Saturday, November 26, 2011

More First Mate Activity

Firstly, thanks very much to everybody who wrote in after the previous post. I'm very grateful for the responses, and it helps me a lot in deciding if the information on the blog is relevant to the readership, and gives me some indication about whether I should continue writing the blog.

This week, among other jobs, I've been preparing materials for the hollow "Bird's Mouth" mast for First Mate. The timber I selected is Silver Quandong (Elaeocarpus grandis) which is a light hardwood which grows in the coastal areas of northern New South Wales and in Queensland here in Australia. It is an excellent structural timber for small boat construction, taking both fastenings and glue well, and being suitable for steam-bending. It is the same strength (modulus of elasticity) as Sitka Spruce and is very close to the same weight at 500 kg/cu metre (Sitka Spruce is about 440 Kg/cu metre and Douglas Fir is about 540 kg/cu metre).

The particular stock of Silver Quandong I've got at the moment is unusually dense, and I was concerned that the mast would end up being too heavy. Once the idea got into my head I started having nightmare thoughts, and before long I had convinced myself it was as heavy as Iron Bark and I was ready to throw away two days worth of cutting and machining! In situations like that, the only thing to do is to carry out a test.

All I did was to accurately cut a piece of the material, measure its volume, weigh it, and then calculate the density. The whole thing took me about three minutes using a piece of scrap from the cutting job. The offcut I had on hand was 125.8mm wide, 19.3mm thick, and I cut it to 250mm in length (the sizes are completely abitrary, but must be measured accurately so you can work out the volume). I weighed it on my workshop scales and it came out at 335 grams.

My sample of timber, with the sizes and weight jotted down in red ball-point

0.250m x 0.1258m x 0.0193m = 0.000606985 cu/metre

0.000606985 cu/m = 335gm = 0.335kg

1 cu/metre = 0.335kg/0.000606985cu/m = 552 kg/cu metre

So I discovered that my timber was actually quite light, being about the same as Douglas Fir (Oregon) and lighter than Hoop Pine. Being a very wet 24 hours, the sample was also damp, so its real density is probably a little lower again. The lesson here is not to get fooled by your insecurities - just do a simple test and many problems disappear. I've applied this principle to many things like testing plywood bonds, paint adhesion, bending strengths of masts etc etc. I always use basic equipment for my testing, but the results are fine for the work I do.

Here are a few pictures showing the method I use for cutting "Bird's Mouth" cut-outs: -

Running a mast stave through the table saw for the first cut. Note that I have a homemade finger-board to hold the stave firmly against the wooden saw fence. Accuracy and consistency are important. These staves for First Mate are 16.3mm thick after having been run through a thicknesser.

I finish the cut using a push-stick for safety

This is how the stave looks after the first cut has been completed

Stave has been end-for-ended and the second cut started
The finished cut

Roller-stands are very useful things to have when working alone with long pieces of timber. Ideally, I would like to have a much larger table-saw, or at least a feed-in and feed-out table, but I have neither the money nor the space. However, it is amazing just how much you can do with cheap, second-hand equipment and a buit of determination

1 comment:

  1. Rossco,

    By way of a general comment on both First Mate & Phoenix3,I have been impressed with the speed Paul Hernes achieves in the videos of him sailing in a stiff breeze & the ease with which he handles the boat.I know the sensation of speed appears greater when close to the water,but he is ceratainly clipping along.

    My question is:- are both designs able to break out & plane?

    Al Burke