Tool to measure thickness - Woodworking Talk - Woodworkers Forum
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post #1 of 42 Old 08-15-2020, 09:58 AM Thread Starter
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So let’s say you need to measure the thickness of something where you have access to both sides, but they’re physically separated by partitions and other obstructions.

In my case, I want to measure the thickness of the fiberglass in a boat hull down by the keel. I can access both sides, but there are many obstructions that would prevent the use of any normal types of measuring tools.

I’m thinking of a tool similar to those that are used to measure paint thickness on a car, but of course this will be much thicker.

Is anyone familiar with such a tool? (That’s not insanely expensive)

Last edited by Quickstep; 08-15-2020 at 12:25 PM. Reason: Meant to say both sides instead of not sides.
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post #2 of 42 Old 08-15-2020, 11:05 AM
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I'm a little reluctant to suggest this because I'm neither an expert with fiberglass or boats. But, it seems to me that you could drill a very small hole (maybe 1/32") where you want to measure, insert a fine wire with a stop on one end, mark it, remove it, and measure it. Then, use a hypo to inject some fiberglass resin to seal the hole. Surely this wouldn't do any harm. Would it?
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post #3 of 42 Old 08-15-2020, 11:15 AM
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I would ask why do you need to know this ?
do you have plans for a hull penetrating project ?
let's say - you found such a tool to measure the thickness.
how often would you use it ??

.

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post #4 of 42 Old 08-15-2020, 11:24 AM
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Do not drill holes in boats below the water line.
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post #5 of 42 Old 08-15-2020, 11:26 AM
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This was my first thought also

Quote:
Originally Posted by yomanbill View Post
I'm a little reluctant to suggest this because I'm neither an expert with fiberglass or boats. But, it seems to me that you could drill a very small hole (maybe 1/32") where you want to measure, insert a fine wire with a stop on one end, mark it, remove it, and measure it. Then, use a hypo to inject some fiberglass resin to seal the hole. Surely this wouldn't do any harm. Would it?

Unless you run into a bracing rib, you should get a pretty accurate reading. The hole will be small enough to not cause any issues and if you fill it from both sides, it should seal up quite well. Filling it from one side won't work because of the double layers. Try tapping with a small hammer in the general area where you want the measurement and listen for a change in sound, "hollow" being the best.


If all else fails, use ground penetrating Radar.......

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 08-15-2020 at 12:51 PM.
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post #6 of 42 Old 08-15-2020, 11:54 AM
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Quote:
Originally Posted by Ranger652 View Post
Do not drill holes in boats below the water line.
it should say: people that don't know what they are doing should not drill holes below the waterline.

#1 - the boat must be out of the water on a trailer before drilling - or else:
I have put dozens and dozens of holes in wooden, fiberglass and aluminum boats
below the waterline with no issues.
but - I learned from the experts on maintenance and alterations of boats.
#2 - proceed at your own risk.

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post #7 of 42 Old 08-15-2020, 12:10 PM Thread Starter
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Quote:
Originally Posted by John Smith_inFL View Post
I would ask why do you need to know this ?
do you have plans for a hull penetrating project ?
let's say - you found such a tool to measure the thickness.
how often would you use it ??

.
John,

I’m looking to install a garboard drain in my boat. It will go fairly deep in the bilge where the fiberglass is at its thickest. I need to make sure I can get a garboard drain with a sufficiently long stem. If I can’t, I’ll give up on the project. If it’s not too thick, drilling a hole is a fine way to go. If it doesn’t work out, I’d rather not make a penetration if I don’t have to.

This will be the third time I’ve drilled a (large) hole in the bottom of a boat and it seems like “measure a hundred times drill once“ is a good way to go.
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post #8 of 42 Old 08-15-2020, 12:13 PM Thread Starter
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By the way, my guess is that the fiberglass is probably somewhere between 3/4-1”. I need to make sure it’s not something like 2”.
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post #9 of 42 Old 08-15-2020, 12:20 PM
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ah so - do you have the drain on hand now ? or do you have to get one
that will fit your requirements. I guess you know they come in brass, plastic, etc.
a long stem can be trimmed down to fit the project. one too short will not work.
I normally work with 1/2" or less thickness with good results. a little 3M marine caulk
and it's good to go.

my last "problematic" project was in my old wooden Chris-Craft cruiser.
I had to move the fresh water intake for the engine due to rot around
the fitting. the only place I could move it to went from 3/4" bottom to
1.5". so I not only had to patch the old hole, I had to drill a new one with
a new fitting. you just have to be resourceful when it comes to boats.
(of course the boat was out of the water and on a cradle during this project).

can you share your project with us ??
I for one am very interested in projects like this.

.
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Last edited by John Smith_inFL; 08-15-2020 at 12:28 PM.
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post #10 of 42 Old 08-15-2020, 12:38 PM Thread Starter
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For an underwater penetration, I’m not comfortable with plastic. I’ll be using either bronze or stainless. The longest I’ve found is 1-1/4”, so if the hull is thicker than that, I’ll go another route.
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post #11 of 42 Old 08-15-2020, 12:44 PM
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https://www.google.com/search?q=garb...h1fAPl6SmwA462

On my sailboat, I had a drain which operated when the boat was on the plane or sailing fast.
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post #12 of 42 Old 08-15-2020, 01:22 PM
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Quote:
Originally Posted by Quickstep View Post
So letís say you need to measure the thickness of something where you have access to both sides, but theyíre physically separated by partitions and other obstructions.

In my case, I want to measure the thickness of the fiberglass in a boat hull down by the keel. I can access both sides, but there are many obstructions that would prevent the use of any normal types of measuring tools.
One should be able measure this from each side and then compute the thickness.

(A) establish two parallel planes or lines (string lines will work or two straight boards) located above and below the area you wish to measure. Measure this distance, this will be dimension "A".

(B) cut a template which will allow you to bypass the obstructions. I've shown a simple arc but any shape will do (it can have multiple bends as long as it is a rigid template) and you can verify the distance from end to end (the contact points) of the template. This will be dimension "B".

(C) Measure the distance from the lower string line to the bottom of the hull where you wish to know the thickness. This will be dimension "C".

(D) Using your template, position the lower contact point at the desired location, swing the upper contact point back and forth as required to obtain the shortest measurement between that contact point and the upper string line. This will be dimension "D".

(E) Now you have all the information needed to compute the actual thickness.

Sounds a lot harder than it actually is. This is very similar in how a CMM (coordinate measuring machine) can reach in and around a casting or other object to get measurements. Only difference is you are substituting the template for the CMM capabilities.
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post #13 of 42 Old 08-15-2020, 01:53 PM
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Nothing against "contractions" ^ , but ......

I could drill a straight pin sized hole through the hull, insert a straight pin, snip it off flush, withdraw it and measure the length just about as quickly as I am typing this..... just sayin'


A drop of clear nail polish would seal it or other fast dry adhesive.
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post #14 of 42 Old 08-15-2020, 02:17 PM
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Quote:
Originally Posted by woodnthings View Post
I could drill a straight pin sized hole through the hull, insert a straight pin, snip it off flush, withdraw it and measure the length just about as quickly as I am typing this..... just sayin'


A drop of clear nail polish would seal it or other fast dry adhesive.
Yes anything could be done. Some folks pound nails into the drywall to find a stud. Then hide the holes with a picture frame.,,,, just saying.
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post #15 of 42 Old 08-15-2020, 03:41 PM
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if you use a fitting with brass stem an extension can be brazed onto it, should your hull be too thick..
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post #16 of 42 Old 08-15-2020, 03:43 PM
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Actually, you can drill thru a hull while in the water, and install a through hull or similar. It was explained to me long ago by a diver. A toilet plunger was part of the procedure.
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post #17 of 42 Old 08-15-2020, 04:04 PM
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There is a thickness meter you can buy for under $100, but that's a lot of money for a one time use. Plus I don't know if there's a calibration for fiberglass, mostly used for metals, I believe.

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post #18 of 42 Old 08-15-2020, 06:01 PM
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Check any tool website for a thickness caliper. Bowl turners use them to check the thickness of their turnings.
I made my own , simple actually. Their are two S shaped pieces that are connected in the center. The caliper looks like the figure 8. You use it by opening the caliper, slide over the area to be measured and then close the caliper. The end not on the wall thickness will be open the amount of thickness of the wall.
I made my calipers from scrap Masonite . I used a compass to draw two circles ,one above the other to resemble an 8.
I then drew another pair of circles inside the first, about 3/8" smaller. This represents the caliper width.
I made a pattern of one half of the figure 8. Two pieces were sawed out and connected together with a bolt and nut.
Do a search for thickness calipers, Penn State Industries is one of many websites that show this tool. I have seen this tool made from the wire from a coat hanger. Do not recall how the two pieces were fastened in the center.
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post #19 of 42 Old 08-15-2020, 06:05 PM
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Quick - I think it is high time for some photos of your boat !!!
(and perhaps a photo of the stern with an arrow to where the drain will be).

.
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post #20 of 42 Old 08-15-2020, 07:21 PM
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Quote:
Originally Posted by John Smith_inFL View Post
.

Quick - I think it is high time for some photos of your boat !!!
(and perhaps a photo of the stern with an arrow to where the drain will be).

.
All good John but I want to see a photo of those boat hull thickness calipers being suggested. Yes I do have, and know what bowl thickness calipers are. Just never seen ones designed for a boat hull. Ya I know, don't give up my day job.
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