Solid vs hollow mast

[leaotis]

I've been told that a birdsmouth is stiffer than a solid mast even though the BMs contains less wood.


Does anyone have experience with both types of mast so can offer a comparison?


Thanks, Mack

[kc8pql]

How big a mast and what kind of rig?

[Ian McColgin]

So much of the strength of a mast is in the wall rather than the center that you can almost always use a bird's mouth in place of a solid stick if both are the same outside dimensions and if you follow the standard birds mouth porportions. Many spar makers feel these standards are very conservative and are engineering lighter, depending on the rig and how the stresses are distributed.

It's perfectly possible to make a birds mouth stiffer than a same sized solid stick if the solid stick's grain allignment is not attended to.

G'luck

[leaotis]

How big a mast and what kind of rig?

For example I was advised against building a birdsmouth mast for my 18' sharpie because it wouldn't be flexible enough.

I wanted a birdsmouth because it would be lighter.

Can a BM mast be made so it flexes?

Was I advises wrongly?

[Ian McColgin]

I am a bit surprised that a sharpie would be so carefully engineered that some difference in mast flex matters. Perhaps it would help if I knew more about the actual design. Stayed or (more likely) unstayed mast? Sprit or leg'o mutton or gaff or whatever? And what does the designer call for?

[leaotis]

I am a bit surprised that a sharpie would be so carefully engineered that some difference in mast flex matters. Perhaps it would help if I knew more about the actual design. Stayed or (more likely) unstayed mast? Sprit or leg'o mutton or gaff or whatever? And what does the designer call for?

Thanks Ian,

I think more of a matter of little or no engineering. Just a suitable limber pole stuck in the bow. We're talking 1904 Chesapeake Bay.

Unstayed leg'o mutton rig on a Reuel Parker sharpie modernized for ply/epoxy crab skiff. Reuel said whatever you do don't put shrouds on it. It's supposta flex and spill in gust.

I know that tightening the snotter gives it a nice bow. I wonder if a birdsmouth would as bendy. I still feel the urge to build a birdsmouth mast.

[hokiefan]

Thinking way back to my Strengths of Materials class, and Mechanical Design as well, a tube is generally a stiffer shape than a similar solid bar. In wood, the natural variability and grain layout makes everything well, more variable. Honestly if it were me, I'd build a birdsmouth mast for your boat and try it. You already have a solid mast right? In case you don't like it. To me handling the lighter hollow mast would be much more appealing. Give it a shot.

Cheers,

Bobby

[JimConlin]

Versus a solid spar of the same dimensions, a birdsmouth spar has less material in it and will be lighter, less strong and less stiff.
For the same amount of material, a hollow spar can be stiffer and stronger.

[rbgarr]

I'd think the epoxied or glued seams would make for more stiffness than a one piece spar of similar dimensions. Maybe these guys would have tested results: http://www.pleasantbayboatandspar.com/

[JimConlin]

Dave, if the ribbon of epoxy in the joint is less stiff (i.e. stretchier) than the wood, it doesn't bear much stress. Certainly less per square inch than the wood.

[David G]

Versus a solid spar of the same dimensions, a birdsmouth spar has less material in it and will be lighter, less strong and less stiff.
For the same amount of material, a hollow spar can be stiffer and stronger.

This fits with my understand, as well - except for the 'less stiff' part. The way it was explained to me by a NA was that, because the rigidity is accomplished by the outermost portion of a solid spar... the center can be eliminated - creating a hollow spar - with little (if any) loss of rigidity. Of course... I could have misunderstood him.

[leaotis]

Ideally two equally proportioned mast could be built, one birdsmouth and one solid. Then both tested for flexibility and strength.

I wonder if Wooden Boat magazine would be interested in publishing the building/testing and putting this controversial matter to rest forever.

[P.I. Stazzer-Newt]

The bending stiffness of a circular cross section tube is proportional to the second moment of inertia from the neutral axis

Pi = 3.14...
Do = Outer Diameter
Di = Inner Diameter

I(tube) = Pi x (Do^4-Di^4)/64

This is the fourth Power of the outer diameter minus the fourth power of the inside diameter....
Assuming that the solid and hollow masts are made from the same timber then very little difference in stiffness - the equation is dominated by the fourth power of the larger number.

Taking an imaginary 4" diameter mast with a 2" hollow.
(Do^4-Di^4) = 240.
where the 4" solid gives 256.

Difference so small that two different trees of the same species migh reverse the practical result.

I've been told that a birdsmouth is stiffer than a solid mast even though the BMs contains less wood.
.....
Thanks, Mack

This bit is flat wrong if the two masts have the same outside diameter.

[A_gus]

​ Originally Posted by JimConlin
Versus a solid spar of the same dimensions, a birdsmouth spar has less material in it and will be lighter, less strong and less stiff.
For the same amount of material, a hollow spar can be stiffer and stronger.

Agree with the above

From a theoretical/engineering point of view this is fairly simple. The strength (stress in the material) depends on the sectional modulus of the mast, the stiffness (resistance to bend) on the moment on inertia and finally the weight on the cross sectional area. These are naturally interlinked so I think some examples may illustrate this best.

If we take a solid mast with diameter 70mm and compare it with a hollow mast with outer diameter 70mm and inner diameter of 50mm (wall thickness 10mm). The section modulus (strength) and moment of inertia (stiffness) reduces with 26%, while the cross sectional area (weight) reduces by 51%.

If the inner diameter is 25mm (wall thickness 22.5mm) the strength and stiffness is about the same only a reduction of 1.6%, while the weight reduction is about 13%.

To get the same strength and stiffness as the solid mast you would need to increase the outer diameter. If we make our hollow mast 78mm outer diameter with a wall thickness of 10mm, we would actually get a mast that is 7% stiffer, 4% less strong and 44% lighter.

All of the above assumes you use the same material with same grain direction etc.

/Anders (Sorry but I only do metric....)

[leaotis]

Agree with the above

7% stiffer, 4% less strong and 44% lighter.

Doesn't seem to be a big difference... except in weight.

[David G]

Doesn't seem to be a big difference... except in weight.

Yup - that's the rationale behind a birdsmouth spar. You get the basic working characteristics of a solid spar... but with a good reduction in weight. Less weight aloft. Easier handling of spars. It's worth the extra bit of fabrication hassle.

[hikingchrs]

I would suspect material selection would be more important to flex than building method. I had an undersized Sitka spruce BM mast it flexed allot, my new Douglas fir mast built to compensate for BM weaker nature hardly flexes at all... flex may be important to your design for it spilling ability it however makes keeping the sail shape more difficult. my new stiff mast makes sailing so much better.
Chris