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Old 13-01-2013, 18:30   #1
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Cored Deck and Hull Stories?

Cored decks and hulls are often touted as giving great strength while saving weight. Different types of core materials are said to have different characteristics.

Some express concerns regarding core materials' tendency to soak up water and lose strength as well as gaining weight. Others tell stories of cores that separate from the fiberglass that surrounds them.

What experiences have CF members had with cored hulls and decks?

How serious are problems?

Are they limited to certain boats and/or manufacturers?

What solutions have been tried? With what success?

Should a buyer steer clear of cored hulls and/or decks?
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Old 13-01-2013, 18:44   #2
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30 years in the sea and rain with balsa core hull, topsides and deck. No problems. Raced hard. Well made. See older threads.
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Old 13-01-2013, 18:47   #3
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Re: Cored Deck and Hull Stories?

Cored deck and hulls. No problems.

It'l be interesting to see how many members have had any problems, or if they just "know someone who knows someone..."
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Old 13-01-2013, 18:49   #4
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Re: Cored Deck and Hull Stories?

Have replaced acres of rotten core professionally. Organic materials and water don't mix.
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Old 13-01-2013, 19:09   #5
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Re: Cored Deck and Hull Stories?

Mine could have been a disaster if I hadn't started on/bought it when I did. Had to cut out and replace balsa under a winch and padeye, and reinforce around the keel stepped mast where the halyard blocks were fastened on deck.
But all is well now.
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Old 13-01-2013, 19:17   #6
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Re: Cored Deck and Hull Stories?

Cored hull and deck on 1984 Pearson. Just had the hull pealed and barrier coated. When the bottom paint and gel coat was pealed you could easily see through the transparent glass layers to the balsa core in the hull. There were no dark areas or any visible evidence of water intrusion in the hull, verified by moisture meters both on the original survey and during the peal and seal.

The hull was done properly IE every spot with through hulls or fittings the hull was solid glass.

There are a few spots on the deck, mainly where the stanchion bases are through bolted where there is some moisture in the core but no delamination. I'm in the process of drying, filling and redrilling all the deck fittings to keep it that way.
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Old 14-01-2013, 07:00   #7
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But then there is the boat a few moorings away here in PG. Balsa cored, but some romantic fool put down a teak deck over the perfectly functional FRP deck. At least a few of the 6000 screw holes must have leaked. A disaster. Good employment for the crew....
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Old 14-01-2013, 07:30   #8
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Re: Cored Deck and Hull Stories?

Most cores work well and are longlasting if constructed and treated as per the manufacturers specifications . The majority of problems occur when fittings are incorrectly installed IE holes not cored for screw fixings or holes drilled or cut wthout the edges being cored and filled. or in some instances when fixings are overstressed

It can take hours /days to prepare these products to receive fittings that only take a few minutes to install on other surfaces.

Once a fitting is installed it is sometime difficult to reconise a fault until real damage has occured and any type of damage can allow water ingress through the core, this often results in harder to repair damage as the water heats up it expands in the core often debonding the core material from the finished surface each side.

Most cores organic or otherwise require simular treatment but some perform better when it comes to denting or crushing the surface from day to day use
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Old 14-01-2013, 07:36   #9
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Re: Cored Deck and Hull Stories?

Have had 2 boats with cores with no problems.

I think that probably most boats are cored somewhere, so since most don't have a problem ......
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Old 14-01-2013, 07:59   #10
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Re: Cored Deck and Hull Stories?

Some F-boats have mixed core, some areas are balsa, some are foam. You'd think it was odd, but balsa has superior strength characteristics, as I understand it.

My F-boat had balsa around the area where the chainplate tang runs through the ama deck. Area got soft, let it go more than I should because the chainplate doesn't actually attach there, it just runs through a 'slot' in the deck and bolts to a bulkhead beneath.

Cut the top layer of the laminate, which is fibreglass, off (the skin). Dug out the rot in the central layer (balsa). Replaced rotten balsa with foam core. Laid back in the top layer, faired in. No big deal.

Like anything else, core, whether foam or wood, needs care.
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Old 14-01-2013, 08:21   #11
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Re: Cored Deck and Hull Stories?

Most fibreglass boats have a cored deck. Look for boats that use solid glass around all the penetrations, and /or inspect very carefully.
Personally, for cruising, I would avoid boats with a cored hull. There are too many cases of delamination and/or water intrusion. These are generally very serious problems.
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Old 14-01-2013, 09:24   #12
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Re: Cored Deck and Hull Stories?

If you want to take no risk that you'll ever have a problem, by a boat with a solid fiberglass hull, like an Oyster or a Beneteau or any of the production monohulls.

If you want a boat which is lighter, stronger, and quieter, and with less condensation/heat loss, buy a boat with a cored hull, like Swans, Hallberg-Rasseys, and all catamarans.

My present boat is my first with a cored hull. The outer skin is Kevlar, and the core is made of encapsulated balsa blocks. I was quite nervous about this when I was buying my boat, but the more professional advice I got, the better I felt. Apparently encapsulated blocks is a particularly reliable (and expensive) method which isolates the effect of any leak through the outer skin.

We shall see. I don't think it is without risk -- sometimes such systems just don't work out the way the designer intended. As someone said, if a cored hull goes really bad, you're really screwed. Ask any SeaRay owner from the 1990's.

I can say, however, that the difference in strength is remarkable. My present boat's hull seems like the Brooklyn Bridge -- no flexing, no creaking, groaning, or squeaking even under the most horrendous sea conditions, quite unlike my previous boat with its very heavy solid fiberglass layup (4" thick in the bottom). You could hardly open the doors below in a seaway in that boat.

Also makes a big difference in speed. Cored hull despite being stronger weighs tons less. That gives my boat a racy D/L ratio of about 200, which really shows in the sailing qualities.

A guess a reasonable approach might be to avoid cored hulls in cheaper and older boats, but maybe consider them in higher end boats, after a thorough survey.


As to cored decks, you have little choice. I haven't seen a GRP sailboat made in the last 30 years which didn't have cored decks. Here again quality is key. A well built boat will have plywood reinforcements in the deck in highly stressed areas, and will be solid glass in places where there are penetrations. A poorly built boat, or one which has had holes drilled in the deck in random places, will have problems sooner or later.
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Old 14-01-2013, 09:42   #13
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Re: Cored Deck and Hull Stories?

Owned a Kevlar honeycomb boat for 15 years (30 years old). Many blocks and go-fasts added. No problems. Penetrations were not cored and sealed unless very high load; core type is waterproof.

Own current cored boat (solid below water line, foam for deck and hull, balsa for some bulkheads) for 5 years (16 years old), no problems, no soft spots, no leaks. Most penetrations cored and sealed.
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Old 14-01-2013, 09:51   #14
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Re: Cored Deck and Hull Stories?

We have a NZ boat from the mid 80's. Built properly its fine

Balsa hull below the waterline to a solid glass strip down the center of the hull.
encapsulated blocks of end grain balsa so if water gets in it can't travel. Vinylester on the outside of the hull below the waterline (no gelcoat)

Balsa Deck, solid glass around windows, chain plate holes, stanchion bases etc
plywood core in high stress area's i.e. winches
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Old 14-01-2013, 09:53   #15
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Re: Cored Deck and Hull Stories?

Given the choice. I prefer Foam core as it will not absorb water if drilled improperly.
There is a correct process to seal holes when adding hardware to a boat with coring.

Here is a picture of the foam core in my hulls above the water line only. You can actually see the blocks that form the bridges
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