The weight of plywood and how it effects your boat

[oceansky]

Why is it when we're building boats, the final weight of the boat is always more than it should be?

One of the main causes of increasing weight is the material we're building from.

Typically, you get a set of plans and it says in the Bill of Materials to use Marine Ply. There are so many variations and plywood species, that here alone the weight can vary drastically. Even if the plans specifies a specific plywood species, the weight here can also jump even within the pack.

What I'd like to do here is, show you how to work out the real weight of the ply your using ( not the manufactures figures) and then go through a simple boat example that I'm hoping, everyone will participate in.

PART 1

First we need to determine the Specific Gravity of the material. (Specific Gravity is the difference between the weight of an object and the weight of freshwater that occupies an equal volume)

Step 1/

Cut a length or strip of plywood ( doesn't need to be too long)

In this example I've cut a length of 6mm ( 1/4") Okoume (gaboon ) plywood measuring 300mm long by 39mm wide ( 11" 13/16" by 1" 9/16" or 11.812" by 1.56")

Step 2/

Sand the edges lightly to remove roughness or saw-cut strands that can hold air bubbles, wipe clean.

Step 3/

Float the sample vertically in the container

Step 4/

Mark the waterline

Step 5/Measure the distance from the bottom of the plywood strip to the water tick mark

Here my water mark measurement from the bottom of the plywood strip is 174mm ( 6 - 13/16" or 6.812 decimal inches)

PART 2

Specific Gravity (SG) Calculation.

Formula:

Watermark distance from bottom of plywood strip/ LOA of plywood strip = SG

Using our plywood strip:

Metric users:

174mm water mark / 300mm length of plywood strip = .58

Imperial Users:

6.812in / 11.812 = .58

The Density of the sample can now be calculated:

Metric Users:

.58 SG x 1000 kg/m^3 freshwater = 580 kg/ per cubic metre

Imperial users:

.58 SG x 62.4 lbs/ft^3 fresh water = 36.2 lbs per cubic ft.

Now what's interesting here is, Okoume (gaboon ) plywood is advertised as being

25 - 28 lbs ft^3 ( 400 - 450 kg m^3) Obviously it's not!

Overall, that's a massive increase of 9.7 lbs/ft^3 approx or 155 kg / m^3 !!!

So the question is - what of other plywoods out there? Are you really getting the weight your looking for?

So how does all this apply to a boat your building?

Boat Example:

Well, let's try an example to see what happens.

Here we've got "Robs Rocket ". It's got a fast name, so it must be fast ha ha

We'll assume this boat is built entirely from 6mm or 1/4 plywood and it's all stitched together ready to be glassed. In other words, it's only just plywood here, nothing else.

The total surface area of the boat hull only is 7.935 m^2 (85.42 ft^2)

To calculate the weight of an object, the formula is:

Metric Users:

Thickness in metres x Total surface area in m^2 x Density in kg/m^3 = weight in Kg

Imperial Users:

(Thickness in decimal inches / 12) x total surface area in ft^2 x Density in lbs/ft^3 = weight in lbs

Using the advertised weight (FROM THE MANUFACTURERS), the boats

(supposed )weight should be:

Metric Users:

(6mm / 1000) =.006m ply thickness x 7.935m^2 surface area x 425kg/m^3 = 20.23 kg

Imperial Users:

(.236" / 12) x 85.42 ft^2 x 26.5 lbs/ft^3 = 44.51 lbs

Now lets use the real and actual density we calculated from the plywood strip:

Metric Users:

(6mm / 1000) =.006m ply thickness x 7.935m^2 surface area x 580kg/m^3 = 27.61 kg

Imperial Users:

(.236" / 12) x 85.42 ft^2 x 36.2 lbs/ft^3 = 60.8 lbs

That's a difference already of 7.38kg or 16.29 lbs.

AND THIS IS JUST FOR THE HULL ALONE.

What of frames, bulkhead, seats, false floors or soles, decking… ? The list goes on and all these weights really add up !!!.

So, before you know it, the boat is 20 to 30 kg ( 44lb to 67 lb) if not more, overweight !! And this is just for this little 14 footer !

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Makes you think, doesn't it

What would make an interesting exercise is , if everyone who's presently building a boat, or has spare scrap plywood lying around from a previous build, is to apply the above formula's to determine the "real " Density of their plywood and then apply it to "Robs Rocket", so we can all see how "Robs Rocket" weight moves around.

It doesn't take that all that long. For everyone's benefit, could you also include the plywood species ie Meranti, Hoop Pine, Douglas Fir etc. If someone does for example, Hoop Pine as a test, don't think for a minute that it will apply to all. You can get variations here as well.

So it would be interesting to see even a couple of tests done by 2 different people on the same species and post it up.

I hope you'll join in as it would be fun seeing the results from everyone. I think this would make for a really interesting topic.

[Tom Lathrop]

It is not the density of the wood that is the issue, it's the actual weight. Sometimes the higher density wood is a better choice for total weight reduction than less dense material. The strength and or stiffness of a particular piece of plywood or lumber is a major factor. When I am concerned about weight (which is always), I weight a sample on a digital scale and compute the lbs per cubic ft from that. For plywood I have weighed several full 4X8 sheets and then compute a weight factor that can be used to calculate the weight of all the ply components of the boat. I like to use the lbs per mm per sq ft as the basic constant for the different kinds of ply. For occoume that is 0.102 lbs per mm per sq ft. Using this number it is easy to determine the weight of all the plywood components in the boat.

That part is easy. What is hard is to calculate the weight of epoxy, finishing materials and extras like fasteners and hardware which always seems to grow beyond earlier estimates. That is mainly because you may know how much epoxy you buy but not how much is wasted, how much is sanded off, how much paint evaporates in drying, etc, etc. In the end, some educated guessing must be done and experience is the best teacher for that. It is very helpful to make weight measurements all during the building process to see how you are moving toward the final weight and how much the different stages contribute. For this, I use a bathroom scale if the boat is small and the same scale and a simple balance of moments technique that works up to a few thousand pounds for larger boats.