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Old 16-10-2021, 00:38   #1
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Negatives of full Fibreglass hulls/decks?

I've noticed a few older fibreglass boats being just fibreglass with no timber or ply. Apart from being heavier what other negatives would apply for a cruising boat?
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Old 16-10-2021, 03:31   #2
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Re: Negatives of full Fibreglass hulls/decks?

For a cruising boat hull, not really a negative. You want a heavy hull, and if the glass is thick enough to be stiff without being cored, it's thick enough for the occasional log-bump. I've never seen a solid FG deck--they're always cored in my experience, but it could be done.
Some will say: "the core adds insulation against condensation." Meh. Not usually enough to make a difference--it's better to simply insulate between the hull and hull ceiling (whatever the final liner of the inside of the boat is), and make it good and thick.
The disadvantages to a cored hull, however, are multifarious: lighter skins means more easily pierced and harder to repair. Installing through-hulls and hardware requires extra care and a longer process; if the core is anything other than foam (like timber or ply), water intrusion can be a slow but sure catastrophe.
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Old 16-10-2021, 03:49   #3
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Re: Negatives of full Fibreglass hulls/decks?

Older boats of solid glass were an indication the builder / designer did not yet understand how to build from thermoset plastic.

Pre fuel crisis of the early 70s resin was cheap and the skill set required for complex build techniques was rare so chopped strand solid layups met the time, cost, quality, strength, weight equation compared to more traditional and labour intensive construction methods.

Post fuel crisis the resin was more expensive and formulated differently. This seems to be the era where osmosis became a thing.

By the mid 80's composite construction had evolved to cover a range of construction technics from budget amateur to high end use of exotic materials and techniques e.single use male plugs with foam core hand layed e-glass through to honeycomb cores vacuum bagged and autoclaved using prepreg Kevlar & Carbon often with bias or unidirectional reinforcements

Core selection was important and some of the timber based cores (ply & balsa) have failed the test of time where Airex, Divinycal & Nomex seem to be doing well.

Wood and ply have their uses but the applications where they are preferred over a composite construction are limited.

Diagonal construction sheathed in glass is one application - essentially a composite construction.

Wood does look nice and no matter how much I improve with resin infusion I can't make Carbon Fibre and Kevlar look as good as teak.
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Old 16-10-2021, 03:56   #4
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Re: Negatives of full Fibreglass hulls/decks?

Pretty much exactly like Benz said. An uncored hull is more flexible, and requires more care in design and construction to achieve an adequately stiff structure overall, but it can be done. Stringers, bonded bulkheads, integral furniture, etc all need to be part of the package.

A cored hull is lighter for the same stiffness, but can have its outer skin holed by an impact that would bounce off a sold hull.
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Old 16-10-2021, 04:01   #5
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Re: Negatives of full Fibreglass hulls/decks?

[QUOTE=Rucksta;3502936]Older boats of solid glass were an indication the builder / designer did not yet understand how to build from thermoset plastic.
QUOTE]

If you are suggesting that solid fiberglass has no advantages over cored construction at all and has no place in any boat built today…. Let’s just say that would be a minority opinion among knowledgeable sailors, designers and builders.
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Old 16-10-2021, 06:03   #6
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Re: Negatives of full Fibreglass hulls/decks?

[QUOTE=BillKny;3502939]
Quote:
Originally Posted by Rucksta View Post
Older boats of solid glass were an indication the builder / designer did not yet understand how to build from thermoset plastic.

QUOTE]



If you are suggesting that solid fiberglass has no advantages over cored construction at all and has no place in any boat built today…. Let’s just say that would be a minority opinion among knowledgeable sailors, designers and builders.


Solid grp in decks would be way too bendy and would require wooden boat style supports under it. Which is why it existed for a while as early builders didn’t understand if.

Cored is far more rigid
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Old 16-10-2021, 06:06   #7
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Re: Negatives of full Fibreglass hulls/decks?

[QUOTE=goboatingnow;3502984][QUOTE=BillKny;3502939

Cored is far more rigid[/QUOTE]

If they are lighter, would they also be faster yachts with a nice fin keel to go to windward
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Old 16-10-2021, 06:11   #8
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Re: Negatives of full Fibreglass hulls/decks?

Some early fiberglass boats did have solid glass decks. But as others have said, it's heavy and requires more support underneath for stiffness. Solid glass hulls are more common. They're heavier for a given stiffness compared to cored, but harder to damage, particularly from impacts.
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Old 16-10-2021, 08:29   #9
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Re: Negatives of full Fibreglass hulls/decks?

I owned a solid fibreglass boat and found that the decks were fine and no work to keep clean. No problems at all.,
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Old 16-10-2021, 08:42   #10
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Re: Negatives of full Fibreglass hulls/decks?

[QUOTE=BillKny;3502939]
Quote:
Originally Posted by Rucksta View Post
Older boats of solid glass were an indication the builder / designer did not yet understand how to build from thermoset plastic.
QUOTE]

If you are suggesting that solid fiberglass has no advantages over cored construction at all and has no place in any boat built today…. Let’s just say that would be a minority opinion among knowledgeable sailors, designers and builders.
If you actually read what he's saying it's clear that some places, like decks, solid fiberglass is almost never the best solution. That's not a minority opinion among knowledgeable sailors, designers, and builders....in fact solid uncored fiberglass decks are used in a distinct minority of boats built today. I'm not actually aware of any over 30', are you?
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Old 16-10-2021, 09:04   #11
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Re: Negatives of full Fibreglass hulls/decks?

Quote:
Originally Posted by Benz View Post
For a cruising boat hull, not really a negative. You want a heavy hull, ....
Well, this is an opinion and one I take exception to. You want a strong hull especially below the waterline. But if you want 'heavy' just for weights sake, then steel and fero-cement boats would be much more popular. Heavy does not equal comfort. Comfort, or the boat's motion, depends on where the weight is located as inertia is primarily what determines period & amplitude of motion and weight high, like a deck, is no bueno. Heavy also means slower, which means you are subject to worse weather on passages as you have 1) less ability to get out of the way and 2) will spend more time at sea 'waiting' to get hit. Heavy also means more fuel consumption, and therefor more cost and more pollution. I also believe solid glass layups are more prone to cracking/failing at hard points compared to a cored hull but it really depends on design. Heavy hull to some extent means a bigger sail plan to push it. That means bigger lines, bigger winches, bigger everything (unless you want to go slower)... that means higher cost of maintenance and replacement and more difficulty in handling the bigger rig.
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Old 16-10-2021, 09:10   #12
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Re: Negatives of full Fibreglass hulls/decks?

[QUOTE=Pete7;3502987]
Quote:
Originally Posted by goboatingnow View Post

If they are lighter, would they also be faster yachts with a nice fin keel to go to windward
Sure now you have a faster boat but disadvantage of fragile hull and removable keel.
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Old 16-10-2021, 09:23   #13
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Re: Negatives of full Fibreglass hulls/decks?

[QUOTE=BillKny;3502939]
Quote:
Originally Posted by Rucksta View Post
Older boats of solid glass were an indication the builder / designer did not yet understand how to build from thermoset plastic.
QUOTE]

If you are suggesting that solid fiberglass has no advantages over cored construction at all and has no place in any boat built today…. Let’s just say that would be a minority opinion among knowledgeable sailors, designers and builders.
Note the two words in the quotes highlighted with bold type and you may see the suggestion is yours.

Modern solid construction includes woven, bias and unidirectional material and techniques like bonded or tabbed grid structures.
Resin to glass ratios are better understood and controlled as is the need to eliminate voids in the layup.

Even with these improvements full solid construction is rare.
Solid construction below the waterline and a variety of techniques for hull and deck is more common.

The OP was asking about older solid construction.
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Old 16-10-2021, 09:53   #14
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Re: Negatives of full Fibreglass hulls/decks?

My boat is built with a solid hull and a solid deck and she is an ULDB. The cabintop is cored, though.

The hull has longitudinal stringers and the deck a fiberglass web instead of core. It is not the easiest way of construction, though.

Once upon a time, few boatbuilders cored hulls under the waterline. The only good reason to do this is to save on material. Today it is becoming common among budget builders of cruising yachts. And it probably works as long as the outer hull is not damaged.

I see no drawback to solid hulls at all. And if you have any kind of impact, you will be happy you have one.
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Old 16-10-2021, 10:14   #15
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Re: Negatives of full Fibreglass hulls/decks?

RE: CONDENSATION. Solid fiberglass hull, cored deck (lauan mahogany). No insulation. Winter in Venice, 1999-2000. After years in the tropics and a couple of winters in milder climates (Whangarei, Marmaris), we were naive about winter conditions. Layer of bubblewrap on deck from dodger forward. Ice inside hatches, so newspaper and bubblewrap inside, which helped. Closed off aft cabin and used borrowed "oil heater" (5 amp electrical on dock) to heat the rest of the interior. No refrigeration, so put food in aft cabin (milk froze). Should have used shore power for dehumidifier instead: VERY wet inside. Had to improvise drying compartment in sailing club's work shed to save books. No Dry-Bunk so put newspaper under mattress and changed it every morning. By the time we installed a Webasto heater, it was spring. Condensation is a serious nuisance in an uninsulated boat.
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