mast

i wonder if anyone has an opion on a solid wooden mast square not round made from a single lenght of sitka spruce heart wood no knots., my current mast is square and hollow with about 6 stays and is 27ft long but it is badly in need of replacement as it was made in 1962 and has had a lot of repairs the boat is a steel wild duck buchanan design 21 ft loa .cheers dave

 

Hi Dave

Nothing wrong with solid masts. George Bueler, my favorite designer, uses them. The only thing is is that the ones on his boats are usually round and they are most certainly figured into the stability calculations.
Therein lies the problem.
Whoever designed your boat based his/her calculations on a hollow mast. One that could wiegh little more than half as much as a solid one with the same stiffness. The heavier solid mast is likely to make your boat a wee bit more tender. Or, depending on the design, more than a wee bit more tender.
This could be compesated by making the new (solid) mast about 25% shorter
but your sail maker wouldn't like that.
You might look around for an old aluminum the same length and for a simular size (displacement wise) boat. A friend of mine once replaced his rotted out hollow wood mast that way.
Other than that, I see a lot of cutting, clamping, and gluing in your future.

Bob

 

The only reason to install a square or rectangular section mast is to get some added strength through engineering. Square sticks are stronger then round in the same dimensions, meaning you can down size the square spar a bit, save weight and have the strength you need for the design's requirements.

Solid masts, need much less engineering to make the thing stand up to the tasks envisioned. You pay a dear weight penalty for this and the considerable increase in height of the center of gravity (much more so on smaller yachts then larger) can adversely effect the boat's ability to stand up to a press or even the stability of your craft.

There are ways you can have your cake (solid spars) and eat it too (no uninvited capsizes) though I'd reconsider the squaring off the naturally round stick. You'd gain nothing by this, it looks a bunch worse then a round one and there's a bunch more work involved fitting a round pole into a square shape.

Some quick math of a 27' spar of typical proportions made of solid sika spruce produces a 15 to 22 pound stick depending on intended use, Douglas fur produces a heavier stick of the same dimensions, running 18 to 27 pounds. Both were round sectioned subjects.

Hollow masts for the same uses, work out 50% or more lighter. In small craft, weight savings, especially aloft can be a real performance enhancement, weight increases, a performance killer.

For what it's worth . . .

 

Round wood masts

Solid masts are much stronger if you retain as much of the tree as possible.

Wood is poor in compression strong in tension, when a tree trunk fails it buckles on the lee side leading to fibre rupture on the windward side. The tree grows its outer fibres with in built pre-tension (pre stressed ) the core is in compression .......... Clever

So a solid mast is best left round and get the closest tree to your diameter requirement. It will be considerably stronger.

A square mast should be hollow.

cheers

 

When looking at the moment of inertia, one will see that the middle part of the mast, the solid part will give hardly any more strength to the mast. It is the outer part of the mast that does it because it has a longer arm to the center of the mast and therefore it will be able to give a much higher moment.
The center of the mast, if solid, will be able to give more compression. I mean; if you would put a load on the mast whilest the mast is upright, the mast will off course be able to take a higher load (the surface is bigger which has to carry the tension/compression).

 

Ok I need to replace my 22' spliced douglas fir mast, it was a square 3 1/2x 3 1/2", the corners were radiused 1", the spar stayed that way until it readed the hounds 13' up. then it tapered down to 2 1/2" at the top. Sould i replace itr with a round pine tree or get a pine 4x4 and round it or radius the corners. ???????????????

 

:!: the boat in question is 21ft and twoand a half tons with a full deep keeland about1200 pounds of ballast and 8ft beam