fiberglass hull thickness

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.

 

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.

 

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.

 

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

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

 

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.

 

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.

 

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

 

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.

 

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?

 

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.

 

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.

 

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.

 

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.

 

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...

 

I would not say that "...the argument for no core is only valid under certain build methods..." but better I would say that certain building methods do need of core.
He wants core, ok that's fine but I'd like to know the reason for complicate so much a thin laminate with no heavy loads.