Robi, the damage could have been done weeks or months ago, it just finally let go now. Did you ever have a crew hang on it, even for just a moment?

I agree that the damage has been or occurred weeks or months ago.

Why I agree with that statement?

Becuase I barely made any pressure on the pole, and it just snapped.
I am thankfull, that it did break on the beach and not out while sailing. The carnage could have been worst.

Really guys, that is is non-sense.

I hardly never refer to my myself as a fully schooled mechanical engineer, but I am. And that rivet and hole did not make weaken the pole by any significant amount. That pole is under normal sailing circumstances under compression, in that situation it is practicall impossile to snap it as is show in the pictures.

The break surface is smooth without any deformantion that would occur when it snapped under compression. This pole snapped under tension, which can only mean that the (initial) damage was fully caused by a loading situation unrelated to sailing or the spinnaker. Additionally crack growth can not happen under compression as the crack itself is closed by the compressional forces. If robi broke this pole with only a very small amount of force then a small crack must have been have present already AND the pole was broken in a way you tear a piece of paper. It is very hard to pull part apart when it is loaded in its plane, but by tearing it (skewed loads) it is alot more easier. This directly points to a misloading of the pole as again spi forces while sailing will never lead to unskewed forces. Most likely Robi could have sailed with this pole and have it survive even with the prior damage being there. The spi pole design is actually very well made from the spi loads point of view. Especially with the stabilizing lines from the bridle chainplates to the strut.

This pole was certainly broken (or initial damaged) by a force that was put on it outside of the upper vertical plane. Either a downward force or a horizontal force with a downward component.

The rivet hole, errornously refered to as weakening the pole, is of no consequence under spi loads as the compression forces in the pole do not allow a crack to develop around this hole. Therefor the hole is perfectly acceptable for sailing. Even when it is fitted under prebend. Metal fatique is not an issue either again because of the compression loads and the the fact that there are only very small fluctuations in the loading while sailing. Both of these work again crack growth with the last even delaying crack initiation to many many many fluctuations, properly in the order of 100.000's or millions of them. These will very easily make up 10 or more years of hard use.

The rivet hole is also insignificant because under spi loads the pole is not under bending at all. The tip support lines transform all possible bending into compression loads. Ergo the material around the full circumference of the pole is under loading and not just predominantly the part on the top and bottom as would be the case in bending. The crosssectional area of the whole is ONLY 5 % of the total crossectional area of the ring of material. This is downright negligiable.

However this whole situation changes when the pole is not loaded by the spi, but by a unrelated to sailing force that has a downward component when the spi pole tip is unsupported in that direction by tying the two halyards to eachother and tensioning them up. The support wires will now hang slack and all the load on the pole, with a very significant leverage, must be taking up by bending. It will be as if the pole it just stuck into a wall with 2 mtr sticking up and somebody is pushing down on its end. It will bend downward to same degree, as far as its elasticity will allow, and then bend permanently or break. Many of us use this principle when breaking and dividing a bar of chocolat on the corner of a table or bending waterpipes or steel garden wires.

I can assure everybody that a pole supported by a nacra ring will bend and break just as easily in this situation. So don't blame the rivet hole, it is really not the culprit

Wouter

I think Robi said he used the pole to lever the boat around when it's on the beach. So do I. My first instinct is to gob something up with epoxy and ram it up the pole to the broken spot in hope of preventing the break. How about a piece of pvc?

Pete,

Ever look inside your pole (or boom for that matter)? I think you'll find rivet casings protruding that will prevent you from "stuffing" anything up there that will do any good -- unless you first remove the rivets and re-insert.

The spi' pole shouldn't be very expensive to repair by using the SS collar or even a good machine shop could likely weld it together (not sure of strength)...worst case the spi' pole isn't the most expensive thing you can break. Ask Robi.

Guys,

You can LIFT the pole and wheel your boat around all day long. Just don't push the pole down--that's where it isn't adequately supported and will eventually break unless you change it out for a steel I-beam.

Looks like a fatigue failure to me.

If it failed due to a single overload, I'd expect the tube to cripple (buckle in) at the bottom. Imagine what would happen if you took a thin aluminim tube and bent it - the tube would squish and form a hinge, but you'd have a hard time tearing it. If you've got a cheap source of aluminum tube, you could do the experiment yourself.

If you're pole is pre-bent, it should be under tension at the top when you don't have the spin up. With the spin up the top goes into compression. That certainly would cause fatigue sooner or later depending on the magnitude of the stresses. Corrosion could exacerbate that as does a stress concentration point.

Once it has lost a lot of its strength due to fatigue, it'll be a lot more vulnerable to minor mishandling.

T, I did in fact look down the inside. Didn't see any insurmountable problems.

What an absolute crock!!! In general Wouter, I believe you speak a lot of sense but this is not one of those occasions.

In contrast to you I'm not a certified mechanical engineer but I've been involved in mechanical engineering manufacture for most of my 65 years on the planet and I can state with absolute certainty that the most significant load on the pole is a bending moment. There is a compression loading but this in itself is not significant, but because the pole is not 'in column' while under that load there is a significant bending moment being applied. This is a result of the compression load and the pole being lifted at the middle while the outer end is being pulled/held down causing it to bend and the point at which the bend changes direction, the point of maximum load, is at that saddle. As a result the, while the lower wall of the tube is being compressed, the top is in tension (being stretched) and under these circumstances the rivet hole has a significant effect as it reduces the area of tube wall at that critical point significantly making it considerably weaker.
While I may not be able to back it up with calcs, it's not rocket science, just simple logic. The pole is in essence a simple lever, loaded at the outer end, effort at the saddle, fulcrum at the beam.
Bern

<img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Simple experience tells me that you have a point Berny.....

T, I'm gonna put the tail end of my hotstick inside the tube. I'll keep you posted.

Unless it had a weakend starting point such as a normal drill hole with a torn perimeter.


There can only be a compression load if the rig has a solid connection between the pole and the bridle intersection.
In this case, the pole end will move up and down independantly which will cause it to fatigue at the intersection.

How many spi poles have prebend?

Note the pre-bent pole....

[quote
How many spi poles have prebend? [/quote]

The Blade, as can be seen in Robi's pic

uses a compression strut below the bridle to hang the pole on. The assembly manual spec's out 2" of pre-bend.

I'd still expect a ductile failure, not a rupture, from a single overload even with an ugly drill hole.

Without pre-bend Wouter is correct. In that case there would be negligible tensile stress in the pole. The upward force applied by the spinn should be counteracted by the lines that extend from the bows to the point where the tack of the spin is attached to the pole. Because the pole extends in front of the bows, those lines also exert an inward, compression force on the pole. Resisting that compressive force is the pole's main purpose. Essentially the system is a truss. It would be a pure truss if you had a hinge in the pole at the bridle connection.

With pre-bend the pole is also a cantilever beam between the bridle and the spin tack. Fatigue damage accumulates every time you rig the boat, and when the spin is unloaded (every dowse, gybe etc.).

IMO, fatigue damage is occurring continuously while sailing because of the ongoing, varying intensity of the halyard load and the inability of the lines that extend from the bows to the tack point to resist that load completely due to the acute angles of attachment.

I don't understand the logic behind making the connection between the pole and the bridle, a solid strut. This only exacerbates the problem as I see it.

Folks,

I think there is some other influence outside normal use to cause the failure.

On the prototype I have my saddle inside the pole as the finish is a bit cleaner once assembled. I always move my boat around with the wheels beneath the hull at the rear beam and the end of the pole as a handle.
I have also given the boat a bit of a hiding on the water with some pretty severe nose dives looking for its limits.
For a while the pole has had a bend in it where I hit in one capsize.
Take a look at the holes in my pole. They have not caused any poblem.

Regards,
Phill

assembled

I see the your secret Phill. It's the relationship between the ant and the crawdad. <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

You just have your certification revoked my dear Berny. Look more closely at the total setup of the pole including the bridle strut and mid pole support lines.

The pole is indeed under small bending stresses due to the small amount of prebend, but these are not significantly compared to the other loading.





You are talking about a buckling failure mode here, HOWEVER the pole at the saddle is supported in all directions but the bridle strut, pole itself and the mid pole support wires. Ergo the compression forces put on the pole by the spinnaker are NOT taken up by the pole in bending but by compression loading in the bridle strut and tension loading in the midpole support wires.

The other poster said it right. The setup is essentially a truss setup with a slight prebend in the pole to force to remove any slack and force it to try to bend more against the bridle compression strut. 2 inches of prebend is really not allot of prebend (low stresses) on a pole of overall 3.50 mtr. length.

It is however not a cantilever suetup under spi load. It will be when the owner tries to handle the boat and pushes down on the pole. For that reason it is much more weak in that role, one for which the pole was never designed.




True to some extend but you are still wrong. Buckling failures and truss setups are not commonly understood not even by many engineers. Sorry.

Wouter