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Old 08-30-2017, 10:14 AM   #1
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Tips on cored hulls

Well this is a bit of a bummer. I just found out we have a cored hull. Balsa wood I assume. We bought the boat thinking it was a solid hull. Whether the survey said one way or another, I don't know. I may dig it up later. Soon after purchasing our 2000 Navigator 4200, we put her in the yard to get all the running gear balanced, aligned, and trued. During the time out, I had the yard install a thru-hull for later use as a anchor washdown. I had the yard save the plug for me. Now, this has been almost a year and a half and I stumbled across it just last week in the garage. I decided to throw it under a sander to see how it looked. Low and behold, there is about 3/4" of balsa sandwiched between two layers of something like 3/16" of glass. (not actual measurements)

Let me be clear on this: While we are a little concerned about the idea of having a cored hull (especially since I have, for the past 2 years, lived under the idea we had a thick and solid one), we can't, at this point, do anything about it. Nor will I up and sell it over this discovery. What we DO need to know is what kind of precautions, if any, should we take to prevent future problems?

We are needing to put her in the yard for bottom paint in the next few weeks, so if there is something I could do while she is out or just generally moving forward... I am all ears (or eyes in this case).

Thanks.
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Old 08-30-2017, 10:24 AM   #2
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To give you peace mind, the only thing I would do is remove all the underwater fittings and make sure that the core is sealed with epoxy or glass and reinstall the fittings with good sealant.
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Old 08-30-2017, 10:25 AM   #3
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I would ask the yard if, when they put in the thru hull, they cut back the core and sealed it with thickened epoxy. If not, get that done only this time, cut back any wet core all the way to dry material.
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Old 08-30-2017, 10:38 AM   #4
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Oh yea... good idea.
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Old 08-30-2017, 11:03 AM   #5
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Cored hulls aren't to be feared. That said, every penetration above and below the waterline should be checked annually. Water penetration is much more common above the waterline most likely because builders take it more seriously if it can sink the boat. I have a 40+ year old charter boat with a cored hull that's doing just fine.

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Old 08-30-2017, 11:26 AM   #6
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Again for piece of mind....... A good man with a moisture meter can check around thru hulls, swim platform, and the like for moisture next time the boat is out of the water. The thru hulls don't have to come out to check. If you find signs of moisture you can pull the thru hull, de-core around the perimeter, fill / seal with epoxy, and reinstall. Chances are that you will be ok. The Surveyer should have found any soft / wet spots when doing their "tap test"
As a side note there are boats out there with wet cores that will outlast you and me.
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Old 08-30-2017, 11:42 AM   #7
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If it is dry and well bonded now, it should not delaminate and you should be safe. Have a surveyor check for moisture content if you have any doubts .
Problems if any come from poor bonding between skins and core or from improper accessories installation.
As the others have said, it is very important to pay attention to the core when installing anything on the hull or deck: cut/drill oversize, use high density inserts, fill with epoxy putty (not polyester) and drill/cut to fit. Cored boats delaminate because of improper accessories installation.
See this page that I wrote many years ago for foam cored hulls:
Foam Sandwich Construction - Page 10 | Bateau2
Backing plates etc. are on page 10.
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Old 08-30-2017, 01:56 PM   #8
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Ditto all comments re: verifying that the hull core at through hulls is sealed. Sealed means sealed with an area of solid laminate around the through hull, or with an annulus of epoxy installed into the bore. It's quite possible that the original-to-the-boat through hulls were done that way during construction; the manufacturer would have known where the hull needed to be penetrated. Thus, upon checking (by removing and looking, or querying the mfr) one of the original through hulls and seeing that it's done correctly you might reduce the number of through hulls you remove and check/redo.

Anybody can buy a moisture meter; not just anybody knows what he's reading. Anybody can rap a hull with a hammer; ditto.

We looked at a mid-80's Pearson racing sailboat a good while ago. We could see all the Balsa blocks telegraphing through the hull(!). We took that as a bad sign; most hull skins are thick enough to not telegraph the delamination at bad blocks/core.
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Old 08-30-2017, 08:07 PM   #9
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3/4" balsa with 3/16" skins is a pretty robust laminate, especially for a 'slow' boat. If properly done, of course. Check the detail areas as others have mentioned and enjoy the benefits. Solid hulls aren't the only good ones.
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Old 08-30-2017, 09:29 PM   #10
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Quote:
Originally Posted by chriscritchett View Post
3/4" balsa with 3/16" skins is a pretty robust laminate, especially for a 'slow' boat. If properly done, of course. Check the detail areas as others have mentioned and enjoy the benefits. Solid hulls aren't the only good ones.


Well... I ain't got no slo boat no mo... I am a planing motoryacht now. Either way... I just measured it and that was a good guesstimate by me and those are the exact measurements. I have a pic, but I can't post pics from the app.

So, like I said, we need to go up on the hard. One of the projects will be to pull the two raw water strainers to change the backing plates to replace the wood over to strengthened fiberglass. I will take a look at these factory thru hulls then and see what I find. I may also have our surveyor to come tap the hull and meter it just to give it a checkup.
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Old 08-30-2017, 09:58 PM   #11
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Interesting corporate history of Navigator if anyone is interested.....

http://www.navigatoryachts.com/uploa...-navigator.pdf
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Old 08-31-2017, 01:23 AM   #12
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I would ask the yard if, when they put in the thru hull, they cut back the core and sealed it with thickened epoxy. If not, get that done only this time, cut back any wet core all the way to dry material.

Just like this.

You can also think about refining the base, the new epoxy primer 600-800my the whole bottom, ensuring that water does not pass through it to glass fiber and core filling. It's a big job to do, scrape the old paint out, the easiest syringe to blow soda or sand. First paint the epoxy 600-800 microns of thickness about 6 x overlay and finally antifouling. This work ensures that water does not penetrate the glass fiber.

If you are going to basically renovate the base, all the thrue hull penetrations with epoxy and waterproof flexible mass gaskets.

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Old 08-31-2017, 01:32 AM   #13
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Originally Posted by Easting View Post
Again for piece of mind....... A good man with a moisture meter can check around thru hulls, swim platform, and the like for moisture next time the boat is out of the water. The thru hulls don't have to come out to check. If you find signs of moisture you can pull the thru hull, de-core around the perimeter, fill / seal with epoxy, and reinstall. Chances are that you will be ok. The Surveyer should have found any soft / wet spots when doing their "tap test"
As a side note there are boats out there with wet cores that will outlast you and me.
Infrared Marine Thermography Surveying The heat camera detects the moisture meter more effectively in the moisture content and reveals a potential problem, may be
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Old 08-31-2017, 02:33 AM   #14
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Whew! With a solid steel hull, almost all I need is to worry about is rust, which has a difficult time to hide.
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Old 08-31-2017, 03:47 AM   #15
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Whew! With a solid steel hull, almost all I need is to worry about is rust, which has a difficult time to hide.
I'm sorry, but can also be a bad or poor welding quality steel plate, etc. which ruptures from too much stress.

whether your shipyard NTD has measured all welds?

I've unfortunately seen many poorly welded seams and piping in ships hull. These words do not mean to criticize your ship, it is hopefully well welded and all substantively treated to prevent corrosion. Which materials have their own good and bad points ...

Ship and Navy yards, weld quality - productivity issues with management and engineer resolutions.


And another pretty mystic thing is the microbes that enjoy lunching on weld seams


http://www.google.fi/url?sa=t&rct=j&...mvrE_0PDkLsj6A
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Old 08-31-2017, 05:34 AM   #16
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Not all cored boats have problems. It depends on what core was chosen.

The house plywood in so many TT decks and PH does cause problems if items are not rebedded as a PM,instead after the sealants have failed.

Balsa core with a good builder and yard can be fine , just requires care when creating new holes in the boat , or fastening new items.

A high quality ( read expensive) foam like Airex is a delight with no long term problems.

Find out the core construction before worrying.
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Old 08-31-2017, 05:39 AM   #17
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I'm sorry, but can also be a bad or poor welding quality steel plate, etc. which ruptures from too much stress.

whether your shipyard NTD has measured all welds?

I've unfortunately seen many poorly welded seams and piping in ships hull. These words do not mean to criticize your ship, it is hopefully well welded and all substantively treated to prevent corrosion. Which materials have their own good and bad points ...

Ship and Navy yards, weld quality - productivity issues with management and engineer resolutions.


And another pretty mystic thing is the microbes that enjoy lunching on weld seams


http://www.google.fi/url?sa=t&rct=j&...mvrE_0PDkLsj6A
WOW- Hold the phone, there's a huge huge difference between a Steel vessel under 60 meters when compared to over 60 meters, Under 60 meters theirs normally no real issues with Hogging/flexing longitudinally but over 60 meters it is a real issue (Lloyd's) Under 20 meters it's un heard of!

Pound for pound metal is far stronger than wood or fiberglass. Unlike wood and glass, which have most of their strength oriented along the lay of their grain or fibers, metal is equally strong in all directions. Metal is so tough one needn't worry about wasting strength because of this, and much trouble is saved because the material can be laid down any which way. In a wood or glass boat, by comparison, designers and builders must always take care to ensure that material is aligned along anticipated load paths.

Now re the Bacteria report? In reality here's a few words from that same report Quote: The Microbes do not eat metal themselves! and really only most common in fresh water applications.

To end my short rebuttal please note the picture below from a severe collision with a larger vessel- The steel yacht didn't even leak!!

Cheers Steve(MIIMS-Lloyd's Maritime) and Steel vessel builder of 40 years
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Old 01-17-2018, 11:55 AM   #18
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**UPDATE**

I wanted to update this thread. I WAS WRONG!!! Our hull is NOT cored. A few weeks ago, Bess reminded me that the core slug pictured above was from the install of the anchor washdown deck connection. After a little searching I was able to track down the hull sample from the thru-hull for the same system. Below are the pictures of the CORRECT slug.

Sorry to all the potential Navigator owners that I mislead and asked me offline about potential purchases.

"These aren't the droids you're looking for... Move along."
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