Plywood box section mast

[Dalan]

Hey folks, I'd like to solicit opinions on this method of building a small-boat mast, which I haven't yet seen talked about hereabouts. It certainly seems simple enough: a rectangular section, approx. 2-1/2" x 4", with the lateral (wide) sides made out of 1/4" plywood and the front and back (narrow) sides made out of ~1" solid stock (sitka spruce, Doug fir, etc).

Glen-L shows such a method as an option on many of their sailing dinghy plans.

I'm considering this option for a Bermudan sloop rig for possible use in Iain Oughtred's Gannet.

The advantages as I see them: (1)Half plywood; needs less of the expensive and hard-to-find clear straight-grained solid stock. (2)Easy to build - particularly if I cut 1/4" rabbets in the edges of the solid front and back faces to hold the thing in alignment during glue-up. (3)Easy to fashion solid core sections where needed at the partners, hounds, masthead, etc. (4)Easy to attach mainsail - just screw a sail track to the trailing edge and use slides on the mainsail. (5)By my rough calculations, should be sufficiently light - such an 18' mast would weigh around 18 lbs, only ~4 lbs heavier than a bare Dwyer DM-2 extrusion.

Possible disadvantages: (1)Boxy, less efficient sectional shape; (2)Aesthetics? (3) Strength? Is there any reason to think that this method would be weaker than an equivalent bird's mouth or other hollow section made of solid stock?

This would be for a stayed rig, so I don't see flexibility (or lack thereof) as a major concern.

Thoughts, anyone?

[Ian McColgin]

It would work and the simplicity may be a plus but you've really got a cross section area of 4 square inches solid wood plus your plywood.

A cross section that has the 4 sq in rearranged in a slightly smaller bird's mouth would be aerodynamically and aesthetically nicer. And even lighter.

It would cost more both for the additional epoxy and the fact that there's more wasted wood.

But in a small unit, the cost difference is small compared the the fact that you'll be looking at it alot.

G'luck

[Keith Wilson]

Although I normally argue against traditionalists in favor of plywood, this is a case where plywood may not be the best material. The cross-grain plies contribute little to the rigidity or bending strength of the mast, and the lower density and hence greater thickness for a given weight of solid wood is also an advantage. Trees, after all, have evolved to stand just like a mast and take the same sorts of stresses. OTOH it would probably work fine.

One advantage of a birdsmouth spar is that you can put scarf joints in the staves without any problem, allowing you to cut out knots and bad grain. It's quite feasible to make a very good one out of not-very-good lumberyard spruce/fir, although you'll end up with a lot of kindling.

[John Blazy]

What about brick mould bought for cheap at a lumber yard as the staves in an elongated, hollow octagon (cross section)? That stuff already has the knots finger joined out to be "clear", and is probably cheap. If you go the plywood route, I'd use a little thicker material. - JB

[Dalan]

Originally posted by John Blazy:
What about brick mould bought for cheap at a lumber yard as the staves in an elongated, hollow octagon (cross section)? That stuff already has the knots finger joined out to be "clear", and is probably cheap. If you go the plywood route, I'd use a little thicker material. - JB"Brick mould"? Whassat?

Thicker plywood... hmmm. If I'm picturing the loads on a mast properly, the solid wood fore and aft faces of a ply-box mast would bear all of the transverse shear loads (the plywood sides would be in compression/tension), and the wide plywood side panels would bear the fore and aft loads, where I would think the wall WIDTH would be more significant than the thickness (picture the web of an I-beam). Nay?

Regarding your "elongated octagon" section, would you be talking about some sort of birdsmouth joinery?

[Mike Keers]

I built a 13-foot Glen-L catboat about 15 years ago, and built the box mast as they detail on the plans--as you say, 1/4" ply sides and lumber fore and aft. I was able to incorporate a groove for the luff rope of the sail into the aft side. The mast is long, perhaps 22 feet, and is stayed.

We've used the hell out of the boat, and never had any problems. This plywood box mast method was very common not so long ago, and in fact a friend has a plastic Chrysler Buccaneer, and the factory mast is built the same way. I've also owned other older wooden boats that had ply box section masts over the years.

Bird's mouth spars have been around a while too, but only recently gained such widespread notice and popularity amongst amateurs, perhaps because of epoxy and the publicity.

Diiferent strokes for different folks, it comes down to what you're comfortable with, your skill and tool inventory. But there's no reason a box mast can't be entirely satisfactory, they have been for many years.

I have a short cut-off piece of my Glen-L mast sitting right here on my desk, so here's a pic. This mast measures about 2.5" wide, and 3.5" the other way, exclusive of the mast groove pieces, which add about an inch, so total depth is around 4.25". The ply is rabbeted into the fore side.
http://personal.riverusers.com/~emkay/mast1.jpg

[ 01-07-2003, 12:10 PM: Message edited by: Mike Keers ]

[Dalan]

Mike: thanks a ton for a first-hand account of your experience with this sort of mast. As this would be the first mast I've ever built, this method appears to be the least intimidating of all the possibilities and is suited to the tools I have available and my woodworking skill (or lack thereof!). I'm thinking I could build such a mast without ruining too much nice wood.

A couple questions: Did you taper the mast at all, or is it straight-section for the whole length? If tapered, how'd you do it? Did you use rabbets in the lumber faces to hold the plywood sides in alignment, or did you just butt-join the whole thing? If butt-joined, was it problematic to keep everything aligned and square?

I'm encouraged to hear the mast has served you well. You didn't mention thinking it terribly ugly...

[Mike Keers]

Dalan,
I guess our posts crossed while I was taking and posting the picture.

As noted, the sides are rabbeted into the front piece, about half the thickness of the 3/4" lumber. At the aft side, the ply lays flat on the lumber, and comes out flush. I built the box first, and applied the luff groove pieces after. These pieces are optional, you could use metal sail track, but I enjoyed the challenge of making them.

Only tools used to make this mast were a table saw and router. It's simply nailed together with very small galvanized finish nails, to hold everything in alignment until the (slippery) epoxy dries. There is no jig needed for keeping things square, it's all self-aligning.

The mast is tapered both ways. The foreside is tapered on the upper half by changing the width of the ply side pieces. The width is tapered by tapering the lumber pieces, before rabbeting in the case of the front piece.

There are of course solid lumber filler blocks in the usual places--gooseneck, foot of the mast, truck and at the shroud attachment points--there are no spreaders on this particular mast, the stay'shrouds terminate about 3/4 up the height.

As for ugly, well, I think it looks just fine--the taper certainly makes it look graceful. I'll try to find a pic.

[Mike Keers]

Alright,
Here we are--ugly? I'm afraid this pic doesn't show much of the mast, but the point is, if anyone out there saw this coming at them across the water, and their first thought was "Jee, look at that ugly plywood box mast" I'd be pretty surprised.

BTW, if you go with metal sail track, the construction is much simplified, and since you rabbet both front and back pieces, it goes together even easier.

After mine was completed, I gave it some epoxy coats and then painted it. It is important to keep these masts up--I've repaired a fair share of them over the years for others that were allowed to deteriorate.
http://personal.riverusers.com/~emkay/glenl13.jpg

[Milli]

I would like to put in a plug for the conventional solid mast construction. One or two laminations of spruce or pine or similiar straight grained stock to make the mast blank and then taper and round. I made a 12 ft. mast for my Piccup Pram this way and it was easy and very rewarding to build; the main tools were a circular saw and a hand plane.

When you work it out there isn't that much weight savings for a hollow mast and you don't have to worry about internal blocks where you want to fasten things. With a bit of figuring you can also work around knots in your stock material quite easily. I have looked at the bird-mouth and box-construction methods as well but still come back to the traditional method for the reasons outlined above.

[Mike Keers]

I think before generallities can be made, it's important to consider the size and application of the spars. Milli, your twelve foot long solid mast is probably fine in the application you're using it for.

The mast I described is twenty feet long, and of the dmensions previously described. I'm not qualified to speak to the engineering involved in the size difference between a hollow spar versus a solid one, to arrive at comparable strength and rigidity. But I can comment on the weight issue you bring up.

The box mast section I have just happens to be exactly a foot long, and in weighing that, I found it to weigh just under 1.5 pounds. I did some calculating, and arrived at the following figures--approximate and for comparison purposes only, as they don't take into account mast taper, or solid blocking in a hollow mast--but just for sake of rude comparison and discussion--

A twenty-foot mast, as on my boat, would weigh 29.16 pounds in box section (1.4" ply, 3/4" fir lumber, and with the wooden luff groove).

The same scantlings produce a 38.88 pound solid spruce mast, and a 49.81 pound pine mast.

Now, in a small unballasted sailboat, there is a tremendous difference between a 29 pound box mast and a 50 pound (pine) one, especially as far as righting moment and so forth goes when the stick is twenty feet tall. I forget the general rule of thumb regarding weight at the masthead and weight of ballast, but it's something on the order of a pound at the masthead requires like five pounds of deep ballast to compensate--this is just paraphrasing from a very poor memory, but the point is, minimizing weight aloft is always desirable, and in an unballasted boat even more so.

To return to the original point, solid masts certainly have their place, but there's much to be said against a solid mast of larger proportions in other contexts. For example, a mast of solid timber to the dimesions I described would involve something like two clear twenty-foot 2x4's to start with....if you've priced clear spruce, or can even get it, I'm guessing cost would be a factor. I had to make new spreaders for a client a few months ago, and a modest slab of clear spruce had to be shipped in, and the cost was approaching what I pay for teak. Not everyone has access to quality lumber or a broad selection of materials.

Don't get me wrong, in the right application I'd go for a solid mast myself, I'm just saying the size and use of the mast and boat enter into the discussion of the various forms of construction.

[Bruce Hooke]

Yup, I'd agree with the basic direction others have laid out - a box mast like this is not the height of efficiency and aerodynamics but it should be perfectly functional as long as your goal isn't to win highly competative races.

Regarding brick mould (a type of trim often applied around exterior windows and doors, especially on brick homes) I have seen too many of these finger joints that appeared to be failing to leave me with much confidence in the integrity of such joints. So, I'd stay away from finger jointed wood on boats (unless you make the finger joints and glue them with epoxy). If it doesn't last well on houses it sure isn't going to last well on the water!

[ 01-07-2003, 08:09 PM: Message edited by: Bruce Hooke ]

[SailBoatDude]

Using Skene's I figured (@75 sq. ft.) your stick would work out at;

27.3 lb. (solid unstayed) 14.6 lb. (solid stayed) 10.1 lb. (hollow stayed) for Sitka spruce.

34.4 lb. (solid unstayed) 18.4 lb. (solid stayed) 12.1 lb. (hollow stayed) for Dug fur.

26.3 lb. (solid unstayed) 14 lb. (solid stayed) 9.7 lb. (hollow stayed) for white spruce.

No plywood, just the math. The box section shown is rather over kill for the job needed. I'm sure this was the intent of the designer, knowing it was to be built in a garage, by less then skilled hands and sailed by the same.

Plywood is my friend and I'm always defending her, but mast construction is no place for it. Half the wood is going the wrong way and the weight penalty is large. On the stick is not the place to pay this toll.

Hollow, birds mouth, solid, take your pick, the numbers speak for themselves.

I you give me the rig type, total sq. ft., luff and foot dimensions I can work out the sizes, shapes and weights reasonably quickly, for whatever type stick you need tapers and all. I'll also bet someone here has the cute little software stuff to do it in a fraction of the time I can.

[ 01-07-2003, 09:09 PM: Message edited by: SailBoatDude ]

[ion barnes]

You know, Its amasing how things become new again. I have a reprinted book, Jane's Book of Fighting Aircraft 1918, and in the section about German aircraft, they give all the so-called German secrets, like hot-molded plywood, laminated box beams, and details of solid wood spars. I have found it to be a real eye opener when you think of the kinds of glue they had to work with and metal fabrication too. Look for a copy in the library and compare that to what we are doing. Its like deja vue.