Thread: Mast timber

Ash is a bad choice ... way too heavy.

Hoop pine is a commonly available species of the right density.

MIK

Thanks for your reply MIK, I thought I must be speaking some other language as I hadnt had a reply!!
I will try Hoop Pine.

I think if you take a bit of extra time to make your mast round you will appreciate it in the end. Just an opinion but the sails will work better and it will look nicer.

hi
Sorry for the misunderstanding, the mast will have an external circular cross section but will have an interior hollow square core. Hope that clarifies all

Originally Posted by Tumzara

I think if you take a bit of extra time to make your mast round you will appreciate it in the end. Just an opinion but the sails will work better and it will look nicer.

Howdy,

Regarding square masts ... completely square ones.

It doesn't make any significant difference in this regard. There is some research to say it might work better because the cross section can be reduced significantly.

The arguement ... and a good argument ... can only be made strongly from the point of view of aesthetics.

Michael

While we're talking masts, the original plans for my Starling specify a 27'4" mast with a 4" dia tapering from the forestay to 2 1/8" at the tip. It specifies Sitka Spruce which would probably cost more than the whole boat, so I'd probably use Oregon (unless there's a better substitute in W.A.) and build with birdsmouth joints to get a hollow section.

What sort of cost do you reckon that would pan out to build? Ballpark figure I mean.

I did a quick search in my wood reference and am assuming what you're calling Oregon pine is Douglas fir. If you make this substitution the mast will be considerably heavier, compared to Sitka spruce.

A 27' 4" mast with a taper starting around at around 21' will yield a 38 pound pole. This assumed (again) that you have a 4" diameter stick that tapers to around 2.75 (I know this is bigger then your spec's), beginning at 21'. A Douglas fir (Oregon pine) mast of the same dimensions yields nearly a 50 pound mast. This is calculated on a round, birdsmouth with a 20% stave thickness. Now, if you reduce the stave thickness to 15%, then you're almost down to the weight of a Sitka spruce. 15% is about as thin as I'd go on a stave. If going this thin, don't test your rig in heavy winds. More importantly, make sure the wood you use has very tight grain lines.

I didn't know how much sail area you're carrying on that stick so I went to the heavy side, about 340 sq. ft., fractionally rigged Bermudian. This is why the taper is thicker.

Par, you're a wonder

The plans specify 202 sq ft (145 main and 57 jib) but I think the current sails are a little bigger. This is for the W.Atkin designed Starling.

The problem I have is that I can't seem to get Sitka here in Western Oz, so I'm looking for an alternative. The again, if a wooden mast is too expensive then I may have to swallow some pride and settle for the aluminium.

Look for a replacement species that has a specific gravity of .43 or there abouts. Some of the cedars would work, though not as good in compression, such as port orford or yellow cedar. Red spruce is hard to find but a great spar wood, so is white spruce. Redwood, Western white or lodgepole pine, eastern hemlock, aspen and yellow poplar are also options, each having advantages and disavantages to it's use.

I have a good source for white and black spruce, both of which are lighter then sitka and not so highly prized that it's priced outragiously.

There is a chance of picking up something more suitable but there are not many options in Australia.

Most of our woods are hardwoods with a high density.

In the past the most commonly available timber for masts was oregon (confirm ... it is Douglas Fir) and we basically accepted the additional weight because of the lack of any alternative.

If there was an option to do some extra engineering to reduce the weight - then that option is sometimes taken.

Spruce that comes to Australia has often been woeful stuff in my experience.

In recent years we have been stuck with even more limited choices as the housing industry has moved away from Oregon and Western Red Cedar to local plantation grown timbers. The main resource for boatbuilders were nice boards selected from the bulk of stuff imported for housebuilding.

The main option now for spars and much trim is Hoop Pine (Aruacaria Cunninghamii .. if I remember right), which is about the density of Oregon but suffers from the problems of being quickly grown in plantations (tendency to warp as it dries and a wider grain spacing). On the upside the plantations are managed to keep it amazingly knot free and consistent and avoid damage like shakes. It was initially grown to supply the local plywood industry and the ply is usually very nice indeed but made in an inconvenient size for much small boatbuilding - 2400 x 1200 or a bit under the 8ft x 4ft sheet the rest of the world uses.

I am sure the prices we pay for softwoods here would horrify North Americans ... but it is a result of not having a lot of choice and perhaps overuse of the small softwood resource in earlier times. As an example ... South Australia had one of the first forestry commisions in the world to plant trees on a big scale as almost all useable timber in that arid state was gone by the 1870s. They mostly chose Pinus Radiata which we affectionately call "crapiata" on this forum - it is generally not suitable for boatbuilding at all though clear pieces seem to be fine for PDRacers!

So in summary, it might be possible to find some of the species suggested by PAR and it might be possible to find them at a reasonable price if you do some digging. I would probably suggest working out a timber list and putting it up on the more general subforums on this site as well as doing your own research. However, it is likely that some sort of compromise will have to be struck.

Main thing is that the boat works and is workable by the crew. The status quo of heavier Oregon spars has been well accepted in Australia for the best part of a century now though people with budget or connections have been able to get hold of some of the small amount of spruce that gets here.

Best wishes
Michael Storer

Originally Posted by Darce

It specifies Sitka Spruce which would probably cost more than the whole boat, so I'd probably use Oregon (unless there's a better substitute in W.A.) and build with birdsmouth joints to get a hollow section.

What sort of cost do you reckon that would pan out to build? Ballpark figure I mean.

That seems like a reasonable strategy - the oregon is heavier but the birdsmouth building method can be lighter than the small rectangular hollow in the standard Oughtred masts or the solid mast of the Starling. You will have to increase the diameter of mast very slightly to keep the same stiffness.

Price ... is hugely variable because of the patchy supply meaning there is no standard competitive rate ... so shop around. And ask on one of the well visited forums about general woodworking with some specification of the sizes and quantity you are likely to need.

Michael

Doing a little ligth reading, I came across an aircraft builders site which seemed to put it all quite simply. It directly compares the three most common timbers :
Strength value ......................Sitka spruce ...........Douglas fir....... Hoop pine
Density at 12% moisture content ........27 ......................32.5 ..............32.5
[pounds per cubic foot]

Static bending: ............................6100 .....................6700 .............8160
fibre stress at proportional limit ......42 MPa ................46 MPa .........56 MPa
[pounds/sq. inch and MPa]

Static bending: ...........................10 400 ..................12 300 ...........13100
modulus of rupture ........................72 MPa .................85 MPa .........90 MPa
[pounds/sq. inch and MPa]

Static bending: .............................1460 k .................1550 k ...........1880 k
modulus of elasticity .......................10 GPa ...............10.7 GPa ........13 GPa
[thousand pounds/sq. inch and GPa]

Compression parallel to grain: .......4200.................... 4850............. 4920
fibre stress at proportional limit .........29 MPa............... 33 MPa .........34 MPa
[pounds/sq. inch and MPa]

Compression parallel to grain: ........5500 ....................6500 .............7060
maximum crushing strength ..............38 MPa ...............44.5 MPa .......48.7 MPa
[pounds/sq. inch and MPa]

Compression perpendicular to grain 810 .....................1100................ ?
[pounds/sq. inch and MPa] ................5.6 MPa............... 7.7 MPa

Shearing strength parallel to grain ...1030 ...................1000 ................?
[pounds/sq. inch and MPa] ..................7 MPa .................7 MPa
[Higher value is better]

Hardness
[load required in pounds] .......................500...................... 600 .................?

Tension strength parallel to grain
[= modulus of rupture ] ........................10 400 .................12 300 ............13100
..........................................................72 MPa ...............85 MPa ..........90 MPa

Tension strength perpendicular to grain 131 ..................130................... ?

As you can see Hoop Pine, while being heavier than Sitka Spruce, is the same density as Oregon yet much stronger in every aspect. Unfortunately it is rated as having low durability whereas Oregon is rated medium. Sitka Spruce is also rated as low, so maybe it doesn't matter.

Would I be correct then in choosing Hoop Pine over Oregon? If it is 12.5% stronger than Sitka, could I reduce sectional sizing accordingly and reduce the weight penalty of using Hoop Pine?

This must be a new definition of the word "simply" with which I am not familiar !!!




AJ

From my understanding and dealings, the best oregon bought into the country until recently was graded "No.2 and better" and as MIK has said, sometimes decent spar timber was available if you ratted through a pack. We have had some woeful timber arrive only to be sent back to the supplier. We had a previous supplier (who quality was ok wtih but some sent back) talk to us at the WBF in Hobart last weekend and they are now bringing in No.1 clear oregon for spars at a reasonable price ($3500 a cube plus GST) in lengths up to 6m. I haven't seen any of the new stuff yet but certainly the best of this suppliers No.2 and better was pretty good stuff.

I have no affiliation with these guys but rather than bring commercial stuff in here, send me a PM and I'll pass on the details.

regards,

AD