Wood Density

My fascination with the weight of the kayak I’m building is growing with my surprise at just how light in weight the hull is. I thought I’d do a quick check on a couple of samples of Paulownia and Cedar.

For the wood I’ve bought it works out that Cedar is about 0.40g/cm³ (about 17 lbs/ft³), and the Paulownia is about 0.27g/cm³ (about 25lbs/ft³). So the Paulownia is 2/3rds the density of the Cedar!

Now that has me thinking – I’m mostly building the deck in Cedar – the extra wood weight plus the coming/deck deckplates etc – hmmm, will it be tippy without me in it? I reckon from looking at it that the deck is about 2/3rds the size of the hull so it might will be balanced at about the same weight top half and bottom half!

Black Pearl Hull Completed

Some quick notes on this.

Firstly, must remember to take care during strip gluing to get the glue all the way through – I didn’t and I found that after fibreglassing the outside and removing the forms… the bottom of the hull started to bend out in a couple of places. Remedy… add epoxy on the inside and glue the strips up properly and weigh down with a few clay bricks (sitting on plastic of course).

Secondly, I tried to reinforce the interior chines with a triangular strip of wood height about 5mm and base about 10mm. What a waste of time! Quick calculation after the fact shows that using wood came in at about 100 grams, but I’ve just checked the density of thickened epoxy – I’m getting about 10 to 20% lighter than straight epoxy – and if I’d just used a thickened epoxy bead it would’ve weighed about 200g. The extra 100g would have been worth not having to cut triangular strips!

And finally, I hate fibreglassing the interior – much more difficult than the outside. Fortunately, relatively easy to fix up any ugly bits a day or two later – in my case 3 patches that needed sanding back and redoing along the chines.

Also, I’ve added up the costs and just out of interest it’s looking like about $NZD1100 total for this one – that’s plans, fibreglass, epoxy, tints/dyes/stains/deck ports and loads of sandpaper. If I’d made some smarter choices along the way I certainly could’ve brought the cost down below $1000.

I’ll do a more thorough break down at the end.

And the weight! Before I forget, the hull with one layer of 175g high density fibreglass on the inside, one on the outside and an extra strip on the keel, it totals about 5kgs on my scales. Incredible really! The Paulownia wood is very light and the high density fibreglass certainly takes much less epoxy than the 200g normal fibreglass I used on the last kayak.