Thread: Hollow Masts

Go with what the designer says because that affects the weight and flexibility of the mast.

Oh hang on, you're working with Welsford's plans ...

Richard

Hey Daddles,

The lack of detail in the plans, allows me to apply my own creativity
I've haven't posted my question to JW because I know he'll promote the Birdsmouth method. I'm really hoping that someone will say that the square hollow is equally good. I'm picking up some Oregon this weekend(140x45) and the size is nearly perfect for a square hollow.

Cheers

From a technical stand point you get a stiffer mast with a rectangular section, then a round spar. This is because the corners are farther from the center of gyration, then the uniformly distanced round staves would be.

I prefer round spars for aesthetic reasons. I just hate the look of a box section mast (that noise was L. Francis Herreshoff rolling over).

I've had occasion to design and build oval spars, using the birdsmouth method as well as round. An oval section provides more fore and aft stiffness and internal volume for halyards, wiring, etc.

The biggest issue I have with birdsmouth is most folks don't engineer their spar to suit, they just build a 20 to 25% stave thickness mast, which isn't saving much weight and usually an unnecessary waste. This of course relates to those replacing a solid or boxed mast with a birdsmouth of similar dimensions. I've designed masts with as little as 12% wall thickness, which is saving a substantial amount of lumber.

In the end, you should consider having Michael or someone make a conversion for you if you elect to build a different stick then the plans show. Navigator's 3/4's (2/3's?) rig is simply stayed and appears to have a fairly bendy tip, but it wouldn't be a difficult thing to muster.

Drop Mik a line on the square masts, he's got a good understanding of them. He's designed square masts for the yawl conversion he did for Sixpence

Birdsmouth looks horrid but is reputedly more scary than actually difficult.

There are easier ways of making a hollow mast too, though not as elegant nor as light.

I think it depends a lot on what you're trying to achieve and why. A nicely done birdsmouth mast with careful selection of timber colours could add quite a lot to the look of your boat. My boats tend to be 'work boats' so I look for 'easy' ways to get good looks eg, I don't sand and sand and sand and ... Having said that, a birdsmouth would be an interesting challenge if you're into building stuff.

It does occur to me that a well designed mast would be designed for a particular method of construction ie, it may not be wise to substitute one method for another as the flexibility will vary, but maybe I'm wrong as I'm not knowledgable here, just trying to tie together a lot of things of read and heard.

Richard

oh, I see you've noticed the wealth of detail on the plans

Typically, PAR got in while I was typing. It's good to have two designers on the list

Richard

It's even better having two designers that agree on most things on the list!!!

That is a great bonus!!!

Yes - PAR summarises it nicely. I like the box section because they are fast to build and not particularly fiddly. They save materials as generally they are a bit lighter and a bit less wasteful of the timber resource - so read cheaper.

But PAR's criticism that he thinks that birdsmouth are prettier - I can't argue with that at all!!!

I'm learning a bit from PAR too - he does know his stuff and has played with some things that I haven't (maybe vice versa too). So I'm picking up stuff too.

For example I have drawn up Birdsmouth masts at 15% wall thickness - which is the traditional standard for a lightweight wood mast, but have been a bit nervous of going down to 12%.

Good to know.

Actually, for a long time I resisted square masts ... ugly ... and always wondered why L Francis Herreshoff used them.

After the OZ PDRacer masts ... I now know why. LF was a lightweighting demon - and he sounds so proud of his method for gluing up long masts on a bunch of bookshelf brackets attached to the side of his house!

Cheers to all
Michael Storer

I've built some pretty big birdsmouth sticks (over 40') and the method works well. It's actually quite easy as the notches are self aligning, usually making a reassuring "click" when you have it just right. I build them in two halves, though I glue it up as a single unit, with some taped off seams to permit separation, when the goo cures. The two halves then can be opened, swallow tails and other elements added internally (sheaves, wiring, etc.) and then sealed up. This also provides an opportunity to adjust any distortions or twists that may have developed into the mast during initial construction (mine never have, since I built a dead flat spar table).

I use construction grade white spruce, unless some one wants to spring for Sitka spruce. In the states we have a couple of large "home improvement" stores, that sell lumber. They sell a cheaper version of the common wall stud, called an economy stud. They are cut short, intended to fit without cutting between standard headers and footers in a house. The wood is almost always straight grained, very light and reasonably clear. I can get several staves out of each 2x4 which makes the mast lumber costs very low. Lumber costs on a 16' mast would be about 8 dollars, which is damn hard to beat.

Scarfed together, with well staggered joints, the assembly becomes a round spar pretty easily. I use an asymmetric stave arrangement, which saves some rounding effort and provides a slightly longer glue line inside the notches. If epoxy is used (I don't usually) the joints can be visible. If aliphatic resin is used, then glue lines are all but invisible. The same would be true from urea formaldehyde glues, which I don't trust in anything other then a trailer boat.

Michael, the 12% mast did survive a full season of racing, which was the requirement for the stick. It broke, the following winter when he got caught in a sudden thunder storm. The 45 to 55 MPH winds he experienced would likely have brought down any mast of reasonable stave thickness (the main sheet over rode it's winch, jammed and caused the de-sticking). I built another at 14% which has served him since.

Okay you've got me convinced.
I'll give the Birds Mouth mast a go....Only because I've got a little bit of hair left, and I want to get rid of it

I've been going through my Router-Bit inventory and I've got 2 Birds Mouth bits(never used of cause)
1 bit is a 16 Stave Bit and the other is a 6/12 Stave bit.
Should I try a 12 Stave Mast? It should save a bit of time when I come to sanding.

Anyway, I'm off to get some Oregon. I'll build a smaller "test mast" first just to get my technique right.

Thanks for the advice guys.


Cheers

I would keep things simple and use an eight stave stick. Handling 16 staves can get unwieldy without some spar building experience.

Here are three sections of birdsmouth mast, using 8 staves. The left side is the commonly used symmetrical stave notch layout. It's easy to do as the notch is in the middle of the stave edge, but it requires more rounding effort. The right side is the asymmetric layout I mentioned. It requires more setup to get the notch right, but saves much material and time when you knock it round. Below is an oval mast section of the same athwart dimension as the ones above it. I've just increased the width of the two side staves. Also note the dividing glue line, so I can make them in halves.

The notch can be done on a table saw with two passes, or a single pass using a datto blade. I prefer to use a router table, as I get less "chatter" and more strip control this way, but that just me. A taper jig can be used on the table saw, though I usually just plane the taper into the staves at the bench.

Ooo I like that Oval shape.
Correct me if I'm wrong, but it looks like the oval mast only requires 2 opposing staves to be slight longer than the rest?

Would you recommend epoxy or a glue like Vise/Gorilla glue???

I've been in the garage most of the morning cutting up stringers from some WRC planks. Gee that stuff is so easy to work with. Tomorrow I'm going to tackle the Oregon, hopefully it will be just as easy

Cheers

I don't use epoxy on mast as a rule, for several reasons. I don't use Gorilla glues at all ( I dislike the stuff), especially if the glue line may be visible. On a brightly finished mast I'd use an aliphatic resin, because I can match the wood color with the glue line. On a painted mast resorcinol would be the choice.

Epoxy on a bright mast is subject to UV protection, which often gets neglected, possibly threatening the joints. Mast temperatures can swing pretty high in my tropical sun shine, again threatening the joints. On a painted mast I'd feel better about it, especially if it was a light color.

Of course the depth of the oval shaped mast goes fore and aft. I placed it that way to save file dimension size for the import filter on attached files.

This is one way of dropping some internal halyards inside the pole.

Thanks for all your advice and help PAR. I really appreciate it.
The Oval shaped mast with internal halyards certainly look like an ideal solution.
I would never even attempted such a thing.
You certainly have me thinking well beyond the "Curtain Rod" that I was originally planning to use for a mast.

Again, thanks for your encouragment and help.

Cheers

Mickj

Mickj,

I've got a selection of about 33 photos taken during one of our mast building sessions. The mast was for a beautiful Phil Bolger-designed Harbinger which I built (strip/diagonal construction, with PCB's written approval). 18,000 (!) staples driven and then pulled out - one by one!

The mast is typical of dozens I've made - the staves are proportioned to yield 17% wall thickness at the widest part, and are hand tapered on a bandsaw prior to having their 'bird's mouth' cut. I do it by two passes over a table saw with the blade set at 45 degrees. It is very important to get the blade depth correct, but then the cutting is fast, accurate, and simple.

My masts have solid inserts top and bottom (seen in the photos) and the bottom one always finishes in a long 'cloths pin' V-cut out (swallow tail) so as to avoid a hard spot and the subsequent stress concentration.

If you want the photos just drop me an email to [email protected] and I'll send them to you.

Ross Lillistone www.baysidewoodenboats.com.au

This is an asymmetric layout, for a twelve stave mast. This provides a thinner stave wall with less rounding required compared to a spar using less staves, but you really need to have a good reason to warrant the extra work.



Nothing more then a rubber band holding the staves in alignment.



It helps if you can hold the mast quite straight during the process.



Two mast halves, with a symmetric notch arrangement and the more easily handled 8 stave layout.

I think birdsmouth is faster and easier to build then box section, mostly because it's a self aligning method.

Build a three foot long mast section and try it. You can rip a scrap piece of 2x4 for all the stock.