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Old 18-04-2014, 23:01   #1
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Portlights in a cored hull

Does anyone know if there is a standard amount of core to remove when installing portlights in a balsa cored hull. I'm replacing the portlights in my O'Day 40 with New Found Metal, need to enlarge the openings a little. The old ones didn't have the core cut back, just sealed with epoxy. Not entirely successfully but I've replaced the damp core and don't want a repeat.

Thanks, Neil
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Old 18-04-2014, 23:09   #2
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Re: Portlights in a cored hull

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Originally Posted by bcboomer View Post
Does anyone know if there is a standard amount of core to remove when installing portlights in a balsa cored hull. I'm replacing the portlights in my O'Day 40 with New Found Metal, need to enlarge the openings a little. The old ones didn't have the core cut back, just sealed with epoxy. Not entirely successfully but I've replaced the damp core and don't want a repeat.

Thanks, Neil



For me, the rule of thumb is that the minimum amount to decore is the thickness of the core, ie for 3/4" balsa core decore at least 3/4". However, it is often the case that the hardware you will be fastening down has a fastened flange which is wider than this, and if you decore far enough you won't have to individually decore each screw hole. So put the port light in the cutout and fasten it in place (dry fit), then make sure to decore well past the screw holes. Unless in this case the flange is too wide, making it faster to do individual screw holes.



Here's a pic of decoring 1/2" core with a 1/4" die grinder and a 1/2" x1/2" bit. Takes seconds to decore a cutout to bare glass both skins. I also often use router bits, either in a die grinder or a cordless, for deeper decoring.
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Old 18-04-2014, 23:33   #3
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Re: Portlights in a cored hull

I'm on the last mile of replacing my old aluminum Bomars with NFM port lights. I agree with Minaret, given the way the NFM are installed. That said, that's a lot of epoxy filling. I ended up NOT decoring all the new holes but rather wetted them all out with epoxy, as I have 1.5" cabin sides with two 1/2" ply sheets in there.

I have a few tips that are not covered in their installation video:

1. Do NOT rely blindly on the template(s) that they send for the location of the bolt holes. I installed 8 5x20 ports, and the holes were spaced a little differently on all of them. I ended up checking the bottom two middle ones (which seemed to always fit the template), drilling them, then fitting the port with dowels in those holes to hold it in place and marking the rest of the holes and drilling them individually. I was a bit irked to say the least, as if they send you a template with drill guides around it you would sort of expect them to at least tell you that they might not line up. The only port where they lined up was a small 5x12 one.

2. Clean the ports and finishing rings well with mineral spirits and acetone before installation. A significant amount of machine oil, dirt, and polishing compound will come off of them.

3. Put rings of butyl around the studs on the finishing ring so they seal the countersunk bolt holes from the outside. NFM's method relies on injecting the holes with silicon from the inside, but it is virtually impossible to ensure that you've got it all the way to the studs.

4. Make sure that you have at least 3/16" gap between the port flange that you insert through the cabin wall and the cutout, and check from the outside. Anything less makes it difficult to ensure that you have the butyl sufficiently packed in.

5. Keep the butyl that you're going to add from the outside, and to finish the finishing ring, in the refrigerator, or a cooler, if possible. If it gets warm it will get messy. When it's cold it's easier to work with and does not start to stick to your fingers as quickly.

6. If you get squeeze-out of the butyl back to the inside flange and it starts to come out the bolt holes, just punch a #3 phillips head screw driver in, twist it, and yank. It will punch a hole and pull out the excess.
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Old 19-04-2014, 00:12   #4
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Re: Portlights in a cored hull

I have a slightly different method.
I cut out the hole for the hatch, lay in the hatch and drill the screw holes only to the bottom layer, using the hatch as the drill pattern.

Pull the hatch back out and de-core the holes and fill them with epoxy & 404 filler. As well I scrape out a bit of the balsa in the cutout then paint the core with a wet paste mix of epoxy and filler, this allows the epoxy to saturate into the balsa. It may drip a bit so I put a paper down to catch drips.

After it's all cured, clean out the cutout so the hatch fits and re-drill the holes all the way thru. Pull the hatch again and fill the gaps in the cutout core with a peanut texture of epoxy filler. When it cures then file it so the hatch fits in.

Then seal the hatch and screws with 3M 4200 UV or a butyl tape, depending on its stability.
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Old 19-04-2014, 10:23   #5
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Re: Portlights in a cored hull

Thanks guys, I've only cut away 1/4" of core around the port cutout and bolt holes and wasn't comfortable with that.
Suijin, I too am disappointed with the template/portlight match-up. NFM needs to make it clear that the templates are not accurate. You're technique seems similar to the one set out by Maine Sail and is what I will use.
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Old 19-04-2014, 15:40   #6
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Re: Portlights in a cored hull

When I decore around portlights and hatches, instead of just filling with an epoxy glue/bog mix, I insert an epoxy saturated unidirectional glass tape, then top up with glue/bog. Pretty much eliminates the possibility of the glue cracking and letting water in.
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Old 19-04-2014, 15:58   #7
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Re: Portlights in a cored hull

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Originally Posted by bcboomer View Post
Does anyone know if there is a standard amount of core to remove when installing portlights in a balsa cored hull. I'm replacing the portlights in my O'Day 40 with New Found Metal, need to enlarge the openings a little. The old ones didn't have the core cut back, just sealed with epoxy. Not entirely successfully but I've replaced the damp core and don't want a repeat.

Thanks, Neil
minaret does this work for a living, so if you follow his methods you will get a Pro job
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