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Old 17-11-2020, 10:26   #1
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What diameter of hole saw for corners--cutting an engine hatch

Background: I have to remove my transmission to take it to a machine shop to have some welding done.

I have three options. Remove the engine, try to do it through the cockpit locker (yuck!) or create a hatch in the cockpit (yea!).

A cockpit hatch also solves other problems like accessing the muffler, accessing the stuffing box, accessing hoses, etc. I'm 50 years old; a cockpit hatch makes sense.

I plan to install a high quality aluminum (probably) or steel (maybe) hatch in the cockpit floor. I realize it will have to be fabricated.

I have read lots of other posts on cutting fiberglass and creating cockpit hatches on the forum and this has helped answer most of my questions (Thank you!)

At this point my plan is to use a diamond or carbide hole saw to cut rounded corners in the fiberglass (masking tape + water + wet/dry vac+ slow cutting).

Then use a fence on each side of the cockpit to slowly connect the holes with another power tool (probably a jig saw with a carbide blade). I will use painters tape + well measured and marked lines.

So...here is my question: What size hole saw should I use for the corners of the hatch? I own a 6" diamond grit hole saw, but I think this is too large.

Say the word---If necessary I can take accurate measurements and more pictures of the cockpit floor the next time I am on my boat.

The center of the cockpit floor is trapezoid shaped with about a 1" high and 1" wide channel for water to slosh into the cockpit drains (out of view).

My plan is to cut about 1" in from each edge of the raised trapezoid area. So the hatch will actually be in the raised area.

I will probably cover the hatch with a teak grate.

Thank you for your insights.
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Old 17-11-2020, 10:43   #2
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Re: What diameter of hole saw for corners--cutting an engine hatch

I did an aluminum watertight Freeman hatch in one boat. It was great for access and indestructable + totally sealed. Mine just had 6 big screw like dogs you twisted from above to remove the lid. Covered it with a grate also.

Not a Freeman but this may fit and not too expensive:
https://shop.hamiltonmarine.com/prod...ket-51429.html

On a budget I wonder if you could remove the plexi from a low profile hatch and replace with aluminum plate?
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Old 17-11-2020, 11:03   #3
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Re: What diameter of hole saw for corners--cutting an engine hatch

As small as needed to end the cut. The jig saw blade is somewhat small so possibly only need a regular drill bit, maybe 3/8".

A hatch is a good idea to fill in the new hole in the cockpit. Unless you have a good fabricator, would rethink making the cutout an odd shape and buy something off the shelf as suggested by Cheechako. Rectangular is good.

They are expensive, but well worth getting better dog downs and a heavy rubber gasket. The metal does get hot, so your idea of putting a grate over it is a good one.
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Old 17-11-2020, 11:23   #4
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Re: What diameter of hole saw for corners--cutting an engine hatch

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Originally Posted by Cheechako View Post
I did an aluminum watertight Freeman hatch in one boat. It was great for access and indestructable + totally sealed. Mine just had 6 big screw like dogs you twisted from above to remove the lid. Covered it with a grate also.

Not a Freeman but this may fit and not too expensive:
https://shop.hamiltonmarine.com/prod...ket-51429.html

On a budget I wonder if you could remove the plexi from a low profile hatch and replace with aluminum plate?
Yep. I like Freeman. I have also thought about cast bronze, but I suspect it will be cost prohibitive.

Thanks for Hamilton link and the plexi idea.
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Old 17-11-2020, 11:26   #5
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Re: What diameter of hole saw for corners--cutting an engine hatch

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As small as needed to end the cut. The jig saw blade is somewhat small so possibly only need a regular drill bit, maybe 3/8".
I thought long, round corners were best to keep fiberglass from cracking; that is why I was thinking in inches.
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Old 17-11-2020, 11:32   #6
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Re: What diameter of hole saw for corners--cutting an engine hatch

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They are expensive, but well worth getting better dog downs and a heavy rubber gasket. The metal does get hot, so your idea of putting a grate over it is a good one.
Dog information duly noted.

Yep. I am thinking a double gasket or a double lip-type gasket if possible. Getting pooped and having my engine room fill-up with water from a leaking hatch is something I would like to avoid.
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Old 17-11-2020, 11:38   #7
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Re: What diameter of hole saw for corners--cutting an engine hatch

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Yep. I like Freeman. I have also thought about cast bronze, but I suspect it will be cost prohibitive.

Thanks for Hamilton link and the plexi idea.
Figure out what hatch you'll use before cutting! Yes large radius are best and my Freeman was one of those big ovals. Freeman are about twice or more $ now than when I did it though. Mine was a budget model without the center handle/release... just six cogs to twist with a flat blade on the perimeter. It stuck up about 3/8" off the floor. I don't see that model anymore.
A company that said they make custom shape popped up when I searched.
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Old 17-11-2020, 11:45   #8
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Re: What diameter of hole saw for corners--cutting an engine hatch

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Figure out what hatch you'll use before cutting!
Yep.

New plan: Measure. Pics. Then Freeman Marine for price quotes before I cut anything. I have been looking at their website and an oval hatch in the middle of the trapezoid might just fit the bill.

I also noticed a trapezoid shaped aluminum and acrylic Lewmar that might work (as long as I replaced the acrylic with an aluminum plate).

Baier also has a flush screw manhole that looks good.
https://www.baiermarine.com/shop/hat...t-flush-screw/
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Old 17-11-2020, 11:54   #9
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Re: What diameter of hole saw for corners--cutting an engine hatch

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New plan. Measure. Pics. Then Freeman Marine for price quotes before I cut anything.

+1. Had a round bronze hatch (16-18") in my previous boat and always wished it was a larger rectangular one for engine better access.

IMO the rectangular hatch would be better than oval if the cost was the same. Mine had the center lift and that did make it easier/safer for the fingers to pull up/put back in place.
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Old 17-11-2020, 12:02   #10
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Re: What diameter of hole saw for corners--cutting an engine hatch

Quote:
Originally Posted by esarratt View Post
New plan. Measure. Pics. Then Freeman Marine for price quotes before I cut anything. I have been looking at their website and an oval hatch in the middle of the trapezoid might just fit the bill.

I also noticed a trapezoid shaped aluminum and acrylic Lewmar that might work (as long as I replaced the acrylic with an aluminum plate).

Not sure why you would replace the acrylic? You can walk on most hatches and you will be protecting it with a teak grate?

Build the grate so that it gets support from the surrounding sole and save the effort?


Also once you get access it is relatively easy to remove a transmission so the hatch would not have to be too large.
Good luck!
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Old 17-11-2020, 12:14   #11
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Re: What diameter of hole saw for corners--cutting an engine hatch

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Not sure why you would replace the acrylic?
I was just going with the previous poster's recommendation (in relation to plexi). Wouldn't heat above an engine be an issue for acrylic or Plexiglas?
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Old 17-11-2020, 12:28   #12
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Re: What diameter of hole saw for corners--cutting an engine hatch

I would think a hatch would heat up a lot more in direct sun than over an engine.

The area around my engine does not get hot. I don't think the exhaust elbow gets that warm, but then I have never grab it while motoring?
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Old 17-11-2020, 12:31   #13
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Re: What diameter of hole saw for corners--cutting an engine hatch

Esarrat:

If you mean to be able to get through the hatch, I doubt that a 15 x 23 oval will be big enuff for chap your age. With a supreme effort of concentration I can JUST remember what it was like to be your age :-)!.

You are engaged on a project that needs to be right the FIRST time!

If I were in your boat, I'd blow fifty or a hunnert bux on setting up some sort of dummy involving a sheet of 3/4" plywood with a 15 x 23 oval cut in it to verify that you can, in fact, get thought it. And out again!!

Then if your agility ain't wot it usta be, you can enlarge to the hole till you determine the minimum feasible size.

Good luck!

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Old 17-11-2020, 12:39   #14
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Re: What diameter of hole saw for corners--cutting an engine hatch

P & E Aluminum hatch, custom sizes they say. 24 x 24 is $680
https://www.go2marine.com/P-E-Alumin...4aAsOpEALw_wcB
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Old 17-11-2020, 12:41   #15
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Re: What diameter of hole saw for corners--cutting an engine hatch

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Originally Posted by Gary Mc View Post
Not sure why you would replace the acrylic? You can walk on most hatches and you will be protecting it with a teak grate?

Build the grate so that it gets support from the surrounding sole and save the effort?


Also once you get access it is relatively easy to remove a transmission so the hatch would not have to be too large.
Good luck!
I suggested that only because it's the floor and a huge hole if broken! I've seen 2 or 3 broken up on deck in my time.
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