Rob:
    These numbers put the cost of construction through the roof, and vary radically from the figures you gave me in a previous post.    25mm core material is $217.52 usd per sheet in case lots (12 sheets).... probably about $15k in just foam to build the boat I was planning to build.............


     Below is a quote from a previous post......... your words in red & blue.   The pertinent portion in blue.   You suggest that 6mm foam sheathed with 200 on each side equates to 6mm gaboon and that 10mm sheathed with 600 outside and 400 inside equates to 9mm gaboon.   In the the more recent text below in black, you suggest a 25mm core, gsm of the fabric not specified, but presumably 750 or 800, as replacement for 6mm ply.    The figures in blue and your figures from the more recent post are in such wild contradiction that I find it difficult to make sense of it all...............

                                                                                                                H.W.

Depends what you call "really useful". The weight difference is 526 kgs/1,157 lbs/just over half a ton, using the numbers below. The empty weight of the boat is 1,800 kgs/3,960 lbs, so the saving is 29%.

The materials list for Woods' 8.8m Saturn specifies (ex waste)
10 sheets 6mm gaboon ply @ 9 kgs per sheet =90 kgs for 30 sq m
52 sheets 9mm gaboon ply @ 13 kgs per sheet =676 kgs for 156 sq m

The exterior of the 9mm is sheathed with 300 gsm cloth and 600 gsm resin (includes filler coat and bog) = 156 sq m x 0.9kgs = 140 kgs.
The interior of the 9 and both sides of the 6mm will/should get 3 coats of epoxy at 8 sq m/litre /coat (Gougeons) = (156 + 630 + 30) x .4 = 86 kgs
Total = 992 kgs

If we leave the stringers in and size the foam/glass for the same stiffness as the ply, we have
30 sq m of 6mm foam with 200 glass each side 30 x (0.5+.2+.2+.2+.4) = 45 kgs
and 156 sq m of 10mm foam with 600 gsm glass outside and 400 inside =156 x (.8+.4+.6+.5+.4)= 421 kgs.
The numbers in brackets are the weights per sq m of the foam+ inside glass+outside glass+infused resin at 1:2 resin fibre+resin to wet the foam both sides.
Total = 466 kgs

Sorry about the metric numbers. It is much easier to work this stuff out using them. 200 gsm glass is 6 ounce, 400 12 ounce, 600 18 oz.

There are a number of adjustments which could be made in both directions, almost all of which result in the infused foam/glass becoming lighter again. eg use thicker glass instead of stringers, reduce the laminate in areas which are not walked on or see impacts, use foam and cheap carbon tow instead of wood for the beams (these are not in the above calculation), there are no fillets, doublers or tabbing on the foam or included in the above, etc.

The laminate for the foam in this example would be sufficient for a performance 12m/40' cruising cat so is overkill on an 8.8m/20'ter. The advantages of ply over foam do not start until the ply is down to about 4mm and tortured so it does not need stringers. Even then, it is heavier than foam/glass, but the foam/glass is less dent resistant.

For a 30-35' cat, 15mm foam with 400 double bias inside and 600 outside will be more than enough. Infused, this will weigh about 3.2 kgs per sq m, of which 1kg will be epoxy. Anywhere the plywood is doubled, double the laminate (or add some carbon tow) . Areas that spec 6m ply, use 10mm foam with 400 gsm each side and ditch the stringers or keep them and use 6mm with 300 each side. Build a table (far easier than scarfing ply sheets and the finish is paint ready, so much less filling and sanding) and make full length panels. Keep the wooden keel, gunwhales and chine logs if you want to, or replace them with fillets and tabbing.

9mm Gaboon ply (4.7 kgs per sq m) with 3 coats of epoxy each side (0.8 kgs, if applied diligently), plus 200gsm of glass weighs 5.7 kgs, plus the stringers, say 6 kgs all up.

9mm is almost double the weight of the foam/glass, plus all the edge treatment, wastage, and gluing

On 07/03/2018 07:51 PM, Rob Denney harryproa@gmail.com [harryproa] wrote:

 

​Those "equivalences" bear no relationship to what is actually used.​    

Base laminates. 

3mm ply = a small stitch and glue dinghy.       12mm foam with 12 oz glass each side = a 12m harryproa or lightweight cat hull.

4mm ply = a stitch and glue 28' racing cat with stringers and frames to support it or a sheet ply beach cat.       17 oz either side of 12mm core would be silly.  17 oz either side of 19mm core makes sense and would be used on a 12m cruising cat or a C50 cruising harry.

6mm ply = a lightweight 30' cat built with stringers and frames.       750 gsm either side of 12mm does not make sense, normal would be 25mm.  Steinars 20m harry is 800 either side of 30 (thicker for better insulation)

9mm ply = 30-40' lightweight cats. 1150 gsm/22 oz triax = 50' plus monos, with 25mm core.  

12mm ply = 40-50' cats.  I don't know any non condo multis using 1500 gsm triax, or why they would need to. 

Boat hulls are about stiffness, more than strength.    If a hull is stiff enough, it will be strong enough, generally speaking.  Which is why so few boats break from water loads, but all of them deflect to a greater or lesser amount..  

Stiffness is a linear function of material strength (double the material, double the stiffness), but the cube of thickness (double the thickness, 8 times the stiffness).  Hence, core thickness is critical.  As is matching the core properties to the skins.  Consequently, a 10mm thick ply sheet will be nearly the same stiffness as 10mm H80 foam with 12 oz glass each side.  

Strength requirements are usually local (around masts, beams, chainplates, rudders, etc) and are addressed with extra laminate (glass or tow) which is far lighter than ply doublers for equivalent strength and can be laid exactly in the location and alignment required.  

Ply vs foam/glass at the extremes is not about strength or stiffness.  3mm foam with 100 gsm glass each side would be about the same stiffness as 3mm ply, but would have no impact resistance.  25mm ply has similar stiffness to 25mm core with 1,5000 gsm each side, but is far heavier.  

Bjorn,

Thanks for the table.  

Maybe remove the "equivalent" and adjust the core thicknesses for a given weight of glass to those above.     

Foam core usually requires ~200 gsm per side to wet it out regardless of whether it is hand laid, infused or bagged.   

According to the Gougeons, unglassed ply should have 3 coats of epoxy each side at 8 sqm per litre.  ie 120 gsm x 3 x 2 = 760 gsm.  I am pretty sure no one with an unglassed ply boat that lives outside for more than a couple of years old has not wished it was glassed on the outside.    As plywood quality has slipped, this has become more important..  

Triax is not available in less than 800 gsm (400 lengthwise, 200 at + and - 45), so perhaps adjust the laminates accordingly.  I suspect triax was used in the comparison to lower the glass laminates strength in the off axis direction so that this would be the minimum strength to compare with ply.  

If you want to get complicated and accurate, add stringers and frames, coatings and glue for the sheet ply.  

On Wed, Jul 4, 2018 at 1:09 AM, StoneTool owly@ttc-cmc.net [harryproa] <harryproa@yahoogroups.com.au> wrote:

 

Here are some ply to sandwich equivalency numbers I found interesting.  Note the core thicknesses.

                                                                                    H.W.