Marine Designs
We have boat plans (both yacht designs, catamaran designs and power designs) for all sizes. From the 12ft Mushulu Dinghy stitch & glue yacht tender to the Oceansky 57 catamaran. We design timber boats, plywood boats, strip plank boats, fibreglass composite boats, steel and aluminum boats to very modern styling for both monohull designs or multihull designs, powerboat or sailboat and in round bilge, multi-chine or radius chine hull forms.
Different Plywood weight density's and how it can effect your boat.
Why is it when we're building boats, the final weight of the boat, at times, is more or less 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 extra weights can 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 !
Overall, it does show the variation you can get when building your boat from various species of plywood. It's not a bad thing, as even when purchasing plywood from the same pack, you can get variations in the weight between sheets of ply.
What I'm trying to show here however, is for you to be aware that your boat can vary in it's final weight somewhat, whether it be lighter or heavier than expected. In our case, we endevour to take the middle ground in regards to our calculations as we have no control over the plywood used or it's varying manufactured weight.
Have fun
Regards
Mark Bowdidge (MRINA)
Bowdidge Marine Designs
