How much deflection is normal in Fiberglass Deck

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halsteadmorris

Seaman Apprentice
Joined
Mar 10, 2021
Messages
41
Hello, all!

I purchased a wellcraft 2002 22WA about 6 months ago. This is my first fiberglass boat, but I have been around boats for years.

I’ve noticed a small amount of bounce in the middle of the floor by the stern. It’s not noticeable unless you are feeling for it. I do not see any support underneath this section.

I know what is too much flex/sponginess in a boat, but I’m not sure what would be considered normal? Absolutely solid as a rock no give whatsoever? Or is a slight amount of deflection in the center of the floor normal on some models?

The stringers on this boat are foam so I am not worried about damage to them.

I appreciate all the input. Thanks
 
M

MikeSchinlaub

Senior Chief Petty Officer
Joined
Jan 14, 2025
Messages
795
If it's a deck like this internet pic...
wellcraft-22wa-1597147-330882986.jpg

Then I would expect it to be pretty solid. Is this soft spot in the panel over the gas tank?
 
Texasmark

Texasmark

Supreme Mariner
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Dec 20, 2005
Messages
14,981
Probably, being over the tank, and being a cover probably some flexing would be assumed as there probably isn't much reinforcing. The question I would ask myself would be: Does it matter if it caves in a little under weight? Probably not. Maybe you would stand on the cover and bend your knees and then straighten out your legs rapidly a few times in succession. Listen for cracking noises. If it doesn't flex that much...3/8" for a number, and doesn't make a crackling noise when you exercise it then I wouldn't worry about it. If it does deflect/crackle, I'd glass in some reinforcing horizontal multiple ply plywood (cabinet grade) strips, 2 or 3 and be done with it.
 
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halsteadmorris

Seaman Apprentice
Joined
Mar 10, 2021
Messages
41
Texasmark said:
Probably, being over the tank, and being a cover probably some flexing would be assumed as there probably isn't much reinforcing. The question I would ask myself would be: Does it matter if it caves in a little under weight? Probably not. Maybe you would stand on the cover and bend your knees and then straighten out your legs rapidly a few times in succession. Listen for cracking noises. If it doesn't flex that much...3/8" for a number, and doesn't make a crackling noise when you exercise it then I wouldn't worry about it. If it does deflect/crackle, I'd glass in some reinforcing horizontal multiple ply plywood (cabinet grade) strips, 2 or 3 and be done with it.
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It’s definitely not 3/8”. Maybe 3/16” at the most. Sometimes I will hear a pop - but it’s hard to get it to do so. That definitely concerns me.

Would you glass in the reinforcement from underneath? Not sure I have enough access to do so. Ill take a look into that when I replace the rotten coffin box hatch and have material laying around.

Thank you for the advice
 
Texasmark

Texasmark

Supreme Mariner
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Dec 20, 2005
Messages
14,981
halsteadmorris said:
It’s definitely not 3/8”. Maybe 3/16” at the most. Sometimes I will hear a pop - but it’s hard to get it to do so. That definitely concerns me.

Would you glass in the reinforcement from underneath? Not sure I have enough access to do so. Ill take a look into that when I replace the rotten coffin box hatch and have material laying around.

Thank you for the advice
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"Would you glass in the reinforcement from underneath?" That ought to work and if there is room, install "ribs" of some sort to get some height prior to installing the glass mat. Height in the "deflection" equation goes up as the cube of the parameters, making it your most effective dimension.
 
Bugzilla46310

Bugzilla46310

Petty Officer 3rd Class
Joined
Dec 4, 2025
Messages
81
Texasmark said:
You are showing your ignorance.....

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That price is outrageous. When my coworker ordered his ME handbook around 1980, they sent him 2 by accident so I scooped up the extra one just for be such a great guy! But it still makes my brain hurt also!!
 
dingbat

dingbat

Supreme Mariner
Joined
Nov 20, 2001
Messages
17,164
Bugzilla46310 said:
That price is outrageous. When my coworker ordered his ME handbook around 1980, they sent him 2 by accident so I scooped up the extra one just for be such a great guy! But it still makes my brain hurt also!!
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Many things have changed in the past 40 years….lol

Still have my 8th edition Mark’s handbook in my library, but has long since superseded by subsequent editions. Currently on its 12th edition.

As a general reference, the Machinery Handbook is a far more practical publication for non-engineering types.
 
D

DeepCMark58A

Captain
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Aug 17, 2015
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Texasmark

Texasmark

Supreme Mariner
Joined
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Messages
14,981
DeepCMark58A said:
Isn't is funny how some consider it ignorance when people do not know what you know.
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My initial reply was a result of the comment contents (see his post #8) of the participant's initial response to my post! The subject is not "Witchcraft" and any person involved in any kind of stress mitigation is aware of the relationship of distance, height, and width in their calculations.
 
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