Timber choice for SOF kayak.

[lance.house]

Hi everyone,
I'm a long time lurker (mostly in the home renovation areas actually) but just signed up at long last to ask a question which I havent bee nable to work out myself for sure...
I'm looking at a first time construction of a kayak and am leaning heavily towards the Tom Yost Sea Bee. I was originally looking at some ply kits from the US (Cheasapeak Light Craft WoodDuck) but now I have changed my mind.

I'm going to use it for mucking around and fishing, mostly inshore and local estuaries/etc (around Hobart) and don't need a full ocean goer (I have a double Mirage fibreglassie forthat...) and there are 2 main values which are driving this project : (1) I am a major cheapass (2) I dont want to buy lots of new timber and epoxy etc for what is totally an unnecessary plaything. I enjoy learning new things and making stuff myself so dont expect to be taking the quickest or easiest option on this, particularly given my values above :) I'm aiming for a kayak made from as much recycled/reused material as possible!

So I need some advice on timber selection for the stringers/keel/gunwhales etc. I have three options in front of me - Tas oak, celery top or oregon/douglas fir.

My issues are:
Tas Oak - by far the easiest for me to source, I could go to the tip shop and buy enough 10 times over and have it scarfed and ready this weekend. It would be second hand - more work but ticks my 're-use/reduce-recycle' boxes. It is alot heavier than my other options - could I use thinner pieces to make up for this or perhaps route out a groove along the inside as well? I also think its not as durable in the water, but am quite happy to give it a coat or two of epoxy and lacquer if required. From my research its not a widely used boat timber.

Celery or Oregon - these would be coming directly from my local friendly miller who has some logs down the paddock. I would actually have to go down there and help out for a day in return for the timber I need (not a high paying investment but probably an interesting one...) but the timber would then be green and require some considerable time drying out... I would only be getting such small sections that they would effectively be 'offcuts' which partially fulfills my desire for a boat made from scrap but not as much as the tas oak would...

I think the celery or oregon would ultimately be better timbers, but would take alot more time from me as well as drying time, as well as not being quite as recycled. Also to be quite frank, I'm not aiming for world class timber frame kayak construction here!

Are there any major reasons not to use the Tas Oak? Could I lighten a tas oak frame as suggested above or am I asking for destruction?

Any other tips from the experienced?

cheers!

Lance

[m2c1Iw]

Welcome Lance,
Have a look at this (http://www.woodworkforums.com/f32/geodesic-airolite-skiff-111205/#post1092240) thread Peter pops in here from time to time and I'm certain can comment.

Tassie Oak while heavier if it's easy to get have go with that for your first time from memory it can be steam bent if necessary.

If you get really keen you could laminate with shorter bits of celery but again from memory celery at around 650 is not a whole lot lighter than some of the stuff sold as oak..

[Boatmik]

I would choose a timber with a similar density to that suggested by the designer.

All the ones you mention are nicely straight grained which is important. But if higher density than the designer suggests will add some weight. And weight is something that is very nice to keep out of a canoe or kayak.

MIK

[lance.house]

Mmm,
thats a useful point.

the yostwerks site sys he uses 'redwood' for the seebea and western red cedar for some other boats (which are longer and skinnier). Redwood is a bit vague to me, but i've found densities of these two ranging from 350-450 kg/m3. Tas Oak meanwhile has a desity of 700-800kg/m3!

Whoah! that would make the boat twice as heavy...

This being the case, combined with the fact that from my brief searching it seems that neither redwood nor western red cedar are particularly strong, whilst tas oak certainly is, would it be allowable/advisable to use significantly smaller stringers? eg maybe 12mmx14mm vs 18mmx18mm (ie 3/4")

Or is this just asking for trouble? Should I just go the long route for some oregon instead? (which isstill heaps denser than either of the others..)


cheers

Lance

[Boatmik]

Howdy, Strength of timber goes with the density - more or less. So if you choose timber of about the same density it will have the same strength and stiffness.

The smaller stringers of tassie Oak will be too flexible.

Basically the lower the density of the timber the larger the cross section can be and the stiffer the piece will be in terms of bending. Which is how it will be loaded.

For example if you use wood of double the density the cross section will have to be reduced by 1/2.

This means the height and width have to be reduced by 0.70 approx to hit the same weight.

Now to work out stiffness
Stiffness from the timber is proportional to the density = 2 times the stiffness of the original timber.

Bending stiffness from the cross section is proportional to depth cubed x width = 0.7 cubed x 0.7 = 0.25

To get total stiffness you multiply
stiffness material x stiffness cross section
2 x 0.25 = 0.5f

Half the stiffness of the lighter wood.

So going for the stiffer, stronger and heavier timber has doubled the flexibility of your structure.

Stick with something around the same density.

Western Red Cedar and oregon are both tremendously strong if the grain is clear. The big risk with cedar is that the grain can wander about quite badly - including right out the side of your pieces - but if nice straight grained stock - or the bad bits are cut out and it is scarfed together with minimum 6:1 scarfs there will be little problem.

Unless you hit something hard enough to break the original designed structure.

It is often an instinct of builders "to add a bit" when building - but that only adds weight and might even compromise the structure or its usefulness.

A secondary problem with going heavier and smaller cross section is that the piece becomes more vulnerable to small defects - there is just not enough cross section left over when there is even a small defect to prevent breakage.

Best wishes
Michael Storer

[labr@]

Lance, your question about routing a groove made me think of the paddles I've made with hollow Tas Oak shafts. It may be possible to rip the pieces down the middle, hollow them with a router and glue back together. There are a number of issues with this including extra work, extra epoxy, danger of taking out too much etc but I think it could be done. The timber lost in the centre has much less effect on stiffness than that at the surface.

I can't put this into a technical form as Mik has done (good explanation back there Mik) but I believe you should be able to get close to the design weight, strength and stiffness using this method.

Disclaimer: I could be wrong! Comments from the more knowledgable required please :U

[whitewood]

Mmm,
thats a useful point.

the yostwerks site sys he uses 'redwood' for the seebea and western red cedar for some other boats (which are longer and skinnier). Redwood is a bit vague to me, but i've found densities of these two ranging from 350-450 kg/m3. Tas Oak meanwhile has a desity of 700-800kg/m3!

Whoah! that would make the boat twice as heavy...

This being the case, combined with the fact that from my brief searching it seems that neither redwood nor western red cedar are particularly strong, whilst tas oak certainly is, would it be allowable/advisable to use significantly smaller stringers? eg maybe 12mmx14mm vs 18mmx18mm (ie 3/4")

Or is this just asking for trouble? Should I just go the long route for some oregon instead? (which isstill heaps denser than either of the others..)


cheers

Lance

Lance,

I'm no boat builder but I sell Paulownia. I know person who has built a SOF kayak using Paulownia for all framing. I also sold a 'bundle ' to a school for the pupils to build SOK kayaks as part of a school project. I don't know what design etc but if you are interested I would give you the teachers email address and you can take it from there.

The density of WRC is Approx 375 Kg per c^m and Paulownia is 275 kg per ^m but their strengths, bending etc. is close. I hope this helps.

John

[lance.house]

Wow BoatMik that was an amazing reply!! :2tsup: Very very useful as I was definitely leaning towards what might have been a wet noodle...

I think that labr@'s idea is a bit more extreem than I was aiming for though!

Would it make such a huge difference to the stiffness (or the weight for that matter) if I was to rout out a groove along and edge (or two of three or four) ? I could conceivably make an X which to me seems like it should maintain a fair bit of stiffness to it... Round vs V-bits? I wouldn't intend to go very deep anyway...
A fair bit of work given i don't have a table router, but still probably only an hour or two to do the lot...


thanks for all the help already,.

LAnce

[Boatmik]

Hi Lance,

It sounds like more fuss than I would bother with, but I am not the one who is deciding!

People here know just how lazy I am as a boatbuilder!

MIK

[lance.house]

well...

after all that, I headed to the tip shop on friday, and after spending 1/2 hour selecting good straight grained bits of tas oak, I came across 3 x 2.6m oregon beams which i think used to be part of some kind of door assembly!
Theres lots of nails and sheetmetal fascia to remove, plus a fair bit of ripping and scarfing, but I think i can happily say that the tip-shop kayak is now genuinely underway and looking like a reasonable chance of turning out a reasonable paddle! woo!



cheers

Lance