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Tricks to coodinate intenal and external spots when drilling holes?

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Tricks to coodinate intenal and external spots when drilling holes?

Old 06-08-2015, 09:20 PM
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Default Tricks to coodinate intenal and external spots when drilling holes?

Figured this has been thrashed and mastered, but could not come up with the magic search terms.

Need to accuratly coordinate/locate the spot where external and internal holes will be when drilling through the upper hull for an antenna mount. Do not have 12' arms. so that is out. Thinking about magnets and iron stuff, but do not know how well that works.

In stuff that does not matter so much, a few exploratory holes to see how close you are to where you "think" is permissible. No way on a boat.

How do the masters nail down the location of the hole drilling without agonizing over not being in the right spot?
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Old 06-09-2015, 02:57 AM
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I'm far from a master so I'll be honest, I don't follow at all what you are trying to do. Please explain with more detail or take a pic of the area in question.
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Old 06-09-2015, 03:16 AM
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If you can't reach the backside how are you going to reach the cable to pull it through? If you are just talking about the mount and are going to have exposed cable running somewhere else you should try to put it where you can thru bolt it with washers on the back side. Antennas do bounce around and can tear screws out. Screws with adhesive around the mount can work but not as well. If screwing to the top of the gunnel there is rarely anything to hit and you shouldn't use longer screws than necessary.
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Old 06-09-2015, 03:23 AM
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Are you trying to drill thru an inner and outer panel with the 2 holes inline? Long drill bit. Home Depot has them up to about 4' long.

Antenna mounts should be thru bolted.
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Old 06-09-2015, 03:47 AM
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Originally Posted by Robert Mc View Post
In stuff that does not matter so much, a few exploratory holes to see how close you are to where you "think" is permissible. No way on a boat.

How do the masters nail down the location of the hole drilling without agonizing over not being in the right spot?


His drill bit is long enough to come through the other side -

the concern is exactly where its coming through

I've faced that situation - but I have no answer - other than agonizing.

Whoever installed my bow eye was "off" a tiny bit on the angle, drilling through the hull
Ended up without enough clearance inside the anchor locker for the full backup plate,
so he just hacksawed some of it off.

At least I don't have to look at it.
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Old 06-09-2015, 04:12 AM
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If I understand what you are trying to do, a SUPER bright light or laser can shine through some fiberglass. Do this at night or if a cabin boat, from the outside shining into a dark cabin. Don't know if you have just a thin wall of fiberglass though.
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Old 06-09-2015, 04:13 AM
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I think you need to use a laser grid. You set it up so that it will shine on the top and continue to the lower section, the rest is measurements.(in a semi dark environment.)
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Old 06-09-2015, 04:34 AM
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Measure from a known existing point(s) you can reach / see like a cleat bolt or the corner of the windshield etc. Triangulate if necessary.
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Old 06-09-2015, 05:26 AM
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Originally Posted by jcassara View Post
Measure from a known existing point(s) you can reach / see like a cleat bolt or the corner of the windshield etc. Triangulate if necessary.
You can do this, or, you could try rare earth magnets. I've seen them used to locate where to drill holes in floors. You put hold one on one side where you want the hole to come through, put the other on the side you will be drilling from. Sometimes takes 2 people, but, you can use tape to hold the first one in place until you get the second one stuck on.
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Old 06-09-2015, 05:28 AM
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Measure from a known reference point to get close, then use a household stud finder to get to the exact spot. Tape a 3/4" hex nut (not stainless) to one side of the hull, and the stud finder will find it from the other side very accurately.

I had to use this trick on the top hull (cabin roof) of my old boat to install a GPS antenna. Worked like a charm!

Good luck!
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Old 06-09-2015, 05:42 AM
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I think I get what the OP is saying. He wants to "mark" his hole from the top of cap or material, but drill from the other side. He can not reach the mark to drill from the the side that would make it easiest.

I think you might be on to something with the strong magnet/iron shavings idea.
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Old 06-09-2015, 10:33 AM
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Sorry it took a while to reply, time zone differences and all. Should have been more clear. here are a few pics (lousy) to try and clear up the issue.

First is my access inside the boat. Would like to bring the mount and VHF coax inside back in the dark area beyond the wiring. It is dark from the camera exposure and the unfinished hull back there.



This is roughly where I think is the corresponding point is on the exterior of the hull and the point of the question, making sure I hit the correct spot.

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Old 06-09-2015, 04:56 PM
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I used the magnet and iron shavings trick to check before drilling a hole thru the bottom of a hull. It worked for me.
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Old 06-10-2015, 11:38 AM
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Originally Posted by bj View Post
I used the magnet and iron shavings trick to check before drilling a hole thru the bottom of a hull. It worked for me.
That does seem to be the most fool proof way of doing it. Trying to physically measure inside and out looked like it was introducing parallax errors due to hull thickness.

Need to make sure I get behind that transverse bulkhead just a bit.
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Old 06-10-2015, 12:02 PM
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I think i would drill from the inside. At least the pilot hole You just might nick a wire drilling in. You should have a good idea of where the bottom of the console is from the outside.
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Old 06-10-2015, 12:26 PM
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I used a magnet and some u-joint bearing needles to locate my bow thruster hole on each side of hull skin. Worked like a champ.

On that deck cap, it might be thin enough for a laser pointer to shine through. Or tape a washer too it and a super bright light.
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Old 06-10-2015, 12:51 PM
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You might could fabricate something - imagine a square picture frame, where you remove the middle 1/3 of one side, and that middle you removed is a gap large enough to fit over the windshield and the frame would then wrap around inside the compartment, you'd be left with 2 stubs pointing directly at each other.

This could be pretty easily done with a large piece of cardboard and a couple strips of wood to ensure it remains straight.

This is hard to describe in words, hopefully you get the idea. Magnets sound easier though.
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Old 06-10-2015, 01:37 PM
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Here is a question. Can you see the windshield fastening bolts? I assume they are thru bolted. If so that cold give you a good reference to measure from.

It might also be easier to put the antenna further aft and might even look better. If you did that are there locations where you have better access to a flat mounting location on the gunnel and then just run the antennae cable forward inside the gunnel.

The magnet idea sounds interesting also.
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Old 06-10-2015, 02:21 PM
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Just looked for the windshield mountings yesterday and could only see the ones at the extreme ends. Looks like the glass gets bolted onto the mount last somehow.

Biggest issue mounting further aft is my extreme lack of flexibility working down there. Reaching even further back for handling the nuts over the bolts "might" be possible through that small access, but my 67 yr old bones ache at the thought of it. That is getting to the point of paying someone to do it for me.

Oh yeah, going much further aft with the antenna looks like it would cause interference with the Bimini when raised up.

Going to do some more internal and external measuring. The spot where the cleat is in a recess is very visible inside the compartment, so measuring back for there to the opening behind that bulkhead should show me clearly about where that is on the exterior. Will check for flatness of the surface as well.
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Old 06-10-2015, 02:32 PM
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I can't help you...but I can tell you how lucky I was once when I had to drill a hole through a bank vault. it was 18" and I only had a 12" bit.

The closest point of reference was the vault door and it was 8' away...

I figured the best way for me to hit the hole from the outside was to drill down or up or sideways..not try to hit the hole head on. It only took two holes from the outside and the second one hit it!
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