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Old 05-21-2016, 07:16 AM   #1
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Cored and Solid Fiberglass Hulls

I posted the below with pictures and a bit more on my blog, but it is simply too difficult to upload pictures to TF with crappy Wi-Fi.

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Cored and Solid Fiberglass Hulls

Knowing almost nothing about fiberglass, other than it’s made of fiber + glass, I have been talking to Gary Mooney, the GRP (fiberglass) expert of the area who has been working on Dauntless this winter and has a lifetime of experience with it on boats and all sorts of other objects.

We’ve talked about the repairs he made on Dauntless, first there were two problems in the hull:

1. The four-foot-long hairline crack that I put in the hull the past July in Finland.
2. An older, badly repaired, thru-hull fitting, also in the forward bilge, that was haphazardly done and allowed water into the hull and was the source of the water in the amidships-forward compartment bulkhead.

So this got us talking about the Krogen hull, in particular, which is a cored, also called sandwich, hull:

1. there is a layer of fiberglass,
2. then the core, in this case, a white non-water absorbing Styrofoam like stuff,
3. then another layer of fiberglass.
4. This is then covered by a gelcoat layer, making the fiberglass impervious to water.
5. Then a two-part epoxy coat is put on to protect the gel coat, Dauntless gets two coats of that,
6. A “Tie-coat” comes next, this tie-coat allows the anti-foul paint to adhere to the epoxy,
7. And lastly comes the anti-foul coating. I am going to try a semi-hard coating, purposely made for very slow boats like Dauntless. It’s said to last 5 years and be smooth enough to slightly reduce fuel consumption. I’ll be happy if it lasts three years and doesn’t hurt fuel consumption.

This boat yard really caters to the commercial boats, so things like the anti-foul, are all things the fishing boats and trawlers (real ones) use and like.

So, talking of hulls with Gary, I asked him about solid fiberglass hulls.

It’s clearly touted in the USA as a “better” meaning safer solution. He scoffed at that, saying that most of the fishing boats here use solid hulls to make them stronger in terms of cargo and heavy equipment, but it also makes them more fragile.

A cored hull has much more flexibility, thus I could hit a rock as I did and the hull flexed enough to crack both the inner and outer layers of fiberglass. Had the hull been solid fiberglass, it’s likely it would have broken in big chunks leaving a meter-long hole in the hull.

This happened recently to a FV just off the coast. Had they not been minutes from shore, they would have sunk. I on the other hand, carried on for another 3 months totally oblivious!

A reliable source tells me that Jim Krogen was always a proponent of the cored hull (sandwich construction) and only succumbed to public perception in the mid-90’s when they changed to making solid fiberglass hulls, below the waterline. Besides better shear strength (as my encounter with the rock showed), a cored hull also provides better acoustical and thermal insulation, when compared to solid fiberglass.

This past winter, sitting outside in the wind and rain, Dauntless was dry as a bone inside, while many other boats with solid hulls, had condensation running off the walls forming little lakes. My storm windows also helped in that regard.

Dauntless was no. 148 in the 42-foot series and was made in 1988. Newer isn’t always better.

Our hull above the rub rail to the cap rail, the gunnel, also has sandwich or cored construction, but in this case, the core is much thicker, made of blocks of balsa wood and has an inner and outer wall for added strength.

Also, cored hulls do provide additional buoyancy, if nothing else, slowing the sinking.
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Old 05-21-2016, 07:36 AM   #2
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Comparing the durability of cored and non cored hulls based on two accidents of different type boats is anything but scientific. Way too many variables to draw any conclusion. Destructive scientific testing of two identical boats (except for cored and non cored) would be the only way to remove all the variables.

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Old 05-21-2016, 08:20 AM   #3
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If I owned a cored hull it would be best too. But I own a solid GRP hull that the best designers of the time feel is the way to go, so that is best.

But wait, I have toyed with buying an Al vessel because many smart builders feel that is best. Now a fellow I know is selling his gorgeous Outer Reef solid GRP and has purchased a larger steel hulled vessel.

Oh oh, I am watching a very large cored hull Westport go through a rebuild and refit. The hull after about 20 years is pristine. I have heard all sorts of positive first hand reports from owners of Bayliners, Pacific Mariners and other Westport's stating no water intrusion.

But my first hand information from owners of KK42 cored hulls is opposite Orin Edson cored hulls. Whether KK owners are less attentive, boats are older, materials and design sub par I have no idea. But I do know two things. One is KK cored hulls were built in China whereas the other cored hulls mentioned above were built in the PNW.

Two, Jim Krogen built a KK 42 for himself. It is a solid hull. Alas he sold the vessel to a good friend of mine. BTW KK solid hulls are alive and well as are many older cored KKs.
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Old 05-21-2016, 08:47 AM   #4
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I once saw a KK in the yard for a very long time because the balsa core was saturated. IMO the boat was probably totaled but the entire bottom was removed and rebuilt. I have no idea of the cost.


It is my strong opinion that balsa coring does not belong anywhere on a boat. Synthetic coring that will not rot and wick water from a failure site to elsewhere is a different story.


I agree with the above statement that the two crashes don't prove anything about either type of construction. I have seen solid FG smashed and pushed in and it didn't break into pieces like glass but did leak.


You could argue that the bottomless KK I saw doesn't provide any proof either however it was apparent that rot had migrated across large areas of balsa. Supposedly with proper wetting out the balsa blocks are separated by resin but that puts us back to the manufacturing process control again.


There is great value for an engineering standpoint to cored construction. Like an I beam it is stronger and lighter than a solid structure, in normal use.


The big problem with coring is proper control of the manufacturing process and what must be greater expense of synthetic core vs balsa.


Personally I would never buy a boat with balsa core below the water line.
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Old 05-21-2016, 09:25 AM   #5
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Hull coring is used for several reasons. Those reasons include saving weight, building an adequately stiff panel with a minimum of fiberglass, saving money, stiffening large flat surfaces, etc. The basis of a cored panel is that there are two skins separated by some distance but connected to one another. This sort of construction gives a stiffer panel than an equivalent panel with the same material as the two skins. The connecting material between the two skins can be anything that is light. Core materials include balsawood, plywood, closed cell foam, and various honeycomb materials (paper, aluminum, plastic, etc.). The stiffness of a cored panel increases as the distance between the two skins increases. The "strength of a cored panel is limited by the shear strength of the core material and the bond between the skins and the core material.

Cored fiberglass panels generally have less penetration resistance than solid glass panels made from an equivalent weight of fiberglass. In that respect, when a boat hits a rock it is more likely to be holed if the hull is cored than if it is solid glass, assuming the two hulls were built to the same stiffness specification.

Cored panels lose their stiffness advantage if the skins become delaminated from the core material. Consequently, the details of the construction of the cored panel are very important in determining the longevity and quality of the panel. A properly built cored panel has zero air bubbles in the core-skin bond. In addition the bond should be made with laminating resin not some sort of filler which has less shear strength than resin since the overall strength of the panel depends critically on the strength of the skin-core bond. The best cored hulls I have seen had the core bonded to the outer and inner skins by vacuum bagging. In addition, the details of the glass reinforcements in the skins are important. A quality panel uses long fiber, glass reinforcements with the glass fibers oriented in directions appropriate to the anticipated flexural loads on the panel. A cheap panel with skins made from chopped strand mat is always an inferior panel.

As far as hull construction goes, there is generally little reason to save weight below the waterline. Consequently, the only reason to core a hull below the waterline is to make the hull stiffer. Note that stiffer is not the same as stronger. Weight saving become important above the waterline, so coring the hull above the waterline as well as coring the decks and cabin house components can give significant stability benefits.

As noted, cored hulls do have insulation benefits over solid glass hulls. However, wood hulls have the same benefits. On that basis you could argue that a wooden hull is superior to a solid glass hull. In fact, a properly made cold molded wooden hull does indeed have significant benefits over an equivalent strength solid glass hull. There are negatives too.

In summary, there is nothing wrong with a well made cored hull. The same applies to a solid glass hull. The cored hull is different from a solid glass hull, but is not intrinsically better. The quality of a hull (solid glass or cored) is all in the way the hull was built. Personally I would take a well built cored hull over a poorly built solid hull and vice versa.

By the way, "fiberglass" is made from glass fibers bonded together by a resin. The common resins are all plastics and include polyester, vinylester and epoxy (in order of increasing cost. Vinylester is better than poly ester and epoxy is better than vinylester. That said, there are reasons for using the different resins. In production boat building polyester is most commonly used because it is the cheapest of the three resins. It also has a much shorter cure time than epoxy which means that the hull can be built more quickly. Vinylester and epoxy resins have lower water permeabilities than polyester resins. That makes those resins better choices for boat hulls. In fact it is common for higher cost/quality boats to have outer layers laminated with vinylester resins for just that reason.
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Old 05-21-2016, 10:06 AM   #6
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Cored and Solid Fiberglass Hulls

It has been my experience that whether cored or not, it is 'aftermarket' hull penetrations and unrepaired (or incorrectly repaired) below waterline fractures that cause this argument/discussion.

A properly laid up hull, whether cored or solid with correctly constructed hull penetrations is just as preferable to a hull that is solid glass.

I specifically looked for a 'non cored' hull when I was boat shopping. Not because hulls with cores are inherently bad, but because you have NO idea what a PO or yard did to the hull in the 30 (or so ) years that made my particular boat affordable for me to purchase.
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Old 05-21-2016, 10:29 AM   #7
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About 20 years ago SeaRay began coring below the waterline on some larger models. Best I recall some vessels were bought back by the builder due to factory installed improper hull penetrations.

Google SeaRay cored hulls for some less than pleasant details.i believe our own boatpoker has some professional thoughts on balsa cored vessels
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Old 05-21-2016, 10:32 AM   #8
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Our last 2 boats have had cored hulls. A Slocum 43 designed by Stan Huntingford (10 years) and our KK42 (9 years). We have dealt with some of the issues associated with cored hulls, the biggest being poor hull penetrations, either from the builder or from previous owners. We've been fortunate in that all the early KK42's were built with Airex as was our Slocum and neither boat had been stored on the hard, in freezing climates.

Hull penetrations take a few additional steps to seal and strengthen the area. The Airex also adds additional hull thickness (~3/4"/20 mm) which means that you need longer transducers and thru hull stems.

If you have a cored hull with any moisture and blisters, it complicates the blister repair because now you have 2 mediums to dry out.

That being said, we've been very happy with both boats. You take the good with the bad.

Here's the hull layup for the KK42 before they went to solid fiberglass hulls below the water line.
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Old 05-21-2016, 11:44 AM   #9
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Indeed poor workmanship by the builder or subsequent work is the root failure of most cored hulls. How you verify prope workmanship long after the fact is beyond me. In my experience most failures can be traced to somebody not doing something right somewhere. A solid hull eliminates most of those concerns. Synthetic core at least contains the problem of moisture and is rot proof. Freezing problems after water penetration are another story.
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Old 05-21-2016, 11:48 AM   #10
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The Bayliner 4788 has a cored hull.

Unlike some famous builders, there has never been a hull failure of a Bayliner 4788 hull.

It's all in how they are built and the materials used.
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Old 05-21-2016, 11:58 AM   #11
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Quote:
Originally Posted by bayview View Post
I once saw a KK in the yard for a very long time because the balsa core was saturated. IMO the boat was probably totaled but the entire bottom was removed and rebuilt. I have no idea of the cost.


It is my strong opinion that balsa coring does not belong anywhere on a boat. Synthetic coring that will not rot and wick water from a failure site to elsewhere is a different story.


I agree with the above statement that the two crashes don't prove anything about either type of construction. I have seen solid FG smashed and pushed in and it didn't break into pieces like glass but did leak.


You could argue that the bottomless KK I saw doesn't provide any proof either however it was apparent that rot had migrated across large areas of balsa. Supposedly with proper wetting out the balsa blocks are separated by resin but that puts us back to the manufacturing process control again.


There is great value for an engineering standpoint to cored construction. Like an I beam it is stronger and lighter than a solid structure, in normal use.


The big problem with coring is proper control of the manufacturing process and what must be greater expense of synthetic core vs balsa.


Personally I would never buy a boat with balsa core below the water line.
Yes, I pretty much agree. One of the reasons i did not look at any Krogens built before the mid 80's. (or whatever the date was) I think that's when they switched from Balsa to Synthetic core.

The for-deck on this boat had been replaced because of core problems. Another reason I liked this particular boat.
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Old 05-21-2016, 12:03 PM   #12
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Originally Posted by Larry M View Post
Our last 2 boats have had cored hulls. A Slocum 43 designed by Stan Huntingford (10 years) and our KK42 (9 years). We have dealt with some of the issues associated with cored hulls, the biggest being poor hull penetrations, either from the builder or from previous owners. We've been fortunate in that all the early KK42's were built with Airex as was our Slocum and neither boat had been stored on the hard, in freezing climates.

Hull penetrations take a few additional steps to seal and strengthen the area. The Airex also adds additional hull thickness (~3/4"/20 mm) which means that you need longer transducers and thru hull stems.

If you have a cored hull with any moisture and blisters, it complicates the blister repair because now you have 2 mediums to dry out.

That being said, we've been very happy with both boats. You take the good with the bad.

Here's the hull layup for the KK42 before they went to solid fiberglass hulls below the water line.
Thanks Larry.

As I said, the cored hull works for me for two primary reasons:

1. higher shear strength (already proven I need that)
2. the thermal insulation is very important for me, in terms of condensation, I have none and the cold water travel I seem to like so much.
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Old 05-21-2016, 12:10 PM   #13
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Originally Posted by cappy208 View Post
It has been my experience that whether cored or not, it is 'aftermarket' hull penetrations and unrepaired (or incorrectly repaired) below waterline fractures that cause this argument/discussion.

A properly laid up hull, whether cored or solid with correctly constructed hull penetrations is just as preferable to a hull that is solid glass.

I specifically looked for a 'non cored' hull when I was boat shopping. Not because hulls with cores are inherently bad, but because you have NO idea what a PO or yard did to the hull in the 30 (or so ) years that made my particular boat affordable for me to purchase.
Yep. And in fact as both you and Larry have pointed out, the hull penetrations can be an issue is not done well.

My long term leak, was caused by the thru hull job and subsequent crappy repair.
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Old 05-21-2016, 12:29 PM   #14
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Yes, I pretty much agree. One of the reasons i did not look at any Krogens built before the mid 80's. (or whatever the date was) I think that's when they switched from Balsa to Synthetic core...
To my knowledge, Krogen never used balsa core below the water line on their 42 model. The drawing that I posted previously was from the Hull Structure Plan #: 353-5, checked and approved May 12, 1976. But then again...
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Old 05-21-2016, 01:28 PM   #15
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I can't answer cored vs. solid as there are way too many variables. To me, it's the quality of the builder and the hull that is important and there are good and bad quality boats both ways. Coring has also changed dramatically over the years. There are far superior materials to core with than balsa. Just as we've seen composites change over the years, we'll continue to see methods of coring evolve.
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Old 05-21-2016, 01:44 PM   #16
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I can assure you 100% Krogen hull #42-61 was synthetic core! I would know, I exposed every inch of the bottom.
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Old 05-21-2016, 01:45 PM   #17
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Coring has a big advantage in planing hull boats. Significant improvement in strength vs weight. That's one reason Carolina cold molded sportfishers have such a reputation for performance. Strong and Light.

But there has to be extra care in making penetrations, all of them. Not hard to do right, but it HAS to be done right.

I built my hull using cold molding, it's performance is way better than similar solid glass boats. I don't think I have any water intrusion issues, but who knows?? I was careful in all the penetrations and have not any hints of a problem, but I know the risk exists. A little nag always in the background...

But in a hull speed trawler, the added weight of solid glass has little burn rate penalty. Solid glass would be fine there. Never understood why KK and others did cored trawlers.
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Old 05-21-2016, 01:50 PM   #18
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I agree with those wise TF sages that have pointed out that cored or solid is less important than construction or the use the boat has seen.

Here is how a solid GRP hull handled hitting a large rock at 10 knots. It survived the encouter, sitting on the rock for a full time cycle, and then being drug off the rock by an overly enthusiastic tow boat operator looking for salvage. I have no problem with solid hulls. Cored is fine as well with the new coring materials.
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Old 05-21-2016, 02:16 PM   #19
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I'm not aware of any KK boats built with balsa core below the waterline. The earliest Manatees were also closed-cell foam. My decks are end-grain balsa cored and seem to offer pretty good resistance to any transmission of moisture through the core. In the case of deck delamination, it's possible for water to flow between glass and balsa layers, thereby affecting larger areas. On at least one plug I pulled from my deck, I found delamination of the glass and balsa core layers without any water intrusion. Once again, construction materials and techniques of the day had their good and bad points. The only water intrusion I've found on my hull has been easily traceable to defective thru-hull or hardware installation techniques.

Given a choice, I would have a quality-controlled cored hull with the latest materials and a single sea-chest. We've had both solid and cored examples and the minimal sound/temperature transmission of cored hulls are our preference.
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Old 05-21-2016, 02:43 PM   #20
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...My decks are end-grain balsa cored and seem to offer pretty good resistance to any transmission of moisture through the core...
This is a good application of balsa for coring IMHO. It sure beats plywood that a lot of our 70 and 80s boats have. On our last boat we had a soft spot by a winch with a balsa core. It was a limited area to repair. I saw a windlass leak that caused most of the foredeck to be ripped up, plywood core.
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