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  #11  
Old 06-12-2012
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Re: De-watering wet core

Quote:
Originally Posted by RichH View Post
As other posters listed ..... open it up and do it 'right'.

The pregnant question is: knowing that you cant completely dry out a deck core with a single stage vacuum pump (6" hg. vac.) and any remaining wetness will prevent the epoxy from bonding .... do you really want to do this knowing that it most probably wont work?
I agree. Open it up with multiple holes on each side and let time and sir dry it, unless there is rot, then you will have to take the core out and start over to have any reasonable result.

There is a lot of confusion in our boatyards about the difference between "wet" core and "rotted" cores. The latter will not "dry out" no matter how long you wait - it will always be soft and spongy, whereas the former will usually dry well and can be sealed and repaired.
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  #12  
Old 06-12-2012
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Re: De-watering wet core

Blue,

The only way to do it right is to cut open one layer of laminate and remove the wet core, replace it, and repair the cutout. Honestly it sounds more daunting that it is. A cutting wheel on an angle grinder will cut out the section pretty quick, the core comes ready to apply, and then you just need to bevel the edges of the old skin and reapply. Use epoxy for the work, and take it slow.

Trying to place a core under vaccume can cause major headachs, and really doesn't work. The thought behind a vaccume is that it will lower the boiling point of water enough that it actually boils in the skin and vaporizes, allowing it to be removed easily. In practice fiberglass doesn't hold a vaccume very well, and the core won't at all. Which means you have to pull a vaccume over the entire inside of the hull. And it has to be a very strong vaccume. Figure 29" or so to get the boiling point of water down to room temprature. This is roughly the same preassure used during vaccume infusion, and requires a very strong pump to hold it. Combined with the fact that every pin hole in the laminate, screw hole, ect can act as a leak, and has to be sealed... It just isn't worth the time.

If the area involved is very small (an inch or two around) you can bore out the core and replace it with thickened epoxy, but over large areas this is poor practice. What you are doing here is replacing the designed in core with an epoxy core. Which will be much stiffer and heavier than balsa/foam. Because of the differences in material properties you will generate major stress risers in the deck. If it has to ever absorb significant blown (like falling off waves) you can actually draw a line on the deck where the epoxy core is, and that is right where the laminate will break.
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Re: De-watering wet core

We are talking about water that took decades to get in and it is sure not coming out overnight
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Re: De-watering wet core

This thread seems to be a potent reminder why solid hand laid glass is the only way to go for a durable hull.

On our new 30 ftr the difference between a solid hull and a cored hull is less than 200 lbs. on a ultra high performance racer like the Schock 40 this extra weight might be a consideration, but on a performance cruiser - we think durability is more desirable.
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Re: De-watering wet core

You need the "Frogwatch Microwave Hull Dryer" guaranteed to drive water out of gelcoat and wet coring. Works just like a microwave oven does where the water molecule preferentially absorbs the microwaves.
Of course, I have not yet built it but i may try it someday. I got the idea when I was trying to dry the gelcoat of a boat once. Use a heat gun around a small hole and you'll be amazed at how much water it drives out.
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Re: De-watering wet core

[QUOTE=BlueBanana34;883608] . . .which seems to have quite a lot of wet core throughout.QUOTE]

Sounds like an extensive repair, not localized problems. Marine surgery likely
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Old 06-12-2012
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Re: De-watering wet core

Quote:
Originally Posted by WDS123 View Post
This thread seems to be a potent reminder why solid hand laid glass is the only way to go for a durable hull.

On our new 30 ftr the difference between a solid hull and a cored hull is less than 200 lbs. on a ultra high performance racer like the Schock 40 this extra weight might be a consideration, but on a performance cruiser - we think durability is more desirable.
The problem with solid glass layup is that regardless (not completely, but close) of the material a structure needs to be a certain thickness in order to provide reasonable stiffness. And fiberglass is heavy, really heavy, tremendously heavy, while core materials are very light weight.

So let's say you need a one inch thickness to get the required stiffness, making that out of fiberglass Will require nominally 25 layers of 24oz glass, with a weight per square foot of .33lbs/sq foot per layer. So 8.25lbs/sq foot of deck.

A cored deck however at one inch thick Would likely use a .75" thick core, with .125 inch skins. The nominal weight then of the skins would be .925 lbs/sq foot plus the core at .5lbs/sq foot (for balsa). So our cored deck weighs in at roughly 2.35lbs/sq foot. Or a little over 1/4 the weight of the uncored deck.

The right answer is not to refuse to use cores, but to treat them properly to prevent core rot problems.
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Sell it, buy a boat without a cored hull.
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Re: De-watering wet core

Quote:
Originally Posted by Stumble View Post
The problem with solid glass layup is that regardless (not completely, but close) of the material a structure needs to be a certain thickness in order to provide reasonable stiffness. And fiberglass is heavy, really heavy, tremendously heavy, while core materials are very light weight.

So let's say you need a one inch thickness to get the required stiffness, making that out of fiberglass Will require nominally 25 layers of 24oz glass, with a weight per square foot of .33lbs/sq foot per layer. So 8.25lbs/sq foot of deck.

A cored deck however at one inch thick Would likely use a .75" thick core, with .125 inch skins. The nominal weight then of the skins would be .925 lbs/sq foot plus the core at .5lbs/sq foot (for balsa). So our cored deck weighs in at roughly 2.35lbs/sq foot. Or a little over 1/4 the weight of the uncored deck.

The right answer is not to refuse to use cores, but to treat them properly to prevent core rot problems.
WDS talked about hulls, not decks.
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Old 06-13-2012
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Re: De-watering wet core

So a solid hand laid FRP boat will weigh about 4x's what a cored FRP boat ?

So if a Schock 35 weighs say 7,500 lbs then a Similar sized boat with a solid hull would weigh 20-25,000 lbs ?

Last edited by WDS123; 06-13-2012 at 01:02 AM.
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