Dense Timber Mast

I am considering making a timber mast for a 9m yacht from relatively dense but also relatively - extremely stiff, strong and durable (when sapwood treated) timber. The timber in question is Rose Gum (Eucalyptus grandis) with a density of 620kg/m^3. I'm not mathematically inept. The general rule with timbers is that their mechanical properties follow a direct relationship with density. Specifically, RG has a modulus of elasticity of 17 Gpa c.f Sitka Spruce 11.2GPa (density 430 kg/m^3). Compressive strengths are, respectively, 66GPa and 40 GPa. Normalised to an equivalent mass, Rose Gum has a 5% greater E mod and a 14% greater compressive modulus. I plan to use cca treated sapwood for extreme durability. Rose Gum macines and glues well (yes, even when treated).

Because of the elevated mechanical properties the wall thickness may reduced to approx 66% of the SS value for the same outside diameter and mass but with both greater strength and stiffness.

Can someone please shoot any logical holes in my reasoning and/ or refer me to examples of where this has been done before. I could quote many other higher density candidate species that could generate lighter, stiffer or stronger spars due to the mathematical advantages inferred by the nature of moment of inertia mathematics. I can only believe that the 'obsession' with low density softwoods is a hangover from the the days of solid spars (where low density IS an advantage) and/ or a lack of modern glues that are effective even in thin walled masts and or a lack of tall sraight hardwood trees?

 

Welcome, Timboz. If the wood is not brittle and prone to cracking, why not?

 

Of course it has been done.

Your line of thinking is very old. Very old. Which makes it good.

It was used to establish the Mongol Empire, which became the largest contiguous empire in history.

The question to ponder is where does the mast need the extremely dense wood that resists compression,versus where does it need a light wood that is elastic.

Start at:
http://en.wikipedia.org/wiki/Composite_bow

In Malaysia I had selected two woods to work with. One was really really heavy and the other light and strong. I ended up going aluminum. Sorry.

Phil

 

Spruce was used even before the 2nd World War in hollow sections on pretty large vessels. Of course after that we got modern glues so hollow sections were standard until Al in the 50s'.

The main reason for soft woods is the modulus versus light weight and righting moment. If you have sailed a dinghy with a modern carbon mast against an aluminium one you would most certainly notice the difference. So much less inertia. The classic engineering problem is that straight beam stiffness in steel, aluminium and wood are all similar. Only in compression and tension do things change.

Apart from obvious reinforcing (locally) a spruce spar it will perform OK if the compression loadings are not high. Aluminium is much better and early carbon masts often suffered as the newer compression high modulus fibres were not available. The Eucalypt should be fine, my only concern is whether that particular one grows with a helical twist which some Eucalypts do. As I don't know the species, your call. There is the maintenance issue as well to consider. Here in the UK there are still spruce masts being made (check Folkboats) and quite a lot use Douglas Fir for spreaders which from my experience is not a million miles from the properties of the Rose Gum Eucalypt.

 

This is really a bad idea unless you look at all of the load conditions and properties. I do a lot of wood structural design as part of my work as an engineer. Very few species of wood have proportional strength relative to their density, even from one species to another you will not find that the different properties relative to each other are similar.

for example, white oak has high cross grain compression strength, as compared to a soft wood like doug fir, but doug fir has a similar bending and grain wise compression strength as the white oak at about a third less weight. and doug fir has a much higher elastic modulus (so it is much stiffer) that white oak, at one third the weight. So it depends on what your are looking to accomplish in terms of load conditions.

Spruce and western red cedar are similar in weight, and in cross grain compression, as they are in pure tension strength. but spruce is much stiffer and much stronger in grain wise compression, and the spruce has the edge in pure bending over red cedar. Cedar also has high rot resistance, but spruce has almost none. Sitka spruce is also much more costly that red cedar, so that might also be considered.

There is no consistent pattern to this difference in properties. What I have to do when there is an option of different woods available, is compare the most critical load conditions, and compare it the properties of the different species. And than also compare the secondary load conditions just to make sure we have not overloaded the wood in another load condition. If there is a cost difference than this may also be considered in the analysis, the next size up western hemlock might cost less than the smaller size dough fir.

You have to compare the compression strength, the bending strength, the shear strength, cross grain compression and its weight (and cost), with the worst case load condition for your installation for EACH of these properties, or you risk a failure in one or the other modes of loading. You have to do something similar if you are changing from a wood to aluminum mast, or aluminum to carbon composite. There is no easy answer.

You just can not design to the load capacity of each of the original wood properties since some of them may not be design critical and you would end up with something much too heavy. you have to know, or be able to determine, the worst case loading on the mast for each of these different properties and compare it to the section properties of your new design and material. It seems very foolish to me to attempt to replace a mast on a yacht this large without doing a proper design review, of course this might not make it economic to use the available wood.

You can try it and take your chances without the analysis, but that is a chance you will have to weigh against the risk.

Good luck.

PS: I do not know what the Mongol Empire had to do with it, and it was not the largest empire-it only lasted 160 years at best, it was the largest land size empire.

 

Mast Timber

Thankyou for everyones contributions.

Petros, I have quoted moduli for Rose Gum and also stated that its specific moduli exceed those of SS so I would not be working blind. Further I can test individual sticks of timber to select the best 'within species' material. As I said I am not mathematically inept so I cant see why I cant use RG with confidence (given its higher specific moduli) when the average boatbuilder 'blindly' uses SS with confidence. With regards to the density relationship you are correct that species (like oaks) with ray parenchyma have lower specific moduli but the vast majority of timbers fall within one SD of each other in specific moduli. I am aware that I would have to design to the minimum modulus - I was more interested in less obvious flaws in my reasoning.

 

The problem with strong but heavy materials in a solid beam of course is the stiffness is proportional to radius to the 4th power and strength to the 3rd power; only tensile strength varies in the same way as weight and that's irrelevant for a mast. You obviously know all that. Things do get better for a hollow mast where you can reduce wall thickness subject to the limitation of Euler buckling, which is hardly likely to be a factor for a wooden mast. Many gums are notoriously difficult to glue, I don't have anything on Rose Gum but that might be a problem and I would certainly make and break test parts to verify the suitability of your choice of glue. Apart from that it might be a good choice, it is known to be tough which is nice to have in a location subjected to shock. I expect construction would be more demanding on accuracy and assembly skills.

 

I'm sure with care it could turn out fine, my concerns would be mainly to ensure the timber included remained stable as to moisture content in regards to the glue lines/stabilty & also the proposal seems to indicate with thinner material that there would be narrower/smaller gluing areas.
Jeff.

 

Compression Moduli

Thanks for the reply Ancient Kayaker but I have to disagree that only tensile strength varies with density. All moduli increase inline with density saving peculiar cellular arrangements such as ray parenchyma (the rays of silky oak etc) which reduce elastic modulus for example. The simple fact is that ALL timber species are made of the same basic structures and are all composed of celluloses. What varies with density is the thickness of cell walls (and greater quantities of extractives) The thicker walls mean more 'stuff' and hence greater moduli. Another determinant of stiffness is microfibril angle (which is analagous to grain slope at a micro level.

Rose gum glues well due to its low extractives levels. Eucalypts glueing difficulties may largely be overcome by wiping with 10% sodium hydroxide to strip out extractives.

 

I understand and agree Timoz. When I wrote "tensile strength varies in the same way as weight" I was referring to the weight of the mast not the density of the material. I should have written "tensile strength of the mast varies as its cross section".

I think we are on the same page. Good luck with the mast.