Hulls

Im not exactly sure of the meaning of " Fully Cored Hulls" as in a Krogan, a lot of talk of blistering. Any info would be helpful. Thanks,BB

Bill: *From what I recall, all Krogen Manatee 36's and many of the earlier 42's were of constructed with closed cell foam core. *While the closed cell type foam touts that it doesn't absorb water or deteriorate, lots of us Krogen owners are learning that if we don't keep up with hull maintenance, blisters will go beyond the little pockets of moisture they start as, and penetrate the hull where moisture will eventually collect in the core. *You can imagine the plethora of things that can occur when this moisture can't escape, not the least of which is mold and de-lamination. *Side by side 42 Krogens with cored and solid glass hulls are both good if maintained. *But if not, a top notch peal and re-glass job with a warranty could run 20K or more on the cored version. *Not a welcome surprise. *If you are looking at a Krogen, particularly a cored hull Manatee or 42*(or any cored hull, for that matter),*work with a surveyor that loves his moisture meter and hammer. *It will save you a bundle. *My two cents.

belizebill wrote:

Im not exactly sure of the meaning of " Fully Cored Hulls" as in a Krogan, a lot of talk of blistering. Any info would be helpful. Thanks,BB

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Ok, a cored hull has a layer of core material between the inner and outer fiberglass layers. Think of it as a sandwich with the core in the middle. The core material was originally Balsa wood and later switched to closed-cell PVC foam. The difference is that if you have water intrusion, Balsa will absorb water and rot. Closed-cell PVC foam isn't supposed to absorb water and doesn't rot. The issue with cored hulls has to do with delamination between the core and the outer layer. Also severe blistering may allow water to permeate the outer fiberglass and enter some core materials. If this water freezes, it leads to delamination by the expansion of the ice.


The value of a cored hull is that it makes the hull thick gererating strenth without the added weight and expense of a solid fiberglass hull of the same thickness.

The issues with cored hulls revolve around construction and fiberglass layer thickness. In the early days of cored hulls, fiberglass was still new technology and techniques hadn't been refined to allow the fiberglass layers to be thinner. As a result, the layers and amount of fiberglass were much thicker. The boat in my avatar is a 1976 Bruno & Stillman with a balsa cored hull, bulkheads, trunk cabin, and forward deck. There have never been any blisters in the hull, water intrusion, or delamination. The only water intrusion into the cored material occurred where the railing stations were though bolted through the deck. Clearly cored hulls make a lighter strong hull when propperly constructed. Unfortunately the issues with thinner fiberglass layers and / or poorly bonded cored hulls may take years to decades to materialize.

Regarding penetrations through cored hulls, there a very good techniques that add significant time to installations. If you are through bolting through a cored deck, you first drill the mounting holes. Then you drill the holes oversize (1/4" to 7/16"). Then you cover the bottom of the whole with tape. Next coat in the inside of the whole with epoxy. While the hole is wet , then fill the hole with epoxy putty (epoxy + fiber and fairing additives). When the epoxy is hard, remove the tape, sand smooth, redrill the mounting holes, and mount the item through the epoxy plug. For less structurally significant items (light fixture), you can drill a hole, tape the bottom, fill with epoxy only, pull the tape to let it drain when the epoxy starts to set. The purpose in both cases is to seal the core to avoid water intrusion from moving horizontally through the cored area, if the bedding compound fails on the item you are attaching. Below the waterline through hulls such as transducers and seacocks are different. If you were going to install a 2" seacock, you would first mark out an area maybe 8" square on the inside of the hull. Next cut though the upper fiberglass layer and the core layer, and remove all this material until only the lower fiberglass layer remains. Then you fill this area with layers of fiberglass or epoxy cloth or mat material until the square hole is built up level with the surrounding hull. Then the area is covered with a layer of cloth or biaxial material maybe 12" square. What you end up with is a solid fiberglass area in a cored hull to mount your seacock through. This area can't be crushed when the through hulls are tightened.* Also, the area around the through hull can be drilled if the seacock has mounting bolt holes to additionally secure the seacock to the hull. Some hulls (such as my Bruno) have these solid areas built in during hull fabrication.


If you buy a cored hull, you want to get a very through survey too make* sure there is no water intrusion.* Also, after purchase its a good idea to pull and likely replace all through hulls to make sure they go through solid glass areas........or you could wait to see if a problem sinks your boat at a later date.

*

Ted

healhustler wrote:

Bill: *From what I recall, all Krogen Manatee 36's and many of the earlier 42's were of constructed with closed cell foam core. *While the closed cell type foam touts that it doesn't absorb water or deteriorate, lots of us Krogen owners are learning that if we don't keep up with hull maintenance, blisters will go beyond the little pockets of moisture they start as, and penetrate the hull where moisture will eventually collect in the core. *You can imagine the plethora of things that can occur when this moisture can't escape, not the least of which is mold and de-lamination. *Side by side 42 Krogens with cored and solid glass hulls are both good if maintained. *But if not, a top notch peal and re-glass job with a warranty could run 20K or more on the cored version. *Not a welcome surprise. *If you are looking at a Krogen, particularly a cored hull Manatee or 42*(or any cored hull, for that matter),*work with a surveyor that loves his moisture meter and hammer. *It will save you a bundle. *My two cents.

Hi,*** I really like the salty look of the Krogen 42, and I do appreciate the practical "beauty" of the Manatee.** What year did Krogen switch from cored to solid glass?** I have had two cored hulls in the past. One an Albin 27 with balsa core and a Eddy & Duff Stonehorse with airex coring.* Neither one gave me any trouble, but just curious when the switch was made.*

PS- Your pilothouse looks awesome.

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*

Gee.* I remember in the 1960s that fiberglass boats were advertised as*"maintenance free."**But that was when comparison was mostly made with wooden boats.

markpierce wrote:

Gee.* I remember in the 1960s that fiberglass boats were advertised as*"maintenance free."**But that was when comparison was mostly made with wooden boats.

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Mark, is your steel hull cored?*


*

Moonstruck wrote:

Mark, is your steel hull cored?

*

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No, but I'm liking the idea of a steel boat more and more.*

*

markpierce wrote:


Moonstruck wrote:

Mark, is your steel hull cored?

*

*

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No, but I'm liking the idea of a steel boat more and more.*

*

Oh yeah, well just wait until your steel hull gets blisters!

*

Won't blister, but could rust, unless the coats of paint, now clearly being applied, are done really expertly with all the right stuff, which I'm sure is the case, Mark.

"The value of a cored hull is that it makes the hull thick gererating strenth without the added weight and expense of a solid fiberglass hull of the same thickness."

In addition it is a great noise barrier as well as at least some insulation.

The problems with cored construction is GOOD core material is not cheap.

So a Franz Mass racing boat will be using very different core material from a TT or other , built to very specific cost restrictions.

The second question is how was the core bonded into the hull when it was constructed?

The number of build techniques was amazing at the start of cored hulls , some worked better than others.

Lots of great information. Did they have cored hulls throught the production. Im looking at a Mannatee ,1984, Thanks everybodt,BB

The Krogen 42's*stopped coring below the water line around 1992 it looks like.

I'm*not sure*blisters would allow enough water*to enter a core to*cause a problem.***If the blisters are that bad, you have other*issues than just a wet core.**And moisture in the core is not just a below the water line*issue.* The biggest culprit of allowing water into the core*is the mounting of hardware above or*below the water line; swim platforms, grounding plates, thru-hulls, rub rails, etc.* We had water in the core of our last boat from the incorrect mounting of*an antenna mount when the boat was in the PNW for 10 years.****


Larry/Lena
Hobo KK42
Zihuatanejo, MX

O C Diver wrote:Ok, a cored hull has a layer of core material between the inner and outer fiberglass layers. Think of it as a sandwich with the core in the middle. The core material was originally Balsa wood and later switched to closed-cell PVC foam. The difference is that if you have water intrusion, Balsa will absorb water and rot. Closed-cell PVC foam isn't supposed to absorb water and doesn't rot. The issue with cored hulls has to do with delamination between the core and the outer layer. Also severe blistering may allow water to permeate the outer fiberglass and enter some core materials. If this water freezes, it leads to delamination by the expansion of the ice.




The value of a cored hull is that it makes the hull thick gererating strenth without the added weight and expense of a solid fiberglass hull of the same thickness.

The issues with cored hulls revolve around construction and fiberglass layer thickness. In the early days of cored hulls, fiberglass was still new technology and techniques hadn't been refined to allow the fiberglass layers to be thinner. As a result, the layers and amount of fiberglass were much thicker. The boat in my avatar is a 1976 Bruno & Stillman with a balsa cored hull, bulkheads, trunk cabin, and forward deck. There have never been any blisters in the hull, water intrusion, or delamination. The only water intrusion into the cored material occurred where the railing stations were though bolted through the deck. Clearly cored hulls make a lighter strong hull when propperly constructed. Unfortunately the issues with thinner fiberglass layers and / or poorly bonded cored hulls may take years to decades to materialize.

Regarding penetrations through cored hulls, there a very good techniques that add significant time to installations. If you are through bolting through a cored deck, you first drill the mounting holes. Then you drill the holes oversize (1/4" to 7/16"). Then you cover the bottom of the whole with tape. Next coat in the inside of the whole with epoxy. While the hole is wet , then fill the hole with epoxy putty (epoxy + fiber and fairing additives). When the epoxy is hard, remove the tape, sand smooth, redrill the mounting holes, and mount the item through the epoxy plug. For less structurally significant items (light fixture), you can drill a hole, tape the bottom, fill with epoxy only, pull the tape to let it drain when the epoxy starts to set. The purpose in both cases is to seal the core to avoid water intrusion from moving horizontally through the cored area, if the bedding compound fails on the item you are attaching. Below the waterline through hulls such as transducers and seacocks are different. If you were going to install a 2" seacock, you would first mark out an area maybe 8" square on the inside of the hull. Next cut though the upper fiberglass layer and the core layer, and remove all this material until only the lower fiberglass layer remains. Then you fill this area with layers of fiberglass or epoxy cloth or mat material until the square hole is built up level with the surrounding hull. Then the area is covered with a layer of cloth or biaxial material maybe 12" square. What you end up with is a solid fiberglass area in a cored hull to mount your seacock through. This area can't be crushed when the through hulls are tightened.* Also, the area around the through hull can be drilled if the seacock has mounting bolt holes to additionally secure the seacock to the hull. Some hulls (such as my Bruno) have these solid areas built in during hull fabrication.

If you buy a cored hull, you want to get a very through survey too make* sure there is no water intrusion.* Also, after purchase its a good idea to pull and likely replace all through hulls to make sure they go through solid glass areas........or you could wait to see if a problem sinks your boat at a later date.

*Ted

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Back in the 70's and*80's when C&C was the original company they built cored hull sailboats.

One of the thinks I remember about my boat was*there were places in the hull that were laid up different than the rest of the hull.* These places were where the factory had installed through hull fittings (engine pickup, paddle wheel water speed, Dept finder)*and there were a couple of others where the owner could do the same for say an A/C pickup.* At these places in the hull you could see where the end grain balsa*core material*had been left out and more glass mat had been laid up.* There was no mistaking the locations of these places.* The inner hull had a dimple (for lack of a better word) at that location.

I don't remember hearing of blister problems on the C&C cored hulls.* Mine had none.*

*

A core , even a budget choice does not cause blisters.

The quality of the resin , how clean the shop , and the production techniques used are the main cause of blisters.Curing moisture and temperature also count.

Quality counts , but since it can not be seen at new boat sales time , its Caviat Emptor!

I think most people will agree that cored below the waterline is a BAD idea.** Right or Wrong?

Egregious wrote:

I think most people will agree that cored below the waterline is a BAD idea.** Right or Wrong?

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My guess is it's like everything else.* It will depend on how well the hull was designed, the quality of the materials used, and how well the hull was manufactured.* And I suppose you could add in how well the boat was maintained by owner(s).

It's possible to screw up the manufacturing of even*a solid fiberglass hull.* American Marine did it with their Grand Banks boats.* "Screwed up" is probably far too harsh a*term, but after Howard Abbey left the company in mid-1974 (because AM was going bankrupt and Howard was worried about getting a paycheck), the company began having problems with its hulls.* Not super-significant ones but problems enough so that a few years later they asked Howard to come back and straighten out their manufacturing problems.* He did, and in an interview with him that I read, he said he found American Marine's hull-layup process to be " a disaster."*

The reasons are too much to go into here, but suffice it to say that poor manufacturing process, lower quality components, and so on can make any manufacturing method-- solid*fiberglass, cored, wood, steel, aluminum, composite---*less than desirable.

Egregious wrote:

I think most people will agree that cored below the waterline is a BAD idea. Right or Wrong?

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Wrong.

I think you can compare cored hulls to boats made of steel. Designed, built, and maintained correctly, they both have advantages and can have long life expectancies. As an example my Bruno & Stillman is 35 years old, and likely has run 3,000+ charter days in the Atlantic Ocean. The hull is still in great shape without core or blistering issues.

In contrast, there are cored boat models that are known to have issues either in fiberglass thickness or general construction. It is well known that some fiberglass hulls are likely to blister and others almost never do.

It's good to know if the boat you are interested in is known to have issues or not. In this economy with prices way down, a hull that's known to have issues can be had at a price that makes peeling and reglassing viable. One of the models I'm looking at is PVC cored and often needs to be peeled and reglassed. Cost runs between $20K and $30K. The end product is a hull that is significantly stronger than the original. If you can steal the boat for a great prices, it's a very viable option. I see this in the same way as buying a boat that needs to be repowered. If the savings on the boat is equal to much of the repower cost, you end up with a nice 20+ year old boat with a new engine.

Ted

Quality builders are moving to fully cored hulls.* Grand Banks new 41 is fully cored.* Here is a quote from a review.


*One of the most notable departures for the 41 was the use of resin infusion and a fully cored hull. Not only is resin infusion a cleaner method of fiberglass construction, the process allows for a more uniform layout with less labor. Quality control managers can also see if any voids or flaws develop during the infusion process. Better yet, it allows Alfaro and his team to control the weight of the build more easily.
This is a company that I would say, up until a decade or so ago, could be viewed as conservative. Yet it has embraced another departure in this model: the inclusion of fiberglass hull liners. In the past, Grand Banks would stick-build their boats. The liner eliminates unnecessary wood while reducing labor hours, a key step when building an "entry-level" vessel. When I worked for Grand Banks in the '90s, we wrestled with trying to continue production of the 36. However, the labor hours on a 36 were darn close to those on the company's 42. It just didn't make sense. With these types of building techniques, lower labor costs mean problem solved.



I believe that Marlo Marine is also doing fully cored hulls.* The new resin infusion and vacuum bagging layups insure good bonding.* The cored hull gets it strentgh not from the core, but by separation of the layers.* Think I beam.* The flange on the I beam is about as strong as a flitch plate, but the separated flanges give the beam its strength.* The farther apart the better.* When weight is put on the beam, one flange contracts while the other stretches.* You can think of the well laid up and bonded cored hull as a* WF beam or a very wide flange.* It is very strong.* Also quality manufacters are now using vinyl ester and even epoxy resins in the first 2 or so layers of the glass in the hull.* This holds down on water penetration into the lay up.* Thru hull penetrations below the water line are in solid glass areas put in place during the lay up.* It takes skill and quality materials, but you can wind up with a strong lighter weight hull.

Newer boats buit with cored hulls below the waterline might be reliable,
however if you get into a SeaRay or other boat built a while ago then what are you getting into then?* I think there needs to be a distinction between old constuction methods and new, like vaccum bagging.
I am by no means an expert on constuction, but I have read that cored construction below the waterline is a bad idea since one ounce of H20 that soaks into it will compromize your hull.*
If I am wrong no problem.*

Egregious wrote:



Newer boats buit with cored hulls below the waterline might be reliable,
however if you get into a SeaRay or other boat built a while ago then what are you getting into then?* I think there needs to be a distinction between old constuction methods and new, like vaccum bagging.
I am by no means an expert on constuction, but I have read that cored construction below the waterline is a bad idea since one ounce of H20 that soaks into it will compromize your hull.*
If I am wrong no problem.*

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Nothing wrong with your thinking.* Stay with what you are comfortable with.* However like you say the new bonding methods and materials are better.* I would have no problem with a new GB 41, and it has pod drives.

*

The biggest issue with cored hulls, after one assumes it was properly constructed at the factory, is what happens down the road. All it takes is one overly confident owner to try and do a DIY project like a thruhull adjustment or something and poof- the integrity is shot and you have a big problem. I personally would shy away from a cored hull. 2 boats ago I owned a cored hull boat and had no issues (it was a 2003 model year) and it was a great boat for it's purpose but now I would not buy one. Maybe that is b/c I just recored the foredeck and bridge deck of our trawler and saw first hand how bad a deck is impacted by a yahoo not doing things properly when they install something on the decks. Do that below the water line and your boat is shot.
I will have to look into whether Marlow uses a cored hull. If so, sadly, I will have to take them off my list of boats to buy "when I win the lottery."

And Woody- you are right about your statement of old techniques vs. new. Even with the new though you still have the issue of what happens to the boat after it leaves the factory.

"All it takes is one overly confident owner to try and do a DIY project like a thruhull adjustment or something and poof- the integrity is shot and you have a big problem."

This actually depends on the core material selected for the build.

Balsa is always a really poor choice , but used because of price.

With AIREX even poor owner workmanship would not put the hull at risk.

Other core materials might or might not.

Belizbill: *Here's a helpful article about the character (and arguments) about the different cores of decks and hulls. *One builder may condemn one while another swears by it. *The all have different effects on strength, flexibility, performance and durability. *On my other computer (still in transit from Germany) I had a great article written by Shionning Designs on the practical applications of each, but I can't find it on the net anywhere. *I'll send it to you when the rest of my stuff arrives. *Meanwhile, here's the link to this one:

http://www.powerandmotoryacht.com/boat/hard-core-facts/

Thank You HH, Bill