On Sun, Sep 2, 2018 at 4:42 AM '.' eruttan@yahoo.com [harryproa] <harryproa@yahoogroups.com.au> wrote:
Rob, you never answered Chris's question. "How much extra would one of the larger Harrys (say the 50) weigh if plywood was used instead of foam and glass? Would that level of extra weight ruin the handling and performance"
My apologies. Had a bit on, announcement next week. Anything else I have missed in the last couple of weeks, please ask again.
The skins on a 50'ter from ply would be 12mm with 200 gsm glass outside and 3 layers epoxy inside. The foam ones are 20mm with 600 db inside and out.
Boat skins are mostly about being stiff enough, and stiffness increases by the cube. So, twice as thick is 8 times as still. This requirement overwhelms the actual strength of the materials which is why 900 each side of 12mm foam may have -a- similar property to 9mm ply, but is a waste of materials. I have not done the numbers, but suspect the property matched is the lengthwise strength. ie if the boat was towed by two tugs, pulling in opposite directions then 2 x 900 db glass is the same as one x 9mm ply. An absurd property to match, and why Steinars 20m/66'ter is more than strong enough with 800 gsm either side of 25 and 30mm foam.
Adding extra material where it is not wanted is the opposite of "conservative". It is adding extra load to the parts of the boat that are likely to break (rig, attachments), while increasing the required rig, engine and hull dimensions to carry the extra weight.
The panel stiffness requirement means a ply 50'ter needs3 stringers per side and probably 1 along the bottom and deck, plus 8 frames vs 3 for the foam boat.
So, the base weights are:
12mm okoume ply (Boatcraft Pacific): 16.2 kgs per 2.44 x 1.22 sq m = 5.4 kgs per sq m
200 gsm glass, plus resin and filler coat (Gougeons): 0.6 kgs per sqm
3 coats epoxy (Gougeons): 0.4 kg/sm
2 m of 50 x 20mm stringer/frame per sq m of surface: 1.2 kgs
Total: 7.6 kgs per sq m
20mm H 80 foam: 1.6 kgs/sq m
Resin to wet out the foam: 2*0.2 kgs = 0.4 kg/sm
Infused glass and resin: 1.8 kgs/sq m
Total: 4 kgs/sqm.
Surface area of lee hull: 65 sq m. The ply hull weighs 494 kgs, the foam one 260 kgs. Difference is 234 kgs/514 lbs. Close to twice as heavy.
This is not to be sneezed at, but is not as significant as the infusion taking half the time to build. And the effort and danger in the ply build. Intelligent Infusion requires almost zero alignment of build frames, no planing of stringers, no man handling of 16 kg sheets, almost zero waste, exposure to toxic chemicals and dust and no sanding or grinding. A no brainer as any one who has ever built a ply boat, then infused one, will tell you.
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Posted by: Rob Denney <harryproa@gmail.com>
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