Thread: Strip Plank Kayak Weight

Peter

I am not a builder of kayaks but sell paulownia. If you can work out the length of the strips you need, width and thickness then you have the volume of timber needed. Allow for some wasteage, I suppose, and then multiply the volume by 280 - 290 kg for dressed paulownia or 384 - 390 kg for western red cedar.

Hope this helps
John

Basically, you need to know what each species weighs per cubic of favorite dimension. Then with some simple math you can calculate out how much each foot (or meter) of strip will weigh. Personally, I like to work with bigger numbers, so I'll calculate the running length of all the strips (the linear dimension), convert this to cubic somethings (usually feet) then multiply by the wood's cubic weight per unit.

In the USA we'll use the USDA's "Wood Handbook" which tells us the weight of a cubic foot of each species. I suspect you have a similar book in your country. If not, try this link Forest Products Laboratory - USDA Forest Service You can down load each chapter or the whole thing (it's not small). It has most of the common woods and other useful information.

Lets say the strip is 1/2" (to keep the math simple) and we have a species of 30 pounds per cubic foot. We need a 576th of this (there are 576 - 1/2" squares in one square foot), which will equal a 1/2" square strip 12" long. This is .05 pounds. Another example is 1" square strips. These would be .2 pounds per 12" long strip. I mention this to show the classic "mechanical similitude" law of physics. You'd think that a strip 2 twice the size of the previous example would weight twice as much right? Nope, and welcome to the wonderful world of slightly abstract physics (okay, not so abstract if you think about it). It's often referred to as the "law of cubes" by engineers and other perticulay anal folks that fool with these sort of things.

I bring this up, because it one of the most common requests placed on a yacht designer (I had another one today). "Can we do this design, just a little smaller?" Sure we can, but it'll be smaller (or bigger) in ways you might not expect. Simply put, the above mentioned strip is twice as big, but has 4 times the area, 8 times the mass and has 16 times the stability. This means the loads imposed will have equally as dramatic a change on the structure. So, in the above examples, multiply the first figure .05 by 4 and you get .2, because it's weight was based on it's volume (area).

The math is slightly easier in metric, but the logic is all the same. Going quietly mad is the hallmark of a good engineer. It beats steering a wheelchair with your chin.

Fabric weights can vary widely, but a general guide is the fabric will weight about twice it dry weight when wetted out and maybe 2 to 3 times it's dry weight when the weave is filled with resin only (like when doing a clear finish). The resin to fiber ratio swings a good bit, but assuming reasonable care, this is a fairly close approximation. Using light weight fillers on painted surfaces can reduce the total sheathing weight by 30% - 40%.

If you want some real life examples to verify the calculations that have already been mentioned, I can give you a bit of an idea from my experience.

My 4.57 metre TK1 has 4 square metres of surface area and is almost all built from 4mm Paulownia. If you assume an average of half a mm gets sanded off in the process of fairing the kayak, you get the following calculation.

0.0035 thick strips x 4 sq metres area x 290 kg/cubic metre (as suggested by Whitewood) = 4kg.

As it happens, that is almost exactly what the timber part of that kayak weighs. I weighed it before the glass went on.

It has 125gsm glass inside and out, so that adds exactly 1kg.

The seat, rudder and footrest weigh 0.7kg. The cockpit rim weighs about 0.2kg, so there must be about 2.4kg of resin to bring the total up to 8.3kg. So the glass to resin ratio is about 1 to 2.4.

Using a kayak close to the dimensions of the one you mentioned, my 5 metre sea kayak is almost 5 square metres (roughly 53 sq ft). It is made from 5mm Paulownia (sanded to 4.5 probably). So the calculation is 0.0045 x 5 x 290 = about 6.5 kg.

The same thickness of glass (125 gsm) adds 1.25kg and the resin a bit over 3 kg. That brings the bare hull to about 11 kg. The hatches, cockpit and bulkheads add about another 1.5 to 2 kg for an all up weight just under 13 kg.

All that really proves is that the calculations do work and Whitewood's figure for the density of Paulownia is pretty right.

You might notice that even with just one layer of 125 gsm glass and a bit of care with application of the epoxy, the timber contributes just about half the total weight of the kayak if you use something as light as Paulownia. Some people use heavier glass which needs more resin and they have heavier hatches and fittings, so some sea kayaks might weigh three times as much as the timber strips.

Those same two kayaks that I described would be about 1.5 kg and 2.2 kg heavier respectively if built from the same thickness of WRC.

So the calculations work, but remember the glass, resin and fittings will at least double the weight of the kayak.

Thanks John, we've spoken before about getting some Paulownia into Tassie - look out for an order soon...

Thanks PAR - I've put the rusty razor blades back where they belong and thanks to you, had something useful to do with my calculator.....

Thanks anewhouse....for those examples which showed up the error I had made at first.....

I'm now happy to continue down the decision making process and no longer feel "foolish and deluded" .

WHAT A GREAT FORUM!!!!!!!!!!!!