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Old 20-05-2012, 07:41   #1
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Cutting Pilothouse fiberglas, installing A/C

I need some help and clarification on cutting my pilothouse overhead/deck ("the roof") to install an RV-style rooftop air conditioner. It needs a 14"x14" hole for air supply/return into the cabin, and probably reinforcement of the edges so it doesn't sag. The air distribution box and controls mount upwards against it from inside. You've probably all seen these on RVs shoreside. Big rectangular box, 36"x30"x13", faired in front to reduce drag. Dometic makes a marine-grade unit called DuraSea ($2500), upgraded from their Commercial Grade RV/truck unit ($675). Guess which one I bought? I can buy 3 cheaper ones for the same money, and I'm sure the expensive one won't last 3 times longer! Anyway, I need to cut through the FG from outside, then match that inside on the overhead liner, much lighter weight. The pilothouse is "walkable-upon", so it will hold the gross weight, but I suspect will need a framed box to keep the edges of the hole from sagging and leaking. Teak? Marine ply? Starboard? Trex?
Right now the question is how to best cut the hole without ripping/tearing the glas. I've seen Sawzalls, Dremel tools, Skilsaws, angle grinders, and many more recommended. Am I correct that the biggest danger is ripping or fragmenting the edges of the hole? Any experience and cautions to share, please?
Night Watch is Dartsailer 27, a Dutch-built boat, 1980, well laid up and solid for the North Sea. Thanks for your help.
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Old 20-05-2012, 07:52   #2
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Re: Cutting Pilothouse fiberglas, installing A/C

why not just place in a hatch overhead and make a special removable fitting around it to make it work, like everyone else does so your pilot house isnt made irrepairable for later use in weather??
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Old 20-05-2012, 08:30   #3
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Re: Cutting Pilothouse fiberglas, installing A/C

+1 for the adapter to a hatch. You may not want to sail around with that on the cabin top plus when it fails the replacement is practically guarenteed to have different dimensions (at least in my experience). If you do cut it make sure you seal the edges well with epoxy to stop future moisure/rot.
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Old 20-05-2012, 09:28   #4
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Re: Cutting Pilothouse fiberglas, installing A/C

Quote:
Originally Posted by Dutch6246 View Post
I need some help and clarification on cutting my pilothouse overhead/deck ("the roof") to install an RV-style rooftop air conditioner. It needs a 14"x14" hole for air supply/return into the cabin, and probably reinforcement of the edges so it doesn't sag. The air distribution box and controls mount upwards against it from inside. You've probably all seen these on RVs shoreside. Big rectangular box, 36"x30"x13", faired in front to reduce drag. Dometic makes a marine-grade unit called DuraSea ($2500), upgraded from their Commercial Grade RV/truck unit ($675). Guess which one I bought? I can buy 3 cheaper ones for the same money, and I'm sure the expensive one won't last 3 times longer! Anyway, I need to cut through the FG from outside, then match that inside on the overhead liner, much lighter weight. The pilothouse is "walkable-upon", so it will hold the gross weight, but I suspect will need a framed box to keep the edges of the hole from sagging and leaking. Teak? Marine ply? Starboard? Trex?
Right now the question is how to best cut the hole without ripping/tearing the glas. I've seen Sawzalls, Dremel tools, Skilsaws, angle grinders, and many more recommended. Am I correct that the biggest danger is ripping or fragmenting the edges of the hole? Any experience and cautions to share, please?
Night Watch is Dartsailer 27, a Dutch-built boat, 1980, well laid up and solid for the North Sea. Thanks for your help.

Whatever tool you use, try a carbide abrasive blade instead of a regular toothed blade. Makes perfect clean cuts in fiberglass with zero blowout, and you can get blades for almost anything in carbide abrasive.
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Old 20-05-2012, 10:14   #5
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Re: Cutting Pilothouse fiberglas, installing A/C

Zee is right. When I bought my boat, it had been a liveaboard in Miami and the owner had removed the hatch cover (easy to do, and fortunately he kept it) and sat the roof-air unit right on top of it. It was a near perfect fit and worked well, but I couldn't stand the looks of it, removed it and gave it away. Just recently I installed a Mermaid 12K when Defender had them on sale.
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Old 20-05-2012, 10:24   #6
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Re: Cutting Pilothouse fiberglas, installing A/C

If you do the cutout, instead of a framed box if the roof is cored you might be able to remove the core for a few inches and replace it with something much stiffer, like coosa board set in epoxy or G-10. That'll stiffen it right up while also sealing the core. Even good ply in epoxy would work.
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Old 20-05-2012, 10:31   #7
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Re: Cutting Pilothouse fiberglas, installing A/C

I am not familiar with your boat so I will ask "In your normal steering position, will the AC block your vision in any way?"
We steer from the cockpit and that may not be the case on your boat.
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Old 21-05-2012, 04:41   #8
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Re: Cutting Pilothouse fiberglas, installing A/C

Quote:
Originally Posted by zeehag View Post
why not just place in a hatch overhead and make a special removable fitting around it to make it work, like everyone else does so your pilot house isnt made irrepairable for later use in weather??
These units are not carry-ons to be in and out of a forward hatch. These are made to do 75mph down the road on an RV rooftop. I intend to nail this sucker down and not remove it, and plan to add reinforcement over the top/sides as well to keep it there. We're liveaboard in the South, and will use it most of the year. It needs a flat surface to mate with for the weather seal. And, if I do want to put in a hatch, wouldn't cutting the FG be the first step anyway?
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Old 21-05-2012, 05:15   #9
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Re: Cutting Pilothouse fiberglas, installing A/C

Thanks all for the responses. I guess I wasn't clear in the first post; the pilothouse is flat on top, very solid, and the roof is well above sight lines from the emergency tiller in the cockpit. We don't sail from there for fun, cause the pilothouse is in the way; have to look through the ph windows to see ahead. Boom clearance over the potential installation is generous, 6 inches plus, sheeted down tight, so she should tack with no interference. If Night Watch were a trawler, this installation would be standard. I'll attach a pic to clarify. Imagine the unit on the starboard half of the centerline, where the bow of the dink is in this pic. That's an Achilles 9.5', for scale.

Minaret:
Quote:
If you do the cutout, instead of a framed box if the roof is cored you might be able to remove the core for a few inches and replace it with something much stiffer, like coosa board set in epoxy or G-10. That'll stiffen it right up while also sealing the core. Even good ply in epoxy would work.
Not sure what's in between the inner and outer skins of the overhead/deck. When I get the cutout position finalized, I'll probably take out a smaller piece of the interior (thinner) part and see. Pushing up from inside the pilothouse yields a bendable feel that doesn't seem to have anything between it and the actual "deck" (rooftop) of the PH, just seems to be empty airspace. Haven't heard of "coosa" board or G-10, but will research. Ever work with Trex patio decking? It's made from recycled soda bottles, so no rot. Kind of like plastic particle board, much less expensive than Starboard from West. Whatever material, the idea here is to build an analog of the framing box used in a stick-built home ceiling that would reinforce joists as in an attic drop-down stairway installation. I believe you understand, just materials I'm not familiar with. Ply set in epoxy is along the line of what I had in mind.

Again, thanks for the ideas.
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Old 21-05-2012, 05:32   #10
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Re: Cutting Pilothouse fiberglas, installing A/C

Quote:
Originally Posted by minaret View Post
If you do the cutout, instead of a framed box if the roof is cored you might be able to remove the core for a few inches and replace it with something much stiffer, like coosa board set in epoxy or G-10. That'll stiffen it right up while also sealing the core. Even good ply in epoxy would work.
+1 on the above

To be more specific:
  • Remove the headliner and check for wires etc
  • Locate your four corners and drill a TINY hole at each
    This will transfer the points inside to confirm location and interference
  • I like a carbide router or Roto-Zip bit for cutting such openings
    A helper with a Shop-Vac inside will reduce dust
  • Overcut the hole enough to trim out the raw edge with teak etc.
  • While the headliner is down, think about power supply for the AC
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Old 21-05-2012, 05:35   #11
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Re: Cutting Pilothouse fiberglas, installing A/C

Quote:
Originally Posted by minaret View Post
Whatever tool you use, try a carbide abrasive blade instead of a regular toothed blade. Makes perfect clean cuts in fiberglass with zero blowout, and you can get blades for almost anything in carbide abrasive.
Minaret, would you recommend a Skilsaw type circular saw with carbide blade over anything else? Plunge cut, then smooth out the corners, or drill corners first, then connect the dots?
Also, I've been told to set the penetration of the blade to go just barely through the material. Concur?
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Old 21-05-2012, 06:40   #12
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Re: Cutting Pilothouse fiberglas, installing A/C

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Originally Posted by capngeo View Post
+1 on the above

To be more specific:
  • Remove the headliner and check for wires etc
  • Locate your four corners and drill a TINY hole at each
    This will transfer the points inside to confirm location and interference
  • I like a carbide router or Roto-Zip bit for cutting such openings
    A helper with a Shop-Vac inside will reduce dust
  • Overcut the hole enough to trim out the raw edge with teak etc.
  • While the headliner is down, think about power supply for the AC
CapnGeo, Thanks for the nudge @ power and dust; "headliner" is not fabric, but an interior layer of thinner FG with airspace in between. I figure to cut a smaller hole in that first for visual inspection of main pilothouse exterior FG. See previous posts.
How to you keep Roto-Zip or dremeltool straight in the cut? I've seen two suggestions: hotmelt glue to hold down guide boards, or small screws for the same, then fill holes. You have a suggestion? I have a DeWalt version of the RotoZip I've used a lot for drywall, etc., but it's hard to hold on track without a guide plate.
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Old 21-05-2012, 07:00   #13
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Re: Cutting Pilothouse fiberglas, installing A/C

Quote:
Originally Posted by Dutch6246 View Post
CapnGeo, Thanks for the nudge @ power and dust; "headliner" is not fabric, but an interior layer of thinner FG with airspace in between. I figure to cut a smaller hole in that first for visual inspection of main pilothouse exterior FG. See previous posts.
How to you keep Roto-Zip or dremeltool straight in the cut? I've seen two suggestions: hotmelt glue to hold down guide boards, or small screws for the same, then fill holes. You have a suggestion? I have a DeWalt version of the RotoZip I've used a lot for drywall, etc., but it's hard to hold on track without a guide plate.
I make a guide template out of cheap plywood with the appropriate offset for your router base, then weight it down in place. I keep several hundred pounds of lead in the shop of various sizes just for this "Gravity Clamp" use. Lacking lead, anything heavy can be used... even your human helpers if you make the template big enough to stand on! If your DeWalt is the 18V one, it won't have the power required (I have one too)....

If your headliner is non-removable, see if you can contact the builder to confirm there are no structural or electrical elements in the planned cut!

Here is the bit I'd use:
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Old 21-05-2012, 07:59   #14
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Re: Cutting Pilothouse fiberglas, installing A/C

Quote:
Originally Posted by Dutch6246 View Post
Minaret, would you recommend a Skilsaw type circular saw with carbide blade over anything else? Plunge cut, then smooth out the corners, or drill corners first, then connect the dots?
Also, I've been told to set the penetration of the blade to go just barely through the material. Concur?
Yep, a circ saw with carbide blade and a hot glued batten is what I'd use. In this case I'd probably set max depth of cut, to make the finish cuts at the corners as small as possible, but that might be a problem with your "headliner". Finish the corners with jigsaw or sawzall, also with an abrasive blade. Be done in no time. Use a router to remove the first 3/4" or so of core, then if you want to go deeper use a detail sander with blade attachment. Google g-10, its the best option to give you really high strength even in a fairly thin core replacement.
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