Coming soon: An addition to the Northeastern GIS herd.

[Sumbloak]

Ok then, let's do this properly. You know what safety factors you allowed in the original mast design, which I assume was intended to be built in solid oregon or similar, of reasonable but not perfect quality (correct me if I'm wrong). From that it's simple enough to work out the required diameter in any other timber, including just paulownia if anyone wants to try it. Also, from what I recall from Skenes or somewhere similar, a 25% wall thickness is safe for hollow, unstayed spars (assuming overall diameter is suitably chosen).

So, all anyone needs to know is what you allowed for with the original mast.

[Beaton2]

I'd expect a two layer lamination of quartered fir and flat sawn paulownia to cup as well as bow and in the process maybe twist some too. In my experience this can happen while the glue up is in the clamps. While the mast pieces are flexy and a pain to deal with, if properly milled they should be pretty flat. Making a three layer lamination is only one more piece to mill. The pieces are thin to begin with so a lot of clamps and cauls would be required to assemble. I think the other methods you describe are better solutions. But talking about laminations, the booms are being made out of plywood, the mast could be made with less than half a sheet.

[Sumbloak]

Like I said, you can always try a test strip. Easiest way to find out what it will really do. Seems to me that if using epoxy, meaning a glue that doesn't affect the moisture content of the timber, then the strip will only change shape if something else causes the moisture content to change (like changing atmospheric conditions). If there is no change in moisture content, the timber wont move.

The paulownia could be quarter sawn anyway. Turning a flat sawn 8x2 into quarter sawn strips for a mast wouldn't be hard. However, the example video you linked showed an effect caused by longitudinal shrinkage of juvenile wood, which is something that will happen regardless of how it is sawn. Is paulownia susceptible to longitudinal shrinkage?

And the thing about three layers is that you are then putting the high modulus stuff on the inside, which is a waste if you are trying to optimise the weight.

Re plywood: it's a waste of time on a mast. The boom is loaded differently.

[Sumbloak]

Here's a thought: if you want to try a two layer strip but don't want to worry about it bending, just make a slightly undersized paulownia birdsmouth mast, then stick the oregon to the outside of that before rounding it over. Can't warp then, and sticking the extra strips on could be done with a bunch of hose clamps.

[Alan Cohen]

The paulownia could be quarter sawn anyway. Turning a flat sawn 8x2 into quarter sawn strips for a mast wouldn't be hard. However, the example video you linked showed an effect caused by longitudinal shrinkage of juvenile wood, which is something that will happen regardless of how it is sawn. Is paulownia susceptible to longitudinal shrinkage?

And the thing about three layers is that you are then putting the high modulus stuff on the inside, which is a waste if you are trying to optimise the weight.

Re plywood: it's a waste of time on a mast. The boom is loaded differently.

The flat sawn paulownia I have is 2" thick. If I rip 1/4" (or whatever) slices it will be quartersawn.

I understand about an inside layer of glass not affecting the stiffness of each individual strake sandwich, but when glued up as a hollow box mast, the glass will form its own box and will actually be an outer layer to the paulownia box. This should make the paulownia component much stiffer, yet not affect the fir.

Here is some shrinkage info on paulownia:

Shrinkage Coefficient

Species	*Specific Gravity	Radial	Tangential
Cottonwood	0.34	3.0	7.0
NW Cedar	0.31	2.2	4.9
E. W. Pine	0.35	2.1	6.1
Redwood	0.35-0.40	2.2	4.9
Paulownia	0.21-0.27	1.1-2.8	2.1-4.9

* Based on OD weight & 12% MC volume

[Alan Cohen]

A more detailed page on the properties of Paulownia:

http://PAULOWNIASEED.COM/Paulownia_P...roperties.html

[Beaton2]

I've broken the rules on wood movement on projects and have regretted it. My advice is to not break them. Tests would be sensible. I see a lot of links on a google search for plywood masts being made, from the early 20th century until the present. Maybe it's not suitable for a goat. I don't know, I'm no sailor or boat builder.

[Sumbloak]

I understand about an inside layer of glass not affecting the stiffness of each individual strake sandwich, but when glued up as a hollow box mast, the glass will form its own box and will actually be an outer layer to the paulownia box. This should make the paulownia component much stiffer, yet not affect the fir.

Illogical. You could just as well say it will make the fir component slightly stiffer but not affect the paulownia.

Glass would only be worth using if it was unidirectional, and then the best place for it would still be on the outside of the mast.

[Sumbloak]

I've broken the rules on wood movement on projects and have regretted it. My advice is to not break them. Tests would be sensible. I see a lot of links on a google search for plywood masts being made, from the early 20th century until the present. Maybe it's not suitable for a goat. I don't know, I'm no sailor or boat builder.

You can find links on ferro-cement boats being built too. That doesn't mean they're the best for strength to weight ratio.

The problem with plywood for masts is that half the grain is running horizontally, which means it contributes almost nothing to tensile strength, as well as the compression strength across the grain being lower than along the grain (can't remember how much offhand). This means you are wasting weight for the same strength and stiffness. The only time plywood works well for masts is if it is very light plywood used for things like the fairing on a wing mast, where you want a stable but light panel over a sizable span.

If you really want a light spar, I'd be looking at a bit of carbon. IIRC the Gougeon's had good results using a mix of 92% timber and 8% carbon in laminates. Unidirectional carbon tape isn't that expensive.

[Alan Cohen]

Illogical. You could just as well say it will make the fir component slightly stiffer but not affect the paulownia.

Glass would only be worth using if it was unidirectional, and then the best place for it would still be on the outside of the mast.

So let's say I built a hollow box mast out of paulownia and glassed the outside, or even used unidirectional carbon fiber. Wouldn't it be stiffer? All I'm adding is another layer of fir on the outside of the glass. I would think the glass sandwich would have to be stiffer than just the paulownia and fir alone, no?

[Sumbloak]

Yes, but it's still not as effective as it would be on the outside. The basic principle is to put the highest modulus material as far as possible from the neutral axis if you want to maximise strength and stiffness for a given weight.

To put it another way, if you made the same mast without the glass, worked out the weight of the glass, then got a thin hardwood veneer (purpleheart? ) of the same weight and laminated that to the outside of the mast, it would be stiffer and stronger than the same mast with glass between the fir and paulownia.

[Sumbloak]

Ok, here's an idea just for fun. I dug around in this board a bit and found out that the stock Goat mast is apparently a hollow box section. It figures Mik would make things easy, but I wasn't previously aware of this fact since I don't have a Goat handy and am mainly interested in this stuff in case I build myself a lug rig at some point.

Anyway, if you are happy with the aesthetics of a basic box section but want to go lighter, what you could do is build it out of paulownia but with smallish square section purpleheart strips up each corner of the box. This is the furthest you can get from the neutral axis, so is good engineering, and would also help provide added dent and wear resistance to the corners, which is something paulownia wont be so good at.

Obviously you wouldn't have to use purpleheart. I just said that for fun. The general idea is basically sound though.

[davlafont]

...what you could do is build it out of paulownia but with smallish square section purpleheart strips up each corner of the box.
...
Obviously you wouldn't have to use purpleheart. I just said that for fun. The general idea is basically sound though.

That does sound intriguing. I would suggest a tongue-and-groove or rabbet or other joinery between these purple corners and the staves. At 16 feet, it would be tough to keep the pieces lined up while the goop is wet.

If *I* had purple heart, that what *I* would do... [/instigation]

[Sumbloak]

I was thinking, as far as I had thought it through, that the go would be to take your two full width staves and rebate the outer corners of them to take the extra strips, then glue the strips into the rebates. I'd even be inclined to do this first, just so there was less goop and fewer things trying to escape when gluing up the whole mast. Would have to make for a neater and easier job IMO.

[MiddleAgesMan]

.... I dug around in this board a bit and found out that the stock Goat mast is apparently a hollow box section....

The "stock" mast is a box section?

That's news to me.

The original masts were all solid rounds, tapered from around the middle to the top, 3 1/2 inch at the bottom and something around 2 inches at the top.

But I guess MIK could have changed the "stock" mast without my knowledge.