PVC for spinnaker pole

Has anyone here ever used or considered using PVC for a spinnaker pole? I'll try anything once. Well, almost.

_________________

Happy Sailing,

David

I see that ending poorly. But, most of the load is taken up by the lines from the end of the pole, so who knows. Might work. I'd be shocked if it survived a big stuffing though.

I thought about that, Karl. Out of curiousity, I went to Lowe's to research the idea. I discovered that the plumbing PVC (black) is a lot thicker (2 to 3 X) than the irrigation (white) PVC. I'm adding a spi system in which I already have an aluminum pole but just wanted to give the PVC a shot. I'll let you know how it goes. If you don't hear from me again, you know what happen.

_________________

Happy Sailing,

David

I would not drill any holes in it. Maybe for where it connects to the front crossbar, but everything else I'd say that'd be a no no.

Intuitively, I don't think this will work. PVC is just too flexible.

/dredges up knowledge from college engineering courses . .


Here are the physical properties of rigid grade PVC:
Tensile Strength, Ultimate 29.0 - 48.3 MPa 4210 - 7000 psi Average value: 41.6 MPa
Tensile Strength, Yield 31.0 - 55.0 MPa 4500 - 7980 psi Average value: 43.3 MPa
Elongation at Break 20.0 - 230 % 20.0 - 230 % Average value: 119 %
Elongation at Yield 5.00 - 6.00 % 5.00 - 6.00 % Average value: 5.46 %
Modulus of Elasticity 2.34 - 3.10 GPa 340 - 450 ksi Average value: 2.67 GPa
Flexural Modulus 2.28 - 2.96 GPa 331 - 429 ksi Average value: 2.52 GPa

Here are the physical properties of 6061T6 Aluminum:
Ultimate Tensile Strength 310 MPa 45,000 psi AA; Typical
Tensile Yield Strength 276 MPa 40,000 psi AA; Typical
Elongation at Break 12.0 % @Thickness 1.59 mm 12.0 % @Thickness 0.0625 in AA; Typical
17.0 % @Diameter 12.7 mm 17.0 % @Diameter 0.500 in AA; Typical
Modulus of Elasticity 68.9 GPa 10000 ksi AA; Typical; Average of tension and compression. Compression modulus is about 2% greater than tensile modulus.
Notched Tensile Strength 324 MPa 47000 psi 2.5 cm width x 0.16 cm thick side-notched specimen, Kt = 17.
Ultimate Bearing Strength 607 MPa 88000 psi Edge distance/pin diameter = 2.0
Bearing Yield Strength 386 MPa 56000 psi Edge distance/pin diameter = 2.0
Poissons Ratio 0.330 0.330 Estimated from trends in similar Al alloys.
Fatigue Strength 96.5 MPa
@# of Cycles 5.00e+8 14000 psi
@# of Cycles 5.00e+8 completely reversed stress; RR Moore machine/specimen
Fracture Toughness 29.0 MPa-m½ 26.4 ksi-in½ KIC; TL orientation.
Machinability 50 % 50 % 0-100 Scale of Aluminum Alloys
Shear Modulus 26.0 GPa 3770 ksi Estimated from similar Al alloys.
Shear Strength 207 MPa 30000 psi

Aluminum is roughly ten times stronger than PVC, although PVC is considerably more flexible and will bend much further before it breaks.

You could design a PVC pipe that has the same bending properties of an aluminum tube. But it would have to have a much higher second moment of area to compensate for the difference in tensile strength. You can calculate that for a tube with the following formula:

Where Do is the outside diameter, Di is the inside diameter; ro is the outside radius, ri is the inside radius.

Then calculate the stress:

Where y = the perpendicular distance to the centroid of the cross-sectional area - roughly ri+(ro-ri)
M = the bending moment
Ix = the second moment of area about x-axis

Translation - bigger diameter, thicker walls.

OK, I'll go away now.

Sprechen Sie Englisch?
¿Hablas inglés?
Parlez-vous anglais?
Czy mówisz po angielsku?
Quack?

No Matt, don't go away. You either Karl. These are the types of responses I was looking for. Of
course, you know I'm gonna do it anyway but I need voices of reason to tell me why I shouldn't.

You see, my thinking is like the design of a large radio tower. For all practical purposes, the diagonal
support channel between the columns could be made out of cable as the required strength of this item
would be in the retractive capacity. Of course, that wouldn't look very strong so something that
appears to be more solid is put in it's place.

I read Matt's technical data and he's absolutely right. However, I believe that the aluminum pole is
over-kill for the job for which it is designed. On my boat, the distance from the end of the spi pole to
the point of it's closest point of support is approximately 4' (4 feet). Cables are attached to the end
of the spi pole which are anchored at each bow tang. It is at these two points that nearly all of the
tension placed on the spi pole will be disbursed. In addition to the negative bend in the pole and it's nearest
point of primary support being 4 feet from the end, I think PVC w/1/4 inch walls is adequate for serving
it's purpose as a spi pole. I jumped up & down on a 4 foot length of PVC and it didn't even look like it
thought about breaking, although it did bend a lot. But remember, the center of the 4 foot length is not
where the pressure will be applied. It will be on the anchored end.

Well, is this gonna work???? I have no idea but I'll never know til all the elements are placed against it. You guys are probably right. Like I told Karl,...if you don't hear from me again, you know what happened. More input please........

_________________

Happy Sailing,

David

I'd never do it. PVC is brittle; and the last thing I'd want is to be facing the jagged end of that pole as I fly forward in a pitchpole. BTW the loading on the spinnaker pole is buckling, not bending.

_________________
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Like I said, don't drill any holes in it. You'll have to use an internal tack line. I'd make a cap from UMHW, and have that slip into the end of the pole. Drill two holes in the cap near the edges for the bridle to go into it. The pressure pulling backwards on the pole will hold it in there. If you've got a combination line like I've got there's a block mounted on the pole, that'll have to be moved to the front beam anyway.

-Tape your snuffer on.
-The line to prebend the pole will just have to wrap underneath the pole and go back up to the forestay for the mast.
-You'll probably want to make a 2nd bridle that is in line with the bridle for the mast, but going from the tang to the pole.

I agree with AntonLargiader, PVC explodes when you put too much pressure on it. Your biggest worry will probably be compression, in which case it'll probably shatter leaving nice jagged shards hanging out there to impale you, or worse, your spinnaker.

I disagree on both points. PVC has a much higher elongation at break (as much as 230%) than aluminum (12%). That means it pulls like taffy before it finally breaks. Aluminum will leave a jagged edge as well. In either situation, I don't envision them breaking immediately - they'd fail catastrophically by bending too far and collapsing the tube. You'd be able to see this happening as the spinnaker loaded up and have enough time to blow the sheet / halyard before things got too pear-shaped.

The pole is a beam, albeit one that expereiences forces from more than two directions (up / down). You'd have to model it by breaking down the forces in the x / y / z axes using vector diagrams / trig / calculus or an FEM program. It's an interesting engineering problem, but I'm far too removed from the classes I had in college (almost 30 years) to analyze it numerically. Much easier on the brain to just do it emperically. Just watch the deflections, Dave!

Don't waste time or money, the load is compression towards the beam. Once the bendy plastic bounces our of colum, the pole will bend and collapse. I used a carbon fiber windsurfing mast on an oversized chute (masthead on a H-20), it worked great on the beach, once sailing and loaded, the flex top mast (Ampro Red) did just that. My alert crew sheeted out and it snapped back to straight but tried to break again once sheeted.

That's essentially what I was saying when I mentioned the 3 axis analysis. The PVC isn't stiff enough to remain in column to withstand the compression.

Yeah, I saw that Matt. Thanks Hammond. Unless there was a point of support perpendicular to the end of the pole, there might be a problem. There may be a problem regardless. Hmmmmmmm.. You know I've got questions. How flexible was your carbon fiber windsurfing mast and how oversize was your spi? You did say "Masthead." That statement definitely took a little wind out of my sails (no pun intended) but I'm still curious. Just looking for inconsistencies but I don't want a system that I'm going to have to baby-sit. I'll already have enough things I'll have to focus on. Beam load, huh??? Hmmmm. Thanks guys.

_________________

Happy Sailing,

David

A decent carbon windsurfing mast is going to be waaaay stiffer than an equivalent diameter piece of PVC. Not to mention a heck of a lot lighter too.

If I were you, I'd drop the PVC spin pole idea and pick up a used windsurfing mast. Probably around $100 or less. Just trim excess length from the top of the mast rather than the bottom. That way your tack hardware is mounted on a stiffer section of the pole. "FlexTop" windsurf masts will be very thin and bendy for the first several feet and then stiff the rest of their length. "Constant Curve" masts will have a more consistent bend characteristic over their length.

sm

Perhaps a solid pvc pole could take the load. I know on my boats if the spin pole is not secured in the center it will really deflect. I would assume even 4' of plastic tube would deflect under the spin load and once it flexes off it will loose its limited rigidity. I would think there is little weight savings with plastic vs aluminum poles and other than eliminating corrosion what will be the gain?