Hollow Square Section Masts

[marshden]

Greetings, I know that MIK uses hollow square section masts in some of his designs and so I thought I would come here to see if someone could direct me to a resource where I can learn how to build one. If anyone has a book reference or a web link, I'd be grateful. I'm considering building an 18'1" and 17' box section mast in order to avoid using aluminum. No birdsmouth spars for me, I've not got the tools nor money to get the tools for such things. Thanks, Marshall

[PAR]

Building a box sectioned mast is as simple as the name suggests. The glue line can be dramatically increased (always a good thing) if the side staves are rabbited into the fore and aft staves, but most small boats can live without this bit of wood working expertise. If you can build a box, you've got the problem licked. The only real test is insuring it is square and straight.

[Boatmik]

What sort of boat Marshall?

What mast length and what sail area? What type of rig?

MIK

[Papa]

There was a site all about this, but it went with AOL.. to internet heaven. or maybe not...:no::no::no:

I did this from memory and that ain't what it once was,, what was I
talking about??? Oh yes,, the mast thing..

Here it is:

[Boatmik]

Wow Papa - Amazing .. it is is gone from elsewhere then it is good to have it here.

However plywood is not the best choice for wooden masts.

Most of the loads in a mast are lined up with the vertical direction. So that is the direction you want the wood grain to go in.

With a plywood mast precisely half the grain is going in the optimum direction, the other half is going the wrong way. So they are roughly (very roughly) half the strength and stiffness of a mast made out of solid timber.

The result is a ply mast of this type is heavy or large in cross section for a particular boat.

The method that PAR talks about is the best and the most convenient in terms of assembly as it is much more self aligning than the method in my plans.

However it does require a bit of tricky carpentry or extra specialist tools to cut the four long rebates. Similar problem to the birdsmouth.

At the moment my plans only require three power tools.
Sander
Jigsaw or circular saw . jigsaw is safer as mistakes happen more slowly.
Cordless battery drill

If builders do have routers or something similar or have a nice rebate plane then they can do the the classical method that PAR mentions. With bigger boats than I have plans for here, I would recommend it too.

Best wishes
Michael Storer

[marshden]

Par, I can build a box to be sure, but I was trying to find information on the thickness of the wood, stave size, amount of blocking for giving support at partners and the like.

Michael, the masts are 18'1" and 17' respectively. The boat sails a cat-ketch configuration of approximately 120 sq ft. I've made solid spars before, but it strikes me that hoisting a solid spar of 18ft with sail wrapped around it could get a little precarious as there is an awful lot of weight aloft and little control on ground level.

Papa, thanks for the file.

For some reason I am unable to acquire the image of Papa's post. I can obtain images just fine from other topics on this board, but not here. Any idea what the problem might be? Thanks

Marshall

[jmk89]

For some reason I am unable to acquire the image of Papa's post. I can obtain images just fine from other topics on this board, but not here. Any idea what the problem might be? Thanks



Marshall

The problem probably is that you and Papa are on different URLs for the Forums (for historical reasons there are three:
http://www.woodworkforums.ubeaut.com.au/
http://www.woodworkforums.com.au
http://www.woodworkforums.com

If you go to the others, you will likely find the picture on one! Alternatively, log in to all three (separate windows or tabs - I use separate windows and minimise two of them). It seems that when you do that the pictures on all will show on any one of them.

[Papa]

The article about the box masts is gone for ever, but there was
an explanation of how they worked, the stringers give it strength and
the ply just keeps the stringer in line, in fact, they recommend 1/8
ply in the article. But that is hard to find around here in large sheets.

They had a 18' mast with 1/2" stringers and 1/8 ply skin that weighed
11lbs and with a 180lb person sitting in the center with the mast supported
on the ends, had less than a inch of deflection.
The cross sections was 2 1/2"X3 1/2" tapered to 2 1/2"x2 1/2' at the top.

It was unstayed BTW.

[Boatmik]

Howdy Marshden,

I generally use a box with a wall thickness about 20% of the cross section measurement.

I also use this URL for the forum and can see papa's post
http://www.woodworkforums.com/forumdisplay.php?f=29

Generally I work out the sizing of a round spar the traditional way. Then calculate a square section of the same stiffness.

Usually when working out stiffness from scratch you need to know the material stresses and strains ... but as this is a direct comparison you will use the same material for both so you just need to compare the stiffness of the mast cross section.

The measurement is called the second moment of area. Actually quite a simple bit of Algebra (eeeek) ... no harder than calculating the area of a circle or a triangle.

http://farm3.static.flickr.com/2388/1709372400_3ca7160d33.jpg

I = the second moment of area

For hollow spars you work out I for the outside measurement of the mast and then work out the I of the hollow area and deduct it.

Michael

[marshden]

Hi, Mik and all,

So, Gerr says that a hollow spar is 1/90 x length, which for an 18ft mast would be approx 2.5 inches. I could get away with a 2.5 " hollow box section mast for both and reduce the weight by approx 45% and, if my calcs work, maintain as stiff or stiffer spar than aluminum? Marshall

[Boatmik]

It doesn't make sense.

I think Gerr would have a whole bunch of criteria to work out a mast diameter. The loads on a mast are proportional to stability. So the length could be the same on a stable or a less stable boat.

2 1/2 inches might be OK for a stayed mast, but our unstayed masts for the little singlehanded PDRacers are about that size ... and they are an 8ft boat with one person working hard to hold them up.

If your boat is more stable (bigger, particularly wider or more weight or crew) than this ... then the mast will need to be bigger too.

Have a closer look at Gerr's book ... there must be a more complete method for unstayed masts.

Michael

[marshden]

Hi, Mik,

Thanks for the reply. My reference to Gerr was from T.F. Jones' "New Plywood Boats." Jones pointed out that this formula was for aluminum masts, but he found it to be right for hollow spars (box or round) as well.

The Oz PDR flies an 86 sq ft sprit boom leg-o-mutton, as I recall. My main sail will not exceed 70 ft and the mizzen will not exceed 50. I'm thinking that the 2.5 number -- which exceeds the mast width preferred by Gerr (as reported by Jones ref to the formula 1/90xL) might be OK. I am only in the beginning stages of gathering knowledge to be able to make an appropriate judgment about the matter. Your help here is very much appreciated. Thanks

[PAR]

I'm not sure who this T.F. Jones fellow is, but he's nuts to make such broad and dramatic conclusions, not to mention just plain wrong. Is it me, or did that rub you oddly too, Michael?

I know Dave Geer and he wouldn't have made such a grand and sweeping generalization like that. For no other reason then he has too many students, wanting to be NA's counting on better judgment that that.

Rough calculations (in my head, from experience) suggest a 18' mast carrying around 70 sq. ft. of area, on a hollow round mast, with a 20% wall, should be 3" at the base with a continuous taper to 1.75". The mast would be about 6.5 pounds if Sitka or white spruce and around 7 pounds if Douglas fur. I consider this a pretty light stick and rather flexible. This could be controlled some, by starting the taper further up the stick.

[Boatmik]

Howdy PAR and Marshden,

Gerr is a very methodical writer. I would trust him to the ends of the earth.

If TF Jones is making unqualified statements like that then I wouldn't trust the book in this case. His advice is wrong and he has misunderstood Gerr.

The design of the mast is always related to the stability of the boat ... not the sail area.

There are some shortcut methods that don't rely on full stability information, but they do rely on factors like weight and beam etc.

Gerr's book would have them.

What is your hull length, weight and beam and maximum crew number?
Also how high is the top of your mainsail from the mast partner?

I will whack it through the spreadsheet for you.

Michael

[Boatmik]

Howdy Papa.

Sounds like a good bench. <grin>

The traditional methods balance stiffness, strength, cross section and weight.

Also the traditional methods provide the right amount of bend to get some automatic depowering of the rig (for leg of mutton sails). The measurements you gave look very appropriate for a masthead rigged yacht. The top will be much too stiff for most 3/4 rigged boats - this is why it didn't bend much. Under high load the PDRacer masts. Don't deflect much in the bottom half, but are quite flexi in the top third.

This is the approximately optimum for the characteristic of the bend. But we did make these masts too soft so built a second set a little bigger diameter that were perfect. The sail needs to be cut to take account of this bend.

http://farm1.static.flickr.com/142/384010227_a974e47f49.jpg

If you do an engineering comparison of a mast made of plywood it will come off second best to a mast made of wood for the reasons above. It either has to be heavier or a larger cross section.

There is quite a lot of wind in this shot. Pure stiffness is not the solution ... it inevitably results in a mast that is heavier than needed and boat performance in stronger winds will usually be less than something lighter and more flexible. Controlled use of the flexibility allow a much greater sail area to be carried into much stronger winds.

http://farm1.static.flickr.com/173/459817181_5228eb151c.jpg

http://farm1.static.flickr.com/204/459817053_f153402877.jpg

None of this is quite as important with a four cornered sail as some of the bending duties are passed to the gaff or yard. The four cornered yard misses out on this function completely as everthing is triangulated.

The ply box is worthwhile for a boom for larger boats than we are talking about here ... because there is not much of a downside for a boom that is bigger in cross section. But a better engineering way is to use ply for the sides of the box boom and solid timber at the top and bottom where the load is greatest. So I am not saying it is completely bad.

The box section out of timber is not too much harder to make and lighter.

The round masts are prettier but probably heavier than the ply mast you suggest.

The birdsmouth are a lot more complication, but pretty and reasonably light.

Best wishes
Michael

[marshden]

Well, it seems as if things have become more complex than slapping pieces of wood together to form a box. Don't take it out on Jones, however, I could have missed something important that I should have gotten rather than the other way around.

Anyway, the boat is 15 ft long and weighs about 300 lbs. Max load is 4 people. Mik, I appreciate your running the information through the spread sheet. Best, Marshall.

[Boatmik]

What is the measurement from the partner to the top of the sail. Doesn't need to be super accurate ... just within a couple of inches (to the closest 50mm)

MIK

[marshden]

Hi, Mik,

The beam of the boat is 5'6" (just in case you needed that).

The main: 15'4 1/2"
The mizzen: 13' 10"

Marshall