restoration planking material

[saltytheseadog]

I am doing repair/restoration work on my 1950 maurice griffiths 26 ft.canoe yawl. The original planking is african mahogany.So far I have removed the garboards and 3 other planks and there are others I would like to replace. I
would like to use old growth VG red cedar as I have a quite a bit in my lumber rack.My other option is Dougfir.The pro's are a plank in the rack is worth 2 in the bush.Cons?

[pcford]

Don't have the engineering numbers in front of me, but I would be leery. I'd think there is significant difference.

African mahogany is readily available with good quality and relatively reasonable cost.

[Bob Smalser]

African Mahogany (Khaya sp.) is three times as hard as WRC and the third harder than DF.

The softwoods can't compete....I suspect a WRC plank fastened between two mahog planks will incur 20 years of compression set in one season.

[Nicholas Carey]

Cons: douglas fir and/or western red cedar are much different woods than african mahogany. They vary in strength, wood movement and hardness. WRC is also a somewhat brittle wood.

Let's look at shrinkage. African Mahogany khaya spp.), from green to oven-dry has these shrinkage characteristics:</font> 2.5% radial shrinkage</font> 4.5%tangential</font> 8.8%volumetric</font>In contrast here's douglas fir:</font> 4.8% radial</font> 7.6% tangential</font> 12.4% volumetric</font>And Western Red Cedar:</font> 2.4% radial</font> 5.0% tangential</font> 6.8% volumetric</font>Looking at strength metrics, African Mahogany (khaya spp.) is 25% stiffer than WRC and 28% less stiff than Douglas Fir. Modulus of elasticity (kPa) is 9700, 7700, 13400, respectively. Higher numbers indicate stiffer materials.

Khaya is 43% stronger than WRC and 16% stronger than Douglas Fir. Work to maximum load (kJ/m³) is 57, 40, 68. Higher numbers indicate stronger materials.
Hardness. Khaya is 131% harder than Western Red Cedar. It's 16% harder than douglas fir. Side Hardness (N): 3700, 1600, 3200 respectively. Higher numbers indicate harder materials.

Since khaya is readily available at [relatively] reasonable prices, I'd go with replacing "like with like".

Try West Wind Hardwoods (http://www.westwindhardwood.com/) in Sydney. They stock Khaya at CA $6.50/bd ft, WRC and douglas fir (VG boat lumber) are the same price, or higher.

[ 10-25-2005, 03:53 PM: Message edited by: Nicholas Carey ]

[saltytheseadog]

Thanks all for the input. I've crunched the numbers and it appears WRC has shrinkage qualities similiar to Khaya,while dougfir has similiar strength qualities but greater shrinkage.I've calculated that a 10 in. VG khaya plank would shrink .250in. as opposed to .480in. forVG DF.This is over the full range from green to oven dry,conditions not likely encountered in service.Bob, I'm not sure what compression set is but I'm thinking it is crushing of the wood fibres at the edges over the service life of the plank.
Its good to know that khaya is available close to here but the cost relative to the WRC in my stack or to DF around here is quite extreme.What i'm wondering ,if you'll be so kind as to humour me, is what do you think would be the consequences of mixing these woods above and below the waterline.

[formerlyknownasprince]

What i'm wondering ,if you'll be so kind as to humour me, is what do you think would be the consequences of mixing these woods above and below the waterline. The consequences would be a sub-standard repair. Whether it causes you or a future owner problems, only time will tell. Would you fit different width / make / compound tyres on your car? You probably won't have a problem - but if you do, it could be a doozie. Above the waterline - the consequences are more visual than anything.

[TimothyB]

Honestly, I would not even consider using cedar or fir to replank a Khaya planked boat. If I couldn't afford Khaya, I would try to find the very closest match I could in a domestic wood first, and even do laminated planks (if I couldn't find thick stock) before going to softwood. I would be concerned that the cedar might split. Unlikely, but still...

Using cedar, or even fir, as a spline on a hardwood planked boat is ok because you want it to be soft, but using it to plank up repairs seems to be asking for trouble.

Try looking at possible alternatives, like different varieties of locust, oak, etc and compare the figures (oak is probably not good because it moves alot with moisture, but check the numbers!). Or maybe someone here will give you some ideas. smile.gif I'll bet you can find something very close that isn't 6.50 a bd/ft.

[saltytheseadog]

Would you fit different width / make / compound tyres on your car? I would and I have ,I just drive slower.

If I couldn't afford Khaya, I would try to find the very closest match I could in a domestic wood first Thats what I'm trying to do.Does anybody have any practical experience in mixing planking material or am I a pioneer in this field?
I agree its not an ideal situation but I think I can find enough planks to replace to break the bank(8 and counting so far).Not to mention the problem finding quality planks.

[Bob Smalser]

Quartersawn Doug Fir heartwood is your closest local match, and won't crush near as badly as cedar. The only potential problem I see is if there are steep bends to make with it in matching the thickness of the more flexible Khaya. It's not as strong, will compression set faster (crushed plank edges that shrink more and more each year), and won't last as long in theory....but it'll work.

There's no real harm in replacing with DF for now to get the boat back in the water, as the work is reversable in the future.

[ 10-27-2005, 12:43 AM: Message edited by: Bob Smalser ]

[formerlyknownasprince]

All we are talking about is say 8 x $6.50 x say 15'? Thats less than one boat unit. I imagine this is one of the lesser costs in this sort of rebuild, isn't it? Why not wait till you've got the cash there to do it right the first time?