# fiberglass hull thickness

Discussion in 'Boat Design' started by Gianf1041, Dec 18, 2019.

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### Gianf1041Junior Member

Yes, ....... yes you're right, everything is still preliminary, I'm working on it, like the rest of the project I have in mind; the attachment I posted is a working sketch.

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### Gianf1041Junior Member

I empirically evaluated the hull / deckhouse weight 700 kg to this weight I added the various components: wheelhouse, engine, 50% fuel, 50% drinking water, batteries, kitchen stove, toilet, anchor, chain, windlass, etc. up to about 1400 kg.

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### TANSLSenior Member

Imo, an average thickness of 20 mm, for an 8 m boat, is very much. If you are calculating the weight according to that thickness, it is very likely that you are penalizing the boat too much.

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### fallguySenior Member

Provide your assumptions for the hull so TANSL is not guessing!

If you are estimating 7mm core and 11mm glass; it is still wrong.

Last edited: Feb 6, 2020
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### TANSLSenior Member

I am not assuming, the OP asks us about the weight of a 20mm medium thickness hull. That is a very specific figure that leaves no room for assumptions.
Nor do I see what need the OP might have to use sandwich panels in the hull of an 8 m long boat. But, of course if, for some unknown reason, he had to use thicknesses of 20 mm, the best would be the sandwich panel.

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### fallguySenior Member

TANSL-I am referring to this post where he asks for the stack to achieve 18mm thickness with a 7mm core.

I am only trying to help. Maybe I am not. But a 7mm core and 11mm of glass/gelcoat is absurd. So I am only curious if he has changed these assumptions.

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### TANSLSenior Member

Sorry @fallguy, my mistake and you are right, this sandwich is absourd.

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### Gianf1041Junior Member

If I add 1000/1100 kg of ballast I get closer to the displacement.
As for the thicknesses I'm reviewing them .........
At the beginning of the project I thought of the sandwich in total with thicknesses from 20mm to 9mm, but I think to review them and evaluate thicknesses of 15 mm in solid for the keel region and 10/7 mm for the other regions. For clarity I am attaching an explanatory leaflet not translated into English.

#### Attached Files:

• ###### hold thicknesses.pdf
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337.8 KB
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### TANSLSenior Member

Just to help you. Could you explain to us what criteria you follow to determine that the thickness should be 15 mm, for example, and not 19 mm?

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### fallguySenior Member

You are still using more glass than core in the drawing.

You have sketched a 7mm core with 6mm of glass each side. This is not correct. The core provides stiffness.

A 7mm core is not stiff enough to produce a flat panel except on a mould or table, so building these may also be trouble. In other words; you cannot build it on a jig or stations as the core is so flimsy it would require a nearly solid structure or one with stations at say 1/2 meter intervals and battens with spaces of not more than 6". And I am just guessing. But basically, the jig is like a full boat.

I am sorry I cannot point out your direct misinterpretations. But in general, the core is thicker than the skins.

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### Gianf1041Junior Member

As I reported in my previous post I am reviewing everything; ......... for the calculation of the "average thickness" in monolithic fiberglass I am using a simplified formula derived from "rules of the thumb" by Dave Geer for the sizing of the hulls, the rule is a summary of what Gerr writes.
thick = (3.28 x Loa) raised to 0.5 + (Boa x 3.28 x 1.58) ............. upper topside 15% less than the "average thickness" , lower topside and bottom 15% more than the "average thickness".
Internal structure with at least 5 bulkheads and 5 stringers per side.
I point out a text by an Italian designer: Paolo Lodigiani - "Capire e progettare le barche - materials, construction, dimensionings ".............this designer follows the" false line "of Gerr's texts, unfortunately he is in Italian. I hope I have translated correctly.
The attachment of the previous post describes my evaluations at the beginning of the idea. I am evaluating differently.

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### fallguySenior Member

I am fine with 18mm total thickness, but you would arrive at it by recognizing a thicker core and thinner skins. And then adjusting for nominal core dimensions. For a 12mm core, 3mm glass/gel per side is a reasonable guesstimate.

I think you need to explain your core/glass assumptions for other members.

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### TANSLSenior Member

You will know, gentelmen, how to excuse me for totally disagreeing but I have projected fishing ships (very hard work) of 7.40 m in length that had a 12.5 mm keel and an 8.5 mm bottom / side. So the thicknesses that you are proposing seem to me, with all due respect, nonsense. And, if there are no reasons that I do not know, I would not go to a sandwich type laminate because, with these thicknesses, it is not necessary.
If I am saying something totally out of place, please let me know.

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### fallguySenior Member

well, build method, for one...

it was assumed by me he had some reason to use a core

Rather than adding more confusion; stick with the idea he wants core versus mostly glass and a thin core...or now forgetting core...the argument for no core is only valid under certain build methods...