Thread: Birdsmouth for GIS Main Mast

Clint

How about calculating the birdsmouth pieces on the inside of the mast - then you can taper the pieces while having constant wall thickness? That is, design it so that the inside taper is correct (assuming tapering staves) but make it with constant thickness staves (tapered in width).

Then when the mast is made up, you can plane each stave so that the outside dimensions are correct, ie so that the top is at the correct thickness (thinness?), and it tapers correctly from foot to cap.

To make it properly, so the peak of the v in the joint runts down the middle of the tapered piece, the birdmouth part itself will need to get smaller (shallower) and move towards the inside. I think you may need to use a gentle taper jig on a router table to do this properly (it won't be much of a taper but it will be appreciable). However, the advantage of that is that it will make the planing easier - you just need to plane off the bits that stand pround of the birdmouth joints.

There are two ways to build a birdsmouth mast, an asymmetrical and a symmetrical stave layout. I strongly recommend the asymmetric arrangement. It's stronger, is cleaner on the inside and is easier to smooth on the outside. Frank's staves were arranged in a symmetric layout.

Michael could easily work out the dimensions of a birdsmouth stick for the GIS. If you posted the general dimensions, I could do it (no charge), including the weights for different species and stave thicknesses and tapers.

The taper can be preformed easily with a taper jig on a table saw. I usually just plane the staves as a clamped together unit (all at once) on a bench, but to each their own. I build several birdsmouth masts per year, including the odd flag pole or two. I use stave taper when weight and mast bend is a design consideration. Ditto mast taper.

Does this Duckworks article help at all

http://www.duckworksmagazine.com/08/...outh/index.htm

Brian

Thanks guys. I'll need to have a lot more coffee to figure this all out.

Cheers,
Clint

Howdy Clint,

I do specialise in weak jokes!

With the GIS, specifically, you can change the mast to birdsmouth without increasing the diameter at all to compensate for the slightly less stiff hollow section.

This is not a joke.

To get the taper (don't you DARE put an untapered mast or any untapered spars on one of my boats!!!!) just copy the taper rate for the overall mast as designed. So ... whatever height the mast is 25% thinner than at the partner, then the stave will have to be 25% tapered at that point too.

The don't DARE ... was a little bit of a weak joke. I think NORM who did the Canadian BETH recently used alloy masts with timber inserts at the top.

Actually the taper of the mast is not Quite that simple for two reasons.

There was no joke in that sentence at all that I could detect

1/ The actual taper rate is between the flat face of the stave and the apex of the birdsmouth notch. But .. I don't think that it is that important to be 100% accurate ... maybe I would worry a bit with a bermudan bendy mast rig or a gaff or lug yard ... but for a lug mast it is not very important.

2/ There will need to be a compromise at the top ... the way I do my mast tapers for spars carrying a gaff or yard is to assume the mast is going to the top of the sailplan and do a traditional taper at the normal heights. I then dock off the part of the mast I don't need and put in some extra taper to get the weight out of the tip by bringing it down quickly to the diameter of the top of the mast in the traditional taper. The birdsmouth will be more restricted in this as I think the stave thickness will prevent the taper from coming down quite as far as a specify in the GIS mast.

3/ As far as tapering the wall thickness ... it is too much labour, I think, unless an owner really likes the idea and is prepared to spring for the extra work. My mate Dusty gets a nice taper in the wall thickness by making the staves, tapering them normally and cutting the birdsmouth. Then he tapers the wall thickness by setting a power plane to about a millimetre and taking some equally off the two faces (that define the wall thickness) of the tops of the birdsmouth staves. So maybe he does one run starting from a couple of feet from the top of the stave. Then another run from 3.5ft from the end of the stave etc.

I have used the glue up the whole thing then take something off the outside at the top type method. We used that for the hardwood birdsmouth mast on the Fenwick Williams Catboat.

Did you see how subtle my weak jokes can be ... I gave you three points ... not two (guffaw).

Maybe a good way with the GIS is to glue up use the same wall thickness to the top but make the top a bit larger diameter as you will be forced to do by the birdsmouth method. Then plane the top bit down to something like the original tip diameter making sure it doesn't reduce the wall thickness too much.

Best wishes
MIK
(who hopes you got a giggle)

I was thinking I MIGHT even do a reverse taper, Michael...make the top of the mast about an inch larger and make a little flat spot on top and put my son's little toy goat at the top. The goat might make a good wind vane.

That would be way too labor intensive. And ugly. Like a reverse sheer.

Okay, I was thinking along the last line you said, gluing up the mast with the constant wall thickness and taper the outside down to get that last bit of taper. Perhaps a few swipes of the plane to knock down the stave thickness wouldn't hurt either. Good, sounds like the right plan.

Until later...

Clint

Michael, your buddy Dusty does his tapers similar to mine. I just plane the taper on the notched side, before I notch them. I do them all at once, starting with a power plane to rough them in, then finishing with a hand plane. I can do several feet of taper in a few minutes this way. On large masts (over 30'), I use a tapered ramp, which is little more then a couple of boards, shimmed up to the taper I need and the plane in feed shoe rides on these. These long tapers require more control and the ramps insure I don't get hollows or humps.

There is a point of diminishing returns in regard to how aggressive you can get with the tapers, but generally you can have conventional looking shapes, using this method. I've found on larger sticks, this isn't as true as smaller masts, mostly because you have more material to work with, I suspect.

Heed Michael's advise about sectional and stave dimensions. Scaling a stick to specific loads can be a profound weak leak in a rig. There's not much more exciting then having a catastrophic mast failure while showing off you sailing skills to friends. Having exploded more then just a few poles over the years (there was a season or two that I just seemed to excel at it, for some reason), it's not some thing you really want to experience. Besides the long row back to shore, you have the tattered remains of the tangled gear dangling in and out of the boat. If you're fortunate enough to have not bonked someone on the head with the "event", you still have the painful embarrassment of loading your broken masterpiece, at the normally vacant launching ramp, which of course this time, will have a dozen or so people milling around, most giggling, as you forlornly drag your busted up contraption home.

As a rule, you can safely add 25% to the diameter of a solid or square sectioned mast, to make a hollow birdsmouth. It'll be heavier then necessary, but it will probably not fall down on you. You can also control sectional modulus with species selection to some degree, as well as stave thickness and notch cutout method. An asymmetric notch is slightly stronger then a symmetric arrangement. This can produce a lighter stick for a given bend rate.

In the end, it's best to let the designer engineer diameters and stave dimensions, as the calculations can be rather daunting for a novice. This isn't to say you couldn't do one, you can, but it'll likely be heavier then necessary and this is a performance killer in small craft.

The 25% rule is good conservative advice that won't cause any problems.

Sure both Paul and I will sit down and work it out ... just to save that extra pound or so.

The calc is not really hard.

Michael

Michael is right, the calculations aren't especially hard, unless you get into some close tolerance stuff (racers, highly strung setups, high tech materials, low safety thresholds, etc.). Most folks, I found, have difficulty getting their head around some of the formulas, the math and concepts, a few of which seem abstract (which kind of explains people like Ross, Michael and me don't it).

I'm a weight freak, okay, possibly just a freak, at least she says so. A recent example was deck beams and stringers that I cut lightening holes in, because of materials substitutions. I used a lumber that was almost 40% heavier then what I'd original planed, but it was also much more rot resistant and considerably stronger. My lightening holes reduced the weight to that of the original species, but I gained an additional 15% in strength (so, of course I made them smaller still). A lighter, stronger structure resulted. Anal, you bet . . . though I will carry more weight on the bottom then an equal model built to specs. I made several other "enhancements" to the design, which ultimately could yield a few degrees less heel then the same boat, sailing along side. I'll kick their butt, just because I was a wee tad anal about weight in the design and building process.

I built a popular (in the USA any way) cat ketch recently. The design was from a buddy and a good one. The client expected to met others of the same design at gatherings and didn't want to be at the back of the fleet. Though it was a spartan design, typical of his work, I paired it down a little more. His 17' dry hull weight was about 550 pounds. I got it done at 475 pounds, for a 14% reduction in weight. The first meeting of the same model boats, included the designer in the prototype and a pickup race to a small island for lunch. My client didn't win in the light airs of the day, but was just off the transom of the designer's boat (the winner). Afterwards the designer openly admitted that he'd been beaten, but noted that Ray (my client) wasn't as attentive with sail shape as he should have been, so he took advantage. Sailed better he'd have lost easily. Such is the difference 14% can make and likely why I chase down weight when possible.

Okay, rant over . . .

Let me ask this one: what does a hollow square mast, which Michael has drawn for the Goat, have over solid and B'mouth? I still think the B'mouth is nicer looking, but maybe I should see some more detailed photos...maybe I can be convinced. The simplicity of it is nearly enough...is it better or not than going B'mouth for the Goat? (I also 'feel' like it is less aerodynamic, but only at high speeds?)

Cheers,
Clint

A hollow square section mast is stronger then a hollow round section of the same diameter. Also, amazingly enough the square section has better aerodynamics then a round section, unless the round section has a sock type luff, which then makes is better. A square section is easier to make, but doesn't look as nice as a round stick.

Birdsmouth masts are hollow. Solid masts can be built up from pieces or simply a whittled down tree of suitable diameter. Both of the hollow mast styles will be much lighter (if designed properly).

On the wind is the only time the mast section really effects the aerodynamics of the sail, unless you're up on plane and dragging the apparent wind forward, while off the wind.

In the case of the GIS, mast section isn't nearly as important as it is when the luff is attached.

Personally I'd want a pretty mast for a GIS, which is in direct contrast to Michael's function over form ideas. I relate it to a prospective wife, who could function very well, cooking cleaning, pleasing, though being butt ugly, I'd have difficulty. This forces me to insist on the whole ball of wax and one who cooks, cleans, pleases and is easy to look at too. Interestingly enough, the designs I admire the most, not only perform very well, but also happen to look great doing it. I consider this the hallmark of a great designer, making very functional shapes look good while they're chewing up the competition.

To me it's not enough to function well. An example of point - if you take a regular household door, preferably smooth faced, make a small bracket and hang a 5 HP outboard on it, the thing will get up and scoot. Maybe some of you have seen the photo of the 'fella that has done this to a kitchen table and is blasting along on a lake. Well it functions well, but lacks in the style department, unless you count weird as a good thing. I would have had to use a nice piece of Chippendale or a hollowed out Steinway baby grand, just to satisfy the "form" aspect.

Howdy,

The original mast for the Goat was the traditional solid round one. It is a nice intro to some traditional skills and the use of the spar guage.

However the solid round mast is quite heavy and a fair bit of work.

So along came the square hollow mast which saves about 45% of the weight and often something off the material bill too and saves labour. I believe the construction is completely non scary.

The reason I did not straightaway go for a birdsmouth mast is I considered it too specialised - requiring specific machinery to do the job and had a good chance of causing significant problems in tool use and procedure for most of my target builders.

Assumption was that if someone really wanted one they would either work it out with my help (as above) or if not equipped to do the calcs, give me a push (as also seems to be happening now) and I would draw one up to become a third mast option for the boat.

Also, interestingly, birdsmouth has become the "flavour of the month" and I think many who would not have considered it a few years ago, would consider it a strong possibility now. Perhaps ... this is exactly what we were all hoping about wooden boatbuilding skills not disappearing .. here we have quite a sophisticated, non traditional method becoming reasonably commonplace. Quite exciting I think!!!

So ... 15 years ago we had the solid mast, about 3 years ago we had the hollow square one, next the birdsmouth.

On the technical side, PAR and I are close than brothers, we seem to make similar design decisions again and again - something to do with quite broad boating backgrounds with a lot of racing in them. Probably the slight differences in our approaches also reflects something of our backgrounds. Paul is highly skilled but understand how to make things easy for amateurs and, well, I am really an amateur builder who has done some professional building as well.

From that I learned that I am intrinsically fairly lazy in boatbuilding terms (I did have some inkling before then) ... so this does tend to reflect the types of boats I design.

Like you will never find me designing a Cape Cod Catboat ... and am fairly unlikely to design a multihull ... my resistance is purely one of surface area!!! I have seen enough surface area in my life to not want to see any more of it needing fairing!!!

Anyway ... this is a great thread and some new things are coming out of it. Much better than threads that go round and round endlessly as they do in some other places ... so once again I'd like to thank everyone for their contributions! (to boating and world peace) ... geez .. just thought ... what has happened to Bjarne ... haven't heard from him for a while!

Anyway .. best wishes
Michael

Bjarne is busy building, hopefully!

Michael, well I am still open to the hollow square mast, but would sure like a plan showing details for a B'mouth mast. I am so tired right now that I am lacking the mental energy to think about it, but hopefully that will pass when my current project finishes. I would be VERY interested is obtaining plans for a B'mouth mast plan.

Cheers,
Clint

Howdy Clint,

If you want 'em ... I will do 'em.

ie work out a birdsmouth mast for the GIS.

But next week.

This week is RAID RAID RAID RAID

MIK