# Weight difference between 10m plywood and FG sandwich hull

Discussion in 'Boat Design' started by pironiero, May 24, 2022.

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### pironieroSenior Member

Does anyone know approximate weight difference between same 10m plywood and FG sandwich sailboat hull?

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### fallguySenior Member

Well, so much depends on methods and layups that the question is simply wide open.

You really need to have the two side by side specified hulls to do it well.

A person can easily compare say 9mm plywood with 300gsm each side to say a 12mm M80 foam core with say 800gsm each side. The plywood probably hand laid and the core say done with some vac. But it is a pure shot in the dark as to whether those two would be specified for the same hull(s).

Then okume or meranti? See?

You can do the math rather easily in metric.

Figure a 10m hull at 6m wide, so 60sqm per hull.

1600 gsm glass
70% under vac 1120 gsm resins
1020 gsm for corecell M80

I get hull weights, no tabbing at 448kg, which is, of course severely flawed no tabbing, but will get us a close estimate.

For plywood, 9mm okume is 28 pounds per sheet, after some conversion, check my math, 4.233kgsm, 0.6 kgsm glass and 0.6kgsm resin is about 652kg for same hulls in plywood..

plywood, as arranged, is about 150% foam, as arranged...

corrections welcome, darts not so much

This is not a question for Facebook, as I explained before.

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### gonzoSenior Member

A design optimized for plywood will use largely developable shapes. There could also be some tortured plywood in some sections. A sandwich construction would have convex and concave sections to increase stiffness. Unless you ask about a specific design, there is no answer. Also, it depends on how much local reinforcements are needed and whether the hull has framing or not.

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### fallguySenior Member

Fully agree. My example is rife with problems. If half of the weights in the boat are not changeable and that is half the vessel weight, the hull weights account for not a 1:3 reduction, but more like 1:6, but so up in the air as to be nearly nonsense.

I got into a great argument on Facebook with these gents about it being too open a question. My comment/reply was the best carrot I could offer.

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### jehardimanSenior Member

As has been said, there is too much variability to answer this question. FWIW, up to ~ 3m wood could probably be lighter. From 3m to 7m it would be a toss-up. Over 7m my money would be of FRP to be lighter.

For comparison, lets look at a wooden/FRP Thunderbird 26 vs a FRP Ranger (Mull) 26 or Colgate 26 all having about 310-330 square feet of sail area. The Ranger is a cruiser, the T-bird a weekender, and the Colgate a daysailer.
Thunderbird: Disp 1656 kg, Ballast 694 kg, Hull and outfitting 962 kg either wood or FRP
Ranger: Disp 2658 kg, Ballast 930 kg, Hull and outfitting 1728 kg FRP
Colgate 26: Disp 1179 kg, Ballast 476 kg, Hull and outfitting 703 kg FRP
The Ranger is more boat than the T-bird and a lot more than the Colgate though.

Last edited: May 24, 2022
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### Alan CattelliotSenior Member

Hi,

please find here a snapshot of my database of true EC-certified products in France (more than 1000 boats total).
For comparaison purpose, loadings are those in D category, though each boat could be also EC-certified in higher design categories !
Pink = Bois = Wooden craft (Strip planking, sandwich polyester/epoxy, molded all mixed)
Green = Unknown (not given in the database, but by the names not given here, I can say that this is glass polyester here !)
Red = Composite Verre Polyester
I have no much metallic boats in this region . I skipped it in the legend.

The shaded zone has been choosen to enclose all the EC certified products for a given length or weight. So these are real boats models, that have been examined through A1 B or G modulus, according to the European Directive 2013/53/UE. As you can see, the quality of construction and the navigation program are the main parameters that drives the overall and final weight difference between boats, in this lenghts interval. You may come to other interessant conclusions looking at this representation. Please tell me.

Cheers,

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### philSweetSenior Member

Another really good boat to use as a comparison is the 19' lightning. Designed in 1938 and still being built today, it can be found in double-plank hull, solid glass hull, cored glass hull, and recently cold molded bottom with plywood sides and deck. They are strictly weight controlled to be in class. The best boats of all types will have a small trim ballast, but not much. It mostly represents the weight savings in the modern rigs and sails. The hull bottom of the lightning is not developable, so the (gentle) compound curves provide the grp builds with just enough help in the stiffness department.

Given that each has to hit the same weight target, it is interesting to compare how the revised production plans since 1938 have done that. There are about 20 versions of the boat to compare. I believe the class weight is actually 20 pounds more than it was in the 60s - 700 pounds vs 680 back then IIRC.

<I initially misread the question, I thought it was 10mm (thick) ply, not a 10m boat>

Last edited: May 25, 2022
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### AlanXSenior Member

Did I miss something here?
The weight or displacement or mLDC (maximum displacement) of a boat is a design decision for that boat and not dependent on the construction materials.
Is not the question about the weight of the hull plating and the structural support members for a given design?

Gerr discusses this in his book "The Nature of Boats" in Chapter 47. He declares wood as the winner!
Simplifying his argument he uses Load/Weight as the criterion for the structural efficiency of the material.
For beams and panels (i.e. frames and hull plating) he ends up with this estimate for structural efficiency:
Efficiency is proportional to E^(1/3)/density​

Here is a table efficiency factors that I calculated for materials, this assumes solid skins and frames, etc:

But Gerr does not look at sandwich or composite construction.
It just so happens I did some calcs for an unstayed mast for different construction methods using wood as a core.
The results (unit weight of the mast) are near the bottom of the table.
All cases consider the same mast load (i.e. section modulus).
The table shows a Carbon skin wins even though it was held back by the wooden core, the results would be significantly better for composite construction if the right core material is used:

Regards AlanX

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### TANSLSenior Member

That is absolutely true, you have spoken as a great expert. Perhaps, however, it would be convenient to clarify that, with the same value of the mLDC, a wooden boat will be able to carry less pay load than another one made of aluminum and this one less than one made of PRFV.
But of course, it all depends on how you and D.Gerr measure the "structural efficiency" of a material.

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### jehardimanSenior Member

Yeah, it's a real pain when people only focus on the material properties, and not everything the material brings to the design.

Yes you did miss something. Gerr's estimate for is only valid for homogeneous flat plates in bending selecting for stiffness/weight. It does not consider curved surfaces, built up beams, composite skins (as you note), the shear modulus (G), the bulk modulus (K), or the strength/weight. In fact is says nothing about the suitability of a material for being made into the lightest weight hull. That requires detailed design analysis.
The selection of a particular material for any reason must be based on what it brings to the design. Your mast calculations are a perfect example. Hollow carbon wins, not because it is stronger or stiffer, but because it is lighter aloft.

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### pironieroSenior Member

Thanks for answering everybody, is an interesting read, but can you give me en answer? I know that hull type will change depending on material and it should be taken to account, let me simplify, how much lighter will 10m fg sandwich panel be than 10m plywood panel with same characteristics that would be used for building 10m sailboat hull?

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### gonzoSenior Member

There is no answer to that. A plywood boat is built of flat panels. A composite hull makes sense with compound curves. You need to first specify a design to get an answer. Also, cold molded is technically plywood. Are you also considering it?

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### fallguySenior Member

Provide hull specs and layup schedule.

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### Alan CattelliotSenior Member

Let's consider two hypothetic panels for comparison, together with a stiffener. We require that these stiffened panels should be ISO compliant, we minimize the acceptability factor with respect to the state of the art, and we calculate the weight ratio between the two options for bulk material. That should answer the question, although I doubt of any utility for this study, since very discutable architectural assumptions are made, regarding the general arrangement of the structure and the building process of such hypothetic panels . Disregarding the hull's shape also. I may also be strucked down by fine architects of wooden boats, who will certainly tell that, with a proper manufacturing, the numbers given for the wood are far to be the best.

According to ISO12215 these two panels should be equivalent for a 10m boat, it terms of ISO requirements. I won't precise the boat weight used for calculations, nor the design category of the boat, since, at this point of the conversation, Pieronero, I have no clue of what your are trying to demonstrate, asking this question. But if you could just for me, or for the others, explain your purpose or your point of view. Depending on your answer, it would be a pleasure to make the calculations for your own application (and arrange translations as well). If you are a student making search, please quote.

Estimate of the difference of weight between these two panels,
taking account into the weight added by a fraction of the stiffener's weight, since every stiffener is "shared" between several panels. Long and short stiffener are of the same composition. The weight of the bounding is not given here, and assume to be equal in the two cases. This implies real issues in the manufacturing of such panel in GRP. You've got here very high class GRP !!!

A GRP panel

A plywood panel

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### linda.vrdoljakJunior Member

If you intend to pass 12215 on 10m sailboat, take the (local here) salona 33 sailboat (parameteres 4.95tons 8knots Cat A) equasion is good up to 15knots racer... and 500x240 in grp can be done in 7mm csm (an example) no thiner to have some half decent 1.4x FOS and 7mm GRP with resin comes to 9kg, add 0.5kg gel and a sloppy builder, ~10kg 1m^2.

The birch is 7.6kg for 12.5mm, and 5.3kg for 9.5mm (1m^2), but who's going to maintain the wood and you need to seal it, more weight,
it could be one alternative for those without the mold.

If you think 4.95tons is heavy, german bavaria 9.7 has 5.2tons and lower specs.​

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