Figuring weight for panels

I'm gearing up for another project, possibly a renovation. As always budget is important, but if I can save weight then it might be worth a little more.

This is NOT a wood vs composite as a material thread, we all know they both have their benefits and drawbacks. So please don't tell me wood rots, because as most builder's here know if you use good epoxy resin then the boat will out live you. This thread is only to try and compare the actual weight of flat panels, and to see if it is worth going with composites. I have been told by several builders/designers that on a small boat their is virtually no weight savings, but I wanted to see the math.

I've been trying to work out the math for the weight difference. I am using 1/2" plywood and 1" composite for my figures.

For Plywood:
½” Okoume BS1088 is roughly 40lbs, and Meranti BS1088 is about 50lbs

4’ x 8’ = 32 sq ft / 9 sq ft = 3.556 sq yds of surface area per sheet side.

If I cover each side with a layer of 6oz cloth, as I have in the past, that will be 42.672oz per side assuming a 50/50 ratio of glass and resin.

85.344oz / 16oz = 5.334lbs

So the weight of a 4’ x 8’ sheet would be roughly 45.3lbs for Okoume, and 55.3lbs for Meranti.

For 5lbs per cu ft composite:

1” thick works out to 2.667 cu ft x 5lbs = 13.335 per sheet.

If I cover each side with a layer of 1708 and 6oz cloth (this is what was suggested) that is 31oz x 3.556 sq yards = 110.236. Assuming a 50/50 mix of glass and resin then it is 220.472oz per side.

So a 4’ x 8’ finished sheet would roughly weigh 40.9lbs

Does this all sound close? So using composites would only save me about 5lbs per 32 sq ft of material used over okoume?

 

Lol we are both wrong. I couldn't remember so I looked up the 1708 on bateau and they have it listed as 17oz biaxial with 8oz mat. They must have it listed wrong because everywhere else I look it up it is 17oz biaxial and 3/4oz mat. So assuming that is correct....

If I cover each side with a layer of 1708 (17oz + 3/4oz) and 6oz cloth that is 23.75oz x 3.556 sq yards = 84.455oz.

Assuming a 50/50 mix of glass and resin (by weight) then it is 168.91oz per side.

So a 4’ x 8’ finished sheet of composite would roughly weigh 34.5lbs

I have always done my resin figures by weight for a 50/50 mix when trying to calculate. I know there is a little better then 9lbs, around 150oz +/-, per gallon of the epoxy I use. It seemed to work out very well on my last build as the whole boat only took about 7 gallons.

So if all that is somewhat correct :-?, then I'd save nearly 11lbs per 32sq ft of material.

I'd be using this for decks and bulkheads, if at all, the transom is a different beast. I only used 1/2" ply and 1" composite to make the calculations easier, but in reality I'd probably use 3/8" ply or 3/4" composite. And I bet the weights will be closer using those thicknesses so I might recalculate again.

Can you tell how bored I am without a current build, lol.

 

Ok did a little more figuring and decided to see what the cost effectiveness would be. I redid the figures for 3/8" ply and 3/4" Divinycell.

3/8” Okoume BS1088 is roughly 30lbs and $90 per sheet

3/8” Meranti BS1088 is roughly 37lbs and $70 per sheet

¾” H80 Divinycell is roughly 10lbs and $200 a sheet (I know there are other options but this is what’s locally available to me)

My resin costs are about $0.37 per oz.

If I buy fabric that is 50” wide then I’ll need 2.67yds to cover each side of the 4’x8’ panels.

Again assuming a 50/50 mix of glass and resin then to cover a panel with:

6oz cloth it will cost about $58.50 and add 5.3lbs

1708 cloth (17.75oz) it will cost $113.46 and add 15.78lbs

So my final costs and weights for the finished 4’x8’panels would be around:

3/8 Okoume BS1088 with 6oz cloth - $148.50 and 35.3lbs

3/8 Meranti BS1088 with 6oz cloth - $128.50 and 42.3lbs

¾” H80 Divinycell (5lbs density) with 1708 and 6oz cloth - $371.96 and 31.1lbs

Looking at the actual numbers, at least to me, it doesn't make sense for my projects to more than double the costs, just to save 4.2lbs per 32sq ft of panels used. So over the course of the entire interior renovation I might be looking at only 30-35lbs or so difference spread over the entire boat?

 

Never heard that before, why would they measure CSM in sq ft, when the rest is measured in sq yds?

Well if that is true then that would only add to the weight and overall cost of using composites, making okoume even more attractive to me.

 

Thanks DN, but I built the plytanic out of 17oz biaxial covered with 6oz cloth and still have some left too. The 6oz covered it very well and I never had any major air bubbles or lam issues, and it smoothed out fairly well, not perfect, but well. I can see the 1708 being a bit thirstier, but I don't recall the 17oz biax I used requiring an excessive amount of resin.

Either way, if it does require that much more, that means the composite will be even heavier and more expensive, and thus that much more unattractive.

 

DN where are you getting glass per yard for that price, without buying in bulk? No point in me buying a bunch of glass I'll never use. Also those figures were with the resin added in, not just the glass. I used the local prices I found, just as I did for the coring. I got 1708 for about $12/yd, and $8/yd.

As far as the glass itself, I got the final info on 1708 and my original calculation of 17oz + 8oz mat is correct. Buckwild is correct the mat is measured in square feet, not yards, but since it is random and inconsistent manufacturers estimate that for a square yard it will be around 8oz or so. Hence the label of 1708.

As far as the resin goes, if I use your figures of 1 gallon to wet out 4sq yds of 1708, then that increases my total weight of the 3/4" composite panel, with a layer of 1708 and a layer of 6oz on each side, to 37.78lbs.

So it would weigh more then the 3/8" okoume plywood with 6oz cloth (35.3lbs). Plus the additional resin adds to the price pushing it up to $404.74. Now even if by some miracle I can shave down the price to $350, that is still more then twice the price of the Okoume without any real weight savings.

Time to start making sawdust again!