have held up over the years, in structural uses. Step on it? Heck, you could "riverdance" on it!
 

· Telstar 28
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I'm not a big fan of marine plywood as a core material. It has the worst characteristics of both foam and end-grain balsa as a core material and is heavy to boot. Like end-grain balsa, plywood absorbs water and rots. Like foam, it will allow water to migrate long distances and delaminate very large areas fairly quickly.

I would suggest using end-grain balsa, like ContourKore, or a foam like Airex or Divinylcell. IMHO, it will create as stiff a deck as the plywood core material, but far lighter.
 

· Telstar 28
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End-grain balsa is actually a fairly tough core material. Its major weakness is when it is exposed to water, it rots. It generally has better compression and shear strength than the foams do.

In a cored laminate panel, the core material acts as a stress web, much like the center portion of an i-beam. It allows the two laminate skins to act as a single unit and allows one to be in tension, while the other is in compression, for torsional loads on the laminate panel. This allows it to be stiffer and stronger for far less weight than you could do with solid fiberglass alone.

If you have any doubts about this...find a 1/2" thick fiberglass sheet about a three feet long and support one end of it and see how much it sags. Now do the same with a piece of cored laminate that has a 3/8" balsa core and 1/16" of glass on each side... and see how much that sags. And then weigh one against the other... The thicker the core material, the stiffer the laminate will be as a general rule.

Interesting, thanks. Whenever I read about the balsa wood core on a sailboat, I think of the balsa wood airplanes I made many years ago. You know, the ones that cracked and shattered like a toothpick as soon as you looked at them too hard.

For this repair, I was thinking of biscuit joining (with glue of course) some wood to the existing plywood core, then using MarineTex to seal it in and join it to the fiberglass.

Is balsa wood something that can take a little abuse, or is it used mostly to keep the fiberglass panels away from each other when the boat is built?
 

· Telstar 28
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Don't quote me out of context. I never said balsa core was good for compression loading with bolts... :rolleyes: just that it had better compression and shear strength than most of the foam-core materials used commonly. I am a big advocate of replacing the core material at anyplace hardware is mounted to the deck. I seriously doubt you'll be able to find a single post where I advocate through-bolting hardware to a balsa cored deck without potting the holes with thickened epoxy EVER.

SD, for once I've got to disagree with you about balsa core...it's absolutely awful for compression loading, specifically from through bolts. Last fall, I laid up new primary winch pads on a homebuilt when the new owner wanted to use electric winches.
The bolts had been overtightened and compressed the balsa (like overdriving a wood screw). It was sealed up well so no water got in but the glass was spidercracked all around the fasteners. A leak in the seal could have (in time) pulled the loaded winch right off the deck. Ouch.
An advantage of using marine ply as a hard point in the core is that the veneers have a bit of 'spring', allowing the bolts to stay under tension.
 
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