Here is an actual comparison I made on
the two building systems per square meter for the Woods Eclipse
hulls based on the actual figures. In one case 9mm Okume ply
with stringers, in the other 1/2" (12mm) Divinicell sandwich. I
don't have any way of figuring in the stringers. I converted to
GSM (grams per square meter for convenience)
Okume 9mm 4730 GSM Glass and resin total 1800 GSM, for a total of 6530 GSM (not counting stringers, and assuming a very good layup of 1:1 by weight of glass and resin
12mm Divinicell 976.5 GSM Glass and resin totaling 3600 GSM for a total of 4576 GSM (using the same 1:1 layup... glass and resin equal weight)
Sandwich in this case weighs in at 70% of plywood, not counting stringers
Using infused panels, assuming 2:1 glass resin ratio, we cut out 900 GSM yielding 3675.5 GSM or 56.3% of the weight of ply & glass.
1/2" divinicell with 900 GSM glass on either side is for a structure without stringers, so there is the additional savings of stringer weight, which I have no way of calculating in GSM, but I suspect that we are below 50% total weight per square meter of area when we use infused foam sandwich, as compared to ply / glass. We also have to
This is based on Richard Woods real published figures from the plans, supplied by a builder, and from his bill materials.
The cost of foam sandwich is high compared to ply/epoxy. Probably at least double when everything is taken into consideration, and there are complications. It appears to me that to build entire hull sides on a table will require that a template be made, which would involve constructing temporary bulkheads, stringers and strongback from "sacrifice material" and laying on some sort of light paneling such as used trailerhouse paneling to get the shape. That's a pretty substantial project by itself. Half bulkheads could be used of course, and simply attached to a floor, as all 4 sides will be the same except the cutout for the bridge deck. You need the actual table of sufficient length, and there are the sacrifice materials used for infusion.
I suspect that there would be no real labor savings when everything was taken into account, and double the structural cost. The entire boat, if everything was built in sandwich would end up far lighter, the basic structure not much over 50% of the weight built from plywood. A lighter boat can carry that much more payload without being overloaded, and the less your total weight, the more easily the boat can be driven, calling for a smaller rig, and smaller engines to do the same job. Additionally you get an insulated boat, and a quiet boat, and a boat built from material that cannot rot.
H.W.
On 08/06/2018 10:25 PM, realink@iprimus.com.au [harryproa] wrote:
Okume 9mm 4730 GSM Glass and resin total 1800 GSM, for a total of 6530 GSM (not counting stringers, and assuming a very good layup of 1:1 by weight of glass and resin
12mm Divinicell 976.5 GSM Glass and resin totaling 3600 GSM for a total of 4576 GSM (using the same 1:1 layup... glass and resin equal weight)
Sandwich in this case weighs in at 70% of plywood, not counting stringers
Using infused panels, assuming 2:1 glass resin ratio, we cut out 900 GSM yielding 3675.5 GSM or 56.3% of the weight of ply & glass.
1/2" divinicell with 900 GSM glass on either side is for a structure without stringers, so there is the additional savings of stringer weight, which I have no way of calculating in GSM, but I suspect that we are below 50% total weight per square meter of area when we use infused foam sandwich, as compared to ply / glass. We also have to
This is based on Richard Woods real published figures from the plans, supplied by a builder, and from his bill materials.
The cost of foam sandwich is high compared to ply/epoxy. Probably at least double when everything is taken into consideration, and there are complications. It appears to me that to build entire hull sides on a table will require that a template be made, which would involve constructing temporary bulkheads, stringers and strongback from "sacrifice material" and laying on some sort of light paneling such as used trailerhouse paneling to get the shape. That's a pretty substantial project by itself. Half bulkheads could be used of course, and simply attached to a floor, as all 4 sides will be the same except the cutout for the bridge deck. You need the actual table of sufficient length, and there are the sacrifice materials used for infusion.
I suspect that there would be no real labor savings when everything was taken into account, and double the structural cost. The entire boat, if everything was built in sandwich would end up far lighter, the basic structure not much over 50% of the weight built from plywood. A lighter boat can carry that much more payload without being overloaded, and the less your total weight, the more easily the boat can be driven, calling for a smaller rig, and smaller engines to do the same job. Additionally you get an insulated boat, and a quiet boat, and a boat built from material that cannot rot.
H.W.
On 08/06/2018 10:25 PM, realink@iprimus.com.au [harryproa] wrote:
Polyester is cheaper but it is a greatly different proposition to using epoxy/ply.
Cost vs StrengthPolyester doesnt grip ply nearly as well, where as we know the epoxy bond is greater than the internal strength of the wood itself. Eventually polyester will let go and of course if not caught in time lots of damage ensues. And it can be hard to spot, for the damage is most likely within the plies and cant be seen from the outside because the paint system is preserving the hull.
Weight vs StrengthThe other thing is pound for pound ultimately foam glass structures strength exceeds that of ply or ply glass, and as structures get larger this effect becomes more pronounced. So conversely while the larger boats seem to have the weight tolerance to enable ply they are strength deficient, whereas the lighter designs can afford weight less but strengths are closer to comparable.Therefore if there is a case for a epoxy/ply HP it is with the smaller designs.
Skin vs Weight
A less strong skin needs more framing to guarantee its form which adds to weight and cost. Unsupported distances between frames and intercostals translates better to smaller structures and therefore preferences smaller boats. Failing to attend to that eats into the load carrying budget.
So is there a case for Ply skinsSo whats the case for ply? You dont have to make the materials to build the boat, you simply have to have the facility to scarf and bond sheets of ply together, mark and cut the skin. Hull sides might nearly mirror their opposite sides speeding things up.
You would skin it with glass if you wanted more durability, if the ply thickness you selected was light but not so strong, and if you wanted to fair the hulls with added peel ply afterwards. Otherwise resin coating could have been as simple as flow coat which will definitely add to the parts rigidity anyway.
It makes for a tight strong and fair structure resistant to weather during the remaining build process which will go on far longer than you might like.
__._,_.___
Posted by: StoneTool <owly@ttc-cmc.net>
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