# Thread: how many idiots does it take to locate a point in the hull?

1. Junior Member
Join Date
Jan 2011
Location
Provincetown, MA
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## how many idiots does it take to locate a point in the hull?

trying to get the location of an exterior point in the hull's interior and vice versa. what's the most reliable way to do that? plan for drilling thru-hulls, and we will drill a pilot-hole just to be certain but we want to get a good idea of where things will be coming in/out. magnets and such are having only marginal results.

2. ## Re: how many idiots does it take to locate a point in the hull?

You have to measure from some known landmark that you can identify from both in and out of the hull, say the bulkhead at the back of the cabin. That might work best for the for and aft location. Maybe use a tube filled with water to mark the elevation. Good luck and be patient.

Cheers,

Bobby

3. ## Re: how many idiots does it take to locate a point in the hull?

Doesn't matter where it is on the outside, so long as the inside location is good. I would choose the position from inside to make sure it clears everything it should, then drill a pilot hole. Stick something through the hole to make it easy to find from outside, then finish up from outside in.

4. ## Re: how many idiots does it take to locate a point in the hull?

Originally Posted by outofthenorm
Doesn't matter where it is on the outside, so long as the inside location is good. I would choose the position from inside to make sure it clears everything it should, then drill a pilot hole. .........
One would reasonably think - but there might also be a design waterline issue, depending on what the hole is for (above /below/distance below?) - if so you'd need to establish a level also.

sayla

5. ## Re: how many idiots does it take to locate a point in the hull?

Don't drill the hole while the boat is in the water.

6. ## Re: how many idiots does it take to locate a point in the hull?

You can use the depthsounder`s hydrophone as reference point in and outside hull, work with a laser distance meter from there. Use a small perpendicular metal to catch the beam in and outside to make sure you got the right spot in and outside. Most metering devices have a scale to calculate or observe corning. Say hydrophone is 360 degrees, bearing distance of the shaft seal would be 188degree at 16' on a 33" cavalier. On a imaginary point, measure its relative bearing from a fixed point (hydrophone) perpendicular 90 degrees inside and outside.

I hope this makes sense, atleast this way i found a weak spot on my hull inside which was damaged on the outside
Last edited by Kwadrant; 06-14-2012 at 06:20 AM.

7. ## Re: how many idiots does it take to locate a point in the hull?

Use structural members as a reference--"3 planks up, 2 frames aft" etc. Or " 8-inches from the chine, 16 inches from the transom". Could also use fasteners as landmarks: "3rd plank, second bung..."

I second the advice to ensure clearance inside, and make sure you can position the seacock handle so you cam throw it open and closed.

Kevin

8. ## Re: how many idiots does it take to locate a point in the hull?

Originally Posted by Sayla
One would reasonably think - but there might also be a design waterline issue, depending on what the hole is for (above /below/distance below?) - if so you'd need to establish a level also.

sayla
Jeez you guys make things complicated.

Function comes first - will it be clear of obstacles and operate easily? Don't know where the waterline is? Look at the plans or find a landmark and make a simple measurement, as has been said.

The only design question to ask is if the thing is above or below the line. If it's below, is it deep enough to do its job when heeled? The answer to that is always "the deeper the better", but not too deep to reach easily. If it's above, "the higher the better", so long as you can reach it.

Lasers? Hydrophones? C'mon. This is dead easy.

Norm

9. Senior Member
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## Re: how many idiots does it take to locate a point in the hull?

When I was a kid my Dad gave me my Grand Father's old pocket compass that has a needle. Placing a small magnet inside or out side the hull on the planking and the compass on the other side has worked for me in the past for locating a point. The needle locks right on and when it is squared to the planking I know I am on center. Then I drill a tiny pilot hole from inside first just to make sure I like the posistion.
Jay

10. Junior Member
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## Re: how many idiots does it take to locate a point in the hull?

Great ideas. We have already laminated the exterior and thus couldnt count planks or fastenings, and since our first holes are for cockpit scuppers with enough slope in the tube and high enough off the waterline we only have a few inch window for our holes. we wanted to be certain of the exit hole location relative to the waterline which is most reliably struck on the outside of the hull. I have a pretty good approximation of the waterline from the interior but wanted to be certain. With that in mind we plan on drilling small pilot holes for every thru-hull.

We ended up getting a mechanical stud-finder (essentially a magnet on a swivel in an enclosure) and used a small, powerful magnet on the inside. A 2-person job to calibrate and make sure there isnt a random ferrous washer or something throwing us off. we tested it for a while, moving the magnet around and trying to pick it up with the finder. Always a few nerves when taking a drill to the hull but lets hope she pops out near the right place!

11. Join Date
Sep 2002
Posts
18,963

## Re: how many idiots does it take to locate a point in the hull?

Laminated on the outside? With what?

12. ## Re: how many idiots does it take to locate a point in the hull?

I recommend you commission a detailed laser scan of the interior and exterior of the boat relative to some common reference point. then you can determine precisely which interior and exterior points correspond.

Or, you could try stronger magnets....That worked for me...

13. Senior Member
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May 2000
Location
Cummington
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4,264

## Re: how many idiots does it take to locate a point in the hull?

Jay's method sounds perfect, but I relate this question to a feature of working on wooden boats that I call "PIGS IN SPACE". This involves my observation that whether it is finding corresponding points inside and outside a hull, or fitting a piece, you can not tell where you are or whether the piece will fit until it fits or you drill that hole. My chief consultant said "there are no problems only solutions".

14. ## Re: how many idiots does it take to locate a point in the hull?

Pffft! Just start drillin'! Don't you own any epoxy? Mistakes with a tiny pilot hole ain't no mistake at all. Don't make this harder than it has to be.

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