Solid Masts

[Mikthestik]

In the old days 1780's large masts were made from tree's cut like cheese wedges held together with rope wolding. Now Aluminium is very popular if expensive. I like the idea of unstayed wooden masts, solid or otherwise. This presents a number of problems, a 45ft mast could easily be 12" diameter. Laminating spruce strips is the obvious answer. Even if you can only get 12 foot lengths by staggering the joints almost any length is possible. You could even make ply wood masts and spars. My formula for mast diameter says reduce by 10% for solid masts. My guess is a solid plywood mast would be as strong as a hollow spruce mast of the same diameter. Any thoughts on the subject. Mik
The idea is for a cheap easy minimalist mast for a minimalist boat.

 

[bregalad]

Plywood puts half the grain running in the wrong direction. Not the best choice.

The unstayed mainmast for my Bolger Romp was clear Douglas Fir, 42' long. 7-1/2" heavily rounded square with 2-1/4" wall thickness at the partners. Tapered to about 2-1/2" or 3" with 3/4" - 7/8" wall thickness.

Often used aluminum masts can be found very inexpensively.
My 25' sharpie had a ~26' beautifully tapered Proctor mast. It had once been the top of the mast on a 70-something foot boat that tried to go through a bridge in Ft. Lauderdale that hadn't opened. I think we paid the insurance company $50.

 

[gonzo]

An aluminum mast will be much cheaper than a good quality spruce mast. Price the materials before making those claims.

 

[Steve W]

Lots of used aluminum masts available for not a lot of money.

 

[Stumble]

There is almost nothing as silly as a solid mast no matter the material. And as mentioned don't under estimate the cost of spar quality timber, it is by no means cheap.

 

[Steve W]

nothing silly about it, for some boats a solid wood spar is perfectly appropriate. A friend of mine has an Egret sharpie that is gaff schooner rigged, no "spar quality timber" involved if you are referring to vg sitka spruce. He built his masts out of New Zealand grown radiata pine from a local big box lumber yard here in minnesota, beautiful stuff that they sell as select boards which he glued up to make his solid spars. These are free standing masts that in his 70s he can step by himself without the use of tabernacles or crane of any kind, he just walks them up and drops them in the hole, to unstep he just lifts them out and throws them in the water and then pulls them in with the halyard. Sure he could have made birdsmouth spars that would be lighter but his solid ones are only about 55lbs if I remember right and were a hell of a lot easier to make, we did weigh them on a bathroom scale. What else would be more appropriate?

 

[PAR]

I disagree in that there's an appropriate use for solid spars. Given modern build methods, there's absolutely no reason, to suffer the disadvantages of a solid spar.

Plywood is a lousy spar material, unless it's being used in a wing spar, where it's a stressed monocoque structure.

Birdsmouth is one method, but there are others and that 55 pound spar, could have easily been 25 pounds or less if hollow, depending on built method and species used.

Lastly, if electing to use the old school mast building methods, do it they way they did back then and quarter the tree, then rotate each section 180 degrees, before banding them back together. This places the growth rings in opposition and makes the spar bigger, stronger and stiffer with the same amount of material.

 

[gggGuest]

I disagree in that there's an appropriate use for solid spars. Given modern build methods, there's absolutely no reason, to suffer the disadvantages of a solid spar.

Well, on a small enough boat the weight disadvantages may not outweigh the extra work of fabricating hollow spars, but for my money that balance probably runs out when spars get to around 15 feet long.

But SteveW is right, hard to beat the cost point of a reasonable second hand alloy spar. The tubes tend to outlive boats, so are readily available...

 

[Steve W]

As I said, he could have built hollow spars using the birdsmouth method or others BUT with considerably more effort and the point is they meet HIS SOR just fine (the only one that matters)and have proven themselves over many years. Im sure they could be under 20lbs if he had made them out of carbon too but that would not be appropriate for an inexpensive old plywood sharpie. Others SOR may be different, we put carbon spars on a Mud Hen a few years ago for a customer so each to his own. I personally would not have solid spars either but don't dismiss those who choose to if it works for them.

 

[Mikthestik]

Well I didn't know they rotated the tree. I do know that 50% of the grain runs the wrong way in ply. However I guess a boat requiring half inch would be just as strong as one built in 3/4 ply, a proper engineering analysis would be required to prove or disprove that statement. If the same boat were built in cold molded style with two 1/4 layers it would I think be quite close to 1/2 inch planks in strength. My idea of a cheap mast which could be home built. is made from strips of ply layed out as my drawing with the grain at 90 degrees. Who knows which grains will be in the wrong direction.. The problem remains a proper engineering analysis would be required to check the mast strength. With a hollow mast the two center strips are not needed. Mik

 

[Stumble]

Well, on a small enough boat the weight disadvantages may not outweigh the extra work of fabricating hollow spars, but for my money that balance probably runs out when spars get to around 15 feet long.

But SteveW is right, hard to beat the cost point of a reasonable second hand alloy spar. The tubes tend to outlive boats, so are readily available...

On a boat this small aluminium tubing is even easier and weighs less.

 

[gggGuest]

On a boat this small aluminium tubing is even easier and weighs less.

Well I have used standard grade industrial anodised tubing for a boom, and while its gloriously cheap, proper spar grade alloy is heat treated to be much stiffer for a given weight.
So actually if you're using straightforward commercial tubing its going to end up rather heavier than second hand spars, provided of course at the small end of the range you can find the secondhand spars in a small enough size. Not a problem in the UK with umpteen thousand Toppers about, all prone to bending the ends of the tubes...

 

[serow]

i think it should be quite easy to make a simple wooden mast for a cheapo boat. Does it have to be round and/or tapered?A rectangular box section glued and screwed with the edges generously faired off would suffice. As it's unstayed the mast can be infilled for a distance either side of where it is restrained at the deck to increase the strength and also provide some additional rigidity against crippling.
I see local buckling as a big risk in getting a scrap ally mast and sticking it through a hole in the deck, some form of additional strengthening may be needed.

 

[PAR]

Plywood is only 2/3's as stiff as solid wood longitudinally, so it needs to be 1/3rd thicker. Plywood is also heavier per unit of strength/stiffness, so increasing its thickness to compensate for natural deficiencies, isn't the best approuch from an engineering standpoint. This isn't debatable, it's the nature of plywood.

I compete with aluminum spar makers, using wooden ones. Up to about 20' most use 6061 T-6 tubing and I can kick these sticks butt by about 10% in wood, but this is about the crossover line, where tubing and of course a real extrusion starts to beat wood, but only in certain applications. In well stayed rigs, aluminum beats me, but in particularly bendy rigs, I got it 'till over 26' - 28'. This is because tubing wall thickness and diameter doesn't change, though some using two or three sections do step down. Once carbon enters the picture, it's game over.

Mik, your stave layout can be greatly improved, from what you've shown, while still keeping it single lengths of stock glued together. Below shows the typical methods, though there are others. The top left is as simple as it gets, but is stronger for the material used. It's a 1x4 stave (3/4"x3 1/2") and will produce a 3.5" square or round stick. It's wall is overly thick, but it's simple. Right top is the same thing, except the staves are slightly notched to capture the next side. This makes a huge improvement in strength and stiffness, plus increases gluing area and makes assembly easier too. Lower left is the most common way and just as simple, but a lot lighter, but Herreshoff took it one step further (lower right) by notching the corner braces to remove even more weight, without sacrificing strength and stiffness. If you look at the colored centered areas, you can see how much weight is saved by making a hollow stick. The lower two show a 20% wall thickness, which is pretty common for a light cruiser. 15% would be a racer, while 25% would be a hefty cruiser. FWIW.

 

[Mikthestik]

Thank you Parr Your last post has answered all the questions I had on hollow masts. mik